Encyclopaedia of petroleum science and engineering, Volume 12

ENCYCLOPAEDIA OF PETROLEUM SCIENCE AND ENGINEERING (VOL. 12) S.L. Sah

KALPAZ PUBLICATIONS

ENCYCLOPAEDIA OF PETROLEUM SCIENCE AND ENGINEERING

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ENCYCLOPAEDIA OF PETROLEUM SCIENCE AND ENGINEERING (VOL. 12)

S.L. Sah

lH

KALPAZ PUBLICATIONS DELHI·110052

Encyclopaedia of Petroleum Science and Engineering

@S.L. Sah

ISBN :978-81-7835-618-1

All rights reserved. No Part of this book may be reproduced in any manner without written permission. Published in 2007 in India by Kalpaz Publications C-30, Satyawati Nagar, Delhi-110052 E-mail: [email protected] Phone: 9212729499 Lasser Type Setting by: Quick Media, Delhi Printed at : Salasar Imaging System, Delhi

Dedicated to the Geophysicists, Geologists, Engineers, Scientists, Universities, Organisations, Teachers, Students, and other working in different disciplines ofpetroleum science and engineering

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"Help us to harness the wind, the water, the sun, and all the ready and renewable sources of power. Teach us to conserve, preserve, use wisely the blessed treasures of our wealth-stored earth. Help us to share your bounty, not waste it, or pervert it into peril for our children or our neighbours in other nations. You, who are life and energy and blessing, teach us to revere and respect your tender world" Prayers of Thomas J. Carlisle

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(CONTENTS) Preface

11

1. Seismic Earth Modeling

15

2. Seismic Earth Imaging

78

3. Three Dimensional (3-D) Seismic Exploration, Processing and Interpretation

81

Appencfuc.-A : Utilization of Natural Gas for Power Generation

210

Appendix-B : Environment and Pollution Control

209

Appendix-C : Ozone Depletion and Greenhouse Effect

217

Appendix-D : Role of Electronics in the Industrial Growth

221

Appendix-E : World's Famous Quotations

231

Appendix-F : News in Focus

239

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Preface "Energy and persistence conquer all things. " -Benjamin Franklin India, usA, China, Japan and South Korea have decided to make common cause as the world's major buyers of oil. They make up half the oil conswnption of80 million barrels per day (BPD), while the USA alone accounts for a fourth of world oil consumption. Their coming together, augurs well for the world economy, which could hit a speed-breaker if prices continue to be unstable. This cartel of buyers can coordinate placement of large orders as well as futures contracts to check against price fluctuation. It could also reduce the impact of USA politics in the Persian Gulf on oil prices. At present, non-OPEC producers account for 60 per cent of total output, as OPEC produces below potential for a variety of reasons. When prices are driven up by rising demand as well as political instability, OPEC tries to work this situation to its benefit by reining in output, taking advantage of the inelastic nature of demand for oil. Such cutbacks are not motivated by economics alone; there is also resource nationalism on the part of oil-producing states. OPEC world have uppermost on its mind that the share of imports in USA oil conswnption, now 50 per cent, is on the rise. Oil transactions may lose some of their political edge, once the USA enters the market as part of a team of buyers who deal with OPEC countries on a different political footing. USA 'geopolitics in the Gulf region is, in part, motivated by a desire to keep oil prices low and stable. It has not been very successful in its efforts. India, China, Japan and South Korea should collectively persuade the USA to be less of a bull in a China shop in the Gulf region, in order to leverage maxinwm results as a buyers' cartel. World oil consumption is expected to rise by 38 million BPD between 2006 and 2030 to 118 million BPD, with non-OECD Asia accounting for 43 per cent of this rise. World oil conswnption rose by 1.2 million BPD in 2005, of which the rise in OECD consumption was just 0.1 million BPD. The transport sector is expected to account for half of the future increase in oil consumption. India would have to look at high oil prices and political instability in

12

Encyclopaedia of Petroleum Science and Engineering

West Asia as a long-tenn variable in economic and financial management. Oil imports have been growing at 45 to 50 per cent over the last two years, rising from $29 billion in 2004-05 to $43 billion in 2005-06. At present, our remittances, capital flows and reserves can comfortably take case of a current account deficit of about 3 per cent of our gross domestic product, or aBout $20 billion. Foreign exchange reserves are now at $160 billion, about $40 billion higher than annual imports. While this looks like a comfortable sum today, reserves are growing only at 10 percent per annum, while our imports are growing at 30 per cent in a year. Exports could grow at 15 to 30 per cent, depending on conditions and shocks in the world economy. The global economy grew by 5.1 per cent in 2004 and 4.5 per cent in 2005, despite ·the rise in oil prices. Our reserves act not only as a cushion against uncertainty but also as agents of longtenn growth to the extent that they can absorb a large current account deficit. Part-one of this encyclopaedia gives about the seismic earth modeling. Seismic modeling involves generating travel times and amplitudes of seismic wave propagating through a specified subsurface reflectivity model that is associated with a specified velocity-depth model. It can be done either in a computer or in a laboratory. In theory, any migration algorithm can be driven in the opposite direction to perfonn modeling. In particular, we can think of migration and modeling as extrapolations in depth and time, respectively. Just as there are several approaches to solving the wave equation for migration, there also are several types of modeling techniques. There are modeling techniques based on the Kirchhoff integral, finite-difference, and f-k domain solutions to the wave equation. The algorithms based on the scalar (acoustic) wave equation, which describes P-wave propagation, are suitable for structural mode)ing in which amplitudes are not as important as travel-times. The algorithms based on the elastic wave equation, which describes both P-wave and S-wave propagation, are suitable for detailed stratigraphic modeling in which amplitudes are as important as traveltimes. Modeling based on one-way wave equations does not include multiples, while modeling based on full wave equations include multiples in the solution. Part-two of this encyclopaedia gives about the seismic earth imaging. Strong lateral ·velocity variations associated with complex overburden structures require earth imaging in depth. Earth imaging in depth is

Preface

13

achieved by depth migration. Only depth migration algorit:lum implement the additional thin-lens term that explicitly accountS for lateral velocity variations. A velocity-depth' model is the seismic representation of an earth model in depth. An earth model and the earth image created from it are an inseparable pair of products of seismic inversion. To obtain an earth image in depth, one has to first estimate an accurate earth model in depth. Aside from earth modeling and imaging in depth, depth migration also used to verify and update velocity depth models. Depth migration is used in an iterative way. 3-D prestack depth migration often is done using the Kirchhoff integral or the eikonal equation. The output of prestack depth migration consists of image gathers, which may be linked to moveout-corrected CMP gathers with the vertical axis in depth. Image gathers consist of traces iri, their migrated positions. A stack of image gathers represents the earth image in depth obtained from prestack depth migration. If the velocity-depth model used in prestack depth migration is correct, then, events on an image gather would exhibit a flat character with no moveout. Part-three of this encyclopaedia gives about the three dimensional (3-D) seismic exploration, processing, and interpretation. Subsurface geological features of interest in hydrocarbon surveys are three dimensional in nature. A 2-D seismic section is a cross-section 9f a 3-D seismic response. 3-D migration of 3-D data provides an adequate and detailed 3-D image of the subsurfllce, leading to a more reliable' interpretation. A typical marine 3-D survey is carried out by shooting .closely spaced parallel lines (line shooting). A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to each other and placing the shotpoints in the perpendicular direction (swath shooting). The direction that is perpendicular to the in-line direction in a 3-D survey is called the cross-line direction. The line spacing in 3-D surveys can be 50 m or less. This dense coverage requires an accUrate knowledge of shot and receiver locations. The area extent of a 3-D survey almost always is larger than the areal extent of the object. In 3-D data processing, traces are collected as common-cell gathers (bins). After stacking, the 3-D data volume is often, but not always, migrated in two stages. The 3-D data volume then is available to the interpreter as vertical sections in both the in-line and cross-line directions and as horizontal sections (time slices). The time slices allow the interpreter to generate contour maps for marker horizons. The interactive environment

14

Encyclopaedia of Petroleum Science and Engineering

provides an effective and efficient means for interpretation of the sheer volUme of 3-D migrated seismic data. The aim of this encyclopaedia is to make the interconnections among the different disciplines ofpetrolewn science and engineering like: seismic earth modeling, seismic earth imaging, and three dimensional (3-D) seismic exploration, processing and interpretation. At the end of this encyclopaedia six appendixes have been included. These appendixes will give more information to readers about the petrolewn science and engineering. This encyclopaedia will help to promote understanding and conmnmication among users. It is suitable for geophysicists, geologists, scientists, universities, organizations, teachers, students and other working in different disciplines of petrolewn science and engineering. The another will be grateful for COmnlents and criticism which might help to improve the later edition of this encyclopaedia. Some of the material of this encyclopaedia has been taken from the books and the papers published in different journals. I am thankful to all of them who have contributed to the development of this encyclopaedia.

October, 2006

SL.Sab

1 Seismic Earth Modeling Introduction Seismic representation of an earth model in depth usually is described by two sets of parameters : (I) layer velocities, and (2) reflector geometries. Depth migration is the ultimate tool for delineation of reflector geometries. If layer velocities are determined accurately, reflector geometries can be recovered by iterative depth migration. Difficulties in estimating layer velocities with a required level of accuracy make the earth model estimation a challenging task for the geophysicist. Nearly all of the practical methods of layer velocity estimation are based on ray theory, and more specifically, on inversion of seismic travel-times. Velocity estimation methods include Dix conversion of rms velocities, inversion of stacking velocities, coherency inversion, and analysis of image gathers from prestack depth migration. Velocity variations within the earth may be characterized in two ways: (I) structure-dependent, and (2) structureindependent. A structure-dependent earth model comprises geological formations with interfaces that coincide with distinct velocity contrasts. We encounter structure-dependent earth models in areas with extensional and compressional tectonics, and especially in areas with salt and overthrust tectonics. A structure-dependent earth model usually req~ires a layer-by-Iayer escimation oflayer velocities and delineation of reflector geometries that coincide with the layer boundaries themselves. A structure-independent earth model comprises geological formations with interfaces that do not necessarily coincide with distinct velocity contrast.

DetaDed Encyclopaedia This encyclopaedia is arranged in alphabetical order. The detailed encyclopaedia is given below :-

16

Encyclopaedia of Petroleum Science and Engineering

Calibration to WeD Tops The depth structure maps derived from time-to-depth conversion or layer-by-Iayer inversion invariably will not match the well tops. The sources of discrepancy between the estimated reflector depths and the well tops include limitations in the methods for interval velocity estimation, mispicking of time horizons inputs to depth conversion, and limitations in the actual depth conversion itself within the context of ray tracing through an earth model that includes complex layer boundaries. For the depth structure maps to be usable in subsequent reservoir modeling and sinmlation, it is imperative to calibrate them to well tops. Consider a seismically derived depth structure map z, (x, y) based on time-to-depth conversion, say, for the layer boundary associated with the top-reservoir. Also consider Nw well tops Zw (xi' Y) for this horizon at locations (xp Yj)' i = 1,2, .... , N", Since the velocity-depth model derived from time-to-depth conversion is supposed to be consistent with the input data, i.e.. the time structure map T (x, y) created from the interpretation or'the time migrated volume of data, we have T(X,y) = 2

(x,y) Vs(x,Y)

Zs

...(1)

Where Vs (x, y) can be either the average or rms velocity map associated with the horizon Zs (x, y). We consider vertical rays rather than image rays as in equation (1). There exists a calibration velocity Vc (x, y) such that, at a well location (x,., Yj) it satisfies the relation given by the following equation :T

_ 2 Zw (Xj,Yj) (x.,y.) V.( ) , , c Xj,Yj

...(2)

Combine equations (1) and (2) we get the following equation:-

Vc (Xj,Yj) Vs(x;,yj)

(Xj,Yj) z,(Xj,Yj)

Zw

From the knowledge of the well top derived

Zw

...(3)

(x,., Yj) and the seismically

reflector depths Zs (x,., Yj) at the well location, equation (3) gives a calibration factor c(x" Yj) = Vc (x,. y)IVs (xi' Yj) computed at each of the well locations. Next, apply kriging or some other interpolation technique to the sparsely defined calibration factors c(xi' Yj)' i = 1,2,...... , N w to derive a

17

Seismic Earth Modeling

cahbration factor map c(x, y) specified at all grid locations (x, y). Kriging is a statistical method of determining the best estimate for an unknown quality such as c(x, y) at some location (x, y) using a sparse set of values such as c(x y;) specified at locations (xi' y;) (Sheriff, 1991). The fmal " step in calibration is to scale the depth structure map Zs (x, y) the calibration factor map c(x,y) given by the following equation:-

ze(x, y)

=

c(x, y) ZS' (x, y)

...(4)

Where zc(x, y) is the calibrated depth structure map. From equations (3) and (4), the calibrated depth Zc coincides with the well top Zw at well location (x,,, y;). Calibration to well tops is done only after the completion of model building, and just before well planning and reservoir modeling. When estimating an earth model by following a layer-by-Iayer inversion procedure, depth horizon associated with the (n - l)st layer should not be calibrated before estimating the model for the next layer n. This is because seismically derived layer velocities almost never match with well velocities. The discrepancy between the two is attributable to several factors, including the limited resolution in velocities estimated from seismic data and seismic anisotropy. Additionally, the high-frequency variations in the well velocities are absent from the seismically derived velocities. The calibrated depth maps can be used to create a solid model of the earth. See fig. 1. Each layer is represented by a solid with its interior populated by specific layer parameters. See fig. 2. These may include compressional-wave and shear-wave velocities, densities, and rock physics parameters such as porosity, permeability, pore pressure, and fluid saturation. When populated by the petrophysical parameters, the solid associated with the reservoir layer represents a reservoir model. For the purpose of reservoir modeling, the solid for the reservoir layer usually is downscaled in the vertical direction by dividing it into thin slices with a thickness as small as 1m, i.e. much ,less then the threshold for vertical seismic resolution. Additionally, the solid for the reservoir layer is upscaled in the lateral direction by dividing each thin slice into fmite elements with a varying size upto 250 m on one side. The reservoir model is eventually fed into a reservoir simulation scheme to predict the geometry of the fluid flow from the given reservoir parameters. Depth Structure Maps We have a set of time horizon maps and a corresponding set of internal velocity maps. In principal, a time migrated image can be

18

Encyclopaedia of Petroleum Science and Engineen·ng

(a)

(b) Fig. 1. (a) Depth horizonts derived from image-ray depth conversion ofthe time horizons. (b) Solid model created from the depth horizons in (a) (AfterYelmaz, 2001.

Seismic Earth Modeling

lseOi" '

lmt'-< -

Fig. 2. Explosion of the solid model shown in fig. l(b).

19

20

Encyclopaedia of Petroleum Science and Engineering

converted to a depth section by mapping the amplitudes along image rays. This notation also can be employed to convert time horizons into depth horizons. The process is done layer by layer starting with the shallowest horizon. A comprehensive mathematical discussion on imageray tracing is given by Hubral and Krey (1980). See fig. 3. Consider an image ray .that departs the nth surface at point SII with coordinates (x,., YII, ZII) and emerges at the right angle at the earth's surface at point So with coordinates (xu Yo' Z = 0). Our goal is to determine the coordinates of the output point (XII' YII' ZII) on the depth map from image-ray depth conversion of the input point (xu Yo' til) on the time map. To achieve this goal, we want to trace the image ray from the point of emergence (xu Yo' 0) back to its point of departure (x,., Y,., ZII).

Fij!;. 3. Principles of image-ray depth conversion

Suppose that the first n-l horizons have already been converted to depth, and that next we want to convert the nth horizon to depth. Since the earth model is known for the first n-l layers, then we know the coordinates of the inter-section point Si of the image ray with the frrst layer, (xl'YI' ZI)' where by definition of the image ray, XI =xo andYI = Yo. By using Snell's law, we can determine the direction of the ray as it departs point Si reaching point S2 on the next surface. As the image ray moves from one surface to the next, we add up the time it takes to travel. When it reaches the (n - 1)st layer, the elapsed two-way time tit-I is given by the following equation ; -

t

= 11-1

II-1 M 2~_k

7

Vk

...(1)

21

Seismic Earth Modeling

Where vk is the interval velocity of the Jcth layer and Ils k is the distance between the inter-section points of the image ray, Sk_1 and Sk the (k - 1)st and kth surface given by the following equation : !l~k = [(Xk - Xk-i)Z + (Yk - Yk_I)2 + (Zk - zk_I)21"2 ...(2) Now, we examine the situation when the image ray departs the (n - l)st layer at point Sn_1 on the way to the nth surface. Again, by Snell's law we know the direction of the ray. We also know the elapsed time tn - tn-I from the (n - l)st surface to the nth surface since we know the total elapsed time tn-I from equation (1) and the total elapsed time tn from the input time horizon read at point (xo'Yo' zO>o Finally, we know the internal velocity VII of the nth layer from the interval velocity map. Therefore, we can calculate the elapsed distance IlsII along the raypath as it departs the pint Sk-I on the (k - 1)st surface in the direction dictated by Snell's law. The quantity Ilsn is given by the following equation : -

Ils n =

Wn

"2 + (tn -

tn-I)

••.(3)

Finally, the coordinates of the point sn that we need to know to perform the time-to-depth conversion are given by the following equations :

=

xn-I + Ils n cos n

....(4)

Y n = Yn-I + Ilsn cos J3

...(5)

+ Ilsn cos Y

...(6)

Xn zn

= zn-I

Where n, J3 and yare are directional cosines of the ray at point Sn-I' The directional cosines are known by the application of Snell's law at point Sn_1 with known coordinates (xn-I' Yn-I' zn-I)' Given the depth and interval velocity maps for the frrst n-l horizons, and the time and interval velocity maps for the nth horizon, we can trace an image ray associated with the time tn (xo' yO> on the time map and derive the depth value ZII (x n, YII) on the depth map. The depth maps are compatible with the time maps. There can be subtle differences because of velocity variations that would give rise to the departure of image rays from the vertical. To quantify the differences between the time mps and depth maps, we can calculate the modulus Mil of the lateral displacement vector between the points So and Sn as follows : -

...(7) The most significant displacement between the vertical rays and image rays is at the fault zones. The displacement vector also has a directional azimuth +n which is given by the following equation:

22

Encyclopaedia of Petroleum Science and Engineering

ell

=

tan-I Yn - Yo

n

...(8)

Xn - Xo

eIln is measured from the inline x direction. The most significant azimuthal

variations are along the fault zones.

Earth Modeling in Depth Structure-independent earth models often are associated with low-relief structures or stratigraphic plays involving a depositional sequence with facies changes. A structure-independent earth model may be estimated initially by Oix conversion ofrms velocities without requiring a layer-bylayer analysis. The simplest method for estimating layer velocities is Oix conversion of rms velocities (Oix, 1955). The method requires the rms velocities associated with the layer boundaries that are included in the earth model to be constructed. The rms velocities ideally are estimated by pre-stack time migration. Alternatively, a smoothly varying from of stacking velocities estimated from dip-movement corrected data may be a reasonable substitute for rrns velocities. Less desirably, stacking velocities themselves with a fair degree of smoothing applied may be used in lieu of rms velocities. The Oix conversion formula is valid for horizontally layered earth models with constant layer velocities and small offsets. For an earth model with dipping layer boundaries and layer velocities with vertical and lateral variations, more accurate methods are required such as stacking velocity inversion, coherency inversion and image-gather analysis. Stacking velocity inversion (Thorson et al., 1985) requires time horizons picked from unmigrated CMP-stacked data and stacking velocities at analysis locations. Assume that a velocity-depth model already has been estimated for the first n - 1 layers, and that we want to estimate the layer velocity for the nth layer below a CMP location. For a trial constant velocity assigned to the nth layer, the method involves normal-incidence time-to-depth conversion of the time horizon associated with the base of the nth layer, then modeling of the nonzero-offset traveltimes associated with the CMP reflection event that corresponds to the base of the nth layer and determining the moveout velocity by fitting a hyperbola to the modeled travel-time trajectory. This procedure is repeated for a range of constant trial velocities, and the velocity that yields the minimum discrepancy between the actual stacking velocity and the modeled moveout velocity is assigned to the nth layer below the CMP location where the stacking velocity inversion is being performed Coherency inversion (Landa et aI., 1988) also requires time horizons picked from unmigrated CMP-stacked data. However, in lieu of stacking

Seismic Earth Modeling

23

velocities as for stacking velocity inversion, coherency inversion requires analyzing CMP gathers themselves. Again, assume that a velocity-depth model already has been estimated for the fIrst n - I layers, and that we want to estimate the layer velocity for the nth layer below a CMP location. For a trial constant velocity assigned to the nth layer, coherency inversion involves normal-incidence time-to-depth conversion of the time horizon associated with the base of the nth layer, then modeling of the nonzerooffset travel-times associated with the CMP reflection event that corresponds to the base of the nth layer, and computing the semblance within a CMP data window that follows the modeled travel-time trajectory. This procedure is repeated for a range of constant trial velocities and the velocity that yields the highest semblance value is assigned to the nth layer below the CMP location where the coherency inversion is being performed. Time horizons used in normal-incidence time-to-depth conversion as part of the stacking velocity inversion and coherency inversion procedures are picked from unmigrated CMP-stacked data. Alternatively, time horizons interpreted from the time-migrated volume of data can be unmigrated to obtain the time horizons equivalent to the time horizons picked from the unmigrated data. We circumvent the picking of prestack reflection traveltimeyin coherency inversion by measuring the discrepancy between the modeled and actual traveltimes by way of semblance. Similarly, we avoid the picking of prestack reflection traveltimes in stacking velocity inversion by measuring the discrepancy between the modeled and actual stacking velocities. Stacking velocity inversion and coherency inversion both take into account vertical velocity gradients which may be available from sonic logs. The methods also honour ray bending at layer boundaries. While Dix conversion assumes a hyperbolic move-out for the reflection event that corresponds to the base of the layer under consideration, both interval velocity estimate from coherency inversion and stacking velocity inversion are based on non-hyperbolic CMP travel-time modeling. Both stacking velocity inversion and coherency inversion can be considered accurate for velocity-depth models with smoothly varying reflector geometries and lateral velocity variations greater than the effective cable length associated with the layer boundary under consideration. As for conventional stacking velocity estimation, the accuracy in interval velocity estimation from Dix conversion, stacking velocity inversion, and coherency inversion are all influenced by the reflector depth, magnitude of velocity, and the cable length. SpecifIcally, the deeper the reflector,

24

Encyclopaedia of Petroleum Science and Engineering

the larger the layer velocity above, and the shorter the cable length, the less accurate is the interval velocity estimate. To estimate, update and verify velocity-depth models for targets beneath complex overburden structures, such as those associated with overthrust and salt tectonics, ultimately, we have to do image-gather analysis (Reshef, 2001). An image gather is the output from prestack depth migration and is a true CDP gather at a surface location. Stacking of image gathers yields an earth image in depth. If the velocity-depth model is correct, then events on an image gather are flat. In this respect, an image gather can be considered like a moveout-corrected CMP gather, except the vertical axis on an image gather is in depth. An event on an image gather with a move-out indicates an erroneously too low or too high velocity. By examining a panel of image gathers from the same location but with different constant trial velocities for the layer under consideration, one can pick the velocity that yields a flat event and assign it as the velocity of the layer above. Image gathers also can be used to make residual corrections to velocity estimates at analysis locations. This normally is done by fIrst converting the gather to the time domain, performing residual moveout velocity analysis, and converting back to the depth domain. The resulting residual correction should favourably improve the power of the stack obtained from image gathers and yield an updated velocity-depth model. The model updating based on the image-gather analysis usually is repeated until residual move-outs on image gathers are reduced to mininrum. Generalised Linear Inversion (GLI) If given an observed data set d, then we have to estimate a set of parameters p which are used to construct a model d' of the observed data set d' such that the difference between the observed data set d and the modeled data set d' is minimum based on a specifIc norm. We need a model equation that relates the modeled data with the model parameters to be estimated as given below : -

d' = Lp

...(1)

where d' is the modeled vector, p is the model parameter vector, and L is the matrix that relates the modeled data vector to the model parameter vector. The error vector e is defIned as the difference between the modeled data vector and the observed data vector by the following equation :-

e = d-d' Substitute equation (1) into equation (2) we get

...(2)

Seismic Earth Modeling

e

=

d-Lp

25 ...(3)

The cumulative squared error S is expressed as

S

=

ere

...(4)

Where T is for transpose. By substituting the value of e in equation (4) we have : or

S = (d-Lpl(d-Lp) S = tfId-pTLTd-tfILp+pTLTLp

...(5) ...(6)

Differentiate equation (6) with respect to p and observe the requirement for least-squares minimization which is as/ap = 0, we get as: -tfTL + pTLTL = 0

...(7)

Apply matrix transpose and rearrange the terms we have: (LTL)p = LTd

...(8)

Equation (8) yields the desired least-square solution which is given as:-

...(9) Where LTL is the covariance matrix and (LTLt' LT is the leastsquares (also called generalized linear) inverse of L. Equation (9) represents the generalized linear inverse (GU) solution to the parameter vector p. This solution is widely used in many stages of seismic data analysis, e.g., deconvolution, residual statios corrections, refraction static-s corrections, and the discrete Radon transform. The constrained solution is given by the equation ; p = (LTL + ~l)-' L 7*d

...(10)

Where ~ is called the damping factor and I is the identity matrix. In some applications; the generalized linear inverse problem is formulated in the frequency domain. Then, the unconstrained solution is given by the equation:p = (L'rLt'L7*d

...(11)

and the constrained solution is given by the equation : p = (L'rL + ~l)-' L'rd

...(12)

Where the asterisk (*) denotes complex conjugate, and p, d and L are complex. In geophysical applications, techniques to solve for the parameter vector p in equations (9), (10) or (II), (12) include Levinson recursion, conjugate gradient, Gauss-Seidel and singular-value decomposition.

Encyclopaedia of Petroleum Science and Engineering

26

Generalised Linear Inversion (GLI) Formalism of Deconvolution

Deconvolution is fundamentally a data modeling technique. Specifically, we model a I-D seismogram for a minimum-phase estimate of the source wavelet, to predict multiples, and ultimately obtain an estimate of white reflectivity series. We shall consider designing a leastsquares inverse filter j(t) that converts a wavelet w(t) to a desired form d(t) such that the difference e(t) between the actual outut yet) and the desired output d(t) is minimum in the least-squares sense. The zero-delay unit spike is a special case of the desired output d(t). Other forms of d(t) can also be considered, such as a zero-phase band-limited wavelet. The model equation for deconvolution is given by the following equation : -

Y (t) = w(t) *j(t)

...(1)

Consider the discrete form of equation (1), with w( t) represented by the m-Iength time series (wo' wI' w2' •••••••••••• wm-I)' andj(t) represented by the n-Iength time series (fo,J;,J;, ............... f,,-I). Equation (1) can be expressed in matrix form as : -

Yo YI Y2

Wo

0

WJ

Wo

W2

WJ W2

0

2

fo

fi fz

Wo

WJ ...(2)

W2 Wm-I

0

Wm-I Wm-I

0

Ym+n-I

Wm-I

fn-l

Equation (2) is the complete transient convolution. Define the output vector on the left-hand side by y, the coefficient matrix on the right-hand side by L, and the filter vector by f Equation (2) can take the compact form as : -

Y

=

If

...(3)

To obtain the least-squares solution for the filter vector!, we have:-

(LTL)f = LTd

...(4)

Consider the special case of a three-point wavelet (wo' wI' w 2 ). Set up the L matrix of equation (2) for this special case:-

Seismic Earth Modeling

L

27 Wo

0

wt W2 0

Wo

0

0 0 Wo

wt W2 0

...(5)

wt W2

Its transpose LT is given by the equation:

LT =

(% ~

wt Wo 0

W2

0

WI

W2

Wo

wt

~J

...(6)

Now multiply equations (5) and (6), we have:-

LTL =

(

w5+~+~

wtWo + w2wt

Wowt + wtW2

w5+~+~ Wowt + WI W2

WOW2

Compute the first three lags of the autocorrelation (ro' rl' r2) of the wavelet (wo' wI' w2), where ro w 2 +w 2 +w 2 012

r l = WOWI + WIW2

r2 = wOw2 ...(8) The elements of the covariance matrix LTL given by equation (7) are the fIrst three autocorrelation lags of the wavelet (wo' WL W2) given by the equation (8). For the general case, we have:1j

Ii

rn_1

ro

1j

1j

ro

rn-2 rn -2

...(9)

ro Where (ro' rl' r2, ••••••• rn_ l ) are the fIrst autocorrelation lags of the input wavelet series (wo' wI' w2 ' •••••• wn_ I ). For the special case of a desired output vector d is given by the equation :-

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Encyclopaedia of Petroleum Science and Engineering

d =

do dl d2

...(10)

d3 d4 Then the matrix product LTd is given by:-

J

wodo + l11 dl + W2 d 2 LTd = Wodl + l11 d2 + ~d3 [ Wod2 + l11d3 + ~d4 Now compute the fIrst three lags of the cross-correlation (go' gl' g2) of the desired output (do, di, d2, d3, d4 ) with the input wavelet (wo' wI w2) which are given as:go = wodo + wldl + w2d2 gl = wodl + w ld2 + w2d3

...(12)

g2 = wOd2 + w ld3 + w2d4 The elements of the matrix LTd given by equation (11) are the fIrst three lags of the cross-correlation of the desired output (do, dl' d2, d3, dJ with the input wavelet (wo' wI' w2) given by equation (12). The general case, we have:-

...(13)

gn-I where (go gl' g2' .... , gn-I) are ~ fIrst n lags of the cross-correlation of the desired output d with the inp:ut wavelet w. By substituting equations (9) and (13) into equation (4), we have:

Seismic Earth Modeling

29.

Equation (14) is the discrete form of the classic Wiener-Hopfintegral equation to estimate the least-squares shaping filter f that converts an input wavelet w into a desired form d. Practical implementations of equation (4) often require adding a small fraction E of the zero-lag of the auto-correlation to the diagonal elements of the matrix LTL given by:-

(LTL+EI)f = LTd

...(15)

where E is called the prewhitening factor and I is the identity matrix. A typical length of the deconvolution filter f in equation (14) is between 40 and 80 samples. This makes the size of the autocorrelation matrix LTL to be between 40 x 40 and 80 x 80. If the autocorrelation is computed from an input seismogram represented by a single trace, then the length of the input data vector is typically 1000 samples. The ratio of the length of the input seismogram used in computing the autocorrelation lags to the filter length should be no less than 8.

Interval Velocity Maps The Dix equation, which relates rms velocities to interval velocities, is used to derive interval velocity maps. RMS velocities are most appropriately estimated from prestack time-migrated data. The type of velocity that can be most reliably estimated from CMP data is the velocity used to apply norrnal-moveout correction. To stack the data we also substitute NMO velocities for stacking velocities. The use of NMO velocities as stacking velocities is based on the small-spread hyperbola assumption. To further substitute stacking velocities for rms velocities is only allowed if the CMP data are associated with horizontally layered earth. To justify the use of stacking velocities as rms velocities, we frrst need to correct for the dip effect on stacking velocities by way of dipmov~out (DMO) correction. In the case of a 3-D survey, we also need to correct for the source-receiver azimuthal effects on stacking velocities by way of 3-D DMO correction. This means that it is the stacking velocity field derived from 3-D DMO-corrected data that should be considered as a plausable substitute for the rms velocity field. But then the DMO velocities are associated with CMP gathers in their unmigrated positions. Strictly, we need the moveout velocities not only corrected for dip and azimuth effects but also estimated from gathers in their migrated positions. This is because the rms velocities used in Dix equation are defmed for a horizontally layered earth model. Thus the desired strategy is that velocities derived from 3-D prestack time migration should be substituted for rms velocities.

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Encyclopaedia of Petroleum Science and Engineering

Although prestack time migration velocities are most described to substitute for rms velocities, the interpreter may be compelled to use whatever velocity functions that may be available. These may have been derived from velocity analysis applied to DMO-corrected data or even to eMP data without DMO correction. Under those circumstances, the velocity functions picked at analysis locations need to be edited for any dip effect by either eliminating the suspect functions altogether or by smoothing. Whatever the sources of information, the interpreter starts with a set of velocity functions, each made up of a set of time-velocity pairs and associated with analysis locations over the survey area. The analysis grid typically varies from 500 x 500m to 2 x 2k:m. Hence, there may be as many as 400 velocity functions per 100 k:m.-squared of the survey area. The grided time horizons are intersected with the velocity functions and, for each horizon, velocity modes are extracted from the velocity functions coincident with the horizon times at the locations of the velocity functions themselves. These velocity nodes are then used as control points input to a gridding algorithm to create horizon-consistent rms velocity maps. There may be a need for further editing and smoothing of the rms velocity grids. Finally, the horizon-consistent rms velocity values and the horizon times at each grid point are used in Dix equation to compute the interval velocity values, which are then used to create the horizon-consistent interval velocity maps. Once again, there may be further need for editing and smoothing of the internal velocity maps to remove any geologically implausable velocity variations. Inversion Methods for Data Modeling

In practice seismic inversion has a broader scope of applications which can be grouped in two categories: (1) data modeling, and (2) earth modeling. What we do in seismic data processing is based largely on data modeling. An observed seismic wavefield can be described in two parts: (1) travel-times, and (2) amplitudes. Seismic amplitudes are more prone to the detrimental effects of noise as compared to traveltimes. Hence, in seismic inversion, we almost always treat traveltimes and amplitudes separately. When modeling the observed data, we either model the traveltimes or amplitudes. When modeling the earth, we use the traveltimes, as in structural inversion, or amplitudes, as in stratigraphic inversion. The important points of applications of seismic inversion for data modeling are given below:-

1. Deconvolution is based on modeling a one-dimensional (l-D) seismogram by optimum Wiener filtering for a minimum-phase estimate of the source wavelet, to predict multiples, and obtain an estimate of white reflectivity series.

31

Seismic Earth Modeling

2 We model traveltime deviations on moveout-corrected CMP gathers to estimate surface-consistent shot and receiver residual statics. 3. We model refracted arrival times to estimate surface-consistent shot and receiver intercept time anomalies, and obtain shot and receiver refraction statics. 4. One type of formulation of the discrete Radon transform is by genralised linear inversion. The discrete Radon transform is used to model a CMP gather so as to attenuate multiples and random noise, while compensating for missing data and fmite cable length in recording.

5. We model the seismic signal represented by reflection events assumed to be linear from trace to trace and at attenuate random noise uncorrelated from trace to trace by using spatial prediction filters. 6. Based on the same data modeling concept, we design spatial prediction filters to perform trace interpolation.

7. Data modeling also can be used in the design of a threedimensional (3-D) dip-moveout correction operator which accounts for irregular spatial sampling and undersampling of recorded data. Most data modeling applications are based on the theory of generalized linear inversion. Inversion Procedures for Earth Modeling Practical methods for estimating layer velocities and delineating reflector geometries can be appropriately combined to form inversion procedures to construct earth models in depth from seismic data. See table 1. Four such combinations are given in this table. These

Table 1. A set of inversion proc;edures for earth modeling in depth to estimate layer velocites and delineate reflector geometries. Layer Velocities

Reflector Geometries

Dix conversion of rms velocities stacking velocity inversion coherency inversion image-gather analysis

vertical-ray time-to-depth conversion (vertical stretch) image-ray time-to-depth conversion (map migration) poststack depth migration prestack depth

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Encyclopaedia of Petroleum Science and Engineering

migrationcombinations are ordered from top to bottom with an increasing level of accuracy. Also, for a given combination the methods for layer velocity estimation and reflector geometry delineation are compatible. In practice, you may wish to choose other combinations of the methods from the left-hand and right-hand columns. Also, you may be compelled to apply an inversion procedure that involves multiple combinations.For instance, in areas where salt tectonics has caused formation of diapiric structures the earth model may be estimated in three parts: (1) the overburden above the salt diapir, (2) the salt diapir itself, and (3) the substratum. You may then use coherency inversion combined with poststack depth migration to estimate the overburden model, and imagegather analysis combined with prestack depth migration to defme the base-salt geometry and estimate the substratum model. The primary consideration in the choice for an inversion procedure is the degree of lateral velocity variations and the complexity of reflector geometries. A mild-to-moderate lateral velocity variation is associated with a zero-offset diffraction response that is represented by a skewed, but almost hyperbolic traveltime trajectory. A strong lateral velocity variation is associated with a zero-offset diffraction response that is represented by a distorted, non-hyperbolic traveltime trajectory. A severe lateral velocity variation is associated with a zero-offset diffraction response that is represented by a complex, multivalued traveltime trajectory. A set of inversion procedures for earth modeling in depth are given below:1. Vertical stretch is a combination of Dix conversion of stacking velocities to estimate layer velocities and vertical-ray time-todepth conversion of time horizons picked from a time-migrated volume of data to delineate reflector geometries. This is a procedure appropriate for cases with negligible ray bending at layer boundaries, gentle dips, and lateral velocity variations judged to be within the bounds of time migration. 2. Map migration is a combination of stacking velocity inversion to estimate layer velocities and image-ray time-to-depth conversion of time horizons picked from a time-migrated volume of data to delineate reflector geometries. This is a procedure appropriate for cases with moderate ray bending at layer boundaries, moderate vertical velocity gradients. and moderate lateral velocity variations. 3. Poststack depth migration is a combination of coherency inversion to estimate layer velocities and posts tack depth

Seismic Earth Modeling

33

migration to delineate reflector geometries. This is a procedure appropriate for cases with significant ray bending at layer boundaries and significant vertical velocity gradients, and strong lateral velocity variations with sharp changes in reflector curvatures. 4. Prestack depth migration is a combination of image-gather analysis to estimate and update layer velocities, and stacking of image gathers to delineate reflector geometries. This is a procedure appropriate for cases with significant ray bending at layer boundaries, and severe lateral velocity variations associated with salt and over-thrust tectonics. These inversion methods are used to estimate an initial earth model in depth. Seismic inversion also is used to update the estimated model. A common application of inversion to estimate the errors in the initial model parameters, i.e., layer velocities and reflector depths, is reflection traveltime tomography. Tomographic inversion involves perturbing the model parameters by a small amount so as to match the modeled reflection travel-time with the observed traveltimes. Refraction traveltime tomography and reflection traveltime tomograpy both are based on the assumption that the perturbation required to update the model parameters is very small compared to the spatial variations in the model parameters themselves. In practice, tomography is best used strictly to touch-up a carefully estimated earth model based on some plausable geological constraints. It should never be used by itself to estimate the model. Map Processing

A map is defmed as a 2-D surface g(x, y). Depending on the quantity being mapped, g (x, y) may have many types of units, e.g., gravitation attraction (m Gal), magnetic intensity (gamma), elevation, or times picked along marker horizons from seismic data. The positive x-axis points eastward and the positive y-axis points northward. A discrete map function is represented by a grid of mesh points over the x - y plane. These mesh points are spaced commonly at equal intervals in the x and y directions. For many types of mapping, g(x, y) is a smooth function and such a map can be analyzed in the Fourier transform domain. However, there are situations (e.g., isochron and structure maps) in which the map function has discontinuities that represent faulting. Maps usually are created from irregularly spaced observation valves. Thus, the

34

Encyclopaedia of Petroleum Science and Engineering

map function at a particular grid point must be computed by some fitting procedure. See fig. 4. Void grid points have been filled with the arithmetic

Fig. 4. A time map of a seismic horizon (After Yilmaz, 2001).

mean of the map function. Before map creation, some correction may be applied to observed data, followed by various typFs of editing. Some of the very short wavelengths in the seismic horizon map result from nearsurface effects that are manifested as residual statics. Some moderately long wavelengths correspond to structural undulatjons that exist in the area. Most of the features with different wavelengths are not spatially isolated, but are' superimposed This characteristic is common for all types of map functions, e.g., gravity, magnetic, elevation or time. To separate the effects of different features from each other, we must analyze them in terms of wavelength. Simple 2-D smoothing and wavelength filtering are techniques for separating anomalies. Vertical derivatives and analytic continuation also are useful for enhancing certain anomalies so that they appear more pronounced on the map. The 2-D amplitude spectrum of a map is an excellent tool for recognizing not only the wavelength content, but also the orientation of

35

Seismic Earth Modeling

various components. The most useful display is the colour contour plot of the amplitude spectrum from which various bands of wavelengths are distinguished clearly. Pink represents long, beige represents moderate and yellow represents short wavelength anomalies. For a 2-D real function, such as a map, the amplitude spectrum is anti-symmetric. Thus, only a pair of quadrants (the first and second) of the amplitude spectrum needs to be displayed. A simple 2-D smoothing operation is the easiest way to obtain a map that represents the region anomaly. See fig. 5.

Fig. 5. A smoothed version of the contour map in fig. 4.

Smoothing basically is done by computing the average value of the grid points that fall onto a ring of some desired radius. The center of the ring coincides with the output point. For n concentric rings with m; points over the ith ring, the average value gj of the quality gij that is being mapped is given by:...(1)

.. Thus, the cumulative average g over n rings is given by :-

g

=

1~_

-

£..Jgj

n ;=1

...(2)

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Encyclopaedia of Petroleum Science and Engineering

Weighting factors, which depend on the distance from the center of the rings, often are used in smoothing algorithms. Equation (2) becomes as:n

g

=

L wigi

...(3)

i=1

Where wi are the weights. The residual anomaly is defmed by : -

g

=

go- g

...(4)

Where go is the grid value at the center of the rings. Fig. 6 shows the regional anomaly obtained by using IS rings. In general, the more rings, the more smoothing ofthe data. Gridding involves fitting a locally plane surface to a set of control points around each grid output point. Consider a planesurface fit in the least-squares sense which is given as:-

Fig. 6. a regional anomaly map (After Yilrnaz, 200 1).

g (x,y)

=

ao + a\x + aaY

...(5)

The least-squares error is given by :M

L = L(gj - gj)2

...(6)

i=\

Where g is the observed value at the gird point (x, y), and M is the member of observations at and around that grid point. For local plane

Seismic Earth Modeling

37

fitting, M usually is set to 9 points. We want to fmd a set of (a o' al' a2) for which L is minimum, the condition is given by : -

8L 8ao

_ 8L _ 8L -0 - 8a, - 8a2 -

...(7)

From equations (5) and (6), we have : M

L

=

~)g; -ao -a,x; -a2y;f

...(8)

;=, From equation (7), we get the following set of simultaneous equations : ~g ...(9) ~ao + ~a,x + ~aaY ~oX + ~a,x2 + ~azXY

ug

...(10)

~aoY + ~a,xy + ~azV

~yg

...(11)

r

When put into matrix fonn, we obtain : -

(:

u u2

~

~

~y2

~y

a, a2

J (~g ug J =

...(12)

~yg

Equation (12) is solved for the set of coefficients (ao, a-i, aa). Real data consist of anomalies of various shapes and orientation that are superimposed on each other. In the transfonn domain, the real anomalies can be separated in terms of their wavelength contents and orientations. This cannot be achieved in the space domain. The transfonn domain provides a way to apply various filtering operations to a map. f\ band of wavelengths can be passed regardless of orientation by the radial filter transfer function. See fig. 7. In practice the transfer functions must be tapered in the neighbourhood of cutoff wavelengths for optimal performance. +k,

(a)

(b)

(c)

Fig. 7. (a) Transfer function for band-pass filter. (b) Transfer function for directional filter. (c) Transfer function for directional band-pass wavenumber filter.

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Encyclopaedia of Petroleum Science and Engineering

Once the transfer function is designed to suit the purpose, the filter itself can be applied to the map in the transform or the space domain. In the transform domain, the transfer function is multiplied with the 2-D Fourier transform of the map. Subsequent inverse transformation yields the filtered map. To apply the filtering in the space domain, first inverse Fourier transform the filters transfer function to obtain its 2-D impulse response. Two-dimensional convolution of this impulse response with the map yields the filtered map. Before applying the 2-D filters, the possible bands of the wavelengths present on the amplitude spectrum of the map to be filtered must be determined. The bands that were determined are used as cut-off wavelengths of the radial filters transfer function. A given map can be scanned with a suite of low-pass filters and several filtered maps can be produced, each with a potentially unique interpretational value. As the bandwidth of the filter is increased, more and more of the short wavelength anomalies are included, making the output less and less regional in character. A filter scan is not limited to low-pass filtering only. High-pass, band-pass, and band-reject filters can be applied to maps to achieve a particular interpretational goal. Directional filters scan the map of interest at various angles to emphasize a particular trend that may exist in the data. In some cases, a certain band of wavelengths may have one dominant trend that is different from that of another band of wavelengths. This situation may imply a change in the tectonic setting over the geologic history in the area.

Model Building First time it is not possible to build an earth model in depth correctly. The velocity-depth ambiguity that is inherent to inversion makes it very difficult getting the right answer, i.e., the true geological model at fi~ time. Limitations in the resolving power of the methods to estimate layer velocities that arise from the band-limited nature of the recorded data and fmite cable length used in recording further compound the problem. Finally, travel-time picking that is needed for most velocity estimation techniques and time-to-depth cohversion as well as picking depth horizons from depth-migrated data to delineate reflector geometries are all adversely affected by noise present in the data. In first attempt, we can estimate an initial model and then update this model to get the acceptable final model. We shall consi<;ler two strategies applicable to both 2-D and 3-D seismic data for initial. model building: (1) a time-todepth conversion strategy based on interpretation in the time domain, and (2) a layer-by-Iayer inversion strategy based on interpretation in tlle

Seismic Earth Modeling

39

depth domain. A widely used combination for time-to-depth conversion in Dix conversion to estimate layer velocities and image-ray depth conversion to delineate reflector geometries. Whereas for layer-by-Iayer inversion, a widely used combination is coherency inversion to estimate layer velocities and poststack depths migration to delineate reflector geometries. The time-to-depth conversion strategy involves the following steps :--

1. Interpret a set of time horizons from an image volume derived from time migration; these time horizons are usually associated with layer boundaries with velocity contrast or geological formations of interest. 2. Intersect rms velocity functions picked at specified analysis locations over the survey area with the time horizons from step (1) to drive horizon-consistent rms velocity maps. The rms velocity functions are preferably picked from gathers derived from prestack time migration. 3. Perform Dix conversion of the rms velocity maps from (2) to derive interval velocity maps. 4. Perform vertical-ray or image-ray depth conversion of the time horizons from step (1) using the interval velocity maps from step (3). Depth conversion of time horizons may be performed by one of these three strategies: (1) most commonly applied strategy is based on a combination ofDix conversion ofrms velocities to interval velocities and image-ray depth conversion of time horizons interpreted from the timemigrated volume of data. This is the usual implementation of map migration. Stacking velocity inversion sometimes may be substituted for Dix conversion to estimate interval velocities. (2) alternatively, depth conversion may be performed using vertical rays. This is acceptable only if lateral !lrispositioning because of lateral velocity variations is negligible. Again, the interval velocities are estimated by Dix conversion, and (3) a third option is to use normal-incidence rays for depth conversion (rarely used). Time horizons interpreted from the time-migrated volume of data may first be forward-modeled to derive 3-D zero-offset traveltimes, which are than depth-converted using normal-incidence rays. Layer-by-Iayer inversion involves the following steps:- (1) Interpret a set of time horizons from unmigrated data to be used in lierf of zerooffset reflection traveltimes required by coherency inversion.

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Encyclopaedia of Petroleum Science and Engineering

Altematively, interpret a set of time horizons from time-migrated data and perform forward modeling to obtain the required zero-offset traveltimes. 2 Assume that interval velocities and reflector geometries for the first n - 1 layers have been estimated, and that we want to estimate the same for the nth layer. By using the time horizons from step (I), first apply one of the inversion methods to estimate the interval velocity field

3. Assign the estimated interval velocity field for the nth layer to the halfspace that includes the unknown part of the mode, i.e., the nth layer and the layers below, and perform depth migration. 4. Interpret the depth horizon associated with the base of the nth layer from the depth image and incorporated it into the velocity depth model. The layer-by-Iayer inversion strategy alternates between layer velocity estimation and reflector geometry delineation for each layer starting from the earth's surface and moving down one layer at a time. The layer-by-Iayer estimation strategy facilitates checking the results of inversion for one layer before moving down to the next. As such model updating can be interleaved with model estimation to circumvent accumulation of errors in layer velocities and reflector geometries as the analysis proceeds from the top down. See fig. 8. Superimposed on the

Fig. 8. An unmigrated CMP-stacked section with seventime horizons (After Yilmaz, 200 I).

Seismic Earth Modeling

41

stacked section are the segments of interpreted reflection traveltime horizons. Starting from the top, ill is the water bottom whereas H6 is the top-salt boundary. Input to coherency inversion is the zero-offset reflection traveltimes associated with normal-incidence rays and unmigrated data. Begin with the task of modeling the water layer by normal-incidence time-to-depth conversion of the time horizon H-i using a constant layer velocity of 1500 mls. For each layer H2 through H6, and one layer at a time, the analysis includes the following steps :1. Apply coherency inversion to compute the horizon-consistent semblance spectrum. 2 Pick the interval velocity profile from the semblance spectrum by tracking the semblance peaks.

3. Assign the internal velocity profile to the half space that includes the unknown layer itself and use the known overburden layer velocities and reflector geometries to create the intermediate velocity-depth model. 4. Perform posts tack depth migration using the intermediate velocity-depth model. 5. Interpret the depth horizon that corresponds the base boundary of the layer under consideration.

6. Incorporate the interpreted depth horizon into the intermediate velocity-depth model. 7. Proceed to the next layer below and repeat the above steps. The resolving power of coherency inversion is governed by the cable length, the reflector depth, the layer velocity and bandwidth of the data. In practice, the interval velocity profile picked from the semblance spectrum must exclude rapid fluctuations. Otherwise, the reflector geometry associated with the base of the layer under consideration will be corrupted by geologically implausable variations. We have to interpolate the interval velocity profiles through the zones with missing reflection events. The accuracy in velocity estimation by coherency inversion can be monitored closely by examining the modeled CMP traveltimes and the associated ray paths. The resolution can be attained from the semblance spectra. The sharpness of the spectra decreases with increasing layer velocity, decreasing cable length, and increasing reflector

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Encyclopaedia of Petroleum Science and Engineering

depth. Fig. 9 shows a time-migrated stacked section with a complex overburden structure associated with salt tectonics. The top-salt boundary is represented by horizon He. The section has been interpreted to identify the layer boundaries with significant velocity contrast. For

Fig. 9. (a) A time-migrated stacked sectIOn with a set of Interpreted horizons. (b) The unmigrated stacked section With the zero-offset traveltlmes modeled from the time horizons in 9(a)

Seismic Earth Modeling

43

coherency inversion to estimate interval velocities layer by layer, the required zero-offset traveltimes were modeled from the time horizons interpreted from the time-migrated section. The complex layer boundaries adversely affect the quality of interval velocity estimation using methods that reply on ray tracing. Lateral velocity variations in the layers above cause rapid fluctuations in the &cmblance profile for the lower below. All known practical velocity estimation techniques based on ray theory alone suffer from a d:!gradation of lateral resolution in areas with complex overburden structures. In areas with low-relief structures and moderate lateral velocity variations, a structure-independent inversion strategy can be used to circumvent interpretation of time horizons when deriving an initial estimate of the earth model. Compared to a layer-by-Iayer inversion strategy, it can prove to be robust and less labour-intensive, e.g., a highly developed deltaic depositional sequence. Here the lateral velocity variations are mild to moderate and the dips are gentle and the structures have low reliefs. We may substitute the stacking velocity field for the rms velocity field that we need for Dix conversion. The following steps are:1. Consider a set of fictitious, flat time horizons, and extract the rms velocity profiles along these horizons from the rms velocity section. 2. Perform Dix conversion to generate interval velocity profiles from the rms velocity profiles. For deeper horizons, lateral velocity variations in the layers above will cause oscillation in the interval velocity profiles. 3. Apply lateral smoothing to remove these oscillations and use the edited interval velocity profiles to convert the flat time horizons to depth horizons. 4. Combine the interval velocity profiles with the depth horizons to build an initial interval velocity field. Lateral velocity variations will cause the flat time horizons to transform to non-flat depth horizons. 5. Perform poststack depth migration using the initial interval velocity field and overlay the depth horizons derived in step (3) onto the depth section. The depth horizons do not conform to the geometry of the reflectors inferred by depth migration.

44

Encyclopaedia of Petroleum Science and Engineering 6. Discard the structure-independent depth horizons and replace them with the depth horizons interpreted from the depthmigrated section. 7. Overlay the depth horizons from step (6) onto the intervalvelocity section from step (4).

8. Extract the internal velocity proftles along the depth horizons from the interval velocity section.

9. Eliminate the oscillations from these proftles and combine them with the -ttw depth horizons from step (6) to build a structurally consistent earth model in depth. 10. Perform prestack depth migration and obtain the image section from the image gathers. Events on image gathers, except for the multiples, are mostly flat. This means that the estimated earth model in depth is fairly accurate. In practice, to attain consistency of the estimated model with the input data, depth migration may have to be iterated a few times. This then is followed by model updating with reflection tomography. Model Updating

Limitations in the techniques for velocity estimation and velocitydepth ambiguity inherent to seismic inversion are compelling reasons for the need to update an .estimated earth model in depth. Unfortunately, model updating tools themselves also have limitations in terms of their ability to resolve lateral velocity variations and refme reflector geometries. Again, the cable length and reflector depth dictate the extent that model updating techniques can resolve the velocity-depth ambiguity. Residual moveout corrections applied to image gathers is a local method and reflection tomography is a global method for model updating. Model Representation and Visualization In earth modeling, a surface corresponding to a layer boundary is usually represented by a set of triangles, the size and shape of which vary depending on the complexity of the reflector geometry. See Fig. 10. A velocity-depth model usually is represented either in the form of a gridded or tessellated volume. Gridding means dividing the whole volume into a set of 3-D cells of equal size with appropriate dimensions in the inline. cross-line and depth directions. Tessellation means dividing the volume associated with each layer into a set of tetrahedral, the size and

Seismic Earth Modeling

Fig. 10. (a) Surfaces that represent the reflector geometries. (b) The triangralated form of these surfaces (After Yilmaz, 200 I)

45

46

Encyclopaedia of Petroleum Science and Engineering

Fig. II. (a) Surfaces that represent layer boundaries, and (b) slabs that represent the layers themselves.

47

Seismic Earth Modeling

shape of which depend on the geometry of layer boundaries. In a tessellated velocity-depth model, a velocity and a gradient, if available from sonic logs, are assigned to each comer of the tetrahedra. A velocitydepth model is represented either in gridded or tessellated form depending on the application that needs it as input. For instance, ray tracing in coherency inversion and prestack depth migration may be performed using gridded or tessellated models. On the other hand, were extrapolation in 3-D poststak depth migration based on fmite-difference schemes is performed conveniently using gridded models. Images of layer boundaries included in an earth model in depth can be converted to a physical model using various image construction techniques. See fig. 11. In areas with complex structures, we often have to deal with multivalued depth surfaces. For instance, salt overhangs associated with diapirism and imbricate structures associated with overthrusting cause a surface to fold onto itself. Model with Complex Overburden Structures Fig. 12 shows a complex overburden structure associated with overthrust tectonics. Such structures were formed as a result of the tectonics movement during the Lower Miocene and Upper Cretaceous, o

2 A

3

4

.•

5

6

7

8

9

10 B

11

12

kmt:====::::::J2]~~m~c:::==~~========----~::~~

O'51--_~"7;;. _; 3 300

~_-.

1.0

~

~. - - - - - - 3 2 0 0 3200

3200

1.5

2.0

~5700

2.5~--~~------~~----------------------------~

Fig. 12. A velocity-depth model with complex overburden structure caused of overthrust" tectonics (Afte Yilmaz, 200 I). and are connnon in North America, South America, and the Middle East. The target horizon is the flat reflector at 2.S km. below the imbricate

structures. The velocity-depth model comprises a shallow sequence with

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Encyclopaedia of Petroleum Science and Engineering

a relatively simple structure. Underneath this shallow sequence is a shalemar sequence with a strong vertical velocity gradients (0.5 mls/m). Then, we have the imbricated fault structures of the carbonate sequence. and finally the target level at 2.5 Ian. characterized as the detachment zone that separate~ the incompetent rock layers above from the competent rock layers below. Fig. 13 shows the CMP-stacked section. The traveltime distortions along the deepest reflection are caused by the severe ray bending within the overburden. Time migration whether poststack or prestack, will not resolve the deleterious effect of the complex overburden structure. Suppose that we already have estimated the velocity-depth model down to 1 km depth. The remaining part of the model will be constructed layer-by-Iayer by prestack depth migration. We want to examine image gathers from prestack depth migration for layer velocity examination and the stack of the image gathers for reflector geometry delineation. Below 1 km the unknown part is represented by the halfspace. The velocity assigned to the half-space is that of the shale-mar sequence. This layer has a vertical velocity gradient. Fig. 14 shows selected image gathers from prestack depth migration. The event T associated with the top of the carbonate sequence exhibits a flat character on the image gathers. This indicates that the velocity field for the layer above is correct. Consequently, the depth image which was obtained by stacking the image gathers yields the correct reflector geometry fophe top of the carbonate sequence. Interpret the top of the cartionate sequence from this section and insert it as a layer boundary into the velocity-depth model. Assign the velocity (5700 mls) for the carbonate sequence to the half-space below the top-carbonate boundary. Perform prestack depth migration and obtain the depth image. The image gathers indicate flat events for the top T and bars B of the carbonate sequence. The depth image which was obtained by stacking the image gathers yields the correct reflector geometry for the base of the carbonate sequence. Interpret the base of the carbonate sequence from this section and insert it as a layer boundary into the velocity-depth model. Finally, assign the substratum velocity (5000 mls) to the half-space below the base-carbonate boundary. Perform prestack depth migration and obtain the depth image. The image gathers indicate flat events for the top T and base B of the carbonate sequence, and the flat reflector F within the substratum. Interpret the flat reflector at 2.5 km from this section and insert it as a layer boundary into the velocity-depth model. The flatness of an event on image gathers is an indication of the accuracy of the velocity field associated with the layer above the layer

Seismic Earth Modeling

Fig. 13. (a) CMP-stacked section associated with the velocity-depth model m fig. 12. (b) Postsack time migration.

49

V"o

o

Fig. 14. Image gathers from prestack depth migration using the three velocity-depth models. T. Band F are the events associated with the top-carbonate, base-carbonate, and the undulying tlat retlector, respectively.

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51

boundary that is represented by that event. All events above that event should be flat. The non-flatness of an event on image gathers is detectable only if there is sufficient cable length. The detectability of residual moveout on image gathers is possible only if there is sufficient cable length. In the limit of zero offset, velocity resolution becomes nil. The residual moveout also is influenced by the magnitude of the layer velocity and the depth of the layer boundary. Using the correct velocity for the substratum, the event associated with the flat reflector F exhibits a flat character on image gathers. Not only is this event flat, but so are all the other events above. Flatness of all the events on image gathers is a means of verifying the accuracy of the velocity-depth model used in prestack depth migration. This is a necessary but not sufficient condition for verifying the accuracy of a model. By using nonzero-offset data, we can resolve the velocity-depth ambiguity for a reflector if the data used in inversion have been recorded with offsets greater than the reflector depth. The additional limitations in model verification based on imagegather analysis are the reflector depths and the magnitude of the layer velocities. Specifically, the deeper the reflector or the higher the velocity of the layer above, the less the detectability of a residual moveout on image gathers. Image gathers from prestack depth migration are used to estimate layer velocities in two ways: (1) constant half-space velocity analysis, and (2) residual moveout analysis. Suppose that the velocity-depth model has been established for the first n-l layers, and that we want to estimate the layer velocity for the nth layer. Using the known overburden velocitydepth model for the first n - 1 layers, assign a constant velocity to the half-space below that includes the nth layer. Perform prestack depth migration and output image gathers at some appropriate interval along the line. The image gathers should exhibit flat character for the events associated with the n - 1 layers, but show a residual moveout for the event associated with the nth layer. Repeat the analysis using the same overburden model and a range of constant velocities assigned to the half-space. By analyzing the image gathers from constant half-space velocity scans, we can make optimum velocity picks at analysis locations that best satisfy the flatness criterion for the layer under consideration. Models with Horizontal Layers

First six horizontal layers model is considered. The six horizontal layers 3-denoted by names HI through H 6• Table 2 gives the layer velocities

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Encyclopaedia of Petroleum Science and Engineering Table 2. Parameters of the model with horizontal layers and constant-velocity near-surface layer.

Layer

HI H2 H3

H4 H5 H6

Velocity (mls)

1500 2000 2400-2700 3000-3500 4500 3000

Depth (m)

100 1000 1500 1800 2250 2700

and depths to the base of each layer. The lateral velocity gradients in layer H3 and H4 are about 125 mls and 200 mls over one cable length, respectively. The procedure for estimating the layer velocities and reflector depths using Dix conversion of stacking velocities includes the following steps :-

1. For each of the layers in the model, pick the time of horizon on the unmigrated CMF-stacked data that corresponds to the base layer boundary. These times are used in line of the two-way zero offset times in equation given below : -

v"

=

V,,2Tn - V,,2_ I Tn_1 T.11 - T.,,-1

......(1)

Where v" is the internal velocity within the layer bounded by the (n - l)st layer boundary above and the nth layer boundary below, Tn and T~_l are the corresponding two-way zero-offset times, and Vn and Vn_1 are the corresponding rms velocities. 2. Extract the rms velocities at horizon times. 3. Use equation (1) to compute the internal velocities for each of the layers from the known quantities, i.e., rms velocities and times at top-layer and base-layer boundaries. 4. Use internal velocities and times at layer boundaries to compute depths at layer boundaries. If the input times are from an unmigrated stacked section, use normal-incidence rays for depth conversion. If the input times are from a migrated stacked section, use image rays for depth conversion. The earth model can be constructed by combining the estimated interval velocity profiles and depth horizons. Comparison with the true

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model clearly demonstrates that the interval velocity estimation based on Dix conversion is not completely accurate. The interval velocity profiles derived from Dix conversion exhibit the sinusoidal oscillations caused by the swings in the stacking velocity profiles themselves. The fundamental problem is that the stacking velocity estimation is based on fitting a hyperbola to CMP traveltimes associated with a laterally homogenous earth model. If there are lateral velocity variations in layers above the layer under consideration, and if these variations are within a cable length, then stacking velocities would oscillate in a physically implausable manner (Rocca and Toldi, 1983). As a consequence, the resulting interval velocity estimation based on Dix conversion is adversely affected. The pragmatic approach would be to smooth out the oscillations in the stacking velocities before Dix conversion and smooth out the oscillations in the velocity profiles after Dix conversion. Then, the resulting earth model is expected to be free of the adverse effects of stacking velocity anomalies. . The procedure for estimating layer velocities from coherency inversion requires CMP gathers at analysis locations and horizon times picked from unmigrated stacked data. Alternatively, time horizons picked from time-migrated data can be forward-modeled to d~rive the zero-offset traveltime needed for coherency invers,ion. As for Dix conversion, the velocity estimate from coherency inversion is local, independent of data away from the analysis location. A procedure for velocity-depth model estimation that includes coherency inversion is conducted layer-by-Iayer starting from the surface. Consider the synthetic data set associate~ with the model with six horizontal layers HI through H 6 • We shall adopt the interval velocity profile for the first layer HI estimated from Dix conversion and start the application of coherency inversion with layer H 2• Assume that the velocity-depth model for the first n - 1 layers already has been estimated. For the nth layer, follow the steps below for coherency inversion : 1. For a trial constant velocity assigned to the nth layer, perform normal-incidence traveltime inversion to convert the time horizon corresponding to the base-layer boundary to a trial depth horizon. 2 Given the geometry of the CMP gather at the analysis location, assign a trial velocity to the half space that includes the layer

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yet to be determined and compute the CMP traveltimes using the known overburden velocity-depth model. The modeled CMP traveltime. trajectory that corresponds to the base of the layer under consideration is in general nonhyperbolic, for the ray tracing used to compute the CMP travetines accounts for ray bending at layer boundaries and incorporates vertical velocity gradients within layers above. Fig. 15.

3. Extract data window along the modeled traveltime trajectory. 4. Compute. semblance using the data within the selected window as a measure of discrepancy between the modeled and the actual travel-times. 5. Repeat all the steps above for a range of constant velocities. 6. Pick the constant trial velocity as the layer velocity for which the semblance is the maximum. The results of coherency inversion are used to pick velocity modes at analysis locations. Specifically, the layer velocity at a given location is selected based on the semblance curve and the data window along the modeled traveltime trajectory making sure that the estimated velocities are geologically plausable. Data windows along modeled traveltime trajectories can be examined for flatness criterion to pick an optimum velocity node. In practice, for 2-D data, coherency inversion often is applied continuously along the line. As for horizon-consistent stacking velocity analysis, for each layer, a horizon-consistent semblance spectrum is computed using coherency inversion. For 3-D data, as for conventional velocity analysis, coherency inversion normally is applied at uniformly spaced grid points over the survey area. We now examine the accuracy ofDix conversion and coherency inversion for the model with horizontal layers but with a near-surface layer hh with laterally varying velocities between 800 mls and 1500 mls. Dix conversion has introduced spurious structures into the model, while coherency inversion has introduced a bulk shift in the reflector depths. In each modeling, an error in the form of a distorted reflector geometry is worse than an error in the form of a bulk shift in the reflector depth. While the error in the form of a bulk shift can be corrected for calibrating the estimated model to well tops, the error in the form of a distorted reflector geometry may require a serious revision of the estimated model.

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Seismic Earth Modeling

(b)

(c)

Fig. 15. (a) CMP raypaths, (b) the CMP gather at the analysis location, and (c) the data window that includes the event in the CMP gather in (b) (After Ytlmaz, 2001).

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Model with Low-Relief Structure

We shall review velocity estimation via stacking velocity inversion and coherency inversion in the presence of low-relief structures. Fig. 16 km°r-_______ 2_A___ 3 ____ 4 ___5____6____7____8____9____'0_B ___'_'___ '2-.

2000 mls

HI

0.5L-----~ _ _ _2_400_

1.0

._~

H3

2900 H,4>---.. 3400

_-----,

H5

3200

1.51--_--H7.

2.oL=~--=:::::.::::===...!3!!!!!-::::::::::::::::::::: 3800

4000

2.5L------

Fig. 16. A velocity-depth model with low-relief structures.

shows a velocity-depth model with such characteristics. The models/ imu1ates a transgressive depositional sequence within the fIrst I-Ian depth, a deltaic sequence between 1.5 to 2 krn, and a deeper depositional sequence between 2 to 2.5 km. Our goal is to detect the subtle lateral velocity variations within the individual sequences. Fig. 17 shows the

Fig. 17. The CMP-stacked section associated with the velocity-depth model in fig. 16. With the interpreted time horizons.

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CMP-stacked section with and without the interpreted time horizons. Assuming that the CMP-stacked section is largely equivalent to a zerooffset section, these time horizons correspond to two-way zero-offset time picks which are used in stacking velocity inversion and coherency inversion. Using coherency inversion and stacking velocity inversion to estimate layer velocities, the velocity-depth models are obtained. In both cases, normal-incidence traveltime inversion is used to delineate the reflector geometries. Both techniques are able to detect the existence of velocity variations from one unit to next. The deltaic sequence is delineated by coherency inversion more accurately. Stacking velocity inversion fails to estimate the internal velocity distribution of the deepest sequence between 2 to 2.5 km, correctly. Coherency inversion has at least been able to detect the relative magnitude of the velocity variations within this sequence with reasonable accuracy. The procedure for estimating layer velocities from stacking velocity inversion requires the horizon times picked from unmigrated stacked data and stacking velocities at time horizons that correspond to layer boundaries in the model. Horizon-consistent stacking velocities are estimated by computing semblance for a range of constant velocities continuously along the time horizon picked from the stacked data. The velocity spectrum for each time horizon then is picked to drive a stacking velocity curve along the mid-point axis. As for is coherency inversion, the velocity estimate from stacking velocity inversion local, independent of data away from the analysis location. A procedure for velocity-depth model estimation that includes stacking velocity estimation is conducted layer-by-Iayer starting from the surface. Assume that the velocity-depth model for the first n - 1 layers already has been estimated. For the nth layer, follow the steps below for stacking velocity inversion : 1. For a trial constant velocity assigned to the nth layer, perform

normal-incidence traveltime inversion to convert the time horizon corresponding to the base-layer boundary to a trial depth horizon. 2. Given the geometry of the CMP gather at the analysis location (not the CMP gather itself, but only the source-receiver geometry associated with it), compute the CMP traveltimes. The modeled CMP traveltime trajectory that corresponds to the base of the layer under consideration is in general nonhyperbolic, because the ray tracing used to compute the CMP traveltimes

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accounts for ray bending at layer boundaries and incorporates vertical velocity gradients within layers above. 3. Compute the best-fit hyperbolic traveltime trajectory, and thus determine the modeled stacking velocity for the trial interval velocity. 4. Measure the discrepancy between the modeled and the actual stacking velocities by way of a semblance. 5. Repeat all the steps above for a range of constant velocities. 6. Pick the constant trial velocity as the layer velocity for which the difference between the modeled and the actual stacking velocities is minimum or the semblance is maximum. As was demonstrated by the coherency inversion tests applied to the model with horizontal layers, this observation has an important practical implication with regard to evaluation and use of the results of velocity estimation. Specifically, lateral velocity variations of very shortwavelengths that are much less than the cable length should not be incorporated into a velocity-depth model. Instead, some lateral smoothing of velocity estimates is almost always needed. The maximum of the semblance curve derived from coherency inversion coincides with the optimum choice for the layer velocity. The sharpness of the peak in the semblance curve, hence the velocity resolution, depends upon the depth of the layer boundary and the magnitude of the layer velocity. The velocity resolution also depends on the effective cable length. The sampling interval for the velocity axis in the semblance curves should be chosen by taking into consideration in velocity resolution that can be achieved. For a given horizon, the data window with the flattest event is distinguishable when the velocity sampling is appropriate. Specifically, for shallow horizons with low velocity, velocity increment needs to be small enough to pick a layer velocity, accurately. However, for deeper events with high velocity, the velocity increment does not have to be as small, since event curvature in the data windows becomes indistinguishable. It is only with large velocity increments that we observe a marked difference in event curvature. A good rule of thumb in practice is that the sampling interval in velocity used in stacking velocity inversion and coherency inversion needs to be specified as small as 25 mls for velocities as low as 1500 mls and can be specified as large as 200 mls for velocities as high as 5000 mls.

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Reflection Traveltime Tomography: Reflection traveltime tomography is based on perturbing the initial model parameters by a small amount and then matching the change in traveltimes to the traveltime measurements made from residual moveout analysis of image gathers (Kosloff et a/., 1996). We must do the best we can in building an accurate earth model in depth so that only small changes remain to be made to the model by tomography. Specifically, a tomographic update can be expected to work provided the changes to be made to the initial earth model parameters in terms of slowness and depths at layer boundaries as small compound to the model parameters themselves. We shall assume that the initial model is made up of horizontal layers with laterally invariant model parameters. In the usual implementation of reflection traveltime tomography, the model parameters are perturbed while preserving ihe offset values of the seismic data. The tomographic update Ap to the model parameters that comprise the changes in the showness and depths to layer boundaries is given by the gener and depths to layer boundaries is given by the generalized linear inversion (GLl) solution by the following equation : AP = (LTLtl LTAt

...(1)

Where At denotes the column vector that represents the residual moveout times measured from the image gathers, L is a sparse matrix, i.e., its elements are in terms of the slowness and depth parameters associated with the initial model, and T denotes matrix transposition. Consider the earth model with horizontal layers. We shall make an attempt to update the initial estimate using reflection tomography. The steps are as given below : 1. Generate a set of image gathers from prestack depth migration using an initial velocity-depth model.

2. Convert the image gathers from depth to time using the interval velocity functions extracted from the initial velocity-depth model at the image-gather locations. 3. Compute the horizon-consistent residual moveout for all offsets along events on image gathers that correspond to the layer boundaries included in the model. The vertical axis represents the residual moveout measured at a reference offset, usually the maximum offset. The horizontal axis represents the CMP locations along the line. Since residual moveout can be either

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negative or positive, the vertical axis is in both the positive and negative directions. 4. Pick the residual moveout profiles for all the horizons by tracking the semblance peaks. Any departure from the horizontal axis indicates a non-zero value for residual moveout. 5 Build the traveltime error vector At using the residual moveout times. 6. Define the initial model by a set of slowness and depth parameters and construct the cofficient matrix L in equation (1 ). 7. Estimate the change in parameters vector Ap, by way of the GLI solution given by equation (1).

8. Update the parameter vector p + Ap. Fig. 18. In a tomographic update, we may wish to perturb a subset of layer velocities and / or reflector geometries. This depends on our confidence in the initial model parameters for the layers and the quality of the residual moveout profiles to be used in inversion. By combining the update interval velocity profiles with the new depth horizons, we obtain the velocity-depth model after the first interaction of tomographic update. Following the update, we check for consistency of the new model with the input seismic data. Overlay the depth horizons from the updated model onto the image section derived from prestack depth migration and note that they coincide with the reflectors associated with the layer boundaries included in the model. Also, the modeled zero-offset reflection traveltimes using the updates model coincide with the observed traveltimes of the events on the unmigrated stacked section that are associated with the layer boundaries included in thP- velocity-depth model. Next, compute the residual moveout semblance spectra from the image gathers at selected locations along the line after the model update and compare them with those before the update. While most of the semblance peaks are now aligned with the vertical axis of the spectra, the tomographic update may be repeated to further remove any. remaining residual moveout errors. Proceed to a second interation of tomographic update by picking a new set of residual moveout profiles from the spectra. The changes in interval velocities and" depth horizons that result from the second iteration are smaller compared to those from the first iteration. Combine the new set of interval velocity profiles and depth horizons to create the next update of the velocity-

Seismic Em1h Modeling

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(a)

FIg. 18. (a) The interval velocity profiles, and (b) the depth honzons after the update (After Yilmaz, 200 I).

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Encyclopaedia of Petroleum Science and Engineering

depth model. Verify the updated model by perfonning prestack depth migration to generate an image section and zero-offset modeling of traveltimes associated with the layer boundaries included in the model. Next, complete the residual moveout semblance spectra to examine any remaining moveout errors. Finally, compute the horizon-consistent residual moveout semblance spectra and overlay the residual moveout profiles from the first iteration. The changes from the first to the second iteration are marginal. The extent to which an initial velocity-depth model is perturbed by a tomographic update depends on the accuracy of that initial model, which in turn depends on how it has been estimated. Irrespective of the strategy followed to drive the initial model, the results of model updating need to be verified for consistency with the input seismic data and examined for any remaining residual moveouts to decide whether or not to continue with the iterations of tomographic update. Reftection Traveltime Tomography (Mathematics for Model Update)

Reflection traveltime tomography is an inversion method for estimating the earth model parameters from the reflection traveltimes associated with the observed seismic data. The reflection traveltime from a source at the surface to the reflection point at the subsurface and back to a receiver at the surface is represented by an integral of the traveltime segments along the raypath that depend on the earth model parameters themselves. This makes the direct inversion of the traveltimes to estimate the earth model parameters a non-linear problem. Small changes in reflection traveltimes are linearly related to small changes in earth model parameters (Aldridge, 1994). To develop a theory for the model update, we shall set the following framework for our strategy : 1. Define the earth model parameters in terms of slowness functions and depths at the boundaries of the layers included in the initial model. 2. Assume that the initial model is made up of horizontal layers with laterally invariant model parameters. 3. Use the reflection times from CMP data associated with the layer boundaries included in the model and update the earth model such that the discrepancy between the modeled reflection times and the actual reflection times is minimum in the least-squares sense. 4. Estimate the changes in the model parameters, rather than the parameters themselves.

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5. Perturb the initial model in two parts : (a) slowness perturbation, and (b) depth perturbation.

6. Perturb the model parameters while preserving the offset values of the seismic data. Consider the raypath geometry in a horizontal layered earth model shown in Fig. 19. We want to derive the reflection traveltime equation for the raypath from the reflection point R j in the zn in the subsurface to the receiver location Gj at the surface z = O. This one-way raypath is associated with a CMP gather at midpoint location y and half-offset h. Our earth model consists of n layers above the reflection point Rp The modeled traveltime segment t ~c from A to C along the raypath Rpj within the ktb layer is given by : h

----y~----------------------------~~z=o x--------

--T-DW~------ Zk_1

---t--------~f_----z,. I

I I

I

,,;'"

"

-1Z -.~L~-;------Zn-1 ! 1

---~-------~-----------------------------------~ IR. II '

__-L!_______________________________ ~+I Fig. 19. Geometry of a nonzero-offset ray. ...(1)

or

t ~c =

(zk - zk_l) Sk

a

sec

ak

...(2)

Since AB = zk - Zk_1 and AC = AB sec k• In equation (1), v is the k interval velocity for the kth layer and x is the lateral distance from the midpoint location y. In equation (2), sk = lIvk is the slowness of the kth layer and k is the angle of incidence at the kth layer boundary. The

a

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Encyclopaedia of Petroleum Science and Engineering

modeled traveltime t'n from the reflection point Ri on the nth interface to the receiver Gj is then the sum of the traveltime segments given by equation (2) within n layers between the points R t and G; given by : -

t\

n

=

...(3)

L(Zk - zk-d Sk sec a k k=1

We now derive the expression for the half-offset hn between the midpoint location y and the receiver location Gr The offset segment BC is given by : -

BC

...(4)

BC

...(5)

or The half offset hnis the sum of the lateral segments within the n layers above the reflection point R; given by : n

hn

=

L(Zk -zk_l)tana k

...(3)

k=1

The raypath used in deriving the traveltime and offset equations (3) and (6), is described by the ray parameter S as S

a

= Sk sin k

••• (7)

Which is the horizontal component of the slowness. Consider an initial estimate of the parameter vecotor p:( ........ sm' .........zm' ........ ) along the raypath from R; to Gp where 1 :;; m :;; n. We want to minimize the difference between the observed times tn and the modeled times tnl by iteratively perturbing the initial estimate of the parameter vector. A change Ap in the parameter vector will change the modeled times as : -

[t~ ]modeled = [tn, linitial + [8t~ 8p ]

...(8)

IIp moduled

The error ell in modeling the traveltime is given by the equation : -

en

[tn ]observed

- [

t~

Initial - [

~;

IIp

]

...(9)

moduled

Or

en =

A

8t~

A

uf - - u n n 8p r

...(10)

Where Illn is the difference between the observed traveltimes In and the initial estimate of the modeled traveltime t l n' Equation (10) can be written as : -

Seismic Earth Modeling

65

...(11) where,

...(12)

At'n has two parts: (1) associated with the slowness perturbation, and (2) associated with the depth perturbation. This is given by equation : ...(13) where, At' n (sm) and At' n (zm) are the contributions of the slowness perturbation and depth perturbation, respectively. To compute the traveltime perturbations caused by the slowness and depth perturbations, we shall follow Sherwood et aZ., (1986). The final expression for the slowness perturbation is given by the following equation :-

...(14) The final expression for the depth perturbation is given as : -

...(15)

At' ,,(zm) = (smcos 8m- Sm+1 cos 8m+l ) Azm From equations (14) and (15), we have : At' =

L" (zm - zm_l)sec8 mAsm + L" (sm cos 8m - Sm+1 cos8

" m=1

m +1

)Azm

m=1

...(16) Finally, write equation (16) in matrix form for all the traveltime perturbation to obtain as : -

...(17) Where ...(18) And

Sm = sm cos 8m- sm+1 cosm+1

...(19)

Write equation (17) ~ compact matrix form:At' = LAp

...(20)

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Encyclopaedia of Petroleum Science and Engineering

The generalized linear inversion (GLl) solution of equation (20) is given by :-

...(21) Where ilt denotes the column vector that represents the difference between the observed reflection traveltimes and the initial estimate of the modeled times, and T denotes the matrix transposition. The procedure for the tomographic model update is given below : 1. Perform prestack depth migration using the initial model and generate a set of image gathers. 2 Compute the residual moveout for all offsets along events

on image gathers that correspond to the layer boundaries included in the model,and thus build the traveltime error vector At, e.g., ill for 10 layers, 1000 CMPs with a fold ono is 3,00,000. 3. Define the initial model by a set of slowness and depth parameters, and construct the coefficient matrix L by computing the non-zero matrix elements Zm and Sm' 4. Estimate the change in parameters vector ilp, by way of the GLI solution. 5. Update the parameters vector p + ilp. 6. Iterate steps (1) through (5) as necessary to maintain the discrepancy between the modeled and actual traveltimes. Residual Moveout Analysis

We begin with building an initial velocity-depth model from the data using the time-to-depth ccnversion strategy. Fig. 20 shows the timemigrated stacked section with a set of interpreted horizons that correspond to layer boundaries with significant velocity contrast. Perform Dix conversion of the horizon-consistent rrns velocity profiles to derive the interval velocity profiles. Often we are compelled to apply some smoothing to the rrns velocity profiles before Dix conversion and even additional smoothing to the interval velocity profiles are required afterwards. Then, perfoml inlage-ray depth conversion of the time horizons interpreted from the time-migrated section to generate the depth horizons. Finally, combine the interval velocity profiles with the depth horizons to create the velocity-depth model. Complete the analysis by checking for consistency of this estimated initial velocity-depth model with the depth

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Fig. 20. (a) A poststack tIme-migrated stacked sectIOn with a set of interpreted hOrIzons, (b) the horizon-consistent rms velocity profiles, and (c) the interval profiles derived from Dix conversion of the rms velocity profies.

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Encyclopaedia of Petroleum Science and Engineering

image derived from poststack depth migration and the stacked section. Note that the reflector geometries inferred by the depth image should be in agreement with the depth horizons. Additionally, observe that the actual reflection traveltimes on the stacked section should be in good agreement with the modeled zero-offset traveltimes. We now check for consistency of the initial velocity-depth model with pres tack data and update it by correcting for the residual moveout observed on image gathers derived from prestack depth migration. Image gathers are like moveout-corrected CMP gathers with vertical axis in depth. Unlike in CMP gathers, however, events in image gathers are in their migrated positions. Superimpose the depth horizons from the velocity-depth model onto the image section and make some minor adjustments where necessary by re-interpreting the depth horizons. Now we will make use of the image gathers to update the initial velocity-depth model. First, consider applying to a CMP gather conventional stacking velocity analysis. Compute the velocity spectrum and pick a velocity function. Following the norrnal-moveout correction of the CMP gather using this velocity function events should look flat if the velocity function had been picked correctly. If the picking was done incorrectly, then we would observe events with residual moveout. In principle, this residual moveout can be computed and used to update the initially picked velocity function. If the initial velocity-depth model has been estimated with sufficient accuracy, then the image gathers derived from prestack depth migration using this model should exhibit flat events associated with the layer boundaries included in the model. Any errors in layer velncities and/or reflector geometries should give rise to residual IPoveout along these events on the image gathers. This residual moveout can be determined and used for model updating. Assume that the residual moveout in parabolic and compute the semblance spectrum. The horizontal axis of the semblance plane represents the depth error and the vertical axis represents the depth of the events. The semblance spectrum has two quadrants that correspond to positive and negative residual moveouts, or to positive and negative depth errors. A flat event would yield a semblance peak that coincides with the vertical axis with zero depth error, whereas an event with residual moveout would yield a semblance peak situated either in the left or right quadrant depending on the sign of the depth error. A vertical function that represents the depth-dependent residual moveout can be picked from the semblance spectrum. This function can

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then be used to correct for the residual moveout. The actual steps to make the residual moveout correction are given below : -

1. Extract the interval velocity function from the velocity-depth . model at the image-gather location where the residual moveout analysis is to be done. 2 Convert the image gather from depth to time using the interval velocity function. 3. Assume that the residual moveout of events on the image gather in time is parabolic and compute the semblance spectrum for a range of negative and positive moveouts. 4. Apply the residual moveout correction to the image gather. 5. Compute a new rms velocity function from the results of the residual moveout analysis. 6. Compute a new interval velocity function from the updated rms velocity function at the image-gather location. 7. Convert the image gathers back to depth using the new interval velocities functions. 8. Finally, update the velocity-depth model using the new interval velocity functions.

In principle, residual moveout analysis can be carried out for image gathers at some spatial interval. The residual moveout spectra computed from the image gathers indicate varying degrees of errors in the initial velocity-depth model. An alternative way to measure the residual moveout is by cOII1puting it along the depth horizons themselves. Pick the residual moveout profiles from the semblance spectra and combine them with the depth horizons to create the residual moveout section. This section given an impression of how much residual moveout, thus the range of errors in the initial velocity-depth model, is present on image gathers as a function of depth and distance along the line. When combined, the updated interval velocity profiles and depth horizons yield the updated velocity-depth model. Following the update, the new model needs to be checked for consistency with the input seismic data. The depth horizons associated with the updated model, when superimposed on the image section, coincide with the reflectors that correspond to the layer boundaries. The modeled zero-offset reflection traveltimes using the updated model also are in good agreement with the observed traveltimes

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of the events on the unmigrated stacked section that are associated with the layer boundaries included in the velocity-depth model. To further verify the accuracy of the updated model, compare the residual moveout semblance spectra computed from the image gathers at selected locations along the line after the model update with those before the update. Most of the semblance peaks are now positioned along the vertical axis of the semblance spectra that corresponds to zero residual moveout. The residual moveout errors have been reduced significant after the model update. The residual moveout analysis of image gathers and the update of velocity-depth model should be performed iteratively until the velocitydepth model and the depth image are consistent. Consistency may be achieved with only a few iterations for cases where residual moveouts are small. Sometimes, consistency is not achieved even after several iterations. This often occurs when the'initial residual moveouts are caused largely by significant errors in the initial velocity-depth model. An erroneous initial estimate most likely is a result of rapid lateral velocity variations less than a spread length. In general, updating velocity-depth models based on residual moveout analysis of image gathers yields acceptable results for moderately complex structures associated with compressional and extensional tectonics. However, it may not be suitable for complex overburden structures associated with overthrust or salt tectonics. Resolving Velocity-Depth Ambiguity by Tomography (Limitations) Reflection traveltime tomography can be successful in updating an initial model with significant errors. Consider the velocity-depth model in fig. 21 with constant layer velocities. Perform prestack depth migration using the initial model to derive the image section and compute the residual moveout semblance spectra of the selected image gathers. The significant moveout errors manifested by the semblance peaks which are off the zero moveout centerline on the spectra. Now, compute the horizonconsistent residual moveout semblance spectra along the depth horizons and note the significant departures from the zero moveout line. Pick the residual moveout profiles from these spectra and perform a tomographic update to obtain a new set of interval velocity profiles and depth horizons. Combine the updated interval velocities and reflector geometries to create the updated velocity-depth model. This model is fairly close to the updated models derived from the applications of time-

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(e)

Fig. 21. (a) A velocity-depth model, (b) poststack depth migration with depth horizons in (a), and (c) modeled zero-offset traveltimes overlayed on the unmlgrated stacked section (After YJimaz, 2001).

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to-depth conversion and layer- by- layer inversion strategies. Model verification tests and residual moveout semblance spectra demonstrate the feasibility and accuracy of the updated model. As demonstrated by the extreme case of an erroneous initial model, a tomographic update can steer the model toward an acceptable final model. Whatever the strategy used in the model building, if we start with an initial model with large errors, do not expect tomographic updating to correct for model errors. Iterative tomographic updating does not necessarily result in convergence of an initial model to a final model with zero residual moveout. Instead, the solution may just wobble and never converge. This scenario is especially likely for cases of complex overburden structures. In such cases, additional information, such as well control, check-shot velocities, and geological constraints, is required to derive an acceptable final model.

Threshold forVelocity-Depth Ambiguity Velocity-depth ambiguity states that an error in depth is indistinguishable from an error in velocity. Consider a single, horizontal reflector at depth z in a medium with constant velocity v. The reflection traveltime associated with a CMP recording geometry of a fixed offset 2h is given by the hyperbolic moveout equation as : t(v, z) = 2~(z2 + h 2 ) Iv ...(1) Perturbation of velocity v by dV and depth z by dz causes a change in traveltime t (v, z) by M expressed as : -

M

=

otlav. dv + otloz dz

...(2)

Differentiating equation (1) with respect to velocity v, we have : -

ot/ov =

_2~(z2 + h 2).lIv2

...(3)

Putting the value oft in equation (3), we have

8t18v = tlv

...(4)

Again, differentiating equation (1) with respect to depthz, we have:-

8tloz =

2zlv~(z2 + h 2)

...(5)

Solving equation (5) and (1), we have:...(6) Substitute the partial differentials given by equations (4) and (6) into equation (2), then normalize with respect to traveltime, we have :-

73

Seismic Earth Modeling Atlt = -dvlv + !lz4zlv2t2

...(7)

For the case of zero offset, t = 2z/v, hence, equation (7) becomes: Atlt = -llvlv + 4zlz

...(8)

Equation (8) states: for a zero-offset case, when the perturbation in velocity llvlv is the same as the perturbation in depth dzlz, no change occurs in traveltme. Thus, velocity-depth ambiguity is infinite. Now consider two earth models: (1) an unperturbed model defmed by the variables (z, v), and (2) a perturbed model defmed by the variables (z + llz, v + llv), such that the two modes are indistinguishable at zero offset. This means that the zero-offset times to associated with these two models are identical. Putting the value of zero-offset time to = 2z/v in equation (1), we have:tlv = to ~(l +4h2 It/v 2 ) ...(9) Perturb the velocity v by llv and evaluate the change in time At, we have:...(10)

M = ot/av. llv

Evaluate the partial differential (otlf)v) from equation (9), and substitute this value in equation (10), we have : -

M

1

4h211v

=

---.--;===== 3 v (vj)

4h2

...(11)

1+"22 toY

Expand the equation--< 11) by the Taylor series upto the second order, weget:-

M =

_

4h211v v3to

.(1-

2h2 ) t~v2

...(12)

Finally, retain only the term with power of two in h, nonnalize with respect to traveltime to and rearrange the variables to obtain the expression for the absolute value of the fractional change in velocity llv/v, we get : llv/v = z2lh 2. Atlto ...(13) Where z = vti2. Equation (13) states another rule for velocity-depth ambiguity: for a reflector at depth z, velocity ambiguity defmed by llvlv can be made smaller by increasing the offset h and decreasing the error in the time pick defined by Atlto at offset h.

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Time Structure Maps Time horizons are picked from image volume obtained from either 3-D poststack or 3-D prestack time migration. Aside from improved imaging of conflicting dips with different stacking velocities, the latter offers the advantage of roviding a 3-D rms velocity field that is associated with events in their migrated positions. Fig. 22 shows a 3-D view of the image volume derived from 3-D prestack time migration. The interpreter identifies the time horizons that are associated with depositional sequence

Fig. 22. An image volume of data derived from 3-D prestack time migration.

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7S

boundaries and geologically and lithologically significant layer boundaries within some of the depositional units. Then, reflection times are picked by combining seed detection and line-based interpretation strategies. A simplified form of an interpretation session without explicit fault identification is given below :-

1. Seed points are placed on the events that is being picked at locations with good signal-to-noise ratio. These are then used to derive a seed detection algorithm to pick patches of the time surface around each seed point. Depending on the signal-tonoise ratio and complexity of geometry of the time horizon, the extent of the surface patches varies from horizon to horizon. Some horizons are almost entirely picked by seed detection, some are covered by limited amounts of seeddetected surface patches, and some are not eligible for seed detection. Seed detection has failed especially in intensively faulted areas. 2 To ensure structural control and adequate coverage of the time horizons, additional picking along inlines and cross lines is required. 3. The surface patches derived from seed detection and horizon strands derived from line-based picking are than combined to form the complete set of control points for each horizons. At this stage, a comprehensive editing and repicking are required to ensure consistency in picking. The edited control points are than input to a surface fitting algorithm to create grid points that defme the surface by a map function In (x, y) at every inline and crossline intersection, where the function value In represents the reflection time at the (x, y) location on the nth surface. Since we use rays to perform time-to-depth conversion, it is important to ensure that ray tracing is made stable by applying a carefully measured amount of smoothing to the gridded surfaces. This smoothing also is needed to edit outliers among the control points that have inevitably corrupted the grid points. Finally, the gridded surfaces are usually displayed in the form of contour maps. Thrning-Ray Tomography

Just as reflection traveltime tomography can be used to update an initial estimate of a subsurface velocity-depth model, turning-ray

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tomography may be used to update an initial estimate of a near-surface velocity-depth model (Bell et al., 1994). The steps are given below:-

1. Begin with the picking of the first arrivals that represent the diving waves through the near surface. 2 Defme an initial velocity-depth model by a set of near-surface layers with constant velocities and thickness, and model the first-arrival times by ray tracing. 3. Compute the difference between the modeled and observed firstarrival times. 4. Estimate the change in parameters vector Ap by way of the GLI solution by perturbing the velocities of the near-surface layers only.

5. Update the parameter vector p + Ap to obtain a new near-surface velocity-depth model. 6. Iterate steps (1) through (5) as necessary to minimize the discrepancy between the modeled and actual first-arrival times.

The fmal velocity-depth model resulting from the iterative application of turning-ray tomography is then used to compute the one-way traveltimes through the near-surface model along vertical raypaths. These are then used to apply the necessary source and receiver statics corrections to the prestack data. Fig. 23 shows the statics solution derived from turning-ray tomography applied to a 3-D offshore seismic data set from the Mississippi Delta (Kim and Bell, 2000). The mudflows characterized by the strings of negative statics shifts. See the significant improvement of event continuity in the central part of the section.

Velocity-Depth Ambiguity A fundamental problem with inversion is velocity-depth ambiguity. This means that an error in layer velocity can be indistinguishable from an error in reflector geometry. To solve the velocity-depth ambiguity, we must use non-zero-ofIset data to estimate the layer velocities and reflector geometries. We can never solve the velocity-depth ambiguity completely and obtain the true-depth model from inversion of seismic data. If, there is ample well control, we may get one of the few solutions and declare

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Fig. 23. (a) A map of statics derived from turning-ray tomography applied to a 3-D seismic data set, (b) an inleni stack along the traverse indicated by the horizontal line in (a), and (c) the same line stack with turning-ray tomographic staties corrections (After Kim and BeJl, 2000).

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that as the solution. Since we can never obtain the true representation of an earth model from inversion of seismic data, the plausable strategy is to estimate an initial model and then update it to get a fmal model that may be considered an acceptable approximation to the true model. The important question in practice is, following the model update, how much velocity-depth ambiguity remains unresolved in a fmal velocity-depth model. The ambiguity with zero-offset data is infmite. By using non-zerooffset data, we can hope to resolve the velocity-depth ambiguity upto a certain theoretical limit. An important rule to keep in mind is that, for data with good quality, velocity-depth ambiguity for a reflector can be resolved with an acceptable degree of accuracy if the data used in inversion have been recorded with offsets greater than the reflector depth (Lines, 1993). While an earth model can only be estimated with an accuracy that is within the threshold of velocity-depth ambiguity, it does have to be consistent with the input data used in inversion to estimate the model. Consistency is a necessary condition for an earth model to be certified as an acceptable estimate of the true model. A quick way to check for consistency is by ray-theoretical forward modeling of zerooffset traveltimes associated with the reflector boundaries that are in the earth model itself, and then comparing them with the actual travel times picked from the stacked data. Any discrepancy between the modeled and actual traveltimes is an indication of errors in the earth model parameters : layer velocities and / or reflector geometries. By using nonzero-offset data, we can hope to reduce the many possible solutions to a few. Furthermore, by introducing constraints, we may be able to corverge to a single solution provided the set of constraints ar reliable. One set of constraints is the depth information at well locations. This well-top information can be used to calibrate results of inversion of surface seismic data and obtain a single earth model in depth that not only is consistent with the surface data set itself but also with the borehole data. Aside from consistency, model verification has to include a test of flatness of events on image gathers derived from prestack depth migration. A correct model, again, within the. limitations of velocity-depth ambiguity and accuracy of inversion methods, would yield an accurate image from pres tack depth migration irrespective of the source-receiver offset. Thus, with the correct model, the resulting image gathers would have flat events. An erroneous earth model, on the other hand, would cause residual moveout on image gathers.

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1. Aldridge, D.F., 1994; Linearization of the eikonal equation; Geophysics, Vol. 59, pp. 1631-1642. 2. Bell, M.L., Lara, R. and Gray, W.C., 1994; Application of turning-ray tomography to the offshore Mississippi Delta; 64th Ann. Intemat Mtg., Soc. Expl. Geophys., Expanded Aubstracts, 1509-1512. 3. Dix, C.H., 1955; Seismic velocities from surface measurements; Geophysics, Vol. 20, pp. 68-86. 4. Hubral, P. and Krey, T., 1980; Interval velocities from seismic reflection time measurements; Soc. Expl. Geophys. 5. Kim, K.S. and Bell, M.L., 2000; 3-D turning-ray tomography and its application to Mississippi Delta; 70th Ann. Intemat Mtg., Soc. Expl. Geophysics., Expanded Abstracts pp. 593-596. 6. Kosloff, D., Sherwood, J.W.C., Koren, Z., Machet, E. and Falkovitz, Y., 1996; Velocity and interface depth determination by tomography of depth migrated gath~rs; Geophysics, Vol. 61, pp. 1511-1523. 7. Landa, E., Kosloff, D., Keydar, S., Koran, Z. and Rushef, M., 1988; A method for determination of velocity and depth from seismic data; Geophys. Prosp., Vol. 36, pp 223-243. 8. Lines, L., 1993; Ambiguity in analysis of velocity and depth; Geophysics, Vo!. 58, pp. 596-597. 9. Reshef, M., 2001; Some aspects of interval velocity and analysis using 3-D depth migrated gathers; Geophysics, Vo!' 66, pp. 10. Rocca, F. and Toldi, J., 1983; Lateral velocity anomalies; 53 rd Ann. Intemat, Mtg., Soc. Exp!. Geophys., Expanded Abstracts, 572-574. II. Sheriff, R.E., 1991; Encyclopedic dictionary of exploration geophysics; Soc. Exp!. Geophysics. 12. Sherwood, J.W.C, Chen, K.C. and Wood, M., 1986; Depths and interval velocities from seismic reflection data for low-relief structures; Proc. Offshore Tech. Conf. pp. 103-110. 13. Thorson, J.R., Gever, D.H., Swanger, H.J., Hadley, D.M. and Apsel, R.I., 1987; A model-based approach to interval velocity analysis; 57th Ann. Intemat Mtg., Expanded Abstracts, pp. 458-460. 14. Yilmaz, O.z., 2001; Seismic Data AnaIysis, Vo!' 2; Society of Exploration Geophysicists, Post Office Box 702740, Tulsa, OK 74170-2740.

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2 Seismic Earth Imaging Introduction Strong lateral velocity variations associated with complex overburden structures require earth imaging in depth, e.g., diapiric structure formed by salt tectonics, imbricate structures formed by overthrust tectonics and irregular water-bottom topography. These are characterized as structuredependent lateral velocity variations. There also exist structureindependent lateral velocity variations, often associated with facies changes, e.g., change in lithology from shale to sandstone to carbonate induce lateral changes in acoustic impedance. Earth imaging in depth is achieved by depth migration. The following is a cause-and-efffect relation between various factors with regard to depth migration : 1. Complex overburden structures often give rise to strong lateral velocity variations. In the presence of strong lateral velocity variations, an earth image in time derived from time migration is not accurate. It is imperative to obtain an earth image in depth by depth migration. 2 Strong lateral velocity variations cause significant ray bending at layer boundaries. 3. This ray bending gives rise to non-hyperbolic of reflection times on CMP gathers that correspond to layer boundaries below a complex overburden structure. 4. As a result, amplitudes and traveltimes associated with the reflection events with non-hyperbolic move-out are distorted during conventional CMP stacking which is based on the hyperbolic moveout assumption.

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Encyclopaedia of Petroleum Science and Engineering 5. This causes CMP stack to depart from an ideal zero-offset wavefield. Therefore, when depth migration is needed, it must be dome before stack and not after. 6. Finally, complex overburden structures often exhibit threedimensional (3-D) behaviour. Therefore, when depth migration is needed, it must be done not only before stack but also in three dimensions. & A

Detailed Encyclopaedia This encyclopaedia is arranged in alphabetical order. The detailed encylopaedia is give below : -

Calculation of Traveltimes A direct method to compute traveltimes is ray tracing through the specified velocity-depth model. A bundle of rays emerging from a source location at the surface can be sprayed down into the earth and traced through the subsurface while accounting for ray bending caused by changes in velocity gradient and refraction at layer boundaries with velocity contrast. Reflection points along each of the raypaths are identified as the intersection points of the rays with the layer boundaries. The traveltime from the source location at the surface and a reflection point at the subsurface is then calculated by integrating the elements of distance along the raypath divided by the velocity associated with that element. By applying reciprocity, the traveltime from a receiver location at the surface to a reflection point in the subsurface can be computed in the same manner. Finally, for a given source-receiver pair at the surface and a reflection point in the subsurface, the total traveltime is computed by adding the traveltime from the source to the reflection point to the traveltime from the reflection point to the receiver. Efficient ray tracing through complex velocity-depth modes is not a simple work. Alternatives to two-point ray tracing have been developed and implemented with sufficient accuracy, e.g., paraxial ray tracing (Keho and Beydoun, 1988) and Gaussain beam ray tracing (Cerveny eta!., 1984). There will not always be a raypath combination associated with a sourcereceiver pair and a reflection point. Direct ray tracing is rarely used for traveltime computations required for prestack depth migration. An alterative to two-point ray tracing is wavefront construction (Vinji et. al., 1993), which involves tracing not just one ray but a fan of rays together. As such, the medium represented by the velocity-depth model used in ray tracing is covered adequately by controlling the ray density along

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wave-fronts (Lacomte, 1999). In areas with low ray density, additional ray bundles may be created by paraxial rays.

Earth Imaging In Depth The migration methods based on a layered media assumption make them accurate for situation with mild lateral velocity variations by simple modifications of the basic algorithms, e.g., rms velocities can be varied laterally in Kirchoff migration. In the finite-difference method, as long as lateral velocity variations are mild, the thin-lens term can be dropped, and the velocity function used in the diffraction term can be varied laterally. In the frequency-wave-number methods lateral velocity variations are accommodated by varying the stretch factor between 0 and I, e.g., Stolt migration. When strong lateral velocity variations are encountered, simple algorithmic modification no longer provide adequate accuracy, and depth migration must be done (Schultz and Sherwood, 1980). Only depth migration algorithms implement the additional thin-lens term that explicitly accounts for lateral velocity variations. The output from the migration algorithms that include the thin-lens term is a depth section, thus the term depth migration. Sometimes the complex overburden can be defined by a single layer and the boundary between the overburden and the substratum can be determined in the form of an irregular interface with a significant velocity contrast. In this case, the layer replacement method can be used to remove the deleterious effects of the overburden on the geometry of the underlying reflections. Time migration requires an rms velocity field, whereas depth migration requires an interval velocity-depth model. A velocity-depth model usually is dermed by two set of parameters : (1) layer velocities, and (2) reflector geometries. While an rms velocity field does not contain discontinuities, an interval velocity-depth model can include discontinuities associated with layer boundaries. A velocity-depth model is the seismic representation of an earth model in depth. An earth model and the earth image created from it are an inseparable pair of products of seismic inversion. To obtain an earth image in depth, one has to first estimate an accurate earth model in depth. Aside from earth modeling and imaging in depth, depth migration also used to verify and update velocity-depth models. Depth migration is used in an interactive way. Starting with an initial velocity-depth model, depth migration is performed and results are interpreted for updating layer velocities and reflector geometries. Using the updated velocity-depth model, depth migration is repeated until such time as the velocity-depth

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model matches with the velocity-depth model input to depth migration. To achieve rapid convergence to a final velocity-depth model, only one set of parameters, i. e., reflector geometries, are altered from one iteration to the next. In 2-D migration, we assume that the seismic line is along the dip direction and that the recorded wave-field is two dimensional. Shot-geophone migration, which is based on downward extrapolation of common-shot and common-receiver gathers using the double-square root equation, focuses primary reflection energy to zero-offset. The migrated section is obtained by retaining the zero-offset traces and abandoning the non-zero-offset traces. Shot-profiJe migration is based on migrating each common-shot gather, individually. In this method, the migrated section is obtained by sorting the migrated common-shot gathers into common-receiver gathers and summing the traces in each receiver gather. 3-D poststack depth migration can be performed using a wide variety of a algorithms based on finite-difference, frequency-wavenumber, and Kirchoff integral solutions to the 3-D scalar wave equation. But 3-D prestack depth migration often is done using the Kirchoff integral or the eikonal equation. The output of prestack depth migration consists of image gathers, which may be likened to moveout-corrected CMP gathers with tr.e vertical axis in depth. Image gathers consist of traces in their migrated positions. A stack of image gathers represents the earth image in depth obtained from prestack depth migration. If the velocity-depth model used in prestack depth migration is correct, then, events on an image gather would exhibit a flat character with no moveout. An erroneously too low or too high velocity would cause a residual moveout on the image gathers. The initial velocity-depth model can be updated by analyzing this residual moveout and correcting for it. Eikonal Equation Consider a plane wave function P (x, y, z, t) with a spatially varying amplitude Po (x, y, f') and spatially varying traveltime T (x, y, z) is giveby:P(x,y, z,; t) = Po (x,y, z) exp {-iw[t-1{x,y, z)]}

...(1)

Assuming that the wave amplitude Po (x, y, z) does not vary spatially, but is a constant. Then, the plane-wave solution of equation (1) satisfies the scalar wave equation given below : -

a2 p

(ip

(ip

ax2 + ay2

+ az2

a

=

2p I v 2(x,y,z)' at2

Substitution of equation (1) into equation (2), we have : -

...(2)

Seismic Earth Imaging

85 1 ...(3)

Equation (3) is called the eikonal equation. It is a ray-theoretical approximation to the scalar wave equation (2). While the solution of the scalar wave equation (2) represents the wave-field P (x, y, z; t) at a point in space (x, y, z) and at an instant of time t, the solution of the eikonal equation (3) represents the traveltime T (x, y, z) for a ray passing through a point (x, y, z) in a medium with velocity v (x, y, z). Specifically, T (x, y, z) = constant represents the wavefront of constant phase at an instant of time. The wave is propagated from one wavefront to the next by way of raypaths, which are perpendicular to the wavefronts. When the medium velocity is not constant but is an arbitrary function of space variables v(x, y, z) and the wave amplitude Po (x, y, z) is not constant but also varies spatially, then the traveltime function T (x,y,z) of equation (1) is not a solution to the eikonal equation (3). For a wave function with spatially varying amplitudes, the eikonal equation is a good approximation to the wave equation only at a high-frequency limit. The high-frequency limit is equivalent to small wavelengths. The eikonal equation can be used to compute traveltimes if the velocity-depth model does not contain large velocity gradients. The eikonal equation (3) can be solved using the finite-difference technique. See fig. 1. We want to

T(T" ____

1'(1'"

T(x-+i1x,z+L\Z) (a)

, +,1y,z)

T(.r+L1x,y.z+L1z) (bl

Fig. 1. Finite-difference mesh used for (a) 2-D and (b) 3-D solution to the eikonal equation.

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compute the traveltime Tofthe eikonal equation at grid point (x + !lx, y, z + AZ) using the known traveltimes at grid points (x, y, z), (x + !lx, y, z) and (x + !lx, y + Ay, z). Computing the traveltimes at depth z + Az from those at depth z means extrapolating T in the z-direction. Rewriting equation (3) in the fonn of an extrapolation equation given by : -

aT

vf)z

...(4)

The 3-D eikonal equation (4) can be solved for the traveltime values T (x, y, z) for the propagating wavefront through a velocity field v (x, y, z) in the subsurface using a finite-difference scheme. Fig. 2 shows an

s

Fig. 2. Wavefronts of an expanding wavefrom a source located at surface point 5. The layer above has a lower velocity than the layer below. The dotted lines indicate the wavefronts associated with the head wave that is refracted at a critical angle along the interface of the two layers.

expanding wavefront through a flat interface with a large velocity contrast. The layer above as a lower velocity than the layer below. At the critical angle of refraction, waves travel along the layer boundary with the faster velocity of the underlying layer. Eventually, these waves

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are refracted back into the overlying layer and are recorded in the form of first arrivals. They often are called head waves. The wavefront associated with the head waves tends to smooth out the sharp change in the expanding wavefront as it crosses over the layer boundary with velocity contrast. Ideally, the traveltime contours should pronounce the sharp layer boundaries. While some solutions to the eikonal equation include the head waves, others exclude the head wave. Fig. 3 shows traveltime contours that were computed using a finite-difference solution to the eikonal equation. The velocity-depth model consists oflayers with velocities that vary from 1500 mls to 4000 mls. The eikonal solution, although it is associated only with the fastest arrival, always yield a raypath from a source at the surface to a point in the subsurface through the grided velocity-depth model. Some of the raypaths are indicated by trajectories that have a solid circle at the end. The smooth behaviour of the traveltime contours is associated with the head wave that often is the flrst arrival. In contrast with the eikonal solution, the wavefront construction yields multiple arrivals. Again, some of the raypaths are indicated by trajectories that have a solid circle at the end. The multiple raypaths are associated with a single point in the subsurface. Where a head wave or diffraction develops, the wavefront construction leaves a gap in the traveltime contours. Part 'C' of this figure shows traveltime contours associated with the reflecting boundary represented by the thick curve. The solution from wavefront construction in this case includes arrivals associated with reflected waves as well as d;ansmitted waves. The eikonal solution does not necessarily yield the maximum energy along the single-arrival raypath. In wavefront construction amplitudes associated with multiple arrivals can be included in the summation. By including multiple arrivals in the summation, the likelihood of attaining a complete image of a complex structure also is higher. Fermat's Principle The traveltime along g raypath from one point to another has an extremum value which, for most physical problems, is a minimum. The raypath also defmes the direction of energy flow. Among a bundle of rays from one point to another, Fennat's principle can be applied to discard all but are raypath that corresponds to a minimum time of travel from one point to the other. This practical concept can be used to perform travel-time computations for prestack depth migration (Vesnaver, 1996).

Encyclopaedia of Petroleum Science and Engineering

88 00

2.0

hor~ntal a~km)

80

.2/J

40

2/J

"::--__--,40 (b)

horizontal axis (km)

00~~~~2/J~~~~4D~~~~60~~;=~8O~::~:~1OO

(e) Fig. 3. Traveltime contours derived (a) from a finite-difference solution to the eikonaI equation, (b) and (c) from wavefront construction (After Lecomte, 1999).

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See fig. 4. The velocity-depth model may be split into a set of horizontal slabs with a specified thickness, say 50 to 300m. Assume that traveltimes

S

----~------------------------z=O

Fig. 4. A sketch that illustrates traveltime calculation using Fermat's principle.

from a source at the surface z = 0 to a depth z through a velocity-depth model already have been computed. Now compute the traveltime from the grid point 6 at depth z to each of the grid points at depth z + ~ within a specified aperture, say grid points 1 to 11. An average velocity between the grid points at depth z and z+ ~ along each of the raypaths may be used in the computation. Among the 11 raypaths from the grid point 6 at depth z to grid points 1 to 11 at depth z+ ~, choose that which corresponds to the minimum traveltime. The process may be continued for all grid points at depth z, and then from one depth to the next, and the traveltimes associated with minimum-traveltime raypaths through each of the horizontal slabs may be added to compute the total traveltime from a source or receiver point at the surface z = 0 to areflection point at some depth in the subsurface.

Iterative Depth Migration Depth migration has been used in an iterative manner to obtain an earth image in depth from CMP-stacked data. When perfonned iteratively, depth migration is done using an initial velocity-depth model and the result is interpreted for the layer boundaries included in the model. The velocity-depth model then is modified accordingly and depth migration

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is performed once more. The process is continued until convergence is achieved. It means that what is input to depth migration as the velocitydepth model matches with the velocity-depth model inferred from the output from depth migration. We shall demonstrate that, by way of convergence, the final velocity-depth model from iterative depth migration, albeit not guaranteey to be accurate, can be made at least consistent with the input data. Consistency means that the modeled zerooffset traveltimes match with the observed reflection traveltimes on the stacked data associated with the layer boundaries included in the velocity-depth model. Convergence and consistency are the two necessary, but not sufficient, conditions for an earth model to be certified as a valid, geologically plausible solution from seismic inversion. For a velocity-depth model to be valid, a further requirement is that it also needs to be consistent with prestack data. Iteration with Different Data Types Depth migration of the zero-offset section produces an accurate image of the salt diapir and yields the same output model as the input model in a single iteration. In fig. 5 the salt velocity and the base-salt boundary are specified incorrectly. In practice, often the top-salt boundary is determined with reasonable accuracy by way of time migration. However, accurate delineation of the base-salt boundary is impossible with time migration. Start with Model B as the initial velocitydepth model and perform the depth migration. The result indicates that the geometry of the base-salt boundary and the flat reflector below has changed. Interpret all the layer boundaries, i.e., top-salt, base-salt, and the deeper reflectors, from the result of depth migration and create an updated velocity-depth model. Use this new model and perform depth migration once more. Interpret the new image from depth migration for all the three layers boundaries and obtain an updated velocity-depth model. Finally, perform depth migration for the third time and interpret for the three layer boundaries, once more. After this third iteration, we find that the velocity-depth model does not change from the previous iteration. Hence, convergence is achieved. The final solution has converged to a velocity-depth model that is different from the true model. The result of iterative depth migration is dictated by the parameters of the initial velocity-depth model. True geometry of reflectors can be recovered by iterative depth migration provided layer velocities are correctly specified in the initial velocity-depth model. If the initial model contains significant errors but only in reflector geometries, and layer velocities are specified correctly and are not altered from one iteration to

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91

.. ' . ---!Okm .t . "'.,. . . . -.,r.-'.. --:IIe:l:---'Ho' ~_~ >d. _

Iteration 1

Model B

L.~

--

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2

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•

lie

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Zero-Offset Depth Migration

Fig, 5. Iterative depth migration using the zero-offset section and model B that contains errors in salt velocity and base-salt reflector geometry.

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the next, then convergence to the true velocity-depth model can be achievable. Zero offset traveltime sections are computed from the final velocity-depth models. Compare these traveltime sections with the input zero-offset wavefield section, and note that they all are consistent with the latter. Consistency does not guarantee that the final solution from iterative depth migration yields the true model. In practice, we will never know which one of the final solutions corresponds to the true model. Depth migration of the CMP-stacked section requires two iterations to achieve convergence. See fig. 6. The resulting image and the model IteratIOn 1

11

;

___ -'~-

2.0

2.$

.

--- --,i

' --' ',,----1

---:-------, ., ! Poststack Depth Migration

Fig. 6. Iterative depth migration using the eMP-stacked section and model A is the true velocity-depth model

inferred from it have some inaccuracies. It appears that starting with the true model, iterative poststack depth migration does not exactly reproduce the true model. For the zero-offset section, starting with the time model, convergence was achieved after one iteration and the resulting model Was the same as the input model. The CMP stack is only an approximation to the zero-offset wavefield. The more the stacked section departs from

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the zero-offset wavefield, the more the final model will depart from the true model. If the initial model contains errors in reflector geometries only, and layer velocities are specified correctly, then convergence using the stacked section to the true velocity-depth model is nearly achievable. Depth migration of the prestack data using the true model requires only one iteration to achieve convergence. If the initial model contains errors in reflector geometries only, and layer velocities are specified correctly, then convergence using the prestack data to true velocity-depth model is achievable. The important points to remember are given below : 1. The image from iterative depth migration will converge to the true velocity-depth model. It depends on the type of input data and errors in the initial velocity-depth model. See fig. 7. Iterative depth migration never guarantees that the solution converges to a true velocity-depth model. Iterative depth migration will converge to true velocity-depth model if input velocity-depth model is in error of input data

true

reflector geometries

layer velocities

zero-offset

yes

most likely

no

prestack

yes

very likely

no

stack

nearly

likely

no

Fig. 7. Performance of iterative depth migration for different types of input data aT\d errors in the initial velocity-depth model.

2. It is wrong to discontinue an iterative application of depth migration without achieving convergence. Because the intermediate output does not represent a valid earth image in depth and neither does it infer a valid earth model in depth. Zerooffset traveltimes associated with this earth model are not consistent with the traveltimes in the input section. 3. In iterative depth migration, only one set of parameters, i.e., either layer velocities or reflector geometries, should be modified

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from one iteration to the next. Changing the parameter type at some intermediate iteration will only divert the solution to a different end result and will cause the convergence to that end result to take longer. It is advisable to keep layer velocities unaltered from one iteration to the next, and only modify reflector geometries by interpreting the output image from depth migration. 4. Number of iterations depend on how much the initial model departs from the true model. If the initial model contains errors in layer velocities, only few iterations are required than if the model contains errors in reflector geometries. If the model contains errors both in layer velocities and reflectors geometries, a large number of iteration is required to achieve convergence.

s.

Iterative depth migration converges to a solution, which never corresponds to the true velocity-depth model. Therefore, the final velocity-depth model estimated from iterative depth migration needs to be calibrated to well data.

While the consistency of the modeled and actual zero-offset traveltirnes verifies that the fmal velocity-depth model from poststack iterative depth migration meets the convergence criterion, the model is not guaranteed to be accurate. There exists not just one but many velocity-depth models are consistent with the stacked data. An acceptable model is that which also is consistent with prestack data. Thus, prestack imaging is resolving the uncertainty in the acceptable velocity-depth models and reducing the many possible models to a few that are geologically plausible. Kirchoff Summation Kirchoff's integral solution to the scaler wave equation is given by:-

02p 02p 02p --+--+-ox 2 ay2 az2

1 v 2(x,y,z)

01 2

...(1)

It gives the pressure wavefield P (x, y, z; I) propagating in a medium with velocity v (x, y, z) at a location (x, y, z) and at an instant of time t. The Kirchoff solution is a mathematical statement ofHuygen's principle which states that the pressure disturbance at time t + At is the

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95

superposition of the spherical waves generated by point sources at a time t (Officer, 1958). The discrete fonn of the integral solution to equation (1) as used in practical implementation of Kirchoff migration is givenby:P

= out

ax~Y "" cos e .~ R,.n 47t.i..J vr

at

...(2)

Where ax and ~Y are inline and crossline trace intervals, Pout = P P(XOUf YouI' z; T = 2z/v) is the output of migration using the input wavefield Pin = P (Xin , Yin' Z = 0; T = t - rlv) within an areal aperture A. See fig. 8. The requirements of Kirchoff summation method are : -

n ~~~~~------------------~-..x

A

y

/

z

Fig. 8. Geometry of a point diffractor to derive the Kirchhoff integral solution to the scalar wave equation.

(1) Computing nonzero-offset traveltimes through a 3-D, spatially varying velocity medium, and (2) scaling and summation of the amplitudes along the computed traveltime trajectory based on the Kirchoff integral solution to the scaler wave equation. The scaling of amplitudes before summation includes application of the obliquity factor cos e, the spherical divergence factor l(vr, and the amplitude and phase corrections lrolexp (i7t/2) implied by the derivative operator

ap .Additionally, as for any migration, under-sampling of the at

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input data in x and y directions needs to be compensated for by a suitable antialiasing filter. It is the traveltime computation that poses numerical accuracy and efficiently challenges when implementing the Kirchoff summation method for 3-D prestack depth migration. Lateral Velocity Variations Lateral velocity variations often are associated with steep dip~ Hence, a depth migration algorithm must not only handle lateral velorit} variations but also must image steeply dipping events accurately. The steep-dip implicit and explicit frequency-space migration algorithms are particularly suitable to accommodate lateral velocity variations. In the case of the implicit scheme, the action of the thin-lens term, which accounts for lateral velocity variations, is achieved by a complex multiplication of the wave field in the frequency-space domain with a velocity-dependent exponential term. In the case of the explicit schemes, lateral velocity variations are accounted for by designing a velocitydependent, laterally varying extrapolation filter and convolving it with the wave field in the frequency-space domain. The problem of lateral velocity variations is studied using a point diffractor buried in a medium with different types of velocity-depth models. The image ray behaviour and the quality of focusing determine whether time or depth migration should be performed. If the starting and end points of the image ray have the same CMP location, only time migration is needed. See fig. 9. A small amount of lateral deviation of the image ray usually implies a well-focused time migration result and hence, a good representation of the geometric form of the subsurface. Large image-ray deviations imply grossly incorrect focusing, thus requiring depth migration rather than time migration. Finally, if more than one image ray is associated with a subsurface point, depth migration is imperative. See fig. 10. The image rays associated with this figure are shown in fig. 11. There is no deviation from the vertical along the image rays down to horizon 2. Therefore, depth migration is not needed to image this horizon. On the other hand, the image rays significantly deviate from the vertical as they travel down to horizons 3 and 4. For example, the image ray starting at CMP location 140 reaches horizon 4 approximately beneath CMP location 180, i.e., a lateral shifts of 40 midpoints. Proper imaging of these two horizons is achievable only by depth migration. At each downward-continuation step, the action of the thin-lens term is equivalent to a time shift that depends on spatial velocity variation.

Seismic Earth Imaging "biT SCUTfA(A: (I.ll.

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Fig. 9. The response of a point diffractor buried in a layered medium (a) is approximately a hyperbola (b). Time migration (c) still is adequate for imaging the diffractor.

98

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Fig. 10. The response of a point dim'actor buried in a medium with severe lateral velocity variation (a) is a distorted traveltime curve that implies false structural features (b). time migration (c) no longer is acceptable; instead depth migration is imperative (d)

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99

coy

---

PvIJH8£R

--. -- -~.----

Fig. II. Image rays through the velocity-depth model in fig. 10. Image rays deviate from the vertical as they travel through the complex structure (After Yilmaz, 200 I).

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Since the thin-lens and diffraction tenns are applied in an alternate manner as the wavefield is downward continued in depth, the effects of these two tenns are strongly coupled when the lateral velocity variation is severe. When lateral velocity variation is moderate to strong, these two tenns can be separated and applied consecutively without significant error (Lamer etal., 1981). Full separation implies that a correction for the effects of the thin-lens term can be done either before or after time migration. If the correction is done after time migration, image-ray mapping should be used. If the correction is done before time migration, mapping using vertical time shifts usually is applied In practice, a correction before time migration often perfonns better, since it tends to provide a better focused migration result. It is a common practice to do time-to-depth conversion of time horizons interpreted from time-migrated data by using image rays.

Layer Replacement The problem of a complex overburden that involves only one layer boundary, such as an irregular water bottom, at which significant ray bending takes place can sometimes be addressed by prestack layer replacement followed by NMO correction, CMP stacking and poststack time migration. In both cases, the approaches are based on the philosophy of revising velocity estimates and obtaining an improved unmigrated stacked section. See Fig. 12. Complex geometry of the boundary between the overburden and the substratum and the significant velocity contrast across this boundary causes the severe ray bending. This in turn causes distortions and disruptions of the underlying target reflections. Without the velocity contrast, the rays would not bend and there would be no need for depth migration. Replacing the overburden velocity with the substratum velocity can be a viable alternative to using depth migration to remove the deleterious effects of a complex overburden on the substrata. This technique is known as layer replacement A technique for layer replacement is based on wave-equation datuming (Berryhill, 1984). This technique involves extrapolating a known wavefield at a specified datum of arbitrary shape to another datum, also of arbitrary shape. Wave extrapolation is performed using the Kirchoff integral solution to the scalar wave equation. It incorporates both the near-field and far-field tenns. The velocity used in extrapolation is that of the medium confmed between the input datum and the output datum. Fig. 13 shows a simple case of datuming. A zero-offset section is computed over three point scatterers buried beneath midpoint location

101

Seismic Earth Imaging

CMPNUMBER

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400

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(b) Fig. 12. (a) Velocity contrast between the overburden and the substratum causes raypath bending at the interface between the two. (b) Replacing the overburden velocity with the velocity of the substratum eliminates raypath bending.

-S s

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~

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Fig. 13. Layer replacement by Kirchhoff summation. (a) Input zero-offset section at datum level z = O. Downward continuation of the wavefield fromz = 0 to datum level z = 800 m using a velocity of 2000 mls. (c) Upward contimation of step (b) back to z = 0 using a velocity of2500 mls. (d) The zero-offset section divided independently using interval velocities (After Yilmaz, 200\).

~.

~ 1::1

;:s 1::1..

of;;;. ~

~.

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103

A in a medium with three layered velocity structure. The point diffractors are situated at the layer interfaces at 800, 1300, and 1900m depths. The traveltime trajectory associated with the shallow point diffractor is a hyperbola. The traveltimes associated with the deeper diffractors are nearly hyperbolic. Extrapolate the zero-offset wavefield at z = 0 using the velocity of the first layer (2000 m1s) and compute the wavefield at the frrst interface z = 800m. The hyperbola associated with the shallow point scatterer largely collapses to its apex since this scatter is located at the first interface. Because the receivers now are closer to the other two deeper scatterers, events associated with them also are compressed. The zero-offset section that would have been recorded if the receivers were placed along the frrst interface. The energy from the shallow point scatterer on this section now arrives at t = 0 because the datum for this section is the interface at which the scatterer is located. Now, extrapolate the wavefield at the first interface (z = 800m) back up to the surface (z = 0) using the velocity of the second layer (2500 m1s). With this twostep wave extrapolation, the frrst layer with the 2000 m1s velocity was replaced with the second layer with the 2500 m1s velocity. The Kirchoff summation is more convenient in handling datum surfaces with arbitrary shapes. Datuming produces an unrnigrated time section at a specified datum z(x), which can be arbitrary in shape. Migration involves computing the wave field at all depths from the wavefield at the surface. In this respect, datuming is an ingredient of migration, when migration is done as a downward-continuation process. Migration requires invoking the imaging principle (t = 0). Wave-equation datuming has several practical applications, e.g, horizon flattening, forward modeling of seismic wavefields, and layer replacement. These are performed in either the prestack or poststack mode. The velocity must be halved when doing poststack datuming to conform to the exploding reflectors model. Layer Replacement (poststack) Wave equation datuming to 2-D surface seismic data is applicable to remove the degrading effect of an irregular water-bottom topography on the continuity and geometry of reflections below. This problem is particularly severe in areas with a strong velocity contrast between the water layer and the substratum. Despite the usual 3-D nature of the problem, the 2-D interpretation of the target reflections often can be improved by replacing the velocity of the water layer with the velocity

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of the substratum. Poststack layer replacement involves two extrapolation steps : -

1. The first step in poststack layer replacement involves downward continuing the wavefield at the surface to the water bottom using the water velocity in extrapolation. In this horizon-flattened section, the water-bottom reflection is at t = 0, which means that all receivers are situated on the irregular water bottom. If we specified the over-burden velocity or the water-bottom topography incorrectly, then the water-bottom reflection would not be at t = O. Wave-equation datuming actually can be applied layer by layer for structural model restoration. At each layer boundary, by examining the flatness of the event at t = 0 and observing any arrival-time departures of the event from t = 0, the validity of an estimated velocity-depth model can be verified. 2 The second step in poststack layer replacement involves upward continuation of the intermediate wavefield back to the surface z = 0 using the velocity of the substratum (2000 mls). A zero-offset section can be created from the same velocity-depth model by setting the first layer velocity at 2000 mls. Zero-offset section and the output of layer replacement are largely equivalent. Both layer replacement and depth migration are processes aimed at removing the effects of the complex over-burden. However, layer replacement only requires accurate representation of the overburden while depth migration requires accurate representation of the entire velocity-depth model. Also the output from depth migration is a migrated depth section, while the outout from layer replacement is an unmigrated time section. After eliminating the complex overburden effect, this section only requires time migration.

Layer Replacement (prestack) Poststack layer replacement does not remove the effect of complex overburden entirely, even if its geometry is known accurately, because the input stacked section differs from the zero-offset section. Starting with the common-shot gathers, prestack layer replacement involves the following steps : -

1. Downward continue all receivers to the output datum using the overburden velocity.

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105

2. Sort the data to connnon receivers gathers. 3. Downward continue all shots to the same output datum using the overburden velocity. 4. Upward continue all shots back to the surface using the velocity of the substratum. 5. Sort the data back to connnon-shot gathers. 6. Upward continue all receivers back to the surface using the substratum velocity. This series of operations eliminates the traveltime distortions associated with the water bottom. Just as it is true for any wave extrapolation-based process, the wave-theoretical layer replacement technique suffers from spatial aliasing. With modem data acquisition, we can record with many channels at small shot and group intervals. Therefore, spatial aliasing should not be an issue with more recent data. With this provision, prestack layer replacement followed by poststack time migration is a practical alternative to depth migration before stack in areas where simple geology is overlain by a single-layer overburden with a strong lateral velocity variation, such as an irregular water bottom. For prestack layer replacement each connnon-shot and commonreceiver gather is extrapolated, independently. In particular, the wave field at a point on the output datum is computed using all the traces in the input gather. The output gather should be computed beyond the lateral extent of the input gather to prevent possible loss of steeply dipping events. For prestack layer replacement, the velocity used in the extrapolation is that of the medium between the input datum and the output da¥n. Shot-Geophone Migration

An advantage of presJack migration is its ability to attenuate multiples. The quality of focusing at zero offset by shot-geophone migration depends on the accuracy of the velocity field used in migration. Erroneously too low or too high velocities would cause a partial collapse of the primary energy to zero offset. Fig. 14 shows a model of a salt diapir. A total of 193,\hot records was created by using a nonzero-offset raytheoretical moo'eling procedure (Yitmaz, 2001). The receiver cable is splitspread witl~ an offset range of 50 to 1200m, and 48 receivers at 50m interval. By extracting the zero-offset traces and placing them side by side, we obtain the depth image. The amplitude weakening along the top-

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Encyclopaedia of Petroleum Science and Engineering

Distance (km)

o

2

468

10

12

O~--------------------~ 3000mls

1

!== 2t------

5000

C.

c!

3

4000

-------_ ..-..

¥_

_

__

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--.~

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_ _',.

a

....-"--"

Fig. 14. (a) A velocity-depth model for a salt diapir. (b) Image obtained from prestack depth migration using shot records with no missing traces. (c) Image obtained from prestack depth migration using shot records with missing traces.

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107

salt event is caused by the limitations in the ray-theoretical modeling. A total of 15 percent of the traces from the modeled shot records was discarded arbitrarily and replaced with zero traces. As a result, some shot records contained few zero traces while some contained all zero traces. After prestack depth migration, focusing of the energy to zero-offset and its vicinity has been achieved in a manner comparable to the case of the complete data set. The prestack depth-migrated section derived from the missing data is very similar to the section derived from the complete data set. Shot-ge0l'hone depth migration can accommodate missing data resulting from recording geometry irregularities.

Shot-Profile Migration A method of shot-profIle migration (Reshef and Kosloff, 1986) shall be reviewed using salt diapir model. A wave theoretical modeling scheme based on the two-way acoustic wave equation was used to generate 154 shot records. Shot spacing is 50m and receiver spacing is 50m. Each shot record contains 97 traces corresponding to a split-spread recording geometry with a maximum offset of 2350m (Yilmaz, 2001). Fig. 15 shows three shot records: (1) one located away from the main diapiric body, (2) one on the left flank, and (3) one other on the right flank of the diapir. The same shot records after shot-profile migration using the true velocitydepth model are shown in fig. 16. Each shot record after migration represents partial image of the subsurface within a limited lateral extent. Now, imagine all 154 shot records after migration placed at their corresponding shotJocations along the line. Then, consider one specific receiver location. There will be traces from a number of migrated shot records that will coincide with this receiver location that are common to all. These traces constitute a common-receiver gather after shot'profIle migration. Selected common-receiver gathers sorted from the migrated common-shot gathers are shown in this figure. The number of traces in a common-receiver gather is determined by the recording geometry. If the velocity-depth model used in shot-profile migration corresponds to the true model, then each shot record should yield a correct image of the subsurface, albeit limited in lateral extent Then, traces from various shot records after migration at the same receiver location should represent the identical image below that receiver location. In other words, a common receiver gather should contain flat events if the velocity-depth model used in shot-profile migration is correct. The fmal step of shot-profile migration involves the summation of the traces in

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Encyclopaedia of Petroleum Science and Engineering

(b)

(c)

Fig. 15. (a) Three synthetic shot records over the salt diapir model. (b) Numbers on top of the shot profiles and the velocity-depth model correspond to CMP locations (AfterYilmaz, 2001).

f(l

kmo._____1_____2__A___3____4_____5_____6_____7_____8_____9______________ 10 B 11 12

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2000mls

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Fig. 16. (a) Shot records after shot-profile migrtion using the true velocity-depth model. (b) Selected common-receiver gathers. (c) Final image from shot-profile migration (After Yilmaz, 2001).

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Encyclopaedia of Petroleum Science and Engineering

each receiver gather to create the image in depth. The partial images with limited lateral extent from each individual shot record coincide with the complete image within the lateral extent of the line in its entirety. Examine the common-receiver gathers in this figure. The receiver locations are labeled as 179, 209 and 249. The events associated with the top-salt and base-salt boundaries and the flat reflector below are positioned differently in relation to the receiver location of each gather. In case of a flat layer boundary, the event is positioned symmetrically with respect to the receiver location (179). In the case of a dipping layer boundary, such as the top-salt, the event is positioned to the right of the receiver location (209) or to the left of the receiver location (249), but always in the updip direction. Accuracy of the velocity-depth model used in pres tack depth migration can be checked by examining event curvature on commonreceiver gathers. Fig. 17 shows three common receiver gathers sorted from the shot records that were migrated using the constant overburden velocity above the salt diapir. The top-salt event exhibits flat character at all three receiver locations since the velocity used for migration is the same as the layer velocity above the top-salt, boundary (3000 mls). Whereas the base-salt event and the event corresponding to the flat reflector below do not exhibit flat character since the migration velocity, in this case, is erroneously lower than the true layer velocities. Event curvature on common-receiver gathers can be likened to residual moveout on moveout-corrected common-midpoint gathers caused by incorrect moveout velocities. By measuring the residual moveout, layer velocities can be updated at each receiver location. Residual moveout analysis can be formulated within the context of common-receiver gathers derived from shot-geophone migration (AI-Yahya, 1989), common-receiver gathers derived from shot-profile migration (Lee and Zhang, 1992), or common-depth point gathers (image gathers) derived from common-offset migration (Cox and Wapenaar, 1992). Residual moveout analysis of image gathers derived from common-offset migration is used for updating layer velocities. The deeper the event, the higher is the velocity and the shorter is the cable length, the poorer is the resolving power of curvature analysis for velocity determination. This observation is comparable to the case of conventional stacking velocity analysis, and it also applies to residual moveout analysis of image gathers derived from common-offset migration. Compare the amplitudes on the images from depth migration of the zerooffset and prestack data. Imaging beneath complex structures has certain

Seismic Earth Imaging

111

(a)

(b)

(c)

Fig. 17. (a) Three common-receiver gathers sorted from shot records. (b) Single common-receiver gather sorted from that records. (c) Salt diapir (After Yilmaz, 200 I).

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implications as to the acquisition geometry, specifically, on the choice of the cable length.

Summation Strategies Fig. 18 shows the zero-offset wave field responses of a point x diffractor buried in media with varying degrees of complexity. The traveltime trajectories associated with the point diffractors buried in a constant-velocity medium, beneath an overburden with mild to moderate lateral velocity variations, and beneath an overburden with strong lateral velocity variations, all are single-valued. Therefore, ray tracing through such models would produce unambiguous traveltimes for Kirchhoff summation. The zero-offset traveltime trajectory associated with the point diffractor buried beneath an overburden with severe lateral velocity variations, however, is multivalued. One of the following summation path has to be chosen ; 1. The travelpath that corresponds to the first arrivals, i.e., minimum-tirne summation trajectory.

2. The travelpath that corresponds to the bowties that contain the most significant portion of the energy associated with the zerooffset wavefield response, i.e., maximum-energy summation trajectory. 3. The travelpath that corresponds to the shortest distance between the source or receiver point at the surface and the reflection point at the subsurface, i.e., minimum-distance summation trajectory. 4. The entire multivaued travelpath. The minimum-time strategy may be suitable for cases of moderate to strong lateral velocity variations, whereas the maximum-energy strategy may be imperative for a case of a complex overburden with severe lateral velocity variations. Ideally, it would be desirable not to exclude any portion of traveltime trajectory and use a multivalued summation path. Efficient traveltime calculation and choice of a summation path are important considerations for the 3-D prestack depth migration of large volumes of seismic data (Sethian and Popovici, 1999). In 3-D prestack depth migration, it is important to make a careful choice of aperture width in the inline and crossline directions. While an excessively large aperture unnecessarily increase run time, a small aparture can produce a poor image from 3-D prestack depth migration. Prestack data may be made

113

Seismic Earth Imaging -

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s d Fig. 18. Zero-offset wavefield responds of a point diffractor buried in (a) a constant-velocity medium, (b) beneath on overburden with mild to moderate lateral velocity variations, (c) beneath an overburden with strong lateral velocity variations, and (d) beneath a complex overburden with severe lateral velocity variations.

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spatially unifonn by azimuth-moveout correction (AMO) (Biondi et. al., 1998), thus enabling use of fInite-difference or frequency-wavenumber algorithms for prestack depth migration. When it becomes necessary to use Kirchhoff summation algorithm, operator aliasing is an issue that needs to be dealt with. Fig. 19 shows a low-velocity hyperbolic summation path. The summation along this path should include more than one sample per input trace. There are three different approaches to handle operator aliasing caused by multiple samples per input trace included in the summation: (1) the summation trajectory that represents the kinematics

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of the migration operator may be truncated to exclude the steep flanks that suffer from aliasing, (2) trace interpolation may be used to create additional traces so as to avoid multiple samples per input trace included in the summation, and (3) the frequency components that are aliased at a given dip along the summation trajectory filtered out (Grey, 1992). The fIrst approach is undesirable since truncation is equivalent to limiting the migration aperture which can destroy steeply dipping events. The second approach is costly in the case of prestack data. Trace interpolation also suffers in accuracy when applied to data with dipping events that conflict with one another. The third approach requires multiple copies of the input trace with different bandwidths. This is not possible for very large input prestack data. Three Dimension (3-D) Common-Offset Depth Migration For events with non-hyperbolic move out that calls for depth migration each of the resulting common-offset volumes following the applications of NMO and 3-D DMO corrections would not be a representation of a 3-D zero-offset wavefIeld. A 3-D common-offset volume of data could not be depth-migrated. In practice, the commonoffset strategy for 3-D prestack time migration sometimes can also be applicable to 3-D prestack depth migration. While, following NMO and 3-D DMO correction, each of the common-offset volumes is time-migrated using a 3-D rrns velocity fIeld, depth migration of the common-offset volumes is done using a 3-D velocity-depth model. There are two reasons why we may choose to apply the common-offset strategy to 3-D prestack depth migration: (1) we may wish to use a 3-D zero-offset depth migration algorithm other than the Kirchhoff summation, and thus avoid the troublesome take of computing 3-D nonzero-offset traveltimes needed for the latter, and (2) for line output or for selected image gathers Kirchhoff summation is the appropriate algorithm. For volume output from 3-D prestack depth migration other algorithms can be efficiently applied to common-
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work strictly for a case of complex overburden structure that gives rise to complex, nonhyperbolic move out. The important points for 3-D prestack depth migration are given below : -

Fig. 20. Two inline sections from image volumes derived from 3-D prestack depth migration: (a) using Kirchhoff summation, and (b) using the common-offset technique.

1. We may like to apply azimuth-moveout (AMO) correction to regularize the prestack data, then use a frequency-wavenumber algorithm to perform the 3-D prestack depth migration. 2. We may like to perform 3-D prestack depth migration using the Kirchhoff summation technique. The Kirchhoff summation implicity handles geometry irregularities associated with the prestack data. 3. A third option is based on migration of the decoupled 3-D common-offset volumes of data that have been corrected for geometry irregularities and reduced to zero offset by way of 3D DMO correction. The preferred choice for the algorithm to

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perform the 3-D common-offset depth migration is one based on wave extrapolation. Guidelines for the choice of strategy for 3-D prestack depth migration are given below : -

1. When dealing with a low-relief structure and moderate lateral velocity variations, use 3-D prestack depth migration for conducting image-gather analysis for model verification and update, but not necessarily for imaging in depth. For the purpose of model verification and update, work with a sparse grid of image gathers. The Kirchhoff sununation is the suitable algorithm to produce such a set of image gathers without creating a whole image volume. For imaging in depth, only 3-D poststack depth migration is required. A finite-difference or frequencywavenumber algorithm is the suitable one. 2 When dealing with a complex structure and moderate-to-strong lateral velocity variations, use a 3-D prestack depth migration algorithm based on decoupling of common-offset volumes of data. 3. When dealing with a compley Overburden structure, use of a 3-D prestack depth migration algorithm without decoupling the common-offset data is imperative. Three-Dimension (3-D) Poststack Time Versus Depth Migration Consider a 3-D survey over a hypothetical salt-dome structure. The Al sections in fig. 21 are the cross-sections of the 3-D zero-offset wavefield along the traverses that coincide with the cross-section of the 3-D velocity-depth model. First, we perform one-pass implicit 3-D time migration on the entire 3-D zero-offset synthetic data and display the same lines after 3-D time migration. The migration velocity field is based on the true subsurface velocity-depth model used in computing the 3-D zero-offset wavefield. Important observations are given below : 1. The top of the salt, albeit the vertical scale is in time, has been imaged properly with 3-D time migration, while the base of the salt has not. This is because the salt diapir acts as a complex overburden. 2. The 2-D time migration produced the correct result for the top of the salt only along the center inline. Because there are no sides-wipes on this line. Therefore, there is no need for 3-D migration. However, on a line away from the center line, even the top of the salt has not been imaged property by 2-D time

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migration, let alone the base of the salt. This is because the line contains sideswipes off the flank of the salt dome. 3. The migration velocities that yield an acceptable 2-D migrated section may be quite different from the true subsurface velocity model required by 3-D migration. We now perform one-pass implicit 3-D depth migration of the 3-D zero-offset synthetic data. After 3-D depth migration of the entire volume of the 3-D zero-offset wavefield using the true 3-D velocity-depth model, we display the same lines. Important observations are given below : -

1. The top and base of the salt now have been imaged properly with 3-D depth migration. The complex overburden problem has been solved. 2. The 2-D depth migration produced the correct subsurface model only along the center inline because there are no sides-wipes on this line. However, one a line away from the center line, neither the top nor the base of the salt have been imaged properly. This is because the line contains sideswipes off the flank of the salt dome. 3. By performing 2-D depth migration iteratively to converge to assumingly correct depth model, we may be forcing the model to converge to something very different from the true one. This stems from the treatment of the sideswipes as events that are in the plane of profile. Three-Dimensional (3-D) Prestack Depth Migration Two prominent circumstances that require migration of seismic data before stack and in three dimensions are given below : -

1. In the presence of conflicting dips with different stacking velocities, we need to image the subsurface by migration of seismic data in time, before stack and in three dimensions. 2. In the presence of lateral velocity variations, we need to image the subsurface by migration of seismic data in depth, before stack and in three dimensions. The algorithm of choice for 3-D prestack depth migration mu$t meet the following requirements : .

1. The algorithm must be able to image steeply dipping reflectors in the presence oflateral velocity variations.

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Encyclopaedia of Petroleum Science and Engineering 2. The algorithm for prestack migration must cope with irregularly sampled data. 3. It is often sufficient to generate image gathers at sparse intervals along the line, or along selected lines or even on a sparse grid over the 3-D survey area.

Three algorithms for 3-D prestack depth migration are: (1) Kirchhoff (2) finite-difference, and (3) frequency-wavenumber. Whatever the type of algorithm, requirement (1) cannot be waived for depth migration. Requirement (2) to handle irregular spatial sampling may be fulfilled by azimuth-moveout (AMO) correction (Biondi et. ai., 1998) or inversion to common offset (lCO) (Chemingui and Biondi, 1999). Once data are spatially regularized so that the resulting prestack data have uniform fold of coverage and have been corrected for source-receiver azimuth, then any of the three categories of migration algorithms, i.e., Kirchhoff-summation, finite-difference, or frequency-wavenmnber, can be used to perform 3-D prestack depth migration. While it has the advantage of producing an image in depth along a set of line traverses without having to produce an image volume in depth, Kirchhoff summation technique lacks the rigor to handle amplitudes that the frequencywavenumber techniques can provide. Finally, the finite-difference and frequency-wavenumber migration methods are global methods. They are not suitable to meet requirement (3). The better treatment of amplitudes by the frequency-wavenumber algorithms compared to the Kirchhoff summation technique, has greatly increased their use in practice (Biondi and Palacharla, 1996). ~ununation,

Time Versus Depth Migration Fig. 22 shows time and depth migration of the CMP-stacked section associated with the salt diapir. The top-salt boundary is imaged accurately by both time and depth migration. Time migration fails to produce a correct image of the base-salt boundary and the deeper reflector. Even depth migration fails to image these two reflectors with sufficient accuracy, although the velocity-depth model input to depth migration was the true model. This is because the CMP-stacked section is only a close representation of the zero-offset wavefield in the presence of a strong lateral velocity variations associated with complex overburden structures. Since poststack migration algorithms are based on the zerooffset wavefield theory, application of zero-offset migration to a CMP-

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stacked section would produce less-than-ideal results. To circumvent this deficiency in CMP stacking and to correctly image the substratum that includes the base-salt boundary and the flat reflector, one needs to do pres tack depth migration. See fig. 23. Prestack depth migration produces an image that is free of the distortions observed on the image produced by poststack depth migration. Imaging accuracy is similar to that of the zero-offset section. To minimize traveltime and amplitude distortions caused by non-hyperbolic moveout during CMP stacking, is to use partial stacking. By a simple series of tests, one can judge as to what portion of the cable, e.g., near offset mid-range offsetjor far offsets, provident required optimum stack as input to poststack depth migration. Poststack depth migration may require a stack based on a subset of offsets, prestack depth migration requires all offsets. Ann accurate image from depth migration is attainable only when the velocity-depth model

122

Encyclopaedia of Petroleum Science and Engineering Prestack

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is correctly defmed, independent of the input data type, e.g., zero-offset, stack, or pres tack. An incorrect velocity-depth model causes a poor image produced by not just poststack depth migration, but also by zero-offset and prestack depth migration. Two-Dimension (2-D) Poststack Depth Migration Fig. 24 shows a velocity-depth model for a salt pillow. The aspect ratio of the horizontal and vertical axes is 1. Hence, the diagram exhibits the true shape of the diapiric structure. The model can be treated in three parts: (1) the constant-velocity overburden above the salt, (2) the salt diapir itself, and (3) the substratum that includes the flat reflector below. So far as the flat reflector is concerned, the salt diapir constitutes a

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complex overburden structure with strong lateral velocity variations. Note the significant velocity contrast across the top-salt boundary and the undulating reflector geometry of the base-salt boundary, i.e., both give rise to ray bending that can only be handled by imaging in depth. A total of 154 shot records were modeled along the lateral extent of the velocity-depth model using the two-way acoustic wave equation (Yi1maz, 2001). The velocity depth model, the CMP-stacked and zero-offset sectons have been displayed with the correct lateral position with respect to one . another. Focusing and defocusing of the reflection amplitudes are

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associated with the base-salt boundary on the zero-offset and CMPstacked sections. Shot and receiver group intervals both are 50m, and the trace spacingjofthe CMP-stacked and zero-offset sections are 25 and 50m respectively. Departure of the stacked sections from the true zerooffset section imposes a limitation on the accuracy of the image we get from poststack depth migration. Normal-incidence rays are associated with zero-offset traveltimes and therefore can be used to examine the degree of complexity in velocitydepth models. For a quantitative assessment of lateral velocity variations image rays need to be examined. See fig. 25. By defmition, image rays emerge at the right angle to the surface. The lateral shifts between the point of departure of the image ray at the reflector position and the point of emergence of the image ray at the surface provides a measure of lateral velocity variation. Consider the image rays departing':fi:om the topsalt layer boundary. These rays show no lateral shift, and therefore, imaging the top-salt boundary does not require depth migration. Instead, it can be achieved by time migration. The image rays from the base-salt boundary show significant lateral shifts, especially beneath the flanks of the diapir. The stronger the lateral velocity variations, the more the lateral shifts in image rays. This behaviour of the image rays indicate that the lateral velocity variations caused by the salt diapir require depth migration to image the base-salt boundary, accurately. The image rays associated with the flat reflector below the salt diapir also show significant lateral shifts. Again, this reflector can only be imaged accurately by depth migration, rather than time migration. Image rays do not sample the reflector boundaries uniformly, i.e., there are regions that contain densely and sparsely populated image rays. In principal, an earth image in depth can be obtained by first migrating a stacked seeton in time, then converting the time-migrated section to depth along image rays using the appropriate velocity-depth model (Lamer et. aI., 1981). This raytheoretical two-step depth migration to obtain an earth image in depth is rarely used in practice. However, it is common practice to perform timeto-depth conversion of time horizons using image rays. Specifically, 3-D volume of stacked data first is migrated in time and selected time horizons are interpreted. These time horizons are than converted to depth horizons along image rays, again, using an appropriate velocity-depth model. Creating depth structure maps using this procedure is called map migration. Time migration is adequate for imaging the overburden above the salt diapir. Depth migration is needed for accurate imaging of the base-salt boundary and the subsalt region.

Seismic Earth Imaging

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125

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Encyclopaedia of Petroleum Science and Engineering

Two-Dimension (2-D) Prestack Depth Migration An early method of prestack depth migration is based on downward continuation of sources and receivers. This method is known as shotgeophone migration. A common-shot gather represents a wave field and thus can be extrapolated in depth at discrete intervals. As a result, receivers are lowered from one depth level to the next. By extrapolating a common-receiver gather, sources are lowered from one depth level to next. By alternating 1?etween extrapolation of common-shot and commonreceiver gathers at each depth level, all sources and receivers are lowered from the surface to each of the reflectors in the subsurface. While sources and receivers are lowered vertically downward from one depth level to the next, the recorded waves are back-propagated along the raypaths from source to a reflector back to receiver locations at the surface. When sources and receivers are lowered to the reflector, they coinCide and the traveltime diminishes to naught. This satisfies the imaging condition. When the maximum depth of extrapolation is reached, traces at zero-offset from each of the resulting common-shot gathers are extracted and placed side-by-side to produce the image from prestack depth migration. Nonzero offset traces are abandoned since all primary energy has collapsed to zero-offset provided the velocity-depth model is correct.

Another method of prestack depth migration is based on migration of shot records, individually. A shot record is a wavefield generated by a signle source. This method is known as shot-profile migration. The migrated shot records are then sorted into common-receiver gathers. Finally, traces in each receiver gather are summed to construct the image below the receiver location. By placing the traces that result from this summation side by side, we obtain the image from shot-profile migration. Advantage of shot-profile migration is its ability to handle irregularities in recording geometry. Since each shot recorded is migrated independently, missing shots, duplicate shots or just irregularities is shot spacing are irrelevant. Two-Pass Versus One-Pass 3-D Poststack Depth Migration Given the choice between one-pass and two-pass schemes, 3-D poststack time migration may be done in two passes provided the vertical velocity gradient is not excessively large and dips are gentle. First, we consider two-pass implicit finite-difference 3-D time migration of the saltdone synthetic data set. See fig. 21 again. Start with the 3-D zero-offset wave field and apply time migration in the inline direction and display

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four selected inline sections and four selected crossline sections. After this frrst-pass migration, the imaging of the center inline is complete, since there is no sideswipe energy that needs to be moved out of the plane of this inline. On the other hand, the center has not been imaged at all, because no movement of energy took place in this line as yet after the ftrst-pass migration. On other lines, there is still some more imaging to be done, e.g., the sideswipe energy on inline. Now sort the data into crosslines, perform the second-pass migration on the already migrated data, and display the same inline and crossline sections. These are now the inlines and crosslines after the two-pass 3-D time migration. Since the overburden velocity above the salt layer is constant, the twopass 3-D time migration correctly images the top-salt boundary. The basesalt boundary has not been imaged correctly by either one-pass or twopass migrations, since we have done time migration rather than depth migration. In areas with a complex overburden structure, such as the overthrust belts, usually velocity varies laterally more in the dip direction perpendicular to the thrust fronts than the strike direction. When that is the case, it might be plausible to do time migration in the strike direction with mild lateral velocity variations, followed by depth migration of selected lines in the dip direction with strong lateral velocity variations. Such two-step hybrid strategy may be useful in building the 3-D velocitydepth model for a subsequent, proper 3-D depth migration of the 3-D data. Start with the results of time migration in the inline direction and perform depth migration in the crossline direction. Then, display the same lines and crosslines. After time migration as the ftrst-pass and depth migration as the second pass, we certainly have restored the true geometry of the topsalt boundary correctly. We have not been able to restore the geometry of the base-salt boundary, except for the center crossline, because this salt structure is truly 3-D in character with no dominant strike or dip direction. The top-salt boundary actually has been imaged more accurately by the two-pass migration as compared with the one-pass migration. The base-salt boundary, however, has not been imaged correctly by the twopass scheme, while it has been imaged correctly by the one-pass scheme. The important points are given below : -

1. If the velocity fteld is judged to be suitable for time migration, the two-pass strategy for 3-D time migration may be acceptable

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Time Slices .<

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provided the vertical velocity gradient is not excessively large and dips are not very steep. 2 If the velocity field requires depth migration, the one-pass strategy for 3-D depth migration is imperative. Finally, we compare the performance of the implicit and explicit schemes using the circular symmetric salt-dome model data. The implicit scheme uses the 45-degree extrapolator in a split mode, and the explicit scheme uses a one-dimensional explicit filter combined with the 5 x 5 McClellan filter template. The top-salt boundary is imaged more accurately by the explicit scheme because of the near-circular symmetry of its impulse response. Positioning errors by the implicit scheme implied by its impulse response are better observed on the depth slices. See fig. 26. The top-salt boundary image by the implicit scheme is not circularly synnnetrical. Significant undermigration especially along the two diagonals bas resulted. But the explicit scheme has preserved the circular character of the salt dome. As a byproduct of 3-D migration algorithms based on wavefield extrapolation, 3-D stacked data can be datumed from the surface to a specified depth or time level in a 3-D sense. This capability can be particularly useful in reservoir studies. Basically, the surface wavefield is downward continued to a desired depth without involving the imaging principle along the way. Important points are given below : 1. Since there are no sideswipes on the center line, datuming in 2-D or 3-D sense is identical. 2 There is a significant difference between 2-D datuming and 3-D datuming for lines with sideswipe energy, i.e., those that are increasing farther from the centerfine. The constant datum level should not be a limitation, particularly in reservoir studies. The 3-D stacked data can be datumed to the top of the reservoir level followed by detailed imaging of the target zone only.

l. AI-Yahya, K., 1989; Velocity analysis by iterative profile migration;

Geophysics, Vol. 54, pp. 718-729. 2. BerryhiIl, J.R., 1984; Wave-equation datuming before stack; Presented at the 54'" Ann. Internal. Mtg., Soc. ExpJ. Geopbys.

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3. Biondi, B., Fomel, S. and Chemingui, N., 1998; Azimuth moveout for 3D prestack imaging; Geophysics, Vol. 63, pp. 574-588. 4. Biondi, B, and Palacharla, G., 1996; 3-D prestack migration of commonazimuth data; Geophysics., vol. 61, pp. 1822-1832. 5. Cerveny, v., Klimes, J!.. and Psencik, I., 1984; PardXial ray approximation in the computation of seismic wavefields in inhomogeneous media; Geophys.,1. Roy, Astr. Soc., Vo1.79, pp. 89-104. 6. Chemingui, N. and Biondi, B., 1999; Data regUlarization to inversion to common offset (lCO); 69th Ann. Internet. Mtg. Soc. Expl. Geophysics, Expanded Abstracts, 1398-1401. 7. Cox, H.L.H. and Wapenaar, c.P.A., 1992; Macromodel estimation by common-offset migration and by shot-record migration; J. Sees. Expl., Vol.l, pp. 29-37. 8. Gray, S., 1992; Frequency-selective design of the Kirchhoff migration operator; Geophys. Prosp., Vo1.40, pp. 565-571. 9. Keho, T.H. and Beydoun, W.B. 1988; Paraxial ray Kirchhoff migration; Geophysics, Vol. 53, pp. 1540-1546. 10. Kosloff, D. and Kessler, D. 1987; Accurate depth migration by a generalized phase-shift method; Geophysics, vol. 52, pp. 1074-1084. 11. Lamer, K.L. Hatton, L. and Gibson, B., 1981; Depth migration of imaged time sections; Geophysics, vol. 46, pp. 734-750. 12. Lecomte, I., 1999; Local and controlled prestack depth migration in complex areas,-Geophys., Prosp., vol. 47, pp. 799-818. 13. Lee, W.B. and Zhang, L. 1992; Residual shot-profile migration," Geophysics, vol. 57, pp. 815-822. 14. Officer, C.B., 1958; Introduction to the theory of sound transmission; McGraw-Hill Book Co. IS. Reshef, M. and Kosloff, D., 1986; Migration of common-shot gathers; Geophysics, vol. 51, pp. 324-331. 16. Sethian, lA. and Popovici, AM., 1999; 3-D traveltime computation using the fast marching method; Geophysics, vol. 64, pp. 516-523. 17. Schultz, P.S. and Sherwood, 1. W.C., 1980; Depth migration before stack; Geophysics, vol. 45, pp. 361-375. 18. Vesnaver, A., 1996; Ray tracing based on Fermat's principle in irregular grids; Geophys. Prosp., Vol. 44, pp. 741-760. 19. Vinji, V., Iversen, E. and Gjoystdal, H., 1993; Traveltime and amplitude estimation using wavefront construetion; Geophysics, vol. 58, pp. 11571166. 20. Yilmaz, O. and Lucas, D., 1986; Prestack layer replacement; Geophysics., Vol. 51, pp. 1355-1369. 21. Yilmaz, O.Z., 2001; Seismic data analysis, vol.2, Society of Exploration Geophysicists, Post office Box 702740, Tulsa, OK 74170-2740.

3 Three Dimensional (3-D) Seismic Exploration, Processing, and Interpretation Introduction Surface geological features of interest in hydrocarbon exploration are three dimensional in nature, e.g., salt diapirs, overthrust and folded belts, major unconformities, reefs and deltaic sands. A two-dimensional (2-D) seismic section is a cross-section of a three dimensional (3-D) seismic response. A 2-D section contains signal from all directions, including out-of-plane of the profile. But 2-D migration normally assumes that all the signal comes from the plane of the profile itself. The out-ofplane signal (sidewipe) often causes 2-D migrated sections to mistie. These misties are caused by inadequate imaging Of the subsurface resulting from the use of 2-D rather than 3-D migration. 3-D migration of 3-D data provides an adequate and detailed 3-D image of the subsurface, leading to a more reliable interpretation. A typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3-D survey is done by laying out a number of receiver lines parallel to each other and placing the shotpoints in th perpendicular direction (swath shooting). In merime 3-D surveys, the shooting direction (boat track) is called the inline direction. In land 3-D surveys, the receiver cable is along the inline direction. The direction that is perpendicular to the inline direction in a 3-D survey is called the crossline direction. In 2-D surveys line spacing can be as much as 1 km., the line spacing in 3-D surveys can be as small as 25m. This dense coverage requires an accurate knowledge of shot and receiver locations. The size of the survey area is dictated by the areal extent of the subsurface target zone and the aperture size

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required for adequate imaging of that target zone. The areal extent of a 3-D survey almost always is larger than the areal extent of the objective. A few hundred thousand to few hundred million traces normally are collected during a 3-D survey. In a marine 3-D seismic survey more than 100,000 traces per square km. are recorded. Most 3-D surveys are aimed at detailed delineation of already discovered oil and gas fields. Additionally, 3-D surveys are repeated over the same area at appropriate intervals, say every 5 years, to monitor changes in fluid saturation which may be inferred from changes in seismic amplitudes. By mapping changes in fluid saturation, changes in fluid flow directions may also be inferred and used for planning of production wells. To make use of seismic amplitudes for reservoir monitoring, data from all vintages must be processed consistently using a processing sequence aimed at preserving relative amplitudes. Seismic monitoring of oil and gas reservoirs by using time-lapsed 3-D surveys is called the 4-D seismic method. In 2-D seismic data processing, traces are collected into commonmidpoint (CMP) gathers to create a CMP stack. In 3-D data processing, traces are collected into common-cell gathers (bins) to create commoncell stacks. A common-cell gather coincides with a CMP gather for swath shooting. Typical cell sizes are 25 x 25 m for land surveys and 12.5 x 25 m for marine surveys. Conventional 3-D recording geometries often complicate the process of stacking the data in a common-cell gather. Cable feathering in marine 3-D surveys can result in traveltime deviations from a single hyperbolic moveout within a common-cell gather. For land 3-D surveys, azimuth-dependent moveout within a common-cell gather is an issue. After stacking, the 3-D data volume is migrated. Before migration, the data sometimes need to be trace-interpolated along the crossline direction to avoid spatial aliasing. The migrated 3-D data volume then is available to the geophysicist as vertical sections in both the inline and crossline directions and as horizontal sections (time slices). The interactive environment with powerful 3-D visualization tools provides an effici~nt means for interpretation of the shear volume of 3-D migrated seismic data. Fault correction, horizon tracking, horizon flattening and some image processing techniques can be adapted to the interactive environment to help improve interpretation. Detailed Encydopaedia

The encyclopaedia is arranged in alphabetical order. The detailed encyclopaedia is given below

Three Dimensional (3-D) Seismic Exploration, Processing.....

133

CrossUne Migration 3-D poststack migration can be performed by a two-stage sununation over the surface of the 3-D zero-offset diffraction hyperboloid. In practice, the two-pass 3-D poststack migration involves successive 2-D poststack migrations of the inlines followed by 2-D poststack migrations of the crosslines. Similarly, 3-D prestack migration also may be perfonned in two stages. While two-pass 3-D migration is only an approximation to a one-pass 3-D migration, the computational benefits of the fonner may encourage its use in practice under certain circumstances. The 3-D nonzero offset traveltime t for a zero source-receiver azimuth represents an ellipsoid given by the following equation.

...(1) where x and yare inline and crossline dissections, respectively, v is the medium velocity and h is the half-offset. The horizontal cross-section of this ellipsoid at a depth is an ellipse. See Fig. 1. For a 3-D recording geometry with an arbitrary source-receiver azimuth defmed by angle e, the inline-crossline Cartesian coordinates (x, y) in tenns of the local Cartesian coordinates (x', y') are given by the following equations.

x

=

.x' cos e - sin e

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and

y = x'sine+y'cose

...(3)

By substituting equation (2) and (3) into equation (1), it can be shown

that the vertical cross-section of the ellipsoid along an arbitrary sourcereceiver azimuth e is also an ellipse (Berryhill, 1991). This characteristic of the traveltime ellipsoid makes it possible to perfonn 2-D migration of 3-D prestack data is one direction only, so as to create 2-D prestack data regularised in offset and azimuth (Canning and Gardner, 1996). This one direction is most likely to be the crossline direction for most cases. Because of the irregular offsets and source-receiver azimuths of 3-D prestack data, an appropriate choice for cross line migration algorithm is based on Kirchoff summation. These output gathers from crossline migration along one inline can be processed subsequently and a 2-D migration in the inline direction can be perfonned by one of the following method.

1. 2-D prestack time migration based on a workflow that includes 2-D DMO correction and 2-D common-offset migration to generate common-reflection-point (CRP) gathers for amplitude variation with offset (AVO) analysis.

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~

________-+-----x

z (a)

~~~------T-----X

A (b) Fig. 1. (a) The 3-D nonzero-offset traveltime surface is an ellipsoid. (b) The vertical cross-section of the ellipsoid is an ellipse.

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2. 2-D earth modeling in depth to derive an accurate image in depth by 2-D prestack depth migration in the inline direction that is assumed to coincide with the dominant structural dip direction. See Fig. 2. The image quality of the migrated sections in these figures (b) and (d) is comparable. A cross line migration workflow includes the following steps.

1. Perfonn velocity analysis of unmigrated 3-D prestack data to derive a 3-D velocity field that is appropriate for migration in the crossline direction. Often, a single, vertically varying velocity function is adequate to use in cross line migration. 2 Perfonn crossline migration and create common-cell gathers along selected inline traverses. The resulting gathers will have traces with regular offset distribution and unifonn sourcereceiver azimuth that is coincident with the inline direction. 3. First, perfonn velocity analysis at selected locations along the inline traverses for which data have been migrated in the crossline direction. 4. Apply NMO and 2-D DMO corrections to the gathers from step (2) using the velocity field derived from the analysis in step (3). 5. Perfonn 2-D common-offset migration using an appropriately smoothed fonn of the velocity field derived from the analysis in step (3). 6. Apply inverse NMO correction using the same velocity field in step (4) and repeat the velocity analysis to derive a velocity field from the data which now have been first crossline migrated then 2-D prestacked migrated in the inline direction. 7. Apply NMO correction using the velocity field from step (6), stack the data and unmigrate using the same velocity field that was used in common-offset migration in step (5). The resulting sections represent 2-D zero-offset wavefields, and thus contain no sideswipe energy. 8. Use the velocity field from step (6) to remigrate the stacked sections. _The resulting migrated sections represent the final product from a 3-D prestack time migration based on the twopass strategy. Aside from being an integral component of a two-pass 3-D prestack titne migration workflow, crossline migration has two other applications : (1) by restricting the aperture for Kirchhoff summation to crossline trace spacing, crossline migration can be used to perfonn azimuth moveout

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)0)

.

41>1

.i

10:

(a)

202

303

.

4....

.

Sll!>

(b)

ld)

Fig. 2. (a) A cross-section from the unmigrated 3-D stacked volume. (b) A crosssection from the migrated 3-D stacked volume, along the same inline traverse. (c) An inline stacked section derived from crossline migration. (d) 2-D migration of the section in (c). (Data Courtesy Shengli Oil Field of China National Patroleum Corporation).

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(AMO) correction of 3-D prestack data. The resulting prestack data still require DMO correction, but only as a 2-D process. Crossline migration yields prestack data with regular offset distribution which can be used as input to a 3-D pres tack depth migration that requires uniformly sampled data, and (2) classline migration can be used to re-orient 3-D prestack data along a desired direction. This application is useful in merging data from a number of neighbow'ing or partially overlapping 3-D surveys which have to be conducted using different recording directions. Specifically, data from one survey can be crossline migrated to generate gathers along the inline direction associated with another survey. Crossline-Smearing The common-cell sorting problem is not solved completely by restoring uniformity in the fold of coverage. The centroid of the midpoints may not coincide with the centre of the cell. Theoretically, if the centroid departs significantly from the centre, then placing the stacked trace at the centroid, rather than at the cell centre, may be considered. This destroys equal spacing of the stacked traces, primarily in the crossline direction. 3-D poststack migration based on the Kirchhoff integral method may be used to produce migrated data volume with uniform trace distribution. See fig. 3. The cell is 12.5m in the inline (labeled N

L.

'I Shol Line 3

~.

i I~

SOm

Shot Line 2

Cell under Study

:1-

,'I

I

I

I

I

I

I

b;

i

If

" If:

11",

! ,d r. ~ It!! ~ Ii! I

#

\

Shol Line 1

IIIIolpOlnl.

~!

""I'i' jot!, 1!lh!::,'

• • • • • • • • IIf • •

iiiilliiiii Mldpotnl COordtft.le em)

(a)

(b)

Fig. 3. (a) A single-source, sing\e-cable marine line shooting. (b) An individual cell With midpoint scattering caused by cable feathering (After Yilmaz, 200 ~).

as shot line) direction and 50 m in the crossline direction. Different symbols represent the midpoints that are associated with different shot

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138

limes. This cell contains midpoints from six different shot lines. With common-cell sorting, the traveltimes of the arrivals in a single cell may not follow a single hyperbolic moveout curve (Levin, 1984). Three different shooting directions are : 1. Strike-line shooting, i.e., no dip perceived along the inline direction (maximum cross-dip case). 2 Shooting direction of a 45-degree azimuth with respect to dip direction. 3. Dip-line shooting, i.e., no dip perceived along the cross-line direction. Date from different shot lines contribute to different portions of the traveltime curves. For a single dipping event, the 2-D recording geometry normally yields a hyperbolic moveout curve. As the line orientation becomes more parallel to the reflector strike, crossdip increases and traveltimes deviate from the ideal hyperbolic moveout curve. This ideal hyperbolic corresponds to the case of no cable feathering, in which all midpoints in the cell coincide with the cell centre. The traveltime deviation in worse when shooting in the strike direction. The traveltime deviation increases with increasing feathering angle, cross-dip, and cell dimension in the crossline direction. It is also more significant in the case of large moveout that occurs at lower velocities and shallow depths. If a common-cell stack were done along the base-fit hyperbolic path, then a loss of high frequencies would be expected. See Fig. 4. The effects of midpoint scatter are significant on a particular data set depends on the amount of cross-dip, cable feathering, and desired bandwidth of the stacked data. 10

I S

40

30

1;

J

I

..... 0

20

(

I

10 0

I -11

-10

-I

0

5

10

1.

)

-10

-20

-40

120

0

T1me DIfI.,enc. (ml)

(a)

(b)

Fig. 4. (a) The stacking operator associated midpoint scatter along the crossline direction. (b) The amplitude spectrum. (After Yilrnaz, 200 1).

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Fig. 5 shows the factors that influence cross-line smearing Crosscurrents cause cable feathering, which makes the midpoints scatter within a cell in the crossline direction. If the shooting direction is such that a dipping interface has a cross-dip component, then the traveltimes associated with that interface in a conunon-cell gather deviate from a single hyperbolic moveout curve. This causes amplitude smearing during stacking, which acts as a high-cut filter. The cutoff frequency primarily is a function of the amount of cross-dip, reflection time, and velocity. The lower the velocity, the larger the cross-dip, and larger the cell size the more likelihood of crossline smearing. The most effective way to avoid crossline smear is to have the cell dimension in the crossline direction sufficiently small, which means shooting with close line spacing.

cr?SSCUl'l'ents

.t.

feathering

presence of dipping interfaces

.nline

~

*" dip line

.t. x-line scatter of midpoints

G)velocity

c.ell~

Slze W

crossdip(f)

crossline smear (f) Fig. 5. Factors that influence crossline smearing.

Explicit Methods Extrapolating a 3-D-zero-offset wavefield in discrete depth steps of Az which is given by :

P(kx, k, y Z + Az, w)

=

P(kx' ky' z, w) exp (- i kz Az)

...(1)

The vertical wavenumber kz in terms of the other transform variables is given by the following equation:

k

=

z

2w 1_(Vkx)2 _(Vky)2 v 2w 2w

...(2)

where, v is the extrapolation velocity. Substitute x = ~k; + k~ in equation (2), weget:

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k

=

2w

v

z

~(Vk)2 f-l2;j

...(3)

With the vertical wavemember kz given by equation (3), we now write the desired extrapolation operator D(k) given by the exponential term in equation (1) for a specific ratio of2w (v) as:

...(4) This operator can be inverse transformed back to the frequencyspace domain. Extrapolation described by equation (1) can be achieved by convolution of the wavefield with the extrapolation filter in the frequency-space domain. We want to compute an extrapolation filter hn in the frequency space (w, x, y) domain with a Fourier

k:,

transform H(k = ~k; + w, v) that best approximates the desired transform given by equation (4). Since D(k) is symmetrical with respect to k = 0, the complex filter coefficients are even, i.e. h = h , and the number of filter coefficients 2N + 1 is odd. The Fourier tra'hsfo~ ofhn is given as: N

H(k) =

he + 2 n=\ 1:. hn cos(nk)

...(5)

The actual amplitude spectrum should be IH(k)1 ~ 1 for all k. Amplitudes of the extrapolated wavefield will not grow from one depth . level to another. The recursive formula for the Chebychev polynomials of equation (5) can be written in terms of cos k as : H(k) = ho + 2h J cos k + 2h2 (2cos k cos k - I) +

...(6)

Convolution with the filter h n can be performed by a recursive Chebychev filter structured described by Hale (1991 b). For the design of the 3-D explicit operator, defme the transform G(k) = cos k, we have:

G(~t' k)

=

cos~k; + k:

...(7}

Equation (7) may be approximated as :

G(kx' ky}

= - I + liz (1 + cos k) (1 + cos ky)

°

...(8}

This approximation is exact for kx = and ky = 0, best for small kx and ky' and deteriorates at large kx and kyo The circular symmetry of the McClellan and transform filter can be improved by an alternative approximation to G(~t' k) of equation (8) by :

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G(kx' ky) = -1 + Ih (1 + cos kx) (1 + cos ky) - cl2 (1 - cos 2kx ) (- cos 2 ky)

...(9)

Where c is an adjustable scalar. A typical value for c = 0.0255. An approximate form ofk such as that given by equation (7) is give by :

k

= COS-I G(k k) x' y

...(10)

Where G(kr ky) is given by the approximate form in equation (9). Write equation (4) for the approximate form of the wave number k given by equation (10) as :

...(11) The error in the extrapolation accumulated after nllz depth intervals is then given by :

Therefore, the extrapolated wavefield P(kr kyo nllz, w) given by equation (1) is compensated for the errors incurred by the approximate form of G(kr ky) given by equation (9) by applying the condugate of E (kr kyo w) of equation (12) to the extrapolated wavefield as :

P(kr kyo nllz, w) = E*(kr kyo w) P(kr kyo nllz, w) ...(13) Following Li's correction (1991) adapted to the explicit schemes (Etgen and Nichols, 1999) as given by equation (13), the wave extrapolation is continued with the approximate form (equation-II) until more error is accumulated. Explicit Schemes combined with the McCleUan Transform

One-pass and two-pass 3-D poststack migration algorithms based on implicit finite-difference schemes are now rarely used. The primary reason is the azimuth-dependent positioning errors they incur. Improvements to the one-pass scheme to achieve circular symmetry in the impulse response (Li, 1991) are computationally involved. Explicit schemes provide circular symmetry and efficiency in the design and application of the extrapolation operators (Notfors, 1995). Explicit methods can be formulated to perform both time and depth migration of 3-D eMP

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stacked data. Three aspects to the design and application of 3-D explicit schemes are : 1. Design of a 2-D explicit operator in the frequency-space domain.

2 Transformation of this 2-D operator to a 3-D wave extrapolator. 3. Application of the 3-D wave extrapolator to migrate 3-D CMPstacked data. Explicit schemes that use the McClellan transform for 3-D poststack migration are implemented in the frequency-space (w, X, y) domain. This enables application of the extrapolation filter at each depth level to each frequency component of the wavefield independently. Wave extrapolation in 3-D is described in the frequency-wavenumber domain by following equation: P(kx- ky Z + Az, w) = P(kx- ky z, w) exp.(- i kzAz)

...(1)

Where, kx and ky are inline and crossline wavenumbers, respectively, kz is vertical wavenumber, and w is frequency in units of radians per unit time, and P is the wavefield that is being extrapolated in depth Z using a depth step size Az. The desired extrapolation operator D(k) = exp (- i kz Az) for a specific ratio of 2 w/v is given by the following equation: D(k)

=

.2w expo -z-;[

...(2)

Where ...(3) We want to compute an extrapolation filter hn in the frequency-space (w, x, y) domain with a Fourier transform H (k, w, v) which is given as: N

H(k) =

Ito + 2 n=1 ~ hn cos (nk)

...(4)

That best approximates the desired transform is given by equation (2). Since D(k) is symmetric with respect to k = 0, the complex filter coefficients hn are even, i.e., h-n = hn' and the number of filter coefficients 2N + 1 is odd. As a result, we only need to compute N coefficients. Table 1 gives design and application of explicit 3-D wave extrapolators. Since the desired amplitude spectrum is ID(k)1 = 1 for all k, then we require the actual amplitude spectrum to be IH(k)1 ~ 1 for all k. This will ensure that the operator is stable, i.e., amplitudes of the extrapolated wavefield will not grow from one depth level to another.

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'Dlble 1. Processing parameters for the land 3-D survey data set Shot interval in m Group interval in m Receiver line spacing in m

25 50 500

Number of receiver lines per swath

6

Number of groups per receiver line Binsizeinm x m

25x25

Fold of coverage Number of bins per Jan2 Number of bins in the survey Sampling interval in ms

Maximum tim;: in IllS

ro 12 1600 426,400 4 4(0)

Number of prestack traces

5,116,800

Data volume in gigabytes

205

Following the detennination of the 1-0 extrapolation filter for specific wand v values, they are convolved with the McClellan transform filter template to translate it to a 2-0 filter in the (x, y) domain for each frequency w. The filter is then applied to the appropriate frequency component of the wave-field P(x, y, w, z) in the (w, x, y) domain at depth z to extrapolate it to depth z + Ilz by way of an efficient recursive scheme that is based on Chebychev polynomials. At each depth, the image P(x, y, z + Ilz, t = 0) is obtained by invoking the imaging principle that is equivalent to summing the extrapolated wave components over frequency. In practice, the usual implementation of the McClellan transform method requires equal spatial sampling intervals, Ax and ay, in inline and crossline directions, respectively. When the two sampling intervals are not equal, as in most marine 3-0 surveys, then trace interpolation needs to be done prior to migration to obtain data with equal sampling intervals in inline and crossline directions. Now, examine the impulse response of the 3-D migration operator based on the explicit scheme and the McClellan transform. There are 101 inlines and 101 crosslines, with line 51 being the center line. See Fig. 6. The 1-0 explicit operator hn has 2N + 1 =39 filter coefficients, and the McClellan filter template is 3 x 3 in size. The sable explicit operator hn provides cleaner N vertical cross-sections, free of dispersive noise compared to the implicit scheme. The explicit scheme does not allow the evanescent energy to tum into propagating energy.

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The dip accuracy of the explicit scheme is governed by the length of the filter hn' i.e., the longer the filter the steeper the dip the algorithm can handle. The McClellan transfonn provides the more circular time slice compared to the splitting method. ~ine

15

31

21

Time Slice

100 ms

51

41

200 ms

300ms

400ms ,lt~~ '!--

Fig. 6. Impulse response of a 3-D migration operator. Implicit Methods The two-dimensional scalar wave equation can be adapted to three dimensions are given below:

fi

fi

ox

By

(;2 Oz

1(

2

)

( -2 +-2 +-2 -2"-2 P(x,y,z,t) vat

=

0

...(1)

Equation (1) describes propagation of a 3-D compressional zero-offset wave field P(x, y, z, t) in a medium with constant material density and compressional wave velocity v(x, y, z), where x is the horizontal spatial axis in the inline direction, y is in the cross line direction, z is the depth axis (positive downward), and t is time. The upcoming seismic wavefield is given by P(x, y, z = 0, t), which is recorded at the surface, we want to determine reflectivity P(x, y, z, t = 0). This requires extrapolating the surface wave field to depth z, then collecting it at t = O. The solution to the twodimensional scalar wave equation can be adapted to extrapolate a 3-D zero-offset wavefield in depth by the following equation:

...(2)

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Where, ...(3)

With

vk

vk

X = _.t, andY=-Y ...(4) 2w 2w kx and ky are the wavenumbers in the inline and crossline directions, respectively, and w is the temporal frequency in radians per unit time. The dispersion relation of equation (3) for constant velocity is given by the equation :

wr =

W

2

e

2k 2 v2 Y v.t ------

4

...(5)

4

where wr = vkj2 is the output temporal frequency. 3-D migration can be performed in two phases: (1) 2-D migration in the inline direction, and (2) 2-D migration in the crossline directions. The two-pass approach is based on the idea of full separation (Claerbout, 1985) of migration operators in the inline and crossline directions. When velocities vary spatially, splitting of the inline and crossline operators is necessary. To implement separation or splitting, the inline and crossline wavenumbers need to be decoupled in the 3-D dispersion relation (equation-3). By Taylor series expansion of equation (3), we get the IS-degree dispersion relation by :

k =

2W(I_ X2 _Y2) v

z

2

...(6)

2

When the cross-dip component is zero (Y = 0), equation (6) reduces to its 2-D form. The inline and crossline terms in equation (6) are separable when velocity is slowly variable in z or independent of x and y (Brown, 1983). The impulse response of the 3-D IS-degree migration operator with its dispersion relation given by equation (6) is an ellipsoid. Within typical bandwidth of seismic data, the 3-D IS-degree impulse response departs from the ideal 3-D impulse response at dips greater than 30 degrees. The time slices of the impulse response are circular in shape. The 45-degree approximation to equation (3) introduces cross terms. Another approximation is to represent the single square root by two square roots (Ristow, 1980) as given below:

kz

~

2;[v'I-X +v'I- -IJ 2

y2

.. ~7)

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Assuming a small cross-dip component, more accuracy in the inline direction is achieved bjmaking the 45-degree approximation given by the equation:

k z

~ 2W[I_~_ Y21 v X2 2--

2

...(8)

2 When more accuracy is required in both directions, we can apply the 45-degree expansion to equation (7) in both the inline and crossline directions given by :

kz

~ 2W[I-~-~1 v

X2 2-2

y2 2-2

...(9)

Within the typical bandwidth of seismic data, the 3-D 45-degree impulse response departs from the ideal 3-D impulse response at dips greater than 65 degrees. The time slices of the impulse response are diamond shaped. The response is accurate in both the inline and crossline directions, and that the largest error in the form of undermigration occurs in the two diagonal directions. Separation works for 90-degree, constant-velocity or 15-degree, slowly varying v(z) cases. When there are large vertical velocity gradients or strong lateral velocity variations, splitting must be used. The algorithm for a one-pass implicit frequency space 3-D poststack migration that uses the splitting method is identical to the 2-D algorithm, except for the additional application of the diffraction term in the crossline direction. See Fig. 7. One-pass 3-D Poststack (I)-X-] Migration Loop over z

[

~:over()J Diffract in x Diffract in ] ~

: imaging principle

(I)

Fig. 7. An algorithmic description of one-pass implicit 3-D poststack migration.

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Impulse Response of the One-Pass Implicit Finite-Difference 3-D Migration The ideal impulse response of a 3-D migration operator is a hollow hemisphere, with semi-circular cross-sections in the vertical planes and circular symmetry on time slices. Figs. 8 and 9 show the 3-D 15-degree and 45-degree operator impulse responses for the one-pass approach. Selected lines and time slices are shown to better illustrate the shape of the responses. There are 101 inlines and 101 crosslines (Yilmaz, 2001). The centre line 51 exhibits the elliptical shape of the 15-degree 2-D operator. As we go to lines away from the centre line, the ellipse gets smaller in size. The time slices of the 3-D 15-degree operator retain the

() 400ms

Fig. 8. A 15-degree split 3-D migration operator impulse response.

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400ms Fig. 9. A 45-degree split 3-D migration operator impulse response.

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cIrcular symmetry. But the traveltimes are not quite correct. It causes undermigration in all directions. The time slices of the 3-D 15-degree operator have circular symmetry. But the time slices of the 3-D 45-degree operator have diamond shape. The 15-degree operator causes the same amount of undermigration in all azimuthal directions. The 3-D 45-degree operator causes less undermigration, but the amount of migration error depends on the agimuth, e.g., the largest error is in the diagonal directions. The one-pass algorithm is appropriate to handle lateral velocity variations. Hence, suitable for depth migration as well as for time migration. With the 45-degree 3-D operator, we end up with azimuthal variation-s in migration error that is manifested in the form of diamondshaped time slices in the impulse response. The best way to achieve circular symmetry is by way of the 3-D phase-shift extrapolation operator. To accommodate lateral velocity variations, a residual correction can be applied at each depth level before extrapolating down to the next depth level. This algorithm, known as phase-shift-plus-correction scheme (Reshefand Kessler, 1989). Another way to circumvent the diamond shape is to Fourier transform the wavefield in the direction with the milder velocity variations, say in the crossline direction, and perform 2-D migration for each of the cross line wavenumbers (Black et aI., 1987). Other solution is to compute the exact 3-D phase shift operator in the Fourier transform domain, inverse Fourier transform in the inline and crossline directions, and then truncate to get a finite-size complex operator in the frequency-space domain. 2-D stable explicit extrapolation operators can be designed with minimal truncation errors within a specified dip range (Hale, 1991). Hale devised an algorithm for 3-D poststack migration that circumvents the cost of computing and tabulating 2-D operators, and applying them by direct convolution. Interpretation of 3-D Seismic Data After 3-D migration, a 3-D data volume is suitable for derivation of the 3-D subsurface geological model. Because of the completeness of data within that volume, the interpreter has more information than from the analysis of a 2-D seismic data set. The use of an interactive interpretation workstation is a common way to deal with large 3-D data volumes. An interactive environment is versatile in viewing the 3-D data volume, e.g., vertical se~tions in iuline, crossline, any arbitrary direction, horizontal sections (time slices). An interactive environment also can provide the capability to improve interpretation, e.g., horizon flattening,

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correlation of marker horizons across faults, and some image processing tools to enhance certain features within the data volume. Present 3-D interpretation systems are based on volume visualization and manipulation of amplitudes within the image volume to enhance structural and stratigraphic features of interest. Interpretation involves using both traveltime and amplitude information contained in the entire image volume. Specifically, structural interpretation is primarily based on picking traveltimes that are coincident with geological layer boundaries.and stratigraphic interpretation is based on manipulation of seismic amplitudes to enhance subtle features associated with depositional environment and sedimentology. A time slice contains events from more than one reflection horizon at the same time level. A spatially high-frequency event on a time slice is either a steeply dipping event or a high-frequency event in time. See Fig. 10. We can infer steep dip at location H from the high-frequency

Fig. 10. Selected time slices from a land 3-D survey (Data Courtesy Nederlandse Aardoline Maatschappij B.V.).

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character of the event, and gentle dip at location L from the low-frequency character. Also, contours associated with a reflection horizon can be traced from time slices. If contours narrow between a shallow and a deeper time slice, then the feature is a structural low. If contours widen between a shallow and a deeper time slice, then the feature is a structural high. See Fig. 11. Time slices can be used to generate structural contour maps. Some 2-D smoothing can be applied to time slices to ease contouring. Edge enhancement can be used to better delineate zero crossing. Some image processing tools also can be used to detect and enhance subtle structural features on time slices. One common artifact that is observable on time slice is the presence of horizontal striations. One cause of time-slice striations is the positioning errors present in the navigation data. Besides contouring, time slices also are useful in quality control. Time slices can be used to check for consistency in picking time horizons along inlines and crosslines. Actually, in some areas with very complex structures, time slices can sometimes be used to trace faults and horizon contours.

Fig. 11. Selected time slices form a land 3-D survey (Data Courtesy Nederlandse Aardoline Maatschappij B.Y.)

3-D visualization facilitates combined interpretation of multiple volumes of data, i.e., image volumes, velocity volumes, and attribute

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volumes. Rapid detection of amplitude anomalies and understanding and correlating the geometries of the various layer boundaries and faults are made possible by the power of3-D visualization. Combining visualization with interpretation tools such as opacity removal, seed detection and edge detection enable the interpreter to get the most out of the data volumes. 3-D visualization of seismic amplitudes is based on the concept of representing each sample in the data volume by a 3-D object called voxel. The voxel representation actually is an extension of a 2-D representation of a sample by a pixel. Each voxel is colour coded by the amplitude of the sample associated with it. Representation of basin-edge faults as a set of surfaces in the absence of the depositional unit would only provide a limited understanding of the structural geology of the subsurface. Structural features such as the intensive fault pattern over the surface area can be enhanced by introducing opacity to the time slab from the image volume. An interpretation of a deltaic sequence is made possible by 3-D visualization and amplitude manipulation. The sequence under consideration can be highlighted on a vertical cross-section from a time-migrated volume of data. The isolated deltaic sequence with the individual members of the sequence can be identified by removing opacity from the subvolume that includes the deltaic sequence. Seed detection can be used to interpret horizons with complex geometry without actually picking traveltimes. Based on a crude definition of the horizon geometry, a horizon-consistent time slab is extracted from the data volume used in interpretation. A horizontal slab is isolated from a 3-D poststack timemigrated volume of data. See Fig. 12. The objective is to isolate a bright spot located within the slab. First, remove opacity to obtain the image of bright spot. Next, label one voxel within the interior of the bright spot as if we are implanting a seed. Then, search for all neighbouring voxels that represent amplitudes within a specified range of the seed amplitude. This process results in isolating a set of voxels that are contiguous in the form of a subvolume. The detected subvolume associated with the bright spot is displayed within the context of the surrounding depositional environment. The seed detection can be repeated by adjusting the threshold amplitude associated with the seed voxel to produce a series of subvolumes with increasing size. The subvolume that represents the bright spot can be isolated completely from the entire image volume. By proper colour coding of amplitudes within the subvolume, the potential hydrocarbon bearing feature can be identified.

Three Dimensional (3-D) Seismic Exploration, Processing.....

Fig. 12. Isolate a bright spot located withm the slab.

153

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Land Acquisition Geometry

Land 3-D acquisition commonly is carried out by swath shooting in which receiver cables are laid out in parallel lines (inline direction) and shots are positioned in a perpendicular direction (crossline direction). See Fig. 13. This shows swath shooting with 6 receivers cables, each

.

.

..

".

--

.'

u

~. . . . . .

y

x Source Locations • Receiver Locations Fig. 13. Swath shooting geometry used in a land 3-D survey.

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with 80 receivers groups, 50m apart. The receiver cables are spaced apart from left to right at 100, 200, 100, 200 and 100 m distances. Shooting usually is perpendicular to the swath starting from the far left and moving in and away to the right of the swath. As one shot line is completed, the receiver cables are "rolled along" the swath a number of stations, equivalent to the short-line spacing, and shooting is repeated. The recording geometry in this figure provides a 25 x 25 m bin size. Once one swath is completed, another swath parallel to it is recorded. This procedure is repeated over the entire survey area. The swath shooting method yields a wide range of source-receiver azimuths, which can be a concern during velocity analysis. The source-receiver azimuth is the angle between a reference line, such as a receiver line or a dip line, and the line that passes through the source and receiver stations. The main advantage of swath shooting is that it is economical. A complete survey plan, including the 12 swaths of receivers are shot locations, is shown in Fig. 14. Coverage varies over the survey between 12-fold and 24-fold. Because

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of operating conditions, uniform coverage usually is not achievable over the entire survey area. It is important to leave some receiver cables on the ground to ensure proper coupling of statics between swaths.

Marine Acquisition Geometry A modem marine 3-D survey involves shooting a number of closely spaced parallel 2-D subsurface lines. This is achieved by using nrulticables and multisource arrays. The more common configuration is 4 to 8 cables with dual source arrays. The recording geometry for multicable marine surveys is similar to that of the multireceiver line recording geometry, known as swath shooting, used in land, shallow-water, and transitionzone surveys. See Fig. 15. This shows marine recording geometry that involves 12 cables and dual source arrays. Since each source-cable combination yields midpoint locations along one subsurface line, this recording geometry yields 24 subsurface lines, simultaneously. As a result, multicable recordng increases the productivity in acquisition by greatly reducing the time in the field. Nevertheless, issues with multicable recording arise in relation to large variations of source-receiver azimuths

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in relation to velocity estimation, migration and amplitude variation with offset analysis. In reality, receiver cables are not straight and parallel to one another. Instead, receiver cables are subject to a certain amount of sideways drift, called feathering, from the ideal cable lines. Cable feathering is caused by cross currents. The angle between the actual cable position and the shot-line direction (the boat track) is called the feathering angle. This angle is not always constant. See Fig. 16. As a result of cable feathering,

Fig. 16. (a) a dual-cable and single source marine recording geometry. ( b) A dual-cable and dual-source marine recording geometry. (Courtesy Western Geophysical).

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midpoints associated with each of the source-cable combination depart from a straight subsurface line. Instead, midpoints are scattered within the hatched areas. For a typicall0-degree feathering angle and a 2400m long cable, the midpoint associated with the far receiver is offset more than 200m off the shot line. This is four lines off at a 50m line spacing. The direction and strength of cross-currents determines the crossline width of the midpoint distribution. In the ideal case of zero feathering, midpoint distribution would have zero cross line width. The dual source, dual-cable configuration yields four bin lines. This is conducted by alternate pops produced by the· two sources. Because of significant variations in cable shape during recording, midpoint distribution in the cross line direction can be quite irregular. We must know exactly where each of the receivers is located along the cable as well as the shot-point location. Navigation data collected on the survey vessel normally include the boat location, source location, and cable compass readings. There are 8 to 12 digital compasses along a typical marine cable. Readings from these devices allow computation of the (x, y) coordinates of the cable compasses. Cable shape then is computed based on a curve-fitting procedure that rejects any anomalous measurements. Navigation data are analyzed during processing, and quality control is carried out to derive the fmal shot-receiver locations. Processing of 3-D seismic data requires binning the recorded data into common-cell gathers To perform 3-D binning, a grid is superimposed on the survey area. See Fig. 17. This grid consists of cells with dimensions of half the receiver group spacing in the inline direction, equivalent to the CMP spacing in 2-D processing, and the line spacing in the crossline direction. By precisely determining the shot and receiver coordinates, we can determine the midpoint locationjand gather them into bins (or cells) for stacking and migration. In reality, midpoint distribution within a cell is not necessarily uniform since cable shape varies from shot to shot and line to line. The centroid of the midpoints is not necessarily at the centre of the cell. Midpoints distribution also can very from cell to cell. The grid with dimensions 12.5 x 25 m represents the bins, and the traces at the midpoint locations that fall within each bin constitute a common-cell gather. The member of the midpoints within each bin defmes the fold for that bin. The fold and midpoint distribution vary significantly from one bin to another. Irregularities in recording geometry, i.e., nonuniform fold of coverage, irregular offset distribution per bin, and irregular

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midpoint distribution within each bin, can cause problems in processing of the 3-D data. Lack of uniformity in the fold of coverage causes inconsistency in the accuracy of velocity estimation from one analysis location to another and variations in the stacked amplitudes. Processes such as 3-D dip moveout (DMO) correction, 3-D prestack time and depth migration, and amplitude variation with offset analysis are adversely affected by the acquisition footprint. Low-fold areas in a 3-D surveyomst be fitted in during acquisition by shooting more lines at appropriate locations. Quality control work must be carried out on board to monitor the fold of coverage. A slight translation and rotation of the grid imposed on the survey area sometimes significantly reduces problems associated with binning, e.g., more uniform midpoint distribution within each cell, improve the uniformity of the fold of coverage over the survey area. It is common not to restrict the gridding to cells of equal size. Adjustment to cell size to achieve a uniform fold of coverage and offset distribution is called flexible binning. Need for Imaging in Three Dimensions The subsurface generally dips in many directions in areas where structural traps of interest exist. It is not possible to identify the inline direction as either a dip or strike direction. Three-dimensional migration often produces different sections from 2-D migrated sections. 2-D migration can introduce misties between 2-D lines in the presence of dipping events. Two dimensional migration cannot adequately image the subsurface, while 3-D migration eliminates these misties by completing the image process. 3-D migration provides complete imaging of the 3-D subsurface geology. 2-D migration can yield inadequate results. The difference between 2-D seismic and 3-D seismic is the way in which migration is performed. Dense coverage on top of a target zone, say a 12.5 m inline trace spacing and a 25 m crossline trace spacing, will not necessarily provide adequate subsurface imaging unless migration is performed in a 3-D sense. Cable feathering in marine 3-D surveys and swath shooting geometry in land 3-D surveys give rise to source-receiver azimuthal variations. Stacking velocities become not only dip dependent but also azimuth dependent. Three-dimensional dip-moveout correction accounts for both dip and azimuth effects on stacking velocities. See Fig. 18. This cross-section represents a case of events with conflicting dips, i.e., specifically, the steeply dipping fault plane reflections and the

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Fig. 18. A cross-sectIOn from a 3-D poststack time-migrated volume of 3-D DMO-stacked data.

gently dipping reflections associated with the sedimentary strata. 3-D DMO correction has preserved both of these events. This has enabled 3-D migration to produce a crisp image of the fault planes. 3-D migration of the data with 3-D DMO correction yields a better delineation of the salt dome geometry. The imaging problem in the subsalt region is not within the realm of 3-D DMO correction and 3-D time migration. But it is a problem that needs to be handled-by 3-D imaging in depth. Different migration strategies are required for different types of subsurface geological circumstances. To correct for conflicting dips with different stacking velocities, one needs to do prestack, and not poststack, time migration. However, prestack time migration, when performed in 2D, once again falls short of meeting the accurate imaging requirement. Because of the acoustic impedance changes along the fault planes caused by the juxtaposition of velocities across the fault blocks, it is useful to examine the migrated section also with its polarity reserved. Imaging in 3-D alone would not provide a good image if the input stacked volume of data does not contain the fault-plane reflections. By combining 3-D DMO corrections and 3-D poststack time migration, the imaging quality for the fault-plane reflections is greatly enhanced. The ultimate imaging solution in the time domain to the problem of conflicting dips with different stacking velocities is 3-D prestack time migration.

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Pbase-Shift-Plus-Correction Method The phase-shift method is designed to accommodate vertically varying velocity field for migration only. It can be extended to acconunodate lateral velocity variations (Pai, 1988). Consider a spatially varying 3-D migration velocity field v(x, y, z) and a laterally averaged, but vertically varying velocity function v (z). The basic idea is to fIrSt extrapolate in depth with the phase-shift extrapolator using the vertically varying velocity function v (z). This is followed by the application of a convolutional operator to the extrapolated wave field at the same depth. This second operation is fundamentally a residual extrapolation to account for the difference between the laterally varying velocity field v(x, y, z) and the vertically varying velocity function v (z). The migration algorithm that incorporates such a correction term into the phase-shift algorithm has been known as phase-shift-plus-correction (PSPC) method. Depending on the degree of lateral velocity variations, the PSPC method can be used either as a time or depth migration algorithm. Fig. 19 shows selected crosslines from the 3-D migrated volumes of data associated with the 3-D survey using three different migration algorithms: (1) the implicit one-pass on top (2) the explicit McClellan transform in the middle, and (3) the PSPC at the bottom. Both of the explicit schemes based on the McClellan transform and the PSPC methods have produced comparable and better images of the subsurface in the vicinity of the overthrust between inline location 5200 and 300. The same region of the subsurface appears to be undermigrated by the implicit one-pass scheme. Fig. 20 shows selected time slices from the results of migration using the three different migration algorithms: (1) the implicit one-pass on the left, (2) the explicit McClellan transform in the middle, and (3) the explicit phase-shift-plus-correction on the right. The wobbly behaviour of the contours on the time slices from the implicit one-pass scheme is related to the azimuthal assymetry of its impulse response. The two explicit schemes, i.e., the McClellan transform and the phase-shift-plus-correction, produce comparable images. It is almost certain that in the future, explicit schemes, because of their easy design and implementation, will become standard. Increased computer power will further encourage use of the explicit schemes. Processing of Three-Dimensional (3-D) Seismic Data 3-D seismic data processing is similar to 2-D seismic data processing. Additional complications do arise in 3-D geometry quality control, statics,

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Fig. 19. Selected crosslines from the volumes of migrated data using three different 3-D poststack migration algorithms: (a) the implicit one-pass, (b) the explicit McClellan transforms and (c) the expliCIt phaseshift-plus-correction.

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Fig. 20. Selected time slices from the volume of mIgrated data using three different 3-D poststack migratIon algorithms: (a) the implicit one-pass, (b) the explIcit McClellan transform, and (c) the explicit phase-sh ift -plus-correction.

velocity analysis. and migration. Editing traces with high-level noise, geometric spreading correction, deconvolution and trace balancing, field statics applications for land and shallow-water data are similar as for 2-D surveys. In 3-D processing, traces are collected into common-cell gathers. A common-cell gather coincides with a eMP gather for swath shooting when the lines are straight. Sorting into common-cell gathers introduces some problems. For a dipping reflector, there is the problem of azimuthal variations of the nonnal moveout (NMO) within the cell for most land data and for marine data with significant cable feathering. Fig. 21 is the base map for a land 3-D survey that covers a surface area of nearly 270 km2 (Yilmag, 2001). The bin size is 25 x 25 m, and there are 520 inlines

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and 820 crosslines. The 3-D survey comprises more than five million recorded traces, O-and more than 400,000 stacked traces. Table 2 gives the list of the survey parameters. There are six receiver lines at 500 m intervals, each comprising 80 groups at 50 m intervals. Shots are placed perpendicular to the swath and at the centres of the receiver lines. East is to the right. Shot locations follow the irregular patterns an receiver locations follow the more-or-Iess irregular lines in the east-west direction. Each shot record comprises subrecords each of which corresponds to Table 2. Design and application of explicit 3-D wave extrapolators

2-D design 3-D design 3-D application

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one of the six receiver lines in the swath. Depending on the shot location relative to the receiver lines in the swath, arrival times of refraction and reflection events vary from one subrecord to another. Elevation generally increase from south-west to north-east. The average fold of coverage is 12 over the survey area. Three coverage maps for offset range 0-1000, 1000-2000, and 2000-400Om. The near-offset fold of coverage appears to be fairly uniform over the survey area. The mid-offset fold of coverage appears to be low and exhibits some variations. Finally, the far-offset fold of coverage map shows that there are bins with missing far-offset traces. Coverage maps are essential to quality control in processing and interpretation. Variations in fold of coverage have undesirable effects on velocity estimation, mUltiple attenuation, noise attenuation and amplitude variation with offset (AVO) analysis. Fold of coverage should be examined within the context of source-receiver azimuthal variations. Coverage maps for source-receiver azimuths fall within three ranges: (1) 340-40 and 100-220, (2) 40-100 and 220-280, and (3) 280-340 and 100-160 degrees measured from the north. The near offsets are largely confined to azinmths that fall in range 1, the middle offsetSiare largely confmed to azimuths that fall in range 2, and the far offsets are largely confmed to azimuths that fall in range 3. The information required to apply the field statics corrections are : (1) the weathering velocity, (2) reflector (bedrock) velocity, and (3) depth to bedrock. They are derived from the uphole surveys which are carried out in the field at close spatial intervals. If field statics corrections are judged to be inadequate, then a near-surface model should be estimated from inversion of refracted arrivals. The surface-consistent refraction statics model does not restrict shot and receiver stations to a survey line, but they can be situated anywhere, as in a 3-D survey. Hence, equation can be used to model the intercept time anomalies at all shot and receiver stations over the entire 3-D survey area which is given as : t'ij =

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Where t'ij are the modeled first-break picks associated with a shallow reflector, Tj are the intercept time anomalies at shot stations, T; are the intercept time anomalies at receiver stations, sb is the bedrock slowness and hij the shot-receiver separation. The near-surface model parameters are estimated such that the difference between the modeled times ( and the actual times t picked from frrst breaks is minimum in the least-square sense.

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Levin (1971) developed the equations for moveout velocity and traveltime associated with a dipping reflector and for a source-receiver pair along an arbitrary azimuthal direction measured from the dip line direction. Fig. 22. 3-D moveout equation is given as :

Fig. 22. Geometry for a difpping planer interface used in deriving the 3-D moveout equation.

...(2) 3-D normal moveout velocity is given as : v

...(3) 1- sin 2 ~COS2 9 Where 9 is the azimuth angle between the structural dip direction and the direction of the profile line, ~ is the dip angle, t is the two-way traveltime associated with the non-zero-offset raypath from the source location to the reflection point on the dipping interface back to the receiver location, to is the two-way zero-offset time associated with the normal-incidence ray-path at the midpoint location, vNMO is the moveout velocity, and v is the velocity of the medium above the dipping reflector. Equation (3) describes an ellipse in polar coordinates. See Fig. 23. The radial coordinates is the NMO velocity vNMO' while the polar angle is the azimuth 9. Orientation of the major axis of the velocity ellipse is in the true dip direction. Once the velocity ellipse is constructed for a dipping reflector at a CMP location, then the following parameters can be determined : VNMO =

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Fig. 23. The 3-D NMO velocity is an ellipse in polar coordinates.

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+

This is called the three-parameter velocity analysis. Each trace is the eMP gather is moveout corrected using the velocity along the corresponding shot-receiver azimuth. Three-parameter velocity analysis is not in use now. Presently, 3-D dip-moveout correction is used to account for source-receiver azimuth and dip effect on stacking velocities. Stratigraphic Interpretation A session for 3-D Stratigraphic interpretation may begin with splitting the image volume into subvolumes that correspond to individual depositional units bounded by the time horizons derived from structural interpretation. The Stratigraphic interpretation involves removal of opacity and seed detection. The important steps are given below :1. Removal of opacity is applied to either a horizontal time slab with a specified thickness, typically a few to tens of time samples, or to depositional unit bounded by the time horizons derived from structural interpretation. Images of a complex channel system are enhanced at the water bottom, and the intensive fracture system begins to develop immediately below the water bottom and increases in complexity as we go deeper in the image volume. See Fig. 24.

Three Dimensional (3-D) Seismic Exploration, Processing.....

Fig. 24. PortIOns of the image volume associated wIth each layer bounded by the tIme honzons.

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Encyclopaedia of Petroleum Science and Engineering 2 Now we look inside a specific depositional unit. Remove the opacity within the subvolume under consideration and uncover a remnant of an ancient channel system. See Fig. 25. The fault system has been found by the subsequent tctonic disturbance, disrupts the channel system at several locations.

Fig. 25. (a) OpacIty removal applied to the image volume. (b) A magnified view of a portion of a remnant of an ancient channel system.

3. Apply seed detection to the event that represents the channel within the subvolume and isolate it completely from the rest of the subvolume Fig. 26 shows a close-up view of the structural and stratigraphic interpretation result.

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Fig. 26. (a) A close-up view of the isolated channel and (b) the channel colour-coded for its elevition (After Yilmaz, 2001).

The result of stratigraphic interpretation needs to be checked for consistency with the image volume of data used in the interpretation. This figure of step (3) shows a view of the channel and an inline section from the image volume derive from 3-D prestack time migration. The

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elevation varies considerably because of the disturbance of the channel by the subsequent faulting. Strike Versus Dip Shooting Dip-line shooting and strike-line shooting, both have advantages and disadvantages. Aspects of dip-line shooting are given below: 1. With dip-line shooting, we have a better spatial sampling in the direction that we need most.

2. Since there is no cross-dip, there is no crossline smearing, and we can afford coarser spacing between lines. 3. With dip-line shooting, we have the disadvantage of complex moveout on common-cell data that can cause problems in velocity analysis (Lamer and Ng, 1984). 4. We have the problem of, inline smear because of the reflection point dispersal along the dipping reflector associated with a nonzero-offset recording. But DMO correction handles this problem Aspects of strike-line shooting are given below: 1. With strike-line shooting, we have better alternation of coherent noise because of the move out behaviour of side scatters.

2. Since there is no dip perceived on a strike line, so we do not require DMO correction. 3. With strike-line shooting, we have the disadvantage of perceiving the largest cross-dip, hence we have the problem of crossline smear. 4. Strike-line shooting would require closer line spacing to prevent spatial aliasing of steep dips in the orthogonal direction. Given the choice between the two types of shooting, dip-line shooting must be employed almost always in data acquisition. Modern 3-D surveys are conducted using sufficiently small inline and crossline spacing, therefore these surveys minimize crossline smearing and remove inline smearing by 3-D DMO correction. Structural Interpretation A 3-D structural interpretation session may begin with viewing selected inline and crossline sections to acquire a regional understanding of the subsurface geology. Other orientations, such as vertical sections

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along dominant dip direction, also may be needed to determine the structural pattern. Time slices then are studied to check the structural pattern. These preview may be made dynamic in an interactive environment. Vertical or horizontal sections can be viewed in rapid succession. Any change in structure in space and time can thus be grasped with ease. Fig. 27 shows an image volume derived from 3-D prestack time migration of data from a marine 3-D survey (Yilmaz, 200 I). There are 296 inline and 1300 crossline in the data volume. The time slices represent snapshots from the 3-D visualization session. The complex structure pattern is represented by a principal fault across the survey

Fig. 27. The image volume denved from 3-D pre-stack time migration of data from a 3-D marine survey.

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area from left to right and a series of fault blocks on both sides of this fault. The water bottom and six deeper time horizons are picked using a combination of seed detection and line-based interpretation. The important steps are given below: 1. First, the seismic event for each horizon is identical within the image volume. Then, at locations with good continuity and signal-to-noise ratio, seed points are placed at the seismic event. For consistency, all seed points for a given horizon are assigned identically, i.e., either at trough or peak. By using the seed detection method, the event is tracked away from each seed point as far as possible laterally in all directions. The result of seed detection is a series of surface patches associated with the horizon under consideration. See Fig. 28.

Fig. 28. Picking of a time horizon from the image volume based on seed detection.

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2 Where seed detection failed. control points are picked along a grid of selected inlines and crosslines. During and after an interpretation session, it is imperative to check the consistency of the picked horizons with the seismic data volume that is used in the interpretation. The final step in structural interpretation is time-to-depth conversion of time horizons interpreted from the timemigrated volume of data. This step requires knowledge of interval velocities for each layer, and as such, it implies earth modeling in depth. A structural interpretation session normally includes explicit delineation of fault framework. The objective is the investigation of stratigraphic features within the reservoir zone, and as such, the time horizons are interpreted without the explicit defmition of the fault patterns. Instead. the fault surfaces are interpreted as part of the horizons themselves . ............,.. (3-D) Dlp-MO¥eGUt (DMO) Correction

Because of irregular spatial associated with 3-D recording geomeUia, the 3-D DMO process is best applied in the time-space domain usiDa the integral method. A velocity analysis is then performed following the DMO correction to estimate dip-corrected and azimuth-corrected velocicities. The important points of the 3-D DMO correction are given below: 1. The process of 3-D dip-moveout corrects for the dip and source, receiver azimuth effects on stacking velocities. 2 It preserves conflicting dip with different stacking velocities during CMP stacking. 3. The 3-D DMO stack is a closer representation of a 3-D zerooffset section as compared to a conventional CMP stack volume of data based on norrnal-moveout correction, only. 4. The 3-D DMO stack can be migrated using a 3-D zero-offset migration algorithm with greater accuracy. 5. Conflicting dips with different stacking velocities given rise to multi-valued velocity picks from velocity spectra. Velocity analysis of 3-D DMO-corrected data alleviates this problem and increases the accuracy of picking an unambiguous velocity function from a velocity spectrum. 6. Velocities estimated from 3-D DMO-corrected data are dip and azimuth independent. Therefore, they are more suitable to derive a migration velocity field as compared to velocities estimated from data without 3-D DMO correction.

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Encyclopaedia of Petroleum Science and Engineering 7. 3-D DMO correction is a process of partial migration before stack. Specifically, it maps normal-moveout-corrected data to normal-incidence reflection points in the subsurface. As a result, the midpoint is a variant under DMO correction. 8. 3-D DMO correction removes the reflection point dispersal associated with nonzero-offset recording in the presence of dipping reflectors. 9. By 3-D DMO correction, prestack data can be implicitly regularized into common-midpoint gathers. This facilitates sorting of prestack data into a set of common-offset volumes, each of which can beconsidered a replica of a 3-D zero-offset wavefield. 10. Following 3-D DMO correction, prestack data can be migrated so as to create CMP gathers in their migrated position (next section). This enables us to conduct velocity analysis to derive a migration velocity field with greater accuracy. 11. The CMP gathers from prestack time migration of 3-D DMOcorrected data can be used for amplitude variation with offset anaysis.

Once the data are sorted into common-cell gathers, velocity analysis is performed. A number of common-cell gathers are included in the 3-D velocity analysis to increase the signal-to-noise ratio, e.g., 5 in the inline and 5 in the cross line direction, for a total of 25 common-cell gathers. Velocity analysis are performed at certain intervals, say 0.5 km, along selected inlines that may be as much as 0.5 km. Apart. The result of velocity analysis at selected control points are used to derive the 3-D velocity field for all common-cell gathers over the entire survey. This is achieved by performing a 3-D interpolation of the velocity functions between the control points. The following equation can be used to model the static shifts at all shot and receiver stations over the entire 3-D survey area written as : t'ij =

sj+rj +Gk +4Miz21i

••• (1)

Where t'li are the modeled traveltime deviations associated with a moveout. corrected reflection event within a specified time gate, Sj is the shot residual statics, rj is the receiver residual statics, Gk is the structure term at the kth midpoint location, here k = (i + j)/2, and 4Mkh2 Ii is the residual parabolic move out term, with 2hij being the shot-receiver

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separation. The difference between the modeled time t' and the actual times t picked from moveout-corrected cell-gathers is mininnnn in the leastsquares sense. For 3-D seismic data, an initial pilot trace is built from the common-cell gather of a selected line where the signal-to-noise ratio is high. Pilot traces for other common-cell gathers are computed from both local input traces and neighbouring pilot traces. Land recording geometry must accommodate some overlap between adjacent swaths to ensure statics coupling between the swaths. It is advisable to apply 3-D DMO correction prior to residual statics estimation. Fig. 29 shows a crossline from a 3-D survey with and without residual statics corrections. Improvement in the reflection continuity at times below 2s can be noted. Followed 3-D DMO correction and residual statics corrections, a fmal velocity analysis is carried out using a grid of the size that may be as small as "500 x 500m. Combined analysis of the gather, stacks, and spectrum enables reliable picking of a velocity function at the analysis location. By combining the velocity functions picked at analysis locations over the survey area, a 3-D velocity field is created. A thorough check on the velocity field is essential for stacking and time migration. DMOstacking velocity field usually is smoothed spatially so as to eliminate lateral velocity variations that are judged to be unacceptable for time migration. By using a smooth version of the 3-D velocity field derived from the 3-D DMO-corrected data, the 3-D DMO-stack volume is then time-migrated. Three Dimensional DMO Correction Combined with 3-D Common Offset Migration We shall develop a workflow for 3-D prestack time migration based on 3-D DMO correction combined with 3-D common-offset migration. This workflow yields common-reflection-point (CRP) gathers which can be stacked to produce the image volume from 3-D prestack time migration. The CRP gathers can also be used to derive a 3-D rms velocity field associated with the migrated data (Ferber, 1994). The important points of this workflow are given below:

1. Starting with input prestack data, apply NMO correction using flat event velocities. These are picked from velocity spectra computed over a sparse grid.

2 Apply 3-D DMO correction and sort the data common-offset volumes.

.....:J 00

1-

2-

3-

4-

(a)

(b)

Fig. 29. A crossline from the volume of stacked data associated with the 3-D survey. (a) Without residual statics corrections, and (b) . with residual statics corrections.

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3. Following NMO and 3-D DMO correction, each CORmon-offset volume is assumed to be a replica of a 3-D zero-offset section, and can be migrated usit\J.g a 3-D zero-offset migration algorithm. A spatially averaged but vertically varying velocity function can be used to perform the migration of the cORmon-offset volumes of data. 4. Sort the migrated cORmon-offset volumes into CRP gathers. 5. Apply inverse NMO correction using the velocity field as in step (1) and repeat the velocity analysis. 6. Create a 3-D velocity field by interpolating the vertical function picked from the velocity spectra and spatially smoothing the resulting velocity volume. 7. Apply NMO correction to the common-reflection-point (CRP) gathers using the post-migration velocity field step (5). The CRP stacks constitute the cross-sections of the image volume from 3-D prestack time migration. Compare the cross-sections with those from the two image volumes based on 3-D poststack time migration of the conventional CMP-stacked data volume and that of the 3-D DMO stack volume, and note the significant improvement in the imaging of the complex fault blocks. 8. To obtain the migrated volume with the updated velocity field, first. perform 3-D zero-offset inverse migration (equivalent to 3-D zero offset forward modeling) of the resulting image volume from step (7) using the same velocity function used to perform the 3-D CORmon- offset migration as in step (3). Selected time slices from this modeled data volume are compared with those from the CMP stack and DMO stack volumes. The modeled volume of data may be treated as equivalent to the unmigrated stack volume. 9. The final step involves 3-D zero-offset migration of the modeled volume of data using the 3-D migration velocity field. Compare the results of a 3-D prestack time migration with the results from 3-D poststack time migration of the data with and without 3-D DMO correction. Note the improvement in imaging the fault planes with 3-D prestack time migration. Three-Dimensional (3-D) Migration Velocity Analysis A workflow for 3-D migration velocity analysis is based on a 3-D extension of the velocity-independent imaging technique by Fouler

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(1984). The important points of this workflow are given below: 1. Start with 3-D prestack data P(x, y, h, t) in coordinates of inline x, offset 2h, crossline y, and event time t in the unmigrated position, and apply NMO correction, 3-D DMO correction followed by inverse NMO correction. 2. Create a constant-velocity stack (CVS) volumeP (x, y. to; VDMO ) using a constant velocity VDMO' where to is the zero-offset event time after 3-D DMO correction. Shown in Fig. 30 is a CVS

Fig. 30. CVS volume cuated by using a velocity of 2400 mls.

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volume created by using a velocity of2400 mls. The prestack data are from the same survey as that of the data shown in Fig. 31.

.

lUI

~I!,~

I-51

101 ,

sq;

1-101

..::.

lGl ,

5!~S

1-151 _1...?1_ _..LfCl...2_ _... 1?_l_--""'Il4_ _..Llo'... .'.!...:..'-..=2=O'

Fig. 31. Stacked sections along selected inline traverses of a 3-D smvey data after crossline migration.

3. Migrate the c~nstant-velocity stack (CVS) volume P(x, y, to; VDMO) to create a constant-velocity migration (CVM) volume P (x, y, 't; vDBa)' where T is the event time after migration, using

182

Encyclopaedia of Petroleum Science and Engineering the constant velocity associated with the CVS volume itself and a 3-D constant- velocity stolt algorithm. 4. Repeat steps (2) and (3) for a range of constant velocities. 5. Extract the time slices from the CVM volumes for a specified time 'tj and combine them to form a velocity cube P(x, y, vmia;T~ e.g., n velocity cubes.

6. For 3-D visualization and interpretation, form a super volume that is a composite of the one on top of the other.

n velocity cubes from step (5) placed

7. Based on the maximum-event-amplitude picking strategy, interpret each of the velocity cubes and create velocity strands associated with the constant times 'tj,j = 1, 2, ..... ,N, along selected crossline or inline traverses. 8. Combine the velocity strands for a specific time 'tj and create an rms velocity map associated with that time. Repeat for all times 't,.,j = 1, 2, ...... ,N, and obtain a set of rms velocity maps. 9. Create a 3-D rms velocity field using the rms velocity maps from step (8). The 3-D rms velocity field honours the structural characteristics of the subsurface. The 3-D rms velocity field derived from the workflow given above is associated with events in their migrated positions. Compared to the 3-D DMO velocity field associated with events in their yet unmigrated positions, it is the preferred velocity field with which we would want to migrate our data using a desired 3-D poststack or prestack time migration algorithm. If we wish to extend our analysis from time to depth domain, the 3-D rms velocity field associated with time-migrated data also is the preferred velocity field to derive a 3-D interval velocity field that can then be used to obtain an image in depth. Three-Dimensional Nonzero-offset Traveltime Equation We shall consider a 3-D recording parallel to the inline direction x with zero source receiver azimuth. See Fig. 32. Consider a midpoint M : (x, y, 0) on cross line y that is associated with a source S : (x - h, y, 0) and a receiver R : (x + h, y, 0), where h is the half-offset. The distance along the raypath SD from source S : (x - h, y, 0) to a point D(0, 0, z) in the sub-surface is given by : SD = ~(x - h)2 + y2 + Z2

•• .(1)

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D (O,O,z)

Fig. 32. Raypath geometry for 3-D nonzero-offset traveltime equation.

The distance along the raypath DR from point D : (0, 0, z) to a receiver R : (x + h, y, 0) is given by the following equation: ...(2)

The traveltime t associated with the raypath SDR then given by as :

vt

=

~(x + h)2 + y2 + Z2 + ~(x _ h)2 + y2 + Z2

...(3)

Where v is the medium velocity. Squaring both sides of equation (3), we get:

v2t2 = [(x+hi + y2 +z2]+[(x_h)l + y2 +Z2]+ 2

...(4)

~[(x + h)2 + yl + Z2] X ~[(x _ h)2 + y2 + Z2] Simplifying equation (4), we have:

v; -vt h 44

(v2t2 - 4h2) x2 + v2t2y2 + v2t2z2

=

2 l

l

•••(5)

Rearranging the terms in equation (5), we have:

x2 y2 Z2 2 (vt/2)2 + (vtl2i _ h + (vt/2)2 _ h2

=

1

...(6)

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Equation (6) represents an ellipsoid. The horizontal cross-section of this ellipsoid at a depth z is an ellipse. At z = 0, the ellipse has these three parameters: (1) semi-major axis a in inline x direction is vtl2, (2) semiminor axis b in crossline y direction is ~(vtI2)2 - h2, and (3) distance

.J

from center to either focus a 2 _ b 2 is h. The ellipsoid of equation (6) in the x-y-z volume describes the kinematics of the impulse response of a 3-D nonzero-offset migration operator applied to 3-D prestack data. When equation (6) is specialised to the zero-offset case, i. e., h = 0, we get:

x2

y2

Z2

(vt12)2

(vtl2)

(vtl2)

- - + - - 2 +--2

= 1

...(7)

Equation (7) describes a hallow hemisphere in the (x, y, z) volume for a constant t with a radius vtl2. This hemisphere represents the kinematics of the impulse response of a 3-D zero-offset migration operator applied to 3-D poststack data. When equation (3) is specialised to the 3-D zerooffset case, h = 0, we get:

vt = Z2~X2 + y2 + Z2 ...(8) Equation (8) describes the diffraction hyperboloid of revolution in the (x, y, t) volume for a constant z. Three-Dimensional Phase-Sbift Migration We start with the solution of the 3-D scalar wave equation to extrapolate a 3-D zero-offset wavefield P(x, y, z = 0, t) in the frequencywavenumber domain which is given by the following equation: P(kx' ky, z, w) = P(kx' ky' z, w) expo (- i kz z)

...(1)

We assume a horizontally layered earth model associated with a vertically varying velocity function v(z). By inverse Fourier transforming equation (I), we have: P(x, y, x, t) =

II P(kx' ky' z, w) exp.(- i k;Z) exp.(- i kxx -

i kyy + iwt) .

dkx dky d w ...(2) Where kz is defmed by the following equation adapted to the exploding reflectors model by replacing v with vl2 :

k = 2w 1_(-fkx)2 _(-fky)2 z

w2

°

2w

2w

...(3)

The imaging principal t = then is applied to equation (2) to get the migrated section P(x, y, z, t = 0) as :

Three Dimensional (3-D) Seismic Exploration, Processing.....

P(x, y, z, t = 0) =JJI P(kx' ky

0,

185

w) expo (- ikx x - ikyY - ikzz) dkx' dky dw •••(4)

This is the equation for the phase-shift method (Gazdag, 1978) adapted to 3-D migration. Equation (4) involves integration over frequency and inverse Fourier transformation along inline and crossline axes at each depth step. Three Dimensional (3-D) Poststack Migration To understand 3-D migration, consider a point scatter that is buried in a constant-velocity medium. The zero-offset traveltime curve in two dimensions is a hyperbola. The curvature of the hyperbolic trajectory for amplitude summation is governed by the velocity function. The equation for this trajectory is written as :

t2 =

.2 + 4x2/VZ

rrns

...(1)

The zero-offset response of a point scatter in three dimensions is a hyperboloid of revolution described by the following equation:

.2 + 4(xv~+ l) 2

t2

=

--'---:--=:'-~

...(2)

Where x and Y are the inline and crossline coordinates of the input sample at two-way zero-offset time t. Migration in three dimensions amounts to summing amplitudes at times t over the surface of the hyperboloid and placing the. result at time • that coincides with the apex of the hyperboloid. The simple diffraction summation technique for migration can be improved by making the appropriate amplitude and phase corrections based on the far-field term of the Kirchhoff integral solution to the scaler wave equatio before summation. The output 3-D image Pout (xO' yO' Z = vtl2, t = 0) from Kirchhoff summation at a subsurface location (xO' yO' z) is computed from the 3-D zero-offset wavefield Pin(x, Y, z = 0, t), which is measured at the surface (z = 0), by a summation over an areal aperture in the inline and crossline directions given by the equaion : P

= out

Where =

Vnns

AxAy

4II

Y x

vnns r

...(3)

is the velocity at the output point (x o' Yo, z), and r

~(x - xo)~ + (y - YO)2 + Z2,

oj

~~[cose p(t) * P;n]

which is the distance between the input

(x, y, Z = aJjd the output (xo,yo,z) points. The output image Pout is computed at (xo' Yo, z = vt/2, t = 0) using the input wave field Pin at (x, y

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.z = 0), t - rlv). Equation (3) can be used to compute the wavefield at any depth z. To obtain the migrated section of an output time 't, equation (3) must be evaluated at z = v-r/2 and the imaging principle must be involed by mapping amplitudes of the resulting wavefield at t = 0 onto the migrated section at output time T. The complete migrated section is obtained by performing the summation in equation (3) over an areal aperture and setting t = 0 for each output location. For 2-D migration, as many as 300 traces may be included in the summation. In three dimensions, as many as 70,000 traces may need to be included in the summation. The rho filter p(t) in equation (3) corresponds to the time derivative of the measured wavefield, which yields the 90-degree phase shift and adjustment of the amplitude spectrum by the ramp function w of frequency. Since the rho filter is independent of the spatial variables, it can be applied to the outout of the summation in equation (3). Finally, the farfield tenn in this equation is proportional to the cosine of the angle of propagation (the directivity tenn) and is inversely proportional to vr (the spherical spreading tenn) in three dimensions. The diffraction hyperboloid has a hyperbolic cross-section in any azimuthal direction. A two-stage summation over the surface of the hyperboloid can be perfonned as follows:

1. Sum along the hyperbolic cross-sections in the inline direction and place the summed amplitudes at the local apexes of these hyperbolas. The hyperboloid now is collapsed to a hyperbola that is in the plane perpendicular to the direction of the first summation. This hyperbola comprises the first summed amplitudes at the local apexes and is contained in the plane of the crossline direction. 2 Sum the energy along this hyperbola and place it to its apex, which is also the apex of the original hyperboloid and is where the image should be placed. The sequence of numerical operations in the splitting and separation methods is shown in Fig. 33. If you have strong dependency of velocities on spatial variables in the inline and crossline directions, then splitting is more accurate then separation. So splitting would be the required scheme for depth migration, whereas separation may be acceptable for time migration. 3-D migration based on full separation is known as the two-pass approach. 3-D migration based on splitting is considered a one-

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Splitt...

SepuadoII

3-D Stacked Data Loop lOver Depth Step DowDwant CGlltinue in tbe InliDe dlrectloll. Dowaward COIltinue in tJ¥

[

Cnulhae direction.

AppI, ' ........ principle.

CIDIe Loop t.

Fig. 33. Algorithms for 3-D migration based on the splitting and separation techniques of implementing migration operators.

pass approach. The two-pass approach based on separation is now outmoded by the one-pass approach based on the splitting on the 3-D extrapolation operator with various improvements in accuracy and efficiency. As for 2-D migration, a convenient domain to do 3-D time or depth migration is the frequency-space domain. Since the need for 3-D imaging becomes significant in areas with steep dips and lateral velocity variations, the 2-D frequency-space algorithm adapted for one-pass 3-D migration is used. Extrapolation operators based on rational approximations to the exact dispersion relation are applied to the 3-D stacked volume of data in the inline and crossline directions. Three-Dimensional (3-D) Prestack Time Migration When the reflector geometries that give rise to the problem of conflicting dips with different stacking velocities have a 3-D behaviour, it is necessary to image the subsurface using 3-D prestack time migration. Fault-plane reflections associated with rotated fault blocks are one such case that requires imaging in 3-D and before stack. 3-D prestack time migration produces common-reflection-point (CRP) gathers which can be used for amplitude variation with offset analysis. The robust alternative to 3-D prestack time migration in practice is to apply NMO and 3-D DMO corrections followed by 3-D poststack time migration. It is assumed that following NMO and 3-D DMO corrections, each of the common-offset volumes of data is equivalent to a 3-D zero-offset wavefield, and thus can be migrated using a 3-D poststack time migration algorithm,

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individually. Crossline migration is a process that reduces the imaging problem from three dimension to two dimensions (Canning and Gardner, 1996). 3-D DMO correction preserves the fault-plane reflections on stacked data and the subsequent 3-D post-stack time migration delineates the fault blocks with improved imaging. The kinematics of 3-D prestack time migration can be formulated as an extension of the kinematics of 2D prestack time migration in the same manner as for 3-D DMO correction. The 3-D DMO process can also be used to describe 3-D prestack time migration. Consider a trace from a common-cell gather with no NMO nor DMO correction and map the amplitude at time sample A to neighbouring cells which are coincident with the source-receiver azimuthal direction associated that input trace along the semi-elliptical trajectory that describes the kinematics of 3-D prestack time migration. Repeat the process for all the traces from the same common-cell gather and map the amplitudes in the same manner. For the hypothetical recording geometry with a common-cell gather coincident with a common-midpoint gather and traces in the gather covering a 360-degree source-receiver azimuthal range, but having the same source-receiver separation, the elliptical trajectories associated with all the traces constitute an ellipsoid of revolution. This ellipsoid describes the kinematics of the impulse response of a 3-D pres tack migration operator. For an input trace with a specific source-receiver azimuth and offset, 3-D prestack time migration can be conceptualized either by way of a semi-elliptical superposition or a diffraction summation over the traveltime trajectory. 3-D recording geometries give rise to non-uniform source-receiver azimuthal and offset coverage.and midpoint scattering over the survey area. 3-D DMO correction implicitly regularize the spatial sampling of the prestack 3-D data. As a result, the data can be decoupled and sorted into common-offset volumes each of which is considered a replica of a 3-D zero-offset wavefield. This then enables us to adopt the robust approach for 2-D prestack time migration based on DMO correction and common-offset migration to develop an efficient workflow for 3-D prestack time migration. Three-Dimensional (3-D) Prestack Time Migration (Summary) The improvement in imaging with 3-D prestack time migration may sometimes be marginal compared to imaging with 3-D poststack time migration of 3-D DMO-stacked volume of data. The benefits of 3-D prestack time migration are not limited to the improved image of the subsurface, but it has other advantages which are given below :

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1. 3-D prestack time migration is the appropriate strategy for imaging conflicting dips with different stacking velocities, such as reflections from steeply dipping fault planes with 3-D geometry and gently dipping strata. 2 The 3-D image volume derived from 3-D prestack time migration is used as an input to 3-D zero-offset amplitude inversion to estimate an acoustic independence model of the earth. 3. The CRP gathers from 3-D prestack time migration are used to perform prestack amplitude inversion to derive amplitude variation with offset (AVO) attributes. 4. The 3-D image volume can be inverse migrated by way of 3-D zero-offset wavefield modeling to derive an unmigrated 3-D zerooffste data volume.

5. The 3-D rms velocity field estimated from the 3-D prestack time migrated data can be used to derive a 3-D interval velocity field by Dix conversion. 6. Finally, the 3-D interval velocity field and the 3-D zero-offset wavefield can be used to derive an earth image in depth by 3-D poststack depth migration. The earth image volume in depth can then be interpreted to delineate a set of reflector geometries associated with key geological markers. The interval velocity field combined with the reflector geometries may be used to build an initial earth model in depth. Three-Dimensional Stolt Migration We now consider the special case of constant velocity v. Stolt (1978) devised a migration technique that involves an efficient mapping in the Fourier transform domain from temporal frequency w to vertical wavenumber kz . We get an explicit expression for was: w =

vI2~k; + k.~ + k;

...(1)

By keeping the horizontal wavenumber and ~t and ky unchanged and differentiating equation (1), we get

dw =

v

kz

2~k;+k;'+k;

. dk_

The equation of the migrated section P(x, y, following equation;

...(2)

• Z,

t

= 0)

is given by the

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Encyclopaedia of Petroleum Science and Engineering P(x, y, Z, t = 0) = III P(kx' k"

0,

w) exp.(- ikx x - ikyY - ikz z) dkx dky dw ...(3)

When equation (1) and (2) are substituted into equation (3), we get:

XP[kx,0, vl2~k; + k~ + k;] exp.(-ikxx - ikyY - ikzz)dkxdkydxz ...(4) This is the equation for constant velocity 3-D Stolt migration. It involves

tow operations in the f-k domain. First, the temporal frequency w is mapped onto the vertical wavenumber kz via equation (1). Second, the amplitudes are scaled by the quantity S which is given by the equation: S =

v~ kz 2 kx2 + ky2 +k2z

...(5)

Equation (5) is the equivalent to the obliquity factor associated with Kirchhoff migration.

Three Dimension (3-D) Survey Design and Acquisition The ultimate goal of conventional processing of a 3-D survey data is to obtain a 3-D seismic image of the subsurface. The image quality from time migration depends on stack quality and accuracy in velocity estimation. However, two other factors control the fidelity of migration, i.e., aperture and spatial sampling. They also can dictate the design of the field survey. The actual survey size should be more than the subsurface area to be mapped. The survey area does not have to be extended equally in all directions. The survey area must be extended in the direction of the steepest part. Another consideration in extending the survey area is the required additional length in profile to achieve full-fold coverage over the already extended survey area. A typical subsurface anomaly with a lateral extent of 4 x 4 km. may require a 3-D survey over an area as large as lOx 10 km. The spatial aliasing problem is caused by Spatiial undersampling of the wavefield to be migrated, e.g.; the stacked section. The spatial sampling of stacked data (without trace interpolation) is defined by the

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recording parameters. Therefore, receiver spacing, crossline spacing, and the crossline directio~ in relation to dominant dip direction used in the field must be chosen-carefully. The maximum frequency that is not aliased gets smaller at increasingly steeper dips, lower velocities, and coarser trace spacing. Typical trace spacings in the inline and crossline directions in 3-D surveys are 12.5 to 25 m, and 25 to 50 m respectively. Even if the spacing in the crossline direction is as small as possible, for economic reasons, it usually is greater than that in the inline direction. Because of this, trace interpolation may be required along the crossline direction before migrating the data. Selection of navigation and recording equipment depends on field conditions. The operating environment also JIBlSt be considered. In the marine environment, water depth, tides, currents, sea conditions, fishing and shipping activity, and obstacles such as drilling platforms, wrecks, reefs and fish traps must be considered. Modem marine 3-D surveys are conducted by deploying upto 12 cables and multiple source arrays. On land, environmental restrictions, accessibility, topography, cultivation and demographic restrictiQns are factors that can affect survey design and acquisition. Because of these restrictions, careful planning and some adjustment of nominal shooting geometry often are required to achieve acceptable fold and offset distribution. Accurate surveying is a necessity for 3-D surveys, since data are collected with such spatial sampling. Statics resulting from lineto-line surveying errors can seriously degrade the image quality obtained from 3-D migration. Positioning error is the limiting factor for line spacing in marine 3-D surveys.

'Ii"ace Interpolation A typical 3-D survey has a trace spacing in the crossline direction that normally is coarser than the trace spacing in the inline direction, in some cases, by as much as four times. This coarse spacing can cause spatial aliasing in the crossline direction. The coarser the trace spacing and the steeper the event dip of interest, the lower the threshold frequency at which spatial aliasing begins to take effect. Trace interpolation is a means to circumvent the adverse effect of spatial aliasing in the crossline direction in 3-D migration. Trace interpolation does not create data, it merely unwraps the f-k spectrum of the input data so that aliased frequencies are mapped to the correct quadrant in the f-k domain. Finally, data do not necessarily need to be trace-interpolated in the crossline direction down to the trace spacing of the inline direction.

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Instead, signal bandwidth and subsurface dip can be taken into ·consideration to compute the optimum trace spacing to avoid spatial aliasing. Presently, trace interpolation is done suing one step complex spatial prediction filters (Gulunay, 1986) in the frequency space domain (Spitz, 1991). Consider a 2-D CMP-stacked data P (x. t) to be trace interpolated . such that the trace spacing of the interpolated data is half of the trace spacing of the original data. Pk(w), the discute Fourier transform of P(x. t) in the time direction, may be represented by the following combination of amplitude and phase spectra in the frequency-space domain: N

j~1 Aj(w) exp.(- jwll.1j),k = O,1,2, ... M,

...(1)

Where Aiw) is the amplitude spectrum of the wavelet associated with the jth dipping event, ll.Tj is the time shift along the jth dipping event from trace to trace, k is the trace index, M is the number of traces, and N is the number of dipping events. A one-step complex prediction filter is designed as a trace interpolation operator for each frequency component in the frequency - space domain and applied to the input data associated with the frequency components with twice the frequency. The following are the steps involved in trace interpolation using one-step prediction filter:

1. Start with a 3-D volume stacked data P(x. y. t) that is to be migrated, and assume that the data volume is adequately sampled in the inline direction, but needs to be interpolated in the crossline direction before migration. Apply Fourier transform in the time direction, P (x. y, w). 2. Sort the data into cross lines. 3. Then sort each crossline complex matrix of data P(y, w) into complex arrays P(y) for each frequency component w. 4. Design a one-step prediction filter from the data array P(y) of frequency w/2 and apply it to P(y) of frequency w to obtain the interpolated array Q(y). 5. Interlace the original data array P(y) with the interpolated data array Q(y) to obtain the output array R(y) with twice the number of elements as in the input array P(y).

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6. Repeat steps (4) and (5) for all frequencies. Then, combine the complex arrays R(y) and sort them into their crossline complex matrix R (y, w). 7. Apply inverse Fourier transform to obtain the interpolated crossline section R (y, t). 8. Repeat steps (2) through (7) for all the crossline sections. Fig. 34 shows a CMP-stacked section with trace spacing of 10m and the same stack with every other trace dropped so as to make the trace spacing 20m, 40m, and 80m. The steep flanks of the diffractions and steeply dipping reflections at coarser trace spacing are the increasingly less distinctive character. Migration of the stacked section with sufficiently small trace spacing provides a clear image, albeit in time, of both gently dipping and steeply dipping reflectors. Trace Interpolation (Mathematics) We want to design a complex Wiener prediction filter to interpolate data in the spatial direction. Consider a CMP-stacked data set P(x, t), where x is the CMP axis and t is the two-way zero-offset time axis. Apply Fourier transform in the t direction to decompose this 2-D data set to its frequency components P(x, w). For each frequency component, define a complex array P:P (x, w) in the x direction. Specifically, we want a filter F:F(x) such that, when applied to the input data array P:P(x, w), it yields an estimate of the input array D:P(x + !J.x/2, w), at x + !J.x/2, where D is the desired output array and !J.x/2 is the prediction lag (Spitz, 1991). The spatial prediction filtering is expressed by the following convolution relation (YIlmaz, 200 1) : ...(1) P(x, w) *F(x) = Y(x) Where Y(x) represents the actual output from prediction filtering. To develop the theory for trace interpolation using spatial prediction filters, consider the case of a three-point one-step prediction filter (F0' F I' F2) and a six-point input data array (Po' PI' P2' P3' P4' P5)' We also want the output array Y(x) to be the input array P(x) one unit of distance ahead, (P I' P2' P3' P4' P5)' Equation for this case takes the form

PI P2 P3 P4 Ps

0 0

Po PI P2 P3 P4 Ps

0

0 Po PI P2 P3 P4 Ps

0 0 Po PI P2 P3 P4

(=~J

0 0 0 0 0 0 0

...(2)

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(a)

(b)

(c)

(d)

Fig. 34. (a) A CMP-stacked section with trace spacing of 10m, (b) the same stack as is (a) with every other trace dropped so as to make the trace spacing 20m, (c) the same stack as in (b) with trace spacing 4001, and (d) the same stack as in (c) with trace spacing 80m.

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Write out explicitly the equations associated with rows 3, 4 and 5 which contain, terms with all three fIlter coefficients (F I' F I' F2) as

P3-P2Fo:PIFI-PoF2~ = 0

...(3)

P4-P3Fo-P2FI-PIF2 = 0

...(4)

P5-P4Fo-P3FI-P2F2 = 0 ...(5) Now split each of these three equations into two parts : (1) terms contain the data array elements (Po' P 2' P J and (2) terms contain the data array elements (P I' P3'P5). Write the two parts in matrix form as given below: -Fo -I'; -F2

0J(PoJ-(1'; 1 P2 F2 -Fo P4

°

-1 Fo

I';

...(6)

Equation (6) suggests that given the prediction filter (F0' F I' F2)' the data array (P I' P 3' P 5) can be calculated from the data array (Po' P 2'PJ. The known data array (Po' P 2' P 4) is associated with consecutive traces in a eMP-stacked data set. The unknown array to be calculated (PI' P 3, P 5) would then be associated with the traces halfway between the traces that correspond to the known data array (Po' P 2' P4). Equation (6) can be used to perform spatial interpolation. Equation (6) is written in matrix notation as : FP-FP ~d u u =0

...(7)

Where F u and Fa are the coefficient matrices with their elements in terms of the prediction fIlter coefficients (F0' F I' F2)' and Pd: (Po' P 2' P4) and P u : (P I' P 3' P 5) are the unknown input data array and the interpolated data array, respectively. Now calculate the prediction filter from the known data (Po' P 2' P 4)· Assuming the input data set is made up of M traces comprise a set of N dipping events represented by the following combination of amplitude and phase spectra in the frequencY,space domain (Spitz, 1991) as: N

Piw)

=

~

1=1

Aiw) exp.(-iwkLlTj),k = 1,2,3 ... M, ...(8)

Where Aiw) is the amplitude spectrum ofthejth dipping event, LlTj is the time shift along the jth dipping event from trace to trace, and k is the trace index. Putting as : z~(w) = exp.(- iwk LlT)

...(9)

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Now equation (8) becomes as : N

Pk =

~Ajz;,k=O,I,2, ... M,

...(10)

}=I

When the variable w is omitted for ease. Given the data array (Po' P2' P4) and the one-step prediction filter (F0' F I' F2)' we want to compute the next element of the data array P6' We have as ; P4Fo + Pl I + PJ'2 = P6

...(11)

Equation (11) can be written as : ...(12) Consider a case of three dipping events. Putting N (12) becomes in matrix form as :

[;f~:~

z:(w)

=

3 in equation

;f~:~ !] [~~:}) [;f~:~] =

z;(w)

1

F2 (w)

z:(w)

.

(13)

Write equation for the case of the data array (Po' PI' P2) to compute the next element P3 using the one-step prediction filter (F' 0' F I', F2') as : P2F'0+P IF I'+PJ'2'= P3 Equation (14) can be written as :

...(14)

N

L A .z~

j=1

}

}

...(15)

Putting N = 3 in equation (15) becomes in matrix form as :

[

Z~(W) z;(w) z;(w)

z:(w) z~(w) z!(w)

1)[F~(W»)

1 I;I(W) 1 F~(w) ,

= [z:(W») z~(w)

...(16)

zi(w)

:~-- - ~

From equation (9), we haw : , z~k(w/2)' =

...(17)

z;(w)

With this definition, matrix equation (16) can be written as: Z:(W/2) z~(w/2) [ z:(w/2)

z~(w/2) 1][F~(W») z;(w/2) z;(w/2)

1 I;I(W) 1 F~(w)

=

[Z~(W/2») z~(w/2) z:(w/2)

...(18)

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Writing equation (13) in terms ofw/2 explicitly, we have:

(;f~~~~ z:(w/2)

;~~~~~ !J(:~~;:;~J (;~~:~~~J =

z;(w/2)

1 F2 (w/2)

z~(w/2)

.

(19)

Comparing equation (18) and (19) we fmd that the prediction filter [F'o(w), F1'(w), F2'(w)] that we need to do trace interpolation of the data component with frequency Q) actually is the prediction filter [F0 (w/2), F I (w/2), F2(w/2)] that is computed directly from the uninterpolated data component with half the frequency w/2. Two-Pass Versus One-Pass Implicit Finite-Difference 3-D Migration in Practice The two-pass approach for 3-D migration is valid strictly for a constant-velocity medium (Ristow, 1980). Theoretically, it is true that the two-pass method only is appropriate for velocities that vary slowly in the vertical direction and do not vary laterally. In practice, we find that the two-pass approach yields acceptable results in areas in which dips are small, vertical velocity variations are moderate, and lateral velocity variations are within the limits of time migration. The significant differences between one pass and two-pass 3-D migrations are given below:

1. two pass 3-D migration causes overmigration and one-pass 3D migration causes undermigration of steeply dipping events in a medium with significant vertical velocity gradients. 2 Error in two pass 3-D migration is negligible for small dips. 3. For steep dips, the error is the same order of magnitude as the error caused by a lack of precise knowledge of migration velocities. 4. Largest overmigration with the two-pass approach occurs at around the 45-degree azimuth. No error occurs when the dip is entirely in the inline or crossline directions. 5. Largest undermigration with the one-pass approach also occurs at around the 45-degree azimuth.

6. The two-pass approach translates the dipping event and of its true position. After the two-pass 3-D migration, the dip is correct

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in contrast to overmigration caused by erroneously too high velocities which not only mispositions the event but also changes its dip. 7. The two-pass 3-D migration results depend on which direction the ftrst-pass migration has been preformed. If it is the inline direction, and if the velocity gradient is large enough for the two-pass approach to cause overmigration, then we get more overmigration in that direction. For instance, if we decide to do two-pass 3-D migration in an overthrust area, then we may want to migrate fIrst in the strike direction where the velocity variations may generally be changing less rapidly than the dip direction to minimize the overrnigration effect of the two pass approach. Also, once we complete the migration in the direction of mild velocity variations, we can afford to revise the second pass in the direction of strong velocity variations without going back to the original stacked data.

REFERENCES 1. Berryhill, J.R., 1991; Kinematics of crossline prestack migration; Geophysics, Vol. 56, pp. 1674-1676. 2. Black, J.L. and Leong, T.K., 1987; A flexible, accurate approach to one-pass 3-D migration; 57th Ann. Internal. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 559-560. 3. Brown, D.L., 1983; Applications of operator separation in reflection seismology; Geophysics, Vol. 48, pp. 288-294. 4. Canning, A. and Gardner, GH.F., 1996; A two-pass approximation to 3D prestack depth migration; Geophysics, Vol. 61, pp. 409-421. 5. Claerbout, J.F., 1985; Imaging the earth's interior; B1ackwelII Scientific Publications. 6. Etgen, J.T. and Nichols, D., 1999; Application of the Li correction to explicit depth extrapolation methods; 69th Ann. Internal. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 1366-1369. 7. Ferber, R., 1994; Migration to multiple offset and velocity analysis; <;Jeophys. Prosp., Vol. 42, pp. 99-112. 8. \F'owler, P., 1984; Velocity-independent imaging of seismic reflectors; A 54th Ann. Internal. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 383-385.

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9. Gazdag, 1. 1978; Wave-equation migration by phase shift; Geophysics, Vol. 43, pp. 1342-1351. 10. Gulirnay, N; 1986; F-X decon and complex Wiener prediction filler; 56th Ann. Internal. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 279-281. 11. Hale, D., 1991 a; stable explicit depth extrapolation of seismic wavefields; Geophysics, Vol. 56, pp. 1770-1777. 12. Hale, D., 1991b; 3-D depth migration via McClellan transforms; Geophysics, Vol. 56, pp. 1778-1785. 13. Lamer, K. and N . P., 1984; Strike versus dip shooting; 53th Ann. Internal. Mtg., Soc. Expl.tJeophys, Expanded Abstracts, pp. 256-260. 14. Levin, F.K., 1971; Apparent velocities form dipping interface reflection; Geophysics, Vol. 36, pp. 510-516. 15. Levin, F.K., 1984; The effect of binning on dala from a fealhered slreamer; Geophysics, Vol. 49, pp. 1386-1387. 16. Li, Z., 1991; Compensaling finile-difference errors in 3-D migration and modeling; Geophysics, Vol. 56, pp. 1650-1660. 17. Notfors, C.D.B., 1995; AccuraIe and efficienl explicil 3-D migralion; 65th Ann. Internal. Mtg., Soc. Expl. Geophys, Expanded Abstracls, pp. 1224-1227. 18. Pai, D.M., 1988; Generalisedfk (frequency-wavenumber) migralion is arbilrarily varying media; Geophysics, Vol. 53, pp. 1547-1555. 19. Reshef, M. and Kessler, D., 1989; Praclical implemenlalion of three-dimensional postslack depth migration; Geophysics, Vol. 54, pp. 309-318. 20. Ristow, D., 1980; 3-D downward extrapolation of seismic data in particular by finite-difference methods; Ph.D. thesis, University of Utrecht, The Netherlands. 21. Spitz, S., 1991; Seismic trace interpolation in the f-x domain, Geophysics, Vol. 56, pp. 785-794. 22. Stolt, R.H., 1978; Migration by Fourier Transform; Geophysics, Vol. 43, pp. 23-48. 23. Yilmaz, O.Z., 200 I; Seismic Data Analysis, VoL II, Society of Exploration Geophysicists, Post Office Box 70~740, Tulsa, OK 741702740.

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Appendix-A Utilization of Natural gas for Power Generation Introduction Natural gas is mixture of hydrocarbon confounds end small quantities of various nonhydrocarbons existing in the gaseous phase or in solutiOn with Oil in natural undergound reservoirs at reservoir conditions. The principal hydrocarbons usually contained in the mixture are methane, ethane, propane, butane, pentanes, etc. and typical nonhydrocarbons gases which may be contained in reservoirs natural gas are carbon dioxide, helium, hydrogen sulphide and nitrogen. Natural gas is found in underground rock formations which are usually sedimentary in origin. The natural reservoir are composed of porous rocks which provide space for accumulation of hydrocarbons. Economically recoverable quantities of hydrocarbons occur in the porous reservoir rock where depositional conditions or deformation of the strat have resulted in the formation of traps which terminate undergound migration and cause accumulations of hydrocarbon fluids and gases. Under reservoir conditions, natural gas and the liquefiable portions thereof occur either in a single gaseous phase in the reservoir or in solution with cude oil and are not distinguishable at that time as separate substances. Today, natural gas is the leading energy source in the domestic use in USSR and it is used as a substitute for oil in transportation as compressed natural gas (eNG) in New Zealand, Netherland, Brazil and Italy. The planning commission has set up an advisory group under the chairmanship of Dr. S. Varadrajan to study and evolve a prespective plan for natural gas development and utilisation. Natural Gas Potential Natural Gas reserves have grown five-fold in the last 10 years in the country. At present, the country had about 1117.31 billion cubic meters of natural gas reserves. Large reserves of natural gas have been discov~ in Bombay Offshore, Tripura, Krishna Godavari and to some

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extent in Rajasthan and Andamans. The chances of finding more in these basins as also in Jammu and Kashmir and Himanchal Pradesh are bright. The vast natural gas reserves in the North-East including in Tripura and Krishna Godavari and Cauvery basins, would require pipeline transportation of varying length for proper utilisation of reserves. The natural gas production in the Gujarat region has increased to 4.3. million cubic meters from 2.48 million cubic meters per day during the Seventh Plan period. It is likely to go up to nine million cubic meters by the end of the eight Plan. The inflow of gas from other fields like Panna and Tapti would increase the output in the region to 20 million cubic metres. Estimates of proven world gas reserves were 3615.2 trillion cubic feet in 1986. OPEC and USSR dominate with three fourth of World's natural gas reserves. India's natural gas production in 1986-87 was 9.81 billion cubic metres. National Gas Grid For optimal utilised of gas, it is essential to ensure transportation of gas where there are potential markets. Natural gas can be transported to long distances through pipeline for meeting the users demand. In India, the frrst onshore gas pipeline was laid in 1964 for supplying gas from Ankleshwar to Utran power station. The first offshore 203 km. long and 26 inch diameter gas trunk line was laid between platform of Bombcy High and Uran shore terminal. This gas line was commissioned in June 1978. Presently, the total gas pipeline laid is 1558.27 kms, of which the shore of offshore is 489 kms. The Oil and Natural Gas Commission is planning a massive network of gas disnibution system for domestic and industrial purposes through a national gas grid. The conceptual study envisages implementation of the plan in three phases. Phase I (A) is an extension of HBJ pipeline from Auriya to KapurthaIa, covering a distance of 1325 kms. at a cost ofRs. 1400 Crores while Phase I (B) is a pipeline connecting Bombay South terminal to Banagalore. This phase also envisages setting up of nine power plants generating 2875 MW and two fertilizer plants of 2700 tonnes a day capacity.

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203

In the second phase, extension of the pipeline to Trivandrum and link with northern grid is proposed at a cost of Rs. 4150 Crores covering a length of S61 0 kms. In this phase, 13 power plants generating 3875 MW and five fertilizer plats of 6750 tonnes a day capacity are proposed. The third phase will cover the northeast and eastern part of the gas fields. The oil and Natural Gas Commission has envisaged a gas grid to fully utilise the natural gas available in the eastern region. The scheme envolves compression of moderately high pressure at each group gathering station of Rudrasagar, Geleki and Lakwa Central Tank Farm for further supply to Assam State Electricity Board (ASEB) and the Hindustan Fertilizers Corporation, Narnrup. The government proposed to link gas fields of the Oil and Natural Gas Commission and the Oil India Limited in Assam. It is also considering pooling the surplus gas from this grid through a trunk pipeline for transportation to Jorhat. The Gas Authority of India has already submitted a proposal for construction of this integrated gas grid The total gas proposed to be transported through this trunk pipeline would. be nearly 3.5 MMCMD out of which 2 MMCMD of gas would be consumed by the proposed power plant of Assam State Electricity Board(ASEB). The balance would be given to other consumers upto Jorhat.

Gas Based Power Plant A major policy decision to utilise the enormous gas reserves for power generation in addition to fertilizer has been taken by the Union Cabinet. Huge Investments have been made for production of industrial gas end its transportation. Rs. 1,700 Crores have been spent in Hazira-Bijaipur-Jagishpur (HBJ) pipeline and Rs. 1530 Crores have been spent for establishing gas handling facilities at the Hazira Complex. The Hazira Gas Processing Complex is sweeting sour gas comine from the South Bassein offshore gasfields and supplying it to fertilizer plants and gasbased thermal plants being set up in route to the HBJ pipeline. Natural gas for power generation will be a boon for industrial development. These power plants can also ensure saving in space and utilities and also eliminate transportation and handling of coal. A fresh contract is being given for the spur pipeline to Dadri, Delhi

and Gaziabad The spur line will provide gas for the Dadri 600 MW power

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204

generation plant the Delhi Electricity Supply Undertaking for operating its gas turbines and for limited industrial uses in Faridabad and

Gbaziabad. Natural gas would be supplied to electricity boards of Rajasthan, Tamil Nadu, Andhra Pradesh, Gujarat, Assam and Tripura. While Rs. 260 Crores was allocated for the power sector in the first five year plan, the figure has gone upto Rs. 34/274 Crores in the Seventh Plan. The per capitor consumption of electricity in the country is merely 200 kWH. compared to 12,000 kWH in Sweden and 10/000 kWH in the

U.SA The governments hopes to increase the installed capacity by

221400 MW in the Seventh Plan and add another 38,000 MW in the eighth plan to being the total capacity to 103,000 MW. According to the Chairman of Gujarat Electricity Board Mr. lashwant Meha the Centre had approved schemes for gas_based power plants in Ahmedabad Baroda and Utran. 600 mw gas-based power plant would be set up at Gandhar in addition to the proposed 750 MW plant at Pipavay. On December 27, 1988, a 220 KV substation worth Rs. 5 Crores was commissioned at Keshod. The centre had allocated four lakhs cubic metres per day of the total 4.50 lakhs cubic metres per day available from the Kalol, Balol and Ankleshwar oilfIelds for setting up a 116 MV; power generating unit of the Ahmedabad Electricity Company at Vatva. Similarly the Centre had allocated 4.5 lakh cubic metres per day for the proposed 123 MW unit at Utran and four lakh cubic metres per day to two units at Dhuvaran of 27 MW each. The centre has also accepted in principle the state governments demand for allocating seven lakh cubic metres per day for setting up a 145 MW power generating unit of the Gujarat Industries Power Company and another 22 lakh cubic metres per day for setting up a 600 MW unit. The first phase of the World Bank Funded Anta Gas based Power plant was completed. Anta project is being put up at en estimated cost of Rs. 373 Crores by the National Termal Power Corporation in collaboration with Asea Brown Boveri of West Germany and Hindustan Brown Boveri. It will be a great boon for the strained northern grid and it will considerably enhance availability of power in the northern states . .Auraiya gas-based power project is also coming up within the stipulated time schedule.

A.ppendix-A.

205

The Anta Power Project's installed capacity will be 430 MW. Auraiya and Kawas will have 600 MW capacity each. Mr. Vasant Sathe, the Union Energy Minister, told reporters in Gandhinagar on 28th January, 1989 that he had already plended with the Union Government to drastically reduce the prices of natural gas to make gas-based power project more econ6mical. According to Mr. Sathe, the Petroleum. Ministry should keep a huge stock of natural gas exclusively for power generation as he is in favour of setting up as many gas based power projects as possible. The combined cycle technology for power generation has a number of advantages. The capital investment would be roughly of the order of two third of the conventional coal based power projects and may be of the order 50 percent of the nuclear power stations with similar capacity.

In addition, the water requirements for these plants is as low as one third of the conventional thermal power generation stations. Furthermore, these plants would enable the NTPC to save fuel to he extent of 35 percent. Whet really makes the gas-based power plants an Attractive proposition is the low gestation period compared with thermal hydro and nuclear power plants.

Power Plant Know-How A Bombay-based private company, UNIK Power Development Corporation, has offered the Gujarat Government and the Gujarat Electricity Board a BOT (Build, Operate and Transfer) model of power plant to build a 123 MW gas- based power station at Utran in Surat district, The BOT or Ozal formula permits a private company to raise its own resources to build a power station, Operate it and transfer it to the electricity board after seven or ten years. The board will thus get a power station without investing heavily in the initial period. The electricity board should, however, purchase and distribute the plants generated electricity, during this period. The system in theory is a joint venture of Non-Resident Indians (NRI), Overseas suppliers and contractors forming a consortium The NRIS and overseas operators will also have a major: share in the project till it is transferred to the state electricity board.

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With the phenomenal industrial development in and around Surat city, the power demand has been growing at a faster rate than the supply. The proposed Utran gas-based plant is thus crucial to maintain power supply. Use of Natural Gas in Industries Conventional uses of Natural gas include feedstock for fertilizer, ammonia and petrochemical industry, fuel for power generation, iron and steel industry, cement and ceramic industry, paper industry and for town gas distribution. Unconventional uses of natural gas include gas turbines/ Cogeneration plants for power generationlbarge mounted process plants ( for isolated offshore fields) for ammonia or methanol, vehicular fuel as CNq enhance oil recovery (EOR) through gas injection, fuel cells through methanol for power generation. The Oil and Natural Gas Commission task force has made headway in optimal utilization of the CNG (Compressed natural gas) in the transport sector. Successful trails by the ONGC have been conducted at Rajamundry, Bombay and new trails have been conducted successfully at Baroda. Dynamic testing of one GSRTC bus is in progress. This is a major break through and could usher in an industrial revolution towards optimum exploitation of the natural gas. Trial runs conducted by the ONGC have so far proved to be highly cost-effective besides resulting in a smoother and pollution free vehicle performance. As per the estimate, the switch over to the CNG would being about a saving of about Rs. 50 for every 100 Kms, for a diesel driven truck. The cost of conversion for a truck works out to be about Rs. 10,000. Besides, the CNG reduces carbon monoxide and sulphur oxide Emission to almost zero level. Its high octane content almost eliminates the poisonous dead compounds and is thus, considered significant environmentally. The country's first gas-based sponge iron plant has been set up at Hazira at a cost ofRs. 305 Crores and on completion it will help the nation to save foreign exchange to the tune of 200 Crores a year. This plant would produce 8.80 lakhs tonnes of spong iron a year and would utilise 8.5 lakhs cubic metres of natural gas a day. It will also have a captive power plant of 30 MW so that it would not have to totally depend on the GEB (Gujarat Electricity Board) for power. A new Petro Chemical Complex is coming up in Maharashtra based on gas extracted from CiC3 fractions. Similarly a large complex is being

Appendix-A

207

set up at Bijaipur in Madhya Pradesh for manufacturing propylene and poly-propylene. More gas crackers for the production of ethylene and spong iron plants based on gas are being set up. World Bank Loan for Gas Projects World Bank loan ofS 295 million has been fmalised for the Western Offshore and Onshore gas development projects of ONGC. The ONGC will utilise World Bank loan on four major schemes to make optimum use of natural gas in the Western Offshore gas fields. The schemes are development of South Bassein and Gandhar fields, construction of gas pipeline from Heera Offshore fields to Uran and seimic surveys for Tapti and Hazira fields. The loan will also be utilised for undertaking studies for identifying least cost investment for development and transmission infrastructure for the Western Region and utilisation of gas. Conclusions The centre has decided In principle to allocate natural gas for power generation to accelerate the pace of industrial development in the country. The natural gas production would be 100 million cubic metres per day by the tum of the century. In 1988-89, it is expected to be 35 million cubic metres per day. The power sector would be accorded high priority as there, is abundance of the natural gas. With the gas reserves in the country going up, the government is planning to set up a number of other gas-based power projects. The total capacity of these projects, likely to be located in Gujarat, Andhra Pradesh and on the South West coast, is estimated around 6500 MW. Transport and domestic sectors consumed 56 percent and 29 percent respectively of the oil consumed in the country and if the latter could switch over to gas, huge foreign exchange saving could be effected in the use of oil. Natural gas is a clear and non-polluting fuel. It is used in the form oflean gas, liquified natural gas {LNG) , compressed natural gas (CNG) and liquified petroleum gas (LPG). It is used as an efficient fuel in domestic cooking and it can be used as a fuel in automobiles. It is a potential substitute for replacing the middle distillates such as diesel and kerosene. In order to utilise this vast source of energy, integrated resource planning is required. It is evident that natural gas will playa major role in the energy mix by the tum of century.

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Appendix-B Environment and Pollution Control Introduction Our country has taken up the problem of environmental protection with the seriousness which it deserves. In fact all the countries have realised the importance of this aspect upon which human survival depends. Parliament has enacted the EnvironmentAct, 1986. Environment is total we can retain ecological balance. Protection of forests, wild-life, flora and fauna, water resources and the very air that we breath is not only the responsibility of the Government agencies but more so the sacred duty of all the citizens. We can only succeed-in our march of rapid industrialisation provided we are able to control the danger of pollution in all its facets. With the massive people's voluntary environment movements both in the country side and cities and with positive response from the industries and urban bodies, we should be able to protect and preserve the environment for not only the present but all for future generation to come. Soviet leader Mikhail Gorbachev's glasnost has given rise to a robust ecological movement in the Soviet Union with environmental issues finally being debated in the congress of people's Deputies. A report says that SOO million Soviets are breathing air which is unbreathable and the prevalence of hydrocarbons and carbon dioxides is 10 times the World Health Organisation (WHO) permissible standards. Local residents ,had forced the closure of around 240 industries over the past few years which were found to be polluting the environment extensively. ussr (NOW RUSSIA) SPENT A mere 1.5 per cent of its GNP on pollution control compared with the 5 per cent spent by the western countries.

Is Pollution Problem Insurmolintable Th",only way to command nature is to obey it. We have experienced enough of droughts, floods, acid rain, pollution of rivers and underground waters as also severe air pollution seriously affecting the lives of thousands. The green house effect due to excessive discharge of carbon dioxide consequent to large scale burning of fossil fuels has

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affected the temperature of the earth, increased ocean levels and even brought about adverse climatic changes in many parts of the earth. Depletion of ozone layer too is an apparent adverse phenomena which is threatening the existence of life on earth. Do the industries have no social responsibility to ensure that they control pollution so as not to endanger the life, property and environment? It is time that the municipal corporation and nagar palika give top priority to proper sewage collection and treatment of effluents before discharge into rivers, streams or on land. If it is not done, it can . have very serious repercussions on our very survival, not only for the future generations but even for the present generation. Even prevention .and control of water pollution act, 1974 and prevention and control of air pollution act, 1981 nothing much has done in this direction. Protection of environment is not the Job of the government and burocracy alone but what is most important is that the people themselves have to be convinced that our future survival is totally dependent on the protection of the environment and mother nature which is sustaining us. PoUution Control Steps Standards for the emission from petrol and diesel driven vehicles notified under the amended motor (Vehicles) Act are being strictly implemented from 1st March. 1990. In a conference held in February, 1990, on environmental pollution, it was decided that the state Transport authoritbs and the pollution control boards would launch a campaign on awareness and training along with enforcement. Wherever possible, petrol stations and other testing centres would be encouraged to install testing equipment so that vehicles could be tested. The conference also decided on a progranune of action to deal with the flash generated from thermal power stations, enforcing of standards for paper, distillery, sugar and father tanning industries, rules for the menagement of hazardous chemicals, hazardous wastes, the transport of hazardous chemicals, etc. State pollution control boards have been asked to monitor water and air pollution and prepare status reports. The Ministry of Agriculture has been asked to initiate a programme of monitoring of residues of pesticides in the environment and consider the desirability of banning or restricting the use of some of the harmful pesticides. Central Ministries of Environment and Urabon development has been asked to take coordinated action to identify polluted water bodies and take remedial measures by encouraging schemes for interception, diversion ana treatment of sewage.

Appendix-B

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Pollution control boards would identify industries near rivers and ensure that they treat their effiuents before releasing them into rivers or water bodies. PoUution Control Scenario of India

The Government of India has published Hazardous Wastes Rules, 1989 under the Environment Act, 1986. It covers the total spectrum of hazardous wastes right from its generation, storage, handling, treatment and transportation to final disposal. In our country, some states like Gujarat and Maharashtra have industrialised very fast. However/ the pace of pollution control has not kept pace with the pace of industrial development. While certain measures have been taken to abate air and water pollution, very little has been achieved in respect of solid waste disposal. Besides the Environment Act, the Ministry of Environment and forests have also issued the rules to handle and management of hazardous wastes. The Pollution Control Boards are gearing up to tackle the problem of solid waste. Dr. GM. Oza, general secretary of the International society of Naturalists (INSONA), has welc0!lled the statement of earlier Union Minister of state for Environment and Forests Maneka Gandhi about the Union Government's decision to set up "environment courts" with the judicial powers of high courts to specifically deal with environment violation cases. Such a move, Dr. Oza feels, shall help in checking pollutions on land, water and air and shall go a long way in the alleviation of human sufferings resulting from environmental ills inflicted by industrial complexes. Waste water reuseis an essential factor in water resources meoagement. The industrial consumption forms a significant part of the total water use. So reuse for industry and internal recycle in industrial plants needs priority in planning water supply and resources management. The need of the reclaimed water and its value for the industry prompted installation of advanced treatment processes, thus providing better water pollution control. Mis Enviro Construction Pvt. Limited is water, sewage and industrial waste treatment specialists. A new method for disposing of effiuent produced in an oil field has been introduced by the oil and Natural Gas Commission. One of the major problems encountered in sustaining Oil from old fields is the difficulty in handling ever increasing water effluents which

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are .produced alongwith oil. Effluent production has been increasing in the Rudrasagar Oil field in Assam which was discovered in the early sixties. Initially ONGC set up a treatment plant to counter the problem The plant comprises two modular units of a total capacity of 800 cubic metres a day. More such units are planned. However/after water is treated in these plants, its disposal still poses a problem What ONGC has now done is to re-inject it into the field through disposal wells. This was found to additionally help maintain the natural pressure of the Oil reservoir. The steel Authority of India (SAIL) has embarkad on a Rs. 318 crore plan to control pollution in its units. 54 pollution control schemes involving Rs. 78 crore are underway. A joint venture has been commissioned to formulate and implement a comprehensive environmental management plan for the SAIL plants, assisted by the world Bank. The scheme ,encompasses state of art monitoring facilities for air, water and noise pollution, establishment plants and training of personnel in monitoring' and control of pollutants. India should not go in for nuclear -energy because of the heavy investment involved and the environmental hazards it poses, according to Dr. Rashmi Mayur, well-known environmentalist. Disposing texis nuclear wastes is a major concern all over the world because ofthe ecological implications. Commenting on the Narmada project, Mr. Mayur said he had already met the chairman of the Sardar Sarovar Narmada Development Corporation, Mr. Sant Mehta and discussed with him some of the environmental issues. Mr. Mehta has given assurences that all viable alternatives for better ecological preservation and rehabilitation would be looked into. The world bank had approved in 1985 a loan and credit to support the construction of the Sardar Sarovar dam and credit for its associated irrigation and drainage system The growing member of free-standing environmental projects and environmental components would require a substantial increase in monitoring and supervision efforts by both the bank and its borrowers. Priority attention would have to be paid to assist borrowers in building the institutional capability for environmental monitoring and regulation.

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In the Nandesari Factory of Deepak Nitrite, Baroda, there are ten different plants. The effluent generated from these plants are liquid and gas. The liquid effluent generated from each of these plants is passed through a neutralisation chamber for PH control. It is then sent to the common effluent treatment plant. It consists of oil and grease separator/ chemical dosing facility, sedimentation chamber and surface aeration lagoons. In factory piping arrangement of the plants is made in such a way that no gaseous effluent is allowed to escape into the atmosphere without .being treated in the pollution control tower. The final tail gas when discharged from this tower at a level of 100 feet above the ground is almost free from any harmful constituents. It is a fact that industrial pollution can be reduced but cannot be totally eliminated. Plantation of trees around the factory and in the Industrial Estate helps in reducing air pollution. Besides Deepak Nitrite Limited Baroda, other industries like: GujaratAlkalies and chamicallimited, Enviro Construction (Gujarat) Pvt. Ltd., Kadam Mehta sanghavi & Associates are working in this field. PoUution in Space A new kind of pollution from light, space debris and radio interference is harming astronomers' vision of the sky and has already led to the closure of some observatories. Ground-based and even space-based observatories can no longer have an unhindered view of celestial bodies because of outdoor lighting, growing radio interference and orbital debris. A report of the International Astronomical Union said astronomers had been facing this form of pollution for the last few years, but it had become acute recently. Even photographs taken using the 48 inch achmidt telescope at the MT Palomar observatory, California, were marred by streaks from satellites or space junk. Observations in some bands of the radio spectrum had to be abandoned because of nearly continuous interference. In order to detect the faint cosmic emmissions that radio astronomers want to study, quiet radio bands are required at frequencies ranging from 20MHZ to 6OOMHZ. Many satellite communication and radio transmission also use this radio spectrum, causing interference. Astonomers have now given up operating certain bands because disturbances.

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The sports fields, traffic at night and bill boards all the major source of light pollution. Most of the major optical telescopes suffer from a sky glow level twice the natural background. The amount of man-made space debris exceeds the amount of meteroites and much of space junk is permanently in the earth's orbit, and pose the danger of knocking into space-based telescopes or giving a mistaken view of the sky from the earth. The International Astronomical Union has called for protection of at least some portion of the sky and radio spectrum for astrophysical research and has urged governments to evolve methods towards this end.

Noise Pollution Noise, One of the active pollutants of man's environment, depends upon the extent of the commercial, industrial, social and cultural activities of a city. These are in turn, proportional to the population density of the area. Trains, aircrafts, blaring radios, T. V. sots, pneumatic drills etc. are unfortunately part of our existence today. Sound is commonly measured in decibels (db). 60 db is the normal level of talk, 90-95 decibels may cause irreversible damage to the automatic nervous system. Noise level in offices are around 60-65 decibels. At a level of 80 db, sound is annoying. WHO has fixed 45 db as the safe noise level. Cities like Bombay, Calcutta and Delhi register noise over 90 db. The noise level * six years, by 2000 AD, it is possible that no one above the age of ten will hear normally. This type of environmental degradation has implications for health as serious as air and water pollution. Medical evidence suggests that noise can cause heart attacks in individuals with existing cardiac problems, insomnia, fatigue, headaches, digestive disorders and that continued exposure to loud noises could lead to chronic problems like hypertension or ulcers and of course deafness. Earsplitting noise can even puncture eardrums, sometimes leading to meningitis, and other infections. Anti-noise activists point out that the right to a healthy environment is one of our seven basic rights as citizens. While in the west, compensation is granted in noise pollution, we do not yet in India have specific law shows the alarming situation in big cities. Because the noise level doubles every on noise abatement. To curb this noise menace,

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certain strict measures need to be taken and the centre is now planning to fomrulate a bill on noise pollution.

Threat of Human Impact on Environment The habitability of the planet is being jeopardised by the global consequences of human activities while the scientific understanding of the functioning of the most important ecosystems of all, the earth is still meagre. The international research community has responded by launching the most ambitious coordinated effort to study on a global scale the earth systems science. The International council of scientific Unions (ICSU) has come out with an action plan for its international geosphere biosphere programme (IGBP) a study of global change. The current focus of the programme is to describe and understand the interactive, biological, chemical and physical processes that regulate the earth's environment for life, the changes that are occuring in this system and the manner in which they are influenced by human activities. The IGBP involves a whole range of disciplines and is dependent on data being collected and to be collected at hundreds centres round the world. What the scientists are after is terrestrial data, data from the atmosphere, space. Oceans and from deep inside the crust of the earth. The scientists will dig deeper into the natural achieves for understanding the global changes of the past. The last 10,000 years have been the most significant in the earth's history as an era in which the present ecological conditions, including our present life support system, were developed and in which the human impact on this support system became increasingly visible. First as a result of rapidly expending agriculture and later through widespread industrialisation and urbanisation. These changes are to be analysed to be able to understand the present and anticipate the future. Advances in technology have made it possible to study the earth as an interactive system, scientists today can collect a wide variety of data on a simultaneous and repetitive basis. Conclusions Forests are being destroyed at the very high rate. An enormous hole is opening in the Ozone layer, reducing the earth's ability to protect life from deadly ultra-violet radiation. Living species die at such an unprecedented rate that more than half may disappear within our life

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times. Chemical wastes, is growing volumes, seep downward to posion groundwater and upward to destroy the atmosphere's delicate balance. Huge quantities of carbon dioxide, methane and cholorofluorocarbons dumped in the atmosphere and traping heat and raising global temperatures. Why are these dramatic changes taking place ? Because the human population is surging, because the industrial, scientific and technological revolutions magnify the environmental impact of these increases, and because we tolerate self-destructive behaviour and environmental vandalism on a global scale. Developing nations should maintain a balance between industrial growth and environmental preservation. Developing countries should take benefit from the experience of advanced countries in environmental preservation. In India, the law of pollution and environment has been made quite strict. Project involving deforestation are required to submit plans for planting an equal number of trees before they are cleared. Our major concern is to develop cost effective products with environmental safeguards. Aerial seeding is required for reforestation. Aerial seeding involves spraying seeds from an airplane flying at an altitude of 200 to 300 feet. Even if 100 to 200 trees survive in every hectare seeded! it is adequate to ensure the prevention of soil erosion. Further seeding could be done with better plant species. The advantages of aerial seeding are cost effectiveness and access to physically inaccessible areas. In a review of its environmental strategy and operations, a report noted the international consultations taking place on the establishment of a global environmental facility. Such a facility would finance programmes to address problems such as the depletion of the earth's Ozone layer/the "Greenhouse effect", loss of the planet's biological diversity and ocean pollution.

Appendix-C Ozone Depletion and Greenhouse Effect Introduction The habitability of this planet is endangered by global trends as evidenced by increases in tropospheric concentrations of Greenhouse gases with future effects on climate and decreases of stratosphere

Ozone. One of the main objectives of International geosphere-biosphere programme (IGBP) is to acquire wider improved knowledge on the response of the biosphere to anticipate atmospheric and climatic perturbations that may be caused by among other rectors, atmospheric concentration of greenhouse gases, Ozone depletion, etc. A system of geosphera-biosphere observatories have been set up. Ozone Depletion The Ozone layer is one of the earth's most important life support system. Ozone, a triatonic form of oxygen, is found largely in the stratosphere, a region of the atmosphere that extends from about 8 km at the poles and 17 km at the equator to about 50 km above the earth's surface. This crucial element constitutes less than one part per million of the gases in the atmosphere Ozone absorbs most of the ultraviolet rays from sun, preventing them from reaching the earth. Solar ultraviolet radiation is energetic enough to break a part important biological molecules including DNA leading to an array of problems. It causes sunburn, skin cancer, cataracts and can suppress the immune system, evidence indicates a one per cent decline in the Ozone layer could lead to four to six per cent Increase in the skin cancers. The drastic thinning of the Ozone layer apparently began around 1976. In May, 1985, the atmospheric scientists of the British Antarctic survC¥-at Cambridge found the springtime amounts of Ozone in the atmosphere over Halley Bay in Antarctica had decreased by more than

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40 per cent between 1977 and 1984. Not only that, the Ozone over Antarctica decreased dramatically each September-October and then gradually replenish itself by the end of November, in 1989, Ozone depletion has been detected in arctic. Recent data indicates that Ozone depletion is occuring on a global scale, although nowhere as extensively as over the Antarctica. There are two common explanations for the Ozone depletion. One theory assumes that pollutants are the cause ana other emphasises a natural shift in the air movements that transport ozone-rich air into the polar stratosphere. Among the pollutants, chloroflurocarbons (CFGs) are the main causes. These compounds were ftrst introduced some 60 years ago and are used as coolants for refrigerators ana air-conditioners, propellant for aerosol sprays agents fur producing foam, and cleaners for electronic parts. CFCs are highly stable and unreactive compounds thus non-toxic in the lower atmosphere (troposphere}. Howevem on raising into the stratosphere they are broken down by ultraviolet radiation clearing chlorine. This chlorine, in turn, reduces the amount of Ozone. When a chlorine atom (cl) collides with an Ozone molecule, the chlorine combines with the Ozone atom forming a chlorine monoxide radial (do) and an oxygen molecule. It has been discovered recently that these CFCs gases are the major contributor to the "greenhouse effect". Scientists now predict a major global climatic warming as a direct consequence of the production of CFCs, the combustion of fossil fuels and other human activities. Previously it was thought that carbon dioxide was the major gas involved in the greenhouse effect. However, CFCs are now known to be upto 10,000 times more efficient at absorbing infra-red radiation. Within 30 years, it is anticipated that the effect ofCFCs could overweigh carbon dioxide and other greenhouse gases. Every year, nearly 362,000 metric tonnes of CFCs compounds will linger in the atmosphere till the end of the next century. About 250 scientists from 40 countries and international A. organizations has taken part in the international conference on tropical Ozone and atmospheric changes held in February, 1990. The conference was aimed at pooling the resources of experts and those involved in the studies of Ozone depletion and ultra-voilet rays as

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well as atmospheric changes to create an awareness in the equatorialtropical region on the protection of the environment and ozone layer.

Green House Effect The global warming is caused by pollution, mainly by increased amounts of carbon dioxide and other gases in the atmosphere. The pollution-linked global increase in temperature has been described by scientists, as the "greenhouse effect". There is no innnediate prospect of reducing the emissions into the atmosphere of carbondioxide and water vapour the "green-house gases", which are warming the earth's atmosphere, the United nations environment progrannne has warned on the eve of the World Environment Day. World scientists have now agreed that the "Greenhouse effect" created by these gases will increase mean temperature by 1.5° C and 4.5° C in the next 30-40 years. The main sources of these gases are coal, gas and oil-fired factories. Temperature rise will cause thermal expansion of oceans by as much as one meter inundating low-lying coastal areas. Nearly one third of the world population lives within 60 km of a coastal line. One meter rise could displace 15 million people in Bangladesh and upto 10 million in Egypt. Deltas in Europe and north America and islands in tropical seas are also wlnerable. The cost of protecting coast lines against the one-meter per rise is estimated at billions of dollars. There is evidence too, of changes in atmospheric and ocean circulation and in regional rainfall. There are indications that agriculture could be radically affected. Some areas may become drier adding to desertification! others may get more rains speeding up soil-erosion. The implications of climate change for the social and political stability of the planet are profound. World scientists have an international conference on global warming with an appeal to the Developing countries to impose tax on fossile fuel consumption so that carbon dioxide production which is heating the earth can be reduced.

A VaIeano of Trouble Large volcanic eruptions have been sending vast amounts of corrosive chloride and flouride chemicals shooting into the upper layers

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of the atmosphere thus seriously depleting the world's Ozone layer, hydrogen chloride and hydrogen fluoride are among the most abundant chemicals ejected during an eruption. Eleven million tonnes of hydrogen chloride and six million tonnes of hydrogen fluoride are spewed into the atmosphere every year. Contribution of Continental Self Regions The massive dumping of organic materiall such as fertiliser residues! into the oceans has more environmental effects than previously thought. The fertilizer contant of rivers flowing to the sea mainly nitrogen and phosphorous bad increased by a factor of ten. With respect of global warming/this over fertilization of coastal shallows seems to have a desirable effect at first glance. The marine micro-organisms, such as algee and single-cell organisms/multiply more rapidly and bind lage quantities of carbon dioxide/a leading contributor to the "greenhouse effect," during their metabolic processes. The ocean thus acts as a "biological pump". The continental selfregionslin particular/even take on the characteristics of an "accelerated sink". Most scientists agree on these things. First the global average temperature has gone up by 0.5 0 C since the industrial revolution. Second, atmospheric carbon dioxide has increased by 30 per cent in the same period, and other industries ana agricultural greenhouse gases have shot up too. Third, if carbon dioxide reaches twice its pre-industrial level (which could happen by 2030; global temperatures could be 1.5° C to 4 0 C higher than those today. With the earth slowly and steadily warming up at a rate between 10 to 60 times faster than anything previously inflicted on the living systems, scientists are afraid of what our planet will be like in the year 2070. The effect would be catastrophic. Ozone depletion could lead to an increase in ultra-voilet rays which would ultimately cause adverse effects on the health of human beings, plants, eco-system and agriculture.

Appendix-D Role of Electronics in the Industrial Growth Introduction The world of electronics is surging forward at a tremendous pace, opening up new vistas in human life. The electronic industry in India made its humble begining during the early sixties with the establishment of a few public sector units. This industry has made a remarkable progress during the last 15 years which one could not imagine . The turnover of the electronic industry has risen fromRs. 4100 million in 1976 to Rs. 34600 million in 1986.,Out of the estimated electronics production of Rs. 10860 crores by the end of seventh five year plan, the estimate of Rs. 500 crores would be for component materials. Electronic industry are likely to achieve the highest rate of growth of 15 percent and above per annum during the eighth five year plan. Great growth potential is seen for the electronic industry, as traditional industries like sugar, cement, textiles and others like power, telecommunications, chemicals, petrochemicals, and steel will be supplying electronics in their operations. Sophisticated industries like space, defence and atomic energy have already taken to electronics in a big way to significantly improve their technological capability. In the manufacturing sector, electronics has begun to play an increasingly important role in the areas of instrumentation and command control. Electronic components and sub-assemblies, computer control systems and instrument, computer software, system engineering and consultancy, communication and broadcasting and consumer electronics, have been identified as thrust areas for development. The electronic industry is sub-divided into following groups :1. Consumer Electronics 2. Industrial Electronics 3. Component Industry

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Foreign CoUaboration in Electronics A number of Indian parties have entered into foreign collaboration with US fIrms for various items in the fIeld of electronics, computer and software viz., electronic clocks, uninterruptable power supply systems, floppy diskettes, pay telephones, video magnetic tapes, electronic guns, printed circuit boards, glass shells for colour T.V., power capacitors, micro-ovens, quartz analog watches, mini computer, PC systems, main frame computer systems, Multi users computers, line printer, computer softwares, software for chip design, software engineering, etc. A number of incentives have been extended to the Indian electronic Industry for promoting electronic exports which include 5 year tax holiday to 100% Export Oriented Units (EOUS) and units under Export processing zone (EPZ) and enhancement of the rate of Import Replenishment Licences. Foreign collaboration and lor foreign investment in software development activity for exports and/or domestic markets will be permitted as per provisions of foreign exchange regulation Act. Exports are being made to countries like USA, USSR, UK, West Germany, Bangladesh, Hongkong, Nigeria, Hungary, Poland, Yugoslavia. For expanding the export markets, exhibitions, seminars, market surveys are conducted on ongoing basis. Scenario of Electronic Industry In India, production of electronic goods is being carried out through the public and private sector. The annual data received by the Data Bank and Information Division (DBID) of the Department of Electronics Shows that the public sector units account for roughly 32 percent of the total production and the remaining 68 percent is contributed by the organized and small scale units in the private sector. Exports are being given the highest priority by the government. In recent years, export of electronics products to both the developing and the developed countries have increased. In order to boost the exports of electronic products, 100 percent Export Oriented Units have been set up in domestic Tariff Area (DTA) and in Export Processing Zones.

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About 60 percent of the export is contributed by units in Santacruz Electronic Export Processing Zone (SEEPZ) and Free Trade Zones at Kandla and Falta. One of the single largest export item is computer software. India in 1986 announced a software policy to promote software exports and to capture a sizable share in international software market. Consumer electronics continues to show sustained growth and this

is expected to continue. The significant increase in the production of black & white TV sets, colour TV sets and tape recorders has largely contributed to its growth. New products such as electronic watches and clocks also have been introduced. The rnajor share of consumer electronic production is contributed by the private sector both in the organised and in small scale areas. The total production worth of electronic products in the country during the year 1988 was about Rs. 63 billion as compared to about Rs. 13.6 billion worth in 1983. The growth rate in the area of electronics in India is much higher than the average world electronics growth rate. The production of electronic equipment is planned to grow from Rs. 72,500 million in 1989-90 to Rs. 200,000 million in 1994-95, which represents an average growth of about 22 percent. Through 1990, the growth rate of consumer electronic appliances was to be 6,8 percent, industrial electronic appliances was to be 10 percent and computer related instruments was to be 12 percent. Overseas production of consumer electronic appliances such as TV s and video equipment by Japansse makes will continue to expand from Asia to Europe. The production of industrial equipments such as telecommunication equipment, personal computers, copying machines and facsimiles, however, will develop off-shore, principally in the United States and Europe. Components Industry The components industry in the country is poised for a very substantial expansion and tentative projections indicate that this industry is likely to increase several times by the end of eighth five year plan. A major increase in demand of electronic components has emerged from the black and white TV and tape recording industry where most of the components are supplied from indigenous sources. Components which have shown large growth, are B & W picture tubes integrated circuits/professional grade PCBs and audio magnetic tapes.

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Production of electronic components in the country is being carried out both in organized as well as small scale sectors. In order to build a strong base for the eventual production and supply of materials for manufacture of critical components in particular, the council for Development of Materials for Electronics ( C-DOME) has been established for coordinating and sponsoring projects in related areas. The production of electronic components has increased from Rs. 3200 million in 1984-85 to Rs. 11,250 million in 1988-89. It is estimated that during 1989-90, the production would reach a level of Rs. 16,000

million The growth of electronic components industry has been largely dependent on the consumer industry. Manufacture of electronic components is regulated by the phased manufacturing programme (PMP). The pwpose of setting the PMP is to provide raw materials, components and consumables to the industry in accordance with the Import Policy of the government, Global demand for electronic parts and components in 1990 will reach

$ 73,600 million based on a yearly growth rate of 10 percent. Switching power supplies are expected to grow the fastest with a yearly growth rate of 13.5 percent. Hybrid ICS, PC Boards, connectors, aluminium electrolytic capacitors, tantalum electrolytic capacitors should expand at an annual rate of 10 to 12 percent. On the other hand, resistors, ceramic capacitors, coils and transformers, which are tied to demand of consumer electronic appliances, are expected to register relatively low growth rates. Microelectronics Industry Microelectronics is a vital ingredient of electronic equipment and systems. The continuous increase in the level of integration is leading to entire sub-systems and even systems being fabricated on a chip of a few mm, square size. Microelectronics is thus making possible the revolution in the management and movement of information. Information and knowledge based technologies are expected to be the driving engines of national economics in future. India has the capability to design, develop and manufacture ICS ranging from SSI (small Scale Integration) to VLSI (very large Scale integration) in both bipolar and MOS technologies. It is estimated that import oflCS in 1987 in various forms, viz, chips, chips and boards and chips as part of subsystems was around Rs. 750 million. The total

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percentage of semiconductors in the Indian electronic equipment is around 5 percent as compared to the international average of around 12 percent. Manufacture of MOS ICS is primarily carried out at the semiconductor Complex Ltd. (SCL). Bharat Electronics Ltd. (BEL), Bangalore, primarily manufactures bipolar ICS. In the private sector, ICS of the SSIIMSI complexity are manufactured by Hindustall Conductors Ltd. and Greaves Semiconductors. Spic Electronics Ltd. (SPEL), Madras is essentially as assembly operation based on diffused wafers procured from abroad. In addition to the industrial activity at SCL, BEL and ITI, several R & D laboratories are also active in various aspects of microelectronics. The Central Electronics Engineering Research Institute (CEERI), Pilani has been working on IC processes and design of ASICS. IITs at Bombay, Delhi, Kanpur, Kharagpur, Madras and Banaras, the Indian Institute of Science at Bangalore, Jadavpur University, Calcutta are some of the major academic institutions which are involved in manpower training and R&D on various facets of units processes, chip design and software tools for microelectronics. Presently, use ofICS in India is relatively small. The biggest user of ICS is the computer industry, followed by telecommunication industry. The share of the consumer electronics industry in consumption of ICS is generally small. The development of digital technology can increase the percentage share of ICS in many consumer electronic items like TV and VCRS. To sustain microelectronics industry at a magnitude envisaged, a significant effort in R&D in both industry and R&D establishments would be required. The government will have to commit substantial funds for supporting R&D as part of the catalytic role for the growth of this industry in the country. The R&D activities should also include some joint development programmes with foreign companies/organizations for mutual benefit. The exports of electronic components worth Rs. 10,000 million planned for 1994-95 can be realised if capacity creation in this industry is planned with a greater empasis towards realising exportable surplus. Existing units who have established quality should plan for quick expansion with clear plan for exports of a substantial part of the proposed

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capacity. New units should plan for investments at a capacity level which will make exports competitive. They should ensure that the products are of latest technology and the quality is internationally acceptable. The financial institutions should insist on investments for adequate quality control procedures.

Control Instrumentation and Industrial Electronics The various test instruments manufactured in India include oscillscope, recorders, signal/function generators, analysers, bridges, digital multimeters, synthesizers, VfVMS amplifiers and digital counters. A wide variety of medical electronic equipment such as electrocardiograph (ECG), X-ray machines, hearing aids, pacemakers, defibrillators, foetal monitors, baby incubator, diathermy units, cardiscope, expirograph ultrasonic scanner, electro-consulsive therapy (ECT) units, electroencephalograph (EEC) units are being manufactured indigenously. The industrial electronic system being manufactured indigenously include ACIDC drives, UPS, SMPS converters, disturbance recorders and fault locators. Even mining electronic equipment like environmental monitoring systems, winder control underground communication system are now manufactured locally. Other instruments for special application that are being produced indigenously, include nuclear instruments, gee-scientific instruments, agri-electronic equipment, flood, air pollution, water treatment, and monitoring systems and textile instruments. Regarding the process control instrumentation industry India has achieved expertise in system engineering, production of distributed digital process control hardware and installation and commissioning of instruments systems. Manufacturing covers sensors and indicators, process control converters and transmitters, process controllers, control valves, actuators, control panels, alarm annunciators, transducers, modules and electronic" weighing systems. The microprocessor based control systems being produced include temperature scanners, sequential event recorders, data loggers, programmable controllers and closed-loop controllers data acquisition systems and distributed digital control system (DDCS) also have come ~p through foreign tie ups. The large demands for process instruments I for pressure, temperature, flow, level, etc. used in thermal, cement and steel plants as well as in petrochemical industries and refmeries are being met from domestic sources. In the steel sector, major electronics contracts involving automation, control, computerization and energy management systems, are being

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handled by indigenous manufacturing organizations. The first major blast furnace automation has been completed with indigenous support. The high voltage direct current transmission programme is being promoted, through local expertise and the sub-systems are being developed locally. In the power electronic sector single-to three phase converters have been developed for the railways. AC drives have also been taken up for indigenous manufacture with foreign support. Thyristors with high current and voltage ratings are now being manufactured in India.

Computer Industry Computers are being extensively used in the country in educational institutions, research & development centres, industry, defence, government offices and in business organizations. Rationalized computer policy was announced by the government of India in 1984, There has be~n a substantial reduction in the price of the indigenously manufactured computers in the country after the announcement of a computer policy. A centre for development and production of computer main frames has been set up for achieving self-reliance in this area. A Centre for Development of Advanced Computing Technology (COACT) also is being set up at Pune by the Department of Electronics.

Telecommunication Industry Communication is the backbone of industrial development and also is vital in integrating the rural areas with the main stream of national development. Manufacture of telecommunication equipment in India has been basically confmed to the Central! State public sectors. Manufacture of the entire range of telecommunications equipment is restricted to the public sector and manufacture of telecommunication terminal equipment has been thrown open to the private sector. Also joint sector units with upto 49 percent private equity participation were encouraged to take up the manufacture of the remaining items, mainly transmission equipment and switching equipment. Manufacture of Electronic PABX systems, ha~ been taken up by COOT ( The Centre of Development of Telematics). Manufacture of electronic switching systems is already in progl:ess at Indian Telephone Industries (ITI), Mankapur in collaboration with CIT-Alcatel of France.

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Digital multiplex equipment has been successfully developed by the Telecommunication Research Centre (TRC) of the Department of Teleconununication (DOT) and produced by Indian Telephone Industries Ltd. Manufacture of digital microwave systems and optical fibre conununication systems as well as the optical fibre cables has been taken up by Madhya Pradesh State Electronics Development Corp. (MPSEDC) and the Indian Telephone Industries (lTI)lHindustan Cables Ltd. Satellite Communication equipment is being manufactured at Indian Telephone Industries, Bangalore. Manufacture of low cost satellite conununication terminals is progress at Gujarat Conununication & Electronics Ltd. (GCEL), Baroda, Small aperture micro-earth stations are currently being manufactured by ITI, Equatorial Ltd. Aerospace and Defence Industry The production base in this sub-sector includes radar, Navigational aids, marine electronic equipment, aeronautical communication equipment, avionics electronic equipment, conununication equipment ( for defence and paramilitary forces) and special defence equipment. The requirements of major users like minisuies of Defence, Tourism and Civil Aviation, Home Affairs, shipping & Transport, Departments of Ocean Development, Transport and space are being met by Public Sector Undertakings like Bhart Electronics Ltd. (BEL) HAL and ECIL. There are also several industries in the private sector which are supplying systems and sub systems such as display and data handling equipment for radar. Society for Applied Microwave Electronics Engineering and Research (SAMEER) was set up as an autonomous society for research, design, development and batch supply of special microwave products required by various agencies such as Defence, space and Atomic Energy. The products taken up for development by SAMEER usually have strategic importance, import substitution objective or are required in small volumes and are not conunercially viable. Conclusions The electronic industry has undergone a major revolution all over the world due to major advances made in the semiconductors technology. Because of increasingly large usage of integrated circuits with more and more complixity, many individual discrete components like ~nsistors, resistors and capacitors have been replaced. Morevoer, with

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the introduction of micro-electronic technology, the size of all other discrete components is being miniaturised throughout the world, in order to drive full benefits of these developments. Today, there is no field where electronics does not find its application either directly or indirectly. Electronics technology is used in a host of applications for improving the efficiency and productivity. Electronic products find numerable applications in all sectors such as Industry, Defence, Telecommunications, Education, Mass communications, medicine, agriculture, railways. space, atomic energy controllInstrumentation, power etc. It is because, electronics is considered as the most efficient tool for improving the efficiency, productivity and quality of service. The electronic technology has become all pervasive in advanced countries and acquiring importance in developing countries Electronic industry is free from air, water and noise pollution and whose location and expansion do not affect the large masses of population concentration. This industry attracts and requires highly technical people with a multi-disciplinary engineering and scientific background whose thinking and applications result in spin-offs which are very important and advantageous to all other industries.

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Appendix-E World¢s Famous Quotations To 10-9-2006

1. Personality can open doors, but only character can keep them open.

-Elmer G Letterman

2 Life can be one satisfaction after another if we let it.

-John Schindier

3. Never stand begging for what you have the power to earn. -Miguel de Cervantes 4. Every great and conunanding moment in the annals of the world, is the triumph of some enthusiasm. -Ralph Waldo Emerson 5. If have the belief that I can do it, I will surely acquire the capacity to do it, even ifI may not have it at the beginning.

-Mahatma Gandhi 6. In real love you want the other person's good. In romantic love you want the other person. -Margaret Anderson 7. If you love life, life will love you back.

-Norman Vincent Pleale 8. The tragedy of life is not that it ends so soon, but that we wait -W M Lewis so long to begin it. 9. An expert is someone who knows more and more about less and less until he knows everything about nothing.

-Albert Einstein 10. If I have the belief that I can do it, I will surely acquire the capacity to do it, even ifI may not have it at the beginning.

- Mahatma Gandhi To 1 17-9-2006

11. Those who say it can't be done are usually interrupted by others doing it. -James A Baldwin

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Encyclopaedia of Petroleum Science and Engineering 12. Hope is the word which God has written on the brow of every man. -Victor Hugo 13. The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. -Marcel proust 14. The end comes when we no longer talk with ourselves. It is the end of genuine thinking and the beginning of the final -Edward Gibbon loneliness. 15. Real love is a pilgrimage. It happens when there is no strategy, but it is very rare be-cause most people are strategists.

--Anita Brookner 16. He that never changes his opinion never corrects mistakes and will never be wiser on the morrow than he is today.

-Tryon Edwards 17. You know that you are in love when the hardest thing to do is say good-bye! -Anonymous 18. View life as a continuous learning experience. -Denis Waitley 19. The miracle of man is not how far he has sunk but how magnificently he has risen~ - Robert Ardrey To 1 21-10-2005

20. Problems ... cannot be solved by the level of thinking that created them. -Albert Einstein

. 21. Life is a tragedy for those who feel. and a comedy for those To 116-7-2006

who think.

-La Bruyere

22. Love is the history of a woman's life; it is an episode in man's.

-Germaine De Stael It is a beggar's pride that he is not a thief.

-Anonymous

23. Admiration is our polite recognition of another's resemblance to ourselves. -Ambrose Birce 24. A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort.

-Herm Albright 25. The mode by which the inevitable comes to pass is effort.

-Oliver Wendell Holmes

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26. I believe that ignorance is the root of all evil. And that no one -Molly Ivins knows the truth. 27. Great eaters and great sleepers are incapable of anything else -Henry IV that is great.

-Lord Byron

28. There is no instinct like that of the heart. To 1 27-1-2006

29. When mind are closed they become impervious to reason. -Jawaharlal Nehru To 1 23-3-2006

30. If the wind will not serve, take to the oars.

-Latin Proverb

To 1 6-8-2006

31. They are never alone that are accompanied with noble

-Sir Philip Sidney

thoughts.

32. Friendship with oneself is all important because without it one cannot be friends with anybody else in the world

-Eleanor Roosevelt 33. The most imaginative people are the most credulous, for them everything is possible. -Alexander Chase 34. The enthusiasm of a woman's love is even beyond the biographer's. -Jane Austen 35. I must have a prodigious quantity of mind; it takes me as much as a week sometimes to make it up. -Mark Twain

36. There is nothing so easy to learn as experience and nothing so -Mark Twain hard to apply. . 37. Friendship marks a life even more deeply than love. Love risks degenerating into obsession, friendship is never anything but sharing. -Elie Wiesel 38. We don't see things as they are, we see them as we are.

-Anais Nin To 1 8-1-2006

39. I couldn ~ wait for success, so I went ahead without it. -Jonathon Winters

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Encyclopaedia of Petroleum Science and Engineering To 1 4-9-2005

40. In askingfor miracles, we are not asking for something outside us to change, but for something inside us to change. -Marianne Williamson 41. Creativity is inventing, experimenting, growing, taking risks, breaking rules, making mistakes, and having fun.

-Mary Lou Cook 42. Who bravely dares must sometimes risk a fall. -Tobias George Smollett 43. The first step to getting the things you want out of life is this: Decide what you want. -Ben Stein 44. Destiny is no matter of chance. It is a matter of choice. It is not a thing to be waited for, it is a thing to be achieved.

-William Jennings Bryan 45. You probably wouldn't worry about what people think of you if you could know how seldom they do. -Olin Miller 46. While we read history we make history.

-George William Curtis 47. Education is what survives when what has been learned has been forgotten. -B F Skinner 48. Marriage - a book of which the first chapter is written in poetry and the remaining chapters written in prose. -Beverly Nichols 49. Three can keep a secret if two of them are dead. 50. Never assume the obvious is true.

-Benjamin Franklin -William Safire

To 1 23-1-2006

51. "Freedom implies not only emancipation from political bondage but also equal distribution of wealth, abolition of caste barriers and social inequities and destruction of communalism and religiOUS intolerence. " -Netaji Subhas Chandra Bose 52. Success is the result of good judgment, good judgment is result if experience, experience is often the result of bad judgment.

- Tony Robbins

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53. Two things are infinite: the universe and human stupidity; and I'm not sure about the universe. -Albert Einstein To 1 21-8-2006

54. The journey of a thousand miles begins with one step. -Miyamoto Musashi 55. To love and be loved is to feel the sun from both sides.

-David Viscott 56. Subbue your appetites, and you have conquered human nature.

-Charies Dickens 57. People demand freedom of speech to make up for the freedom of thought which they avoid. -Soren Aabye Kierkegaard 58. While I can run, I'll run; while I can walk, I'll walk; when I can only crawl, I'll crawl. But by the grace of God, I'll always be moving forward. -Cavett Robert 59. The foolish man seeks happiness in the distance, the wise grows it under his feet. -James Oppenheim 60. We all live with the objective of being happy; our lives are all -Annne Frank different and yet the same. 61. This life is worth living, we can say, since it is what we make it.

-William James 62. You cannot be lonely if you like the person you're alone with.

-Wayne Dyer To 1 23-7-2006

63. What we love to do we find time to do.

-John L Spalding

64. There is no greatness where there is not simplicity.

-Leo Tolstoy 65. To love and win is the best thing. To love and lose, the next best. -William M Thackeray 66. Tough times never last, but tough people do.

-Robert Schuller 67. A life spent making mistakes is not only more honorable but more useful than a life spent in doing nothing.

-George Bernard Shaw

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68. Don't walk behind me; I may not lead. Don't walk in front of me; I may not follow. Just walk beside me and be my friend.

-Albert Camus (f).

The best feelings are those that have no words to describe them.

-Michelle Hammersley 70. Son. Always tell the truth. Then you will never have to remember -Sam Rayburn what you said the last time. 71. If you do not hope, you will not find what is beyond your hopes.

-Anonymous 72. Only a life lived for others is a life worthwhile.-Albert Einstein To 1 9-7-2006

73. Small opportunities are often the beginning of great enterprises. -Demosthenes 74. I have witnessed the softening of the hardest of hearts by a -Goldie Hawn simple smile. 75. My motto is : Contented with little, yet wishing for more.

-Charles Lamb 76. The most important thing is to be whatever you are without -Rod Steiger shame. 77. Love is the triumph of imagination over intelligence.

-HLMencken 78. Never explain - your friends do not need it and your enemies will not believe you anyway. -Elbert Hubbard 79. The only thing that overcomes hard luck is hard work.

-Harry Golden BO. Just because you love someone doesn't mean you have to be involved with them. Love is not a bandage to cover wounds.

-Hugh Elliott 81. It's choice, not chance, that determines your destiny.

-Jean Nidetch To 11-5-2006

82. Virtue has its own reward, but no sale at the box office. -Mae West

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237 To 11-5-2006

83. A mind that is stretched to a new idea never returns to its -Oliver Wendell Holmes original dimension. To 1 27-11-2005

84.

Until you make peace with who you are, you'll never be content with what you have. -Doris Mortman

85. Never stand begging for what you have the power to earn.

-Miguel de Cervantes 86. Every great and commanding moment in the annals of some -Ralph Waldo Emerson enthusiam

'irl. Where there is love there is life.

-Mahatma Gandhi

88. When you reach for the stars you may not quite get one, but you won't come up with a handful of mud either.

-Leo Brunett 89. I don't know the key to success, but the key to failure is trying to please everybody. -Bill Cosby 90. The supreme happiness in life is the conviction that we are loved. -Victor Hugo 91. Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great. -Mark Twain To 116-5-2006

92. When thought becomes excessively painful, action is the finest -Salman Rushdie remedy. To 1 4-9-2005

93. The greater the obstacle, the more glory in overcoming it. -Moilere 94. Each of us bas a ftre in our hearts for something. It's our goal -Mary Lou Retton in life to ftnd it and to keep it lit. To 1 21-6-2006

95. The best time to plant a tree was 20 years ago. The second -Chinese Proverb best time is now.

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Encyclopaedia of Petroleum Science and Engineering To 1 26-3-2006

96. Fear not for the future, weep not for the past. -Percy Bysshe Shelley 97. Shoot for the moon. Even if you miss, you'll land among the -Les Brown stars. To 1 18-8-2006

98. TIme present and time past are both perhaps present in time future. - T S Eliot To 1 23-1-2006

99. Strength does not come from physical capacity. It comes from -Mahatma Gandhi an indomitable will. To 1 2-7-2006

100. Plans are only good intentions unless they immediately degenerate into hard work. -Peter Drucker To 1 9-4-2006

101.

It is better to have enough ideas for some of them to be wrong than to be always right by having no ideas at all. -Albert Einstein To 1 30-10-2005

102. The secret of happiness is to make others believe that they are .ru~ifft

~IBd

To 1 12-2-2006

103.

Adversity cause some men to break; others to break records. - William A Ward To 1 20-3-2005

104. The question in life is not whether you get knocked down. You will. The question is, c.re you ready to get up and fight for what -Dan Quale you believe in.

~

Appendix-F News in Focus ONGC TO INVEST RS. 130,000 CR Hun For Oil and Gas In India, Besides Look At Acquisitions Abroad State-owned Oil and Natural Gas Corp (ONGC) plans to invest over Rs. 130, 000 crore during XIth Plan period (2007-12) in domestic oil and gas hunt, over seas acquisition and expansion of Mangalore refmery. "ONGC's domestic oil and gas exploration and produce tion spending will rise by 56% to more than Rs. 80,000 crore in 2007-12 as against Rs. 51,299 crore expenditure in the Xth Plan Period (2002-7)," company chairman and managing director RS Sharma said. On overseas oil and gas properties acquisition, about Rs. 50,000 crore has been earmarked for the XIth Plan as opposed to Rs. 13,500 crore in the current plan period. "Besides, the company would also invest Rs 8000 crore in expanding Mangalore Refmery and Petrochemical Ltd (MRPL) capacity to 15 million tonnes per annum from 9.69 million tonnes currently," he said. Sharma said deepwater and frontier basins are thrust areas for ONGC's domestic E&P. The focus areas would also include technology solutions for improving recovery factor, expeditions development of new as well as marginal fields, refurbishment of surface facilities and technology induction. The Improved Oil RecoverylEnhanced Oil Recovery projects in 15 major fields had arrested the 7% decline in production from the matured fields and reversed the trend. Production from the 15 fields, which was 24.08 million tonnes per annum, fell to 19.56 mtpa in 1999. However, this trend was reversed and production rate increased by 2.25% per annum. Output from the 15 major fields is 22.65 mtpa in 2006. But for IORlEOR the production would have dipped to 12 mtpa this year, he said. "Eight IORlEOR projects worth Rs. 1,703 crore have been completed and another 10 projects (Rs. 12,138 crore) were under implementation,"

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Sharma said. Envisaged oil gain through IORlEOR projects was 110 million tonnes by 2020, he said adding 25 lOR projects have been identified in other medium fields. ONGC plans to invest about Rs. 12,700 crore in de velopment of marginal fields in XIth Plan period. Besides, Rs. 1,263 crore is being in vested in G-l and GS-15 field development, Rs. 507crore in D-l field development, Rs. 1,688 crore in Vasai (East) development and Rs. 2,937 crore in additional development of Bassein field. Sharma said Rs. 2,570 crore is being invested in renewal of Assam oil field and another Rs. 3,492 crore in other revamping projects. NOW, ONGC WIND FARMS TO POWEROILFIELDS New Delhi: For Oil and Natural Gas Corporation (ONGC), the answer is blowing in the wind. The state-run explore is moving ahead with plans to produce 100 MW of electricity from wind generators in Gujarat and Andhra Pradesh to power continuous pumping operations in its oilfields there.

The oil major is estimated to pump Rs. 500 crore into two wind farms, each with a capacity of 50 mW. The farms will be set up in the coastal areas of these states. The units are expected to come on stream by early 2007. The board approved proposal in July. The idea of wind farms was conceptualisd by then ONGC chairman Subir Raha. It is considered the most cost-effective and environmentally clean way of ensuring uniterrupted operations in the oilfields in these two states. Oil from most of these fields has to be pumped up by using electric pumps. This is interrupted due to the erratic supply pattern of the state power grid. Once operation is stopped, it takes time to bring back to normal level even after powers supply is restored. Captive diesel or gas-fired generators are expensive to run and also a major cause of pollution. ONGC, however, is grappling with the problem of dealing in a sellers'market. Its open tender for turnkey contractors has elicited only two responses from Suzlon Enegry and Vestas, the the only domestic firms that have the expertise in the field. The turnkey contractor is to be selected on the basis of best offer on the cost of generating oner uni.

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Thecost-per-KW nonn in India is in the region ofRs 23, depending on parameters such as wind velocity at the location. The bids are under evaluation in 7 -8 months of awarding the contract. Due to fiscal incentives, there has been a spurt in investments in winds farms. Carbon Credits and Depreci ation Benefits apart, the standalone economics of wind farms make them attractive financial propositions in the long tenn due to free fuel and low operation cost. Coupled with softer technology risks and shorter gestations, these factors drive wind power two notches ahead of conventional fossil-fuel (coal or gas) power-generation avenues. The cost of wind power generation is logging lower bottoms every it year, and is less than 30% of what it was three years ago. Comparative analysis with fossil-fuel systems makes wind-systems a clear favourite, especially considering its green features. ONGCINKS OR.PAcrwrm: CUBA New Delhi: ONGC Videsh, the overseas in vestment arm of stateowned explorer Oil and Natural Gas Corporation, has signecf.a production~ sharing contract with Cuba's national oil company Cuba Petroleos (Cupet) for two ecploration acreages in the Gulf of Mexico. ONGC will explore Blocks 'N-34' and 'N-35' in Cuba's exclusive economic zone. ONGC Videsh has already made its presence in Cuba's upstream sector through participation in six offshore blocks. This contract would give major boost to the company's activities in the region, ONGC chairman RS Sharma said after signing of the agreement in Havana on September 9. Cuba Petroleos officials say a total of six firms have signed deals for 16 blocks in the Gulf of Mexico. At home, ONGC also restored full daily gas supply of 40 million cubic metres on Friday from its flood-hit Hazira complex in Gujarat. Gas processing operations at Hazira reached a throughput level of 40 million cubic metres a day, 22 days ahead of the initial schedule. Hazira was shut down on August 7 after being flooded, haloting two-thirds of India's domestic gas output. The plant receives gas from Reliance-BG-ONGC consortium's Panna-Mukta-Tapti fields and ONGC's own Bassein field. GAR. TO INVEST IN ASSAM GAS PROJEcr New Delhi: State-woned GAIL India is planning to invest Rs 5,460 crore in the Assam gas cracker project.

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The project, to be set up as an integrated petrochemical complex at grated petrochemical complex at Lepetkata in Dibrugarh, will have a capacity of 2,20,000 tonne per annum of ethylene and 60,000 tonne per annum of propylene. It will also produce 55,000 tonne per annum of raw pyrolysis gasoline and 12,500 tonne per annum of fuel oil. While the gas would be supplied by OIL and ONGC, naphtha would be sourced from Numaligarh Refinery. The project, which is expected to be completed in 60 months, will be implemented by a joint venture company to be promoted by GAIL with 70% equity participation.

OIL COS TO ACQUIRE CANE F1ELD Govt. Wants Supply of Ethanol To Make Swadeshi Gasohol New Delhi: Oilfields and coal mines are passe. India is now poised to make a go at acquiring sugarcane acreages overseas in search of energy security. The idea is to put in place an assured supply of ethanol, a byproduct of the sugar industry that is mixed with petrol to produce swadeshi fuel or 'gasohol'. Progressiveused of this fuel will reduce the country's oil bill by reducing dependance on imported crude. PM Manmohan Singh is expected to flag the issue of Indian stateowned oil firms acquiring sugarcane field in Brazil during his visit later this month. Brazil is the largest sugarcane producer in the world and global leader in gasohol usage. It allows foreing ownership of sugarcane acreages which are irrigation. The sugarcane farms are highly mechanised and have integrated sugar mills. India and Brazil have held a round of talks on the issue. Brasilia indicated it does not have problem as of now with Indian oil firms acquiring sugarcane acregaes, either on their own or injoint venture with Brazilian state frrms. Several European frrms have acquired acreages and taken up ethanol manufacturing for captive use in home country, which might prompt rethink on foreign ownership. According to a government working paper, existing tieups between Indian oil frrms and Brazil's national hyddrocarbons entity Petrobras can be expanded for canalising ethanol from captive acreages in that country. ONGC Videsh, GAIL and refiner-marketer BPCL have MoUs with Petrobras.

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Efforts to promote gasohol, started by then oil minister Ram Naik during the NDA rule, bas been doddering due to short supply of ethanol. The oil ministry is running pilot projects in nine sugarcane growing states of blending 5 per cent ethanol in petrol. The plan is to makes it mandatory in all states and then move up to 10 per cent blending. The department of chemicals and fertilisers has opposed mandatory blending of petrol, saying domestic production of ethanol is not even meeting demand from chemicals industry and alcohol manufacturers. Any diversion for petrol blending will aggravate the woes of the chemicals industry. NOW, IT'S TIJRN OFOn..REFINlNGOFFSHORING New Delhi: If the past decade saw the world getting Bangalored with BPO (business process outsourcing), it's the of Indian oil to sizzle abroad by ushering in the age of RPO-refming process outsourcing. Several firms are setting up new refineries or ramping up existing capacities to keep motorists across the seven seas tanked up. Leading the charge of India's oil brigade is Mukesh Ambani. His Reliance Petroleum is pumping $6 billion into a 29-million-tonne refmery being built next to parent Reliance Industries' existing 3-million-tonne unit at J amnagar. In neighbouring Vadinar, Essar Oil of the Ruias is setting up a 10-million-tonne plant for $2.4 billion. Following up are state-owned Indian Oil, ONGC and Hindustan Petroleum-all toying with the idea of building plants in Paradip, Mangalore and Vizag, respectively. All these capacities aim at servicing the growing crunch in refming capacity which has failed to keep pace with the rise in demand for evercleaner fuels worldwide. For example,no new refinery has been built in the US since 1976 in the face of tighter Green laws, though over 200 million light vehicles zipping across its highways consume 11 per cent ,of world oil output. If this has US policymakers worried, it spells opportunity for Irtdia to become the world's refmer as a fitting reply to those who spite the BPO industry as a source for "cyber coolies". Reliance will usher iIi the new era in 2008 when its plant starts shipping motor fuels. Reliance intends to become a key driver in the US motoring market with its new plant that is impressive globally in scale and scope. The combined capacity of the new and existing units will turn the Jamnagar complex into the world's largest single-location refinery with a capacity

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of 1.2 million barrels a day, or 20 per cent higher than the current No.1, Venezuela Paraguana. As reported by TOI, a deal with Chevron-Texco is m offing. The US blajor will take stake in the Relance refinery where it will process its crude for feeding its retail networks in USIEU markets The refinery will be more sophisticated than the existing unit, allowing it to produce Green fuels for the USIEU markets from cheaper crudes (heavy) this will help it to make $10-11 on each barrel, which is $3 more than the Asian benchmark. The refinery will also produce a new octanpbooster (alkylates) being progressively used in the US and fetch $5-6/ barrel premium. PLANET CLUB HAS 3 NEW MEMBERS 2003 UB 313, Charon And Ceres Additions To Solar System Prague: Under a rpoposed "Big Bang" expansion by leading astronomers, three more planets would joint the Solar system. Besides reafftrming the status of puny Pluto- whose detractors insist it should not be a planet at all-the new lineup would include 2003 UB313, the farthest-known object in the solar system; Pluto's largest moon, Charon; and the asteroid Ceres, which was a planet in 1800s before it was demoted. The panel also proposed a new category of planets called "plutons", referring to Pluto-like objects that reside in the Kuiper Belt, a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects. Pluto itself and two potential newcomersCharon and 2003 \JIB 13-would be plutons. The provisionally named 2003 UB313 's discoverer, Michael Brown of the California Institute of Technology, nicknamed it Xena after the warrior princess ofTY fame, but it likely would be rechristened something else later, the panel said. Opponents of Pluto, which was named a planet in 1930, still might spoil for a fight. Astronomers also were being asked to get rid of the term ''minor planets", which long has been used to collectively descnbe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as "small solar system bodies". The galactic shift would force publishers to update encyclopaedias and school textbooks, and elementary shcool teachers to rejig the panet

Appendix-F

245

mobiles hanging from classroom ceilings.Far outside the realm of science, atrologers accustomed to making predictions based on the classic nine might have to tweak their formulae. The IAU-has a "watchilst" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits.

N-POWER GENERATES 17 PER CENT OF WORLD'S ELECIRICfIY How is nuclear power produced? Nuclear power is the method of energy production from nuclear reactions, primarily fission, that is the process of atomic nuclei splitting up and releasing energy. Nuclear reactions generate thermal energy or heat, which is converted to kinetic energy by means of a steam turbine and then a generator for electricity poroduction. Nuclear power currently provides about 17 per cent of the world's total electricity and 7 per cent of global energy consumption. The US nuclear share of electricity generation is about 19 per cent while in France, 78 per cent of all electric power is generated by nuclear reactors. What is a nuclear reactor? A nuclear reactor is a device in which nuclear reactor is a device in which nuclear chain reactions are initated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second). The most significant current uses of a nuclear reactor are for the generation of electrical power and for the production of plutonium for use in nuclear weapons. Electricity was generated for the first time by a nuclear reactor on December 20, 1951 at Idaho, USA. On June 27, 1954, the world's first nuclear power plant that generated electricity for commercial use was officially connected to the Soviet power grid at Obninsk, USSR. What is nuclear fission? Fission is when the nucleus of an atom splits into two or more smaller nuclei plus some by-products. THese by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy. Currently all commercial nuclear reactors are based on nuclear fission. What is nuclear fusion?

~6

Encyclopaedia of Petroleum Science and Engineering

Nuclear fusion isa process in which two nuclei join, forming a larger nucleus and releasing energy. Nuclear fusion is the energy source for the stars including the sun and hydrogen bombs are also based on the principle of fusion Controlled nuclear fusion could in princiople be used in fusion power plants to produce safer, cleaner power plants to produce safer, cleaner power, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as of yet none has produced more energy than it consumed. What is the fuel used in nuclear fission? A nuclear fussion reaction requires fissile material, one that is capable of sustaining a chain reaction of nuclear fission. The three most important fissile materials are Uranium-233, Uranium-235 and Plutonium-239. Besides, there are other fissionable naterial which can be converted to a fissile isotope, such as Uranium-238 (which generates Plutonium-239) and Thorium-232(which generates Uranium-233). At the present use rete, there are 50 years worth of low cost known uranium reserves remaining. Since the cost of fuel is a minor cost factor for fission power, more expensive, lower grade sources of uranium could be used in the future. As opposed to current light water reactors, which use Uranium-235 (0.7% of all natural uranium), fast breeder reactors use Uranium-238 (99.3% of all natural uranium). It has been estimated that there is anywhere from 10,000 to five billion years worth ofUranium-238 for use. Another alternative would be to use Thorium as a fission fuel. Thorium is three times more abundant in the earth's crust than Uranium. This is particularly useful for India, which has 32 per cent of global thorium reserves. Why is there opposition to nuclear power form some quarters? Critics of nuclear power point out that nuclear tehnology is often dual-use, and much of the same materials and knowledge used in a civilian nuclear program can be used to develop nuclear weapons. This is what the jargon "nuclear proliferation" refers to. While enriched uranium used in most nuclear reactors is not concentrated enough to build a bomb (nuclear reactors usually run on 4 per cent enriched uranium, while a bomb requires an estimated 90% enrichment), the technology used to enrich uranium could be used to make the uranium could be used to make the uranium needed to build a bomb. To prevent this, safeguards on nuclear technology were published in the Nuclear Non-Prolifer ation Treaty (NPT) of 1968 and monitored by

Appendix-F

247

the International Atomic Engergy Agency (!AEA). Nations signing the treaty are required to report to the IAEA what nuclear materials they hold. They agree to accept visits by !AEA auditors to verify their material reports and physically inspect the nuclear materials in exchange for access to nuclear materials and equipment in the global market. India is not a signatory to the Nuclear Non-Proliferation Treaty. India's oil import bill surges to $40 b: India's oil import bill swelled to $40billion in first llmonths of 2005-06 fiscal on the back of high global oil prices. Inida imported 90.34 million tonnes of crude oil for $34.95 billion (Rs. 154,717 crore) and 10.63 million tonnes of petroleum products for $5.29 billion (Rs. 22.821 crore) in April-February 2005-06, according to latest data with ministry of petroleum and naturla gas. In full year 200405, India had spent $25.98 billion (Rs. 117,006 crore) on import of95.86 million tonnes of crude oil and $3.28 billion (Rs. 14.930 crore) on import of! 0.47 million tonnes of petroleum products. The country exported 19.51 million tonnes of petroleum products for $9.55 billion (Rs 42,098 crore) in April-February 2005-06.

BIG On.. DISCOVERY IS REPORTED DEEP IN GULF Clifford Krauss An announcement by three oil companies of a succesful production test in the Gulf of Mexico, potentially the largest American oil find in a generation, was seen by experts as ushering in a new era in ultra-deep water offishore drilling.

Chevron, Devon Energy and Statoil ASA, the Norwegian oil giant, reported that they had found 3 billion to 15billion barrels in several fields 175miles offshore, 30,000 feet below Gulf's surface, among formations of rock and salt thick. While it is too early to know exactly how big the fields are, the oil companies expressed hope that they had the potential of being ~ven potential of being even larger than those at Prudhone Bay, Alaska. US has reserves of 29 billion barrels, meaning tht at the high end of the estimates, the discovery could increase reserves by 50%. It comes as the output of oil and gas in shallower wells in the Gulf of Mexico, with about one-quarter of American oil reserves, is ebbing and environmental resistance to offshore drilling in areas closer to coastlines remains strong.

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This is frontier stuff, said Daniel Yergin, president of Cambridge Energy Research Associates, nothing that the the discovery is at levels deeper than deep-sea fields in the North Sea and off North Africa. Success at these depths in the Gulf of Mexico would facilltate ultradeep water exploration elsewhere in the world because it will have prover the technology and capabilities. It will take over a year of drilling to confmn the value of the find, and the depth of the water will make extraction extremely expensive will make extraction only if oul prices remain at least $40 a barrel, according to oil industry analysts. The analysts cautioned that there was little likelihood the report would give drivers much reliedf at the pump because full production might not come on line for five years or more. By itself, it also appears that the discovery could make little more than a dent in the country's energy dependence. And given that US uses 20.5 million barrels of crude oil a day, the new areas at most hold supplies that would quench the nation's oil thirst for two years. In addition, there is a shortage of rigs able to drill in deep water, another constraint in exploiting the find quickly. But Chevron and the other companies involved expressed excitement The discovery Nichols, chairman of Devon Engergy, could not have happened in a better place. According to Chevron, the about two years of drilling by the three companies, using seismic and drilling equipment at record depths and pressure. Our strong strategic position in deep water Gulf of Mexico, said George L Kirkland, a Cheveron executive VP, will continue to be a platfonn for future growth for years to come. Shell, BP, Exxon Mobil, Anadarko Petroleum and Petrsleo Brasileiro have leasses on comparable waters in the Gulf, and the successful test is likely to set off a wave of drilling in deep water as well as the building of platforms and the laying of pipelines. It's going to attract deep-pocket and patient investment to work these fields, said Wayne Andrews, oil analyst. These are very expensive wells to drill, and the production facilities also required to produce the reserves are also going to be expensive operations.

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ONGC'S SECOND SUDAN FIELD STARTS PUMPING On. New Delhi: State -owned Oil and Natural Gas Corporation's (ONGC) second oiltield in Sudan has began crude oil production with output expected to reach 50,000 barrels per day year end. Sudan's Block 5A, where ONGC's overseasann ONGC Videsh Ltd (OVL) has 24.12% stake, began oil production on June 26, a company official said. ''Currently, Thar Jath field in Block 5A is producing 38,000 barrels per day (bdp) but we hope to stabilize production to 40,000 bpd very soon. Mala field in the same block would come to production in 2007 with an output of 10,000 bpd, which would rise to 20,000 bpd by 2008. Our share from the entire 5A output would be 15,000 bpd," be said OVL currently has 25% stake in Sudan's Greater Nile Oil Project (Block 1, 2 and 4), which produces 280,000 barrels per day. The Thar Jath field is located about 900 km south of Khartoum within the prolific Muglad Basin. Crude oil from Thar Jath will be exported from Thar Jath through a 172-km export pipeline, which links the Central Processing Facilities (CPF) to the existing GNOP pump station at Heglig. The frrst produced oil from Block 5A will reach Port Sudan in August for its frrst commercial lifting official said.

'On.IRJNGERIS TIlREATENlNGANTARCITCA' Sydney: Declining oil reserves and rising prices could see nations overturning a ban on exploration in the last untouched frontier Antarctica, an oil expert said at scientific meet. Ali Bakhtiari, a former senior adiviser for the National Iranian Oil Company, said at a meeting of international Antarctic specialists in Hobart, Tansmania, "When you have the enormous price increase that I can foresee governments and companies will want to fmd oil anywhere. There is now only one frontier province left and that is Antarctica. OllJSWELL Why the world is not about to run out of the preious fuel How will you pay to run your car? How will you get the children to school? Will cheap flights stay cheap? With oil hovering, at $70 a barrel, almost everyone's been worrying about the black gold running dry. Every

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few weeks, it seems, "Out of Gas," "The Empty Tank" are joined by yet more gloomy titles. But don't hit the panic but-ton just yet; there's good news as well. An article titled 'Steady As She Goes'in The Econ-omist points out that despite today's obsession with the idea of ''peak oil", what really matters to the world economy is not when conventional oil production peaks, but whether we have enough affordable and convenient fuel from any source to power our current fleet of cars, busesand aeroplanes. With that in mind, the global oil industry is on the verge of a dramatic transformation from a risky exploration business into a technology-intensive manufacturing business.And the product that big oil companies will soon be manufacturing us "greener fossil fuels." The race is on to manufacture such fuels for blending into petrol and diesel today, thus extending the useful life of the world's remaining oil reserves. This shift in emphasis may even result in a breakthrough that replaces oil altogether. To see how that might happen, consider this question: is the world really running out of oil? Kenneth Deffeyes, a geologist at Princeton, thought that the peak would arrive late las t year. It did not. In fact, oil production capacity might actually grow sharply over the next few years. Cambridge Energy Research Associates (CERA), an energy consultancy, says the world's oilproduction capacity will increase by as much as 15 m barrels per day (bpd) between 2005 and 2010 - equivalent to almost 18% oftoday's output and the biggest surge in history. Peak-oil advocates remain unconvinced. A sign of depletion, they argue, is that big Western oil firms are finding it increasingly difficult to replace the oil they produce, let alone build their reserves. But a look at the global picture gives a different perspective. The United States Geological Survey (USGS) has concluded that the world has around 3 trillion barrels of recoverable conventional oil in the ground. Of that, only one-third has been prouduced. That, argued the USGS, puts the global peak beyond 2025. And if ''unconventional'' hydrocarbons such as tar sands and shale oil (which can be converted with greater effort to petrol) are included, the resource base grows dramatically and the peak recedes much further into the future. It is also true that oilment will probably discover no more "supergiant" fields like Saudi Arabia's Ghawar (ehich alone produces 5m bpd) but that does not mean an end to discovery altogether. Using ever fancier technologies the oil business is drilling in deeper waters, more difficult

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terrain and even in the Arctic (which, as global warming melts the polar ice cap, will perversely become the next great prize in oil). Now for the second big question: whenever the production peak comes, will it inevitably prompt a global economic crisis? Kenneth Ro goff, a Harvard professor and the fonner chief economic of the IMP, thinks concerns about peak oil are greatly overblown: "As oil production slows, prices will rise up and down the futures curve, stimulating new technology and conservation. We might be running low on $20 oil, but for $60 have adequate oil suplies for decades to come." So don't give up your car just yet-think alternative.

China opens oil blocks to foreign investros: China National Petroleum Corporation (CNPC), the country's largest oil and gas procucer, has for the first time invited foreign companies to bid on exploration rights in nine designated blocks in the oil and gas:-rich Tarim basin. The nine blocks cover an area of 110,000 sq kilometres and the tender will require foreign firms to cooperate with Chinese counterpartS. Under a contract, foreign frrms involved in resource exploration in China will pay entire development cost. Russia, China step forward for landmark energy cooperation Beijing: Russian president Vladimir Putin and Chinese president Hu Jintao agreed on Tuesday to deepen energy cooperation, as Russian gas giant Gazprom said it would look to meet some of China's frustrated energy needs. Putin, who has made energy security the theme of Russia's current presidency of the G8 group of industrialised nations, said in November that diversifying energy export routes was a top priority, with supplies to Asia of paramount importance. But China's top energy planner, Zhang Guobao, has called the slow movement in Russian plans for new pipelines 'regrettable' and criticised the Russian government's unwillingness to support Chinese efforts to invest in Russia's energy sector. Putin arrived in Beijing on Tuesday accompanied by Russia's top energy chiefs, including the chief executive of Gazprom, Alexei Miller. China's Xinhua news agency said the two sides signed three deals on oil and natural gas cooperation. Miller said two pipelines from Russia would eventually supply China widl6Q-80 billion cubic metres of gas a year.

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He said Gazprom had signed a memorandum on gas supplies with CNPC, the Chinese oil and gas conglomerate. Details of the gas deal were not available, but it may go some way to mollifying Chinese leaders, who have ignalled their frustration Russian oil andgas imports. But the much promoted meeting between Putin and Hu produced only broad agreement about energy cooperation. "We support investment in both countries to develop oil and gas resources and the energy potential of Russia and China," the two sides said in a joint statement issued after they met. "The energy organisations and companies of both countries will continue work to advance oil and gas pipeline projects." China has been eyeing Russia's vast oil and gas reserves as its dependence on imports has ballooned in recent years, but it has been unable to pin its neighbour down. Also accompanying Putin were energy minister Viktor Khristenko and resources minister Yuri Trutnev, as well as Sergei Bogdanchikov, the head of state oil firm Rosneft. China wants Russia to firm up a possible 30 million tonne per year oil pipeline deal and natural gas supplies to feed its economy.

1NTERNA1l0NAL Kazakhstan-China oil pipeline s~ operation: China has started receiving crude oil from neighbouring ~tan through a 962 kilometre long cross-border pipeline, marking the beginning of the commercial operation for China's fIrst direct oil import pipeline, the state media reported on Wednesday Crude oil from Kazakhstan poured into a petroleum tank in Alataw pass, northwest China's Xinjiang Uygur Autonomous region through the cross-border pipeline. Currently, the oil flux is only around 120 cubic metres per hour due to the valve failure in a Kazakhstan, ZHU Minjie, a customs offcer at the Alataw pass said. It will take 15 days to fIll up the 50,000 -cubicmeter oil tank before the oil is piped to Dushanzi in Karamay where the country's largest oil refmery plant will become operational in 2008 to produce 5.5 millon tons of refined oil a year, ZHU was quoted as saying.

ONGC, SHELL SIGN DEAL ON JOINT Oll..EXPLORA110N New Delhi: Oil and Natural Gas Corporation (ONGC), India's most valuable company, and Shell, world's third largest oil frrm, on Thursday signed a landmark agreement that threw open the possibility of the open

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the possibility of the Indian major getting a stake in the Dutch gaints overseas projects including Sakhalin-II in Russia. - The Memorandum of Understanding (MoU) between ONGC and Shell Exploration Co BV (a subsidiary of Royal Dutch/Shell) provides for the two companies looking at joint exploration andproduction of oil and gs in India, possibilities of cooperation in international projects, joint development of Coal Bed Methane (CBM) gas, export of petroleum products from MRPL (a ONGC subsidiary), manufacture of hi-grade bitumen and possible cooperation in petrochemicals.

It also opened possibilities of Shell's participation in ONGC-MRPL's in the new 15 million tonnes refmery and petrichemical complex being planned adjacent to Mangalore Refinery and Petrochemicals Ltd, ONGC chairman and managing director Subir Raba told reporters after singning the MoU. The landmark pact, he said, also furthered chances ofONGC picking up a stake in Shell's 660-million dollar Hazira LNG import terminal in Gujarat. The two firms may also look at setting up a C2 extraction plant at Hazira. Raha said ONGC, which has a 20 per cent stake in the Sakhalin-l looking at possibilities of stake in Shelloperated project in Russia, Sakhalin-II and tieing up gas from the two projects to make its shipment viable.

was

"Shell is not present in upstream business in India. This MoU will fulfil the gap," Vikaram Singh Mehta, Chairman, Shell Companies in India, said on the ocassion. TIlE END OF On. VisualiseThis: Five years from now, Iran becomes a renegade state and is preparing for a nuclear standoff. It shuts off oil supplies to the rest of the world. Nigeria collapeses into civil war. Oil production plwmnets by over two million barrels per day, and the price leaps to $150. India is now a hungry tiger, consuming oil insatiabley to sustain the expectations of its impatient population and at this cost of energy, the economy threatens to stall... The opposition is making a lot of noise and people take to the streets. Sounds like a scenario from a Tom Clancy novel? But even if there. is a miniscule chan-ce of some part of this scenario coming true, there would be no need for further argument onthe need to develop alternative technologies. I do not sucribe to the doomsday scenario. A

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great deal of efort is going into oil exploration and into new technologies. I believe in 10 years, India will never be totally self-sufficient. In any evnt, oil is a growing source of vulnerability. Its price volatility affects growth, its emissions affect the environment, and dependence on it constrains political options. If energy security is to be our cornerstone of growth, it's a 'nobrainer' that we must look for avenues for lessening our dependence on oil. It's gratifying how much scope there is for reducing oil dependence by developing alternative technologies. Until 1975, the US was profligate, almost wantion in its use of oil. It was literally cheaper than bottled water, and used accordingly. But the oil shock forced them to look for alternatives-energy-efficient vehicles. As fuel economy rose, oil consumption fell by 17% in spite of a GDP increase of 27%. Even today, inspite of a slackening of focus, America wrings twice as much work from each barrel of oil as it did in 1975. Thanks to society's capacity for self-adjustemnt, relatively small changes in technology can result in huge changes inconsumption patterns. In fact, the threat of alternate technologies puts pressure on those with a vested interest in conventional hydrocarbons to become more efficient. There is an ongoing, countrapuntal sawaal-jawaab between conventional and alternative energies with alternative technology thrwing down a gauntlet that conventional energy is compelled to pick up. In India, we are energy-deficient on all fronts. Our strategy must be to align our alternate energy quest with our physical resources. So the pursuit of CNG that we possess in abundance is an obvious direction for us. We must also look for alternative technologies that have a multiplier effect on other sectors. Bio-diesel extracted from agricultural sources can create a new income opportunity for farmers. Our mountains of garbage could tum out to be a bonanza, with biomass-to-liquid technology. In our company, we are tacking the development of alternatives with missionary zeal. Finally, there is the irresistible challenge of innovation. India has so much need for alternative energy that it opens up the field for a whole new generation of innovators and entrepreneurs. It's a challenge that is yuniquely congruent with our competitive advantage. Our competitive advantage does not lie in low costs. It lies in our unique capacity to come up with solutions to life's daily problems at the loweast cost per unit of innovation. India is destined to become an innovation leader and

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the quest for alternate energy is challenging us to live up to that destiny, consumption patterns and much greater energy security. The author is vice-chainnan and MD, Mahindra & Mahindra

INDIA TO INVEST SLBDllON IN LVORY COAST

Eyes Oil Exploration, Coal Abian: India plans to invest $1 billion in oil exploration and mining in war-divided Ivory Coast in the open new factories as it strengthens trade ties, its top envoy in the country said on Monday. With an increasing need for raw materials and energy as its economy expands, India is following China by stepping up its presence in resoucrce-rich sub-Saharan West Africa, where offshore oil output is steadily rising. Indian ambassador to Ivory Coast Amarendra Khatua took more than 100 Ivorian entrepreneurs and government ministers ministers to an IndoIvorian trade comission meeting in Delhi this month where they drafted dozens of deals to be signed later this year. Chief among them is India's plan to invest in offshore oil exploration on the Ivorian stretch of the prized Gulf of Guinea coastline which isbelieved to hold large, untapped reserves. Ivorian oil production is currently more than 60,000 bpd. "Indian investment in the mining and hydrocarbon sector in this country will be $1 billion over the next five years, " Khatua said. Oil and Natural Gas Commission had invested $12 million to explore one offshore block it was now drilling. Indians were also seeking to mine gold, diamonds, manganese, bauxite, iron ore and chrome, either by starting new mines or by forming partnerships to exploit existing ones, Khatua said. "India and Chinabecause of their population demands, economic growth and increasing prosperity - need energy security, plus they have money to invest," he said. India's plans come at a time when some investors in the former French colony are shying away from new business ventures due to the political uncertainty and insecurity thatare the legacy of a 2002-03 civil

war. A struggling peace process has failed to reunite the country, split between a rebel-held north and government south, while delayed

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elections meant to draw a line under the conflict look unlikely to take place as planned by the end of October. But Khatua was optimistic, pointing out that the mining projects would take time to set up and that the crisis could be resolved by the time these were operating. "India has identified this market and it believes this crisis will be resolved soon and that it will then be able to penetrate deeper into the market," he said. India has granted Ivory Coast a $26.8 million credit line which it has used to buy 400 Tata buses and fann machinery. A second credit line for agriculture, agro-processing, fishing and information technology would follow soon, Khatua said On top of plants to open five plants to process Ivorian cashew nuts locally, Indian pharmaceutical firms are to build two factories producing more affordable medicines in the world's poorest continent "There will be Indian drugs which are world class but cheap ... When (Indianmade drugs) come via western countries it becomes costly. This is about direct access to the market," Khatua said He put Indo-Ivorian annual trade at $360 million and said it rose by a third in 2005. India exports rice, transport, engineering equipment and textiles to Ivory Coast. Ford offers clean-burning hydrogen vehicle: Ford Motor became the first automaker to begin production of a commercially-viable hydrogen engine which emits little but clean water vapor from the tauilpipe. The hydrogen powered intemal combustion engines are destined for shuttle busses and will be ready for delivery later this year, Ford spokesman Nick Twork said. "This engine represents a milestone in Ford's research efforts in hydrogen technology," said Gerhard Schmidt, Ford V-P for research, advanced engineering.

SAFE RETURN In 2005-06, demand for electricity in India exceeded supply by an total requirements The Centre estimatesthat installed generation capacity needs to reach 200,000 mw by 2012 compared to 124, 287 mw as on March 31,2006. The country added 1,253 mw of wind power in 2005 taking cumulative installation to 4, 253 MW.

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ONGC: RICH GAS RESERVES IN MYANMAR BLOCK VERIF1ED New Delhi: ONGC Videsh Ltd (OVL) announced on Wednesday that Myanmar offshore Shwe field Block A-I, in which it holds 20% interest, was verified to have between 2.88 trillion cubic feet (TCF) and 3.56 TCF of gas in place. The gas volume has been assessed by Houston-based consulting

finn Ryder Scott company, OVL said in a statement Apart from OVL, GAIL holds 10% participating interest in this block operated by Daewoo International Corp with 60% stake, while Korea Gas Corp holds the

remaining 10010. The consortium has already started the 2005-06 drilling campaign in Shwe Nilar-I well this month. The Shwe Phyu field will be assessed by three appraisal wells during the current drilling campaign, acording to OVL. The consortium will start an exploration drilling programme in BlockA-3 with the first exploratory well Mya-l in the beginning of the new year. OVL and GAIL together hold 30% stake in this block too, in the same pattern asin Block A-I. "During the 2005-06 drilling campaign, the consortium will drill 6-7 wells in Blocks A-I and A-3, comprising three appraisal wells at the Shwe Phyu field, two or three more exploratory wells to test the Shwe Nilar and Ngwe North prospects in Block A-I and one exploratory well to test the Mya prospect in BlockA-3," OVL statement said. According to ONGC group chairman Subir Raha, "Work on development of the fields has already started and efforts are being made to bring the gas to India." REUANCE SETS SIGHTS ON BRAZIL FOR ElHANOL Arindam Sen Gupta Brasilia: Reliance is looking to buy thousands of hectares of land in Brazil on which it will grow sugar cane to produce ethanol- a biofuel that's catching the world's imagination. Land is cheap here - $1000 (or Rs 45,000) per hectare, abundant water just a dig away, clear ownership titles and a government eager to get it all going. Like the Reliance rep, its president RC Sharma, ther~'s a gaggle of Indian businessmen looking at this sprawling country of 85 lakh sq km like a group of school children looking at a candy ~tore. Distance no longer deters them; what matters is opportunity. Pharma, mining, oil exploration and railway firms feel that possibilities are boundless here.

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Among them, representatives of the Tatas, AV Reddy's and scores of others. Said Habil Khorakiwala ofWockhardt: ''This is my fJISt visit to Brazil and I've decided to stay back for a week. I am looking not only at Brazil but also Argentina, Paraguay, Uruguay and Venezuela. The market is huge and costs a fraction of other overseas markets." Reliance's Sharma isn't staying back. There's clarity inthe company's vision. "The big pressure in coming decade will be on land," he said. "Apart from producing ethanol, we are looking at Brazil as world's food basket of future. As the world's hungry get a taste of food, demand will spiral Only vast and untapped land masses will be able to feed them" But why are the sturdy Sikhs of Punjab selling their land to migrate to Canada and not look at the chep fertile tracts of South America? "Because they don't know about it," said Saroj Poddar, Ficc president. ''You'll see the interest once you guys write about it. No where in the world will you get fertile land that's as cheap. " Will Reliance take its ethanol back to India? "Only if there's demand there," said Sharma. "We will sell wherver there is demand" GSPLPlPELINE WIlL TAKE REUANCE GAS TO REFINERY Ahmedabad:Gujarat State Petronet (GSPL), a subsidiary of Gujarat State Petroleum Corporation (GSPC), has signed an agreement with Reliance Industries for transportation of gas from Bharuch to Jamnagar for a period of 15 years starting from first quarter of fmancial year 2008-09. GSPL, which has already laid down its pipeline up to Rajkot, will lay down 225 km long pipeline linking Rajkot and refinery project of Reliance Industries, which is located near Jamnagar. Though the GSPC's official refused to divulge information, sources said that the pipeline project would be undertaken at an esimated cost ofRs 500 crore. According to sources, Reliance group is setting up 1,385 km long gas pipeline network between Kankinada and Bharuch for utillisation of its huge gas find at KG basin. The Gas Transportation and Infrastructure, a subsidiary of RIL, is setting up a wide network of pipelines under its east-west pipeline project with an estimated cost of over Rs 20,000 crore. the pipeline will have capacity to transport l00mm scmd gas and the pipeline will traverse though the state of Andhra Pradesh, Karnataka, Maharashtra and Gujarat, right up to Bharuch. Sources said that from

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Bharuch, The pipeline will be unlineked with GSPL network in order to provide gas supply to the Reliance's refmery at Jamnagar. Besides Jamnagar, RIL needs gas for its own projects at Nagothance, Dahej, Patel ganga and Vadodara. The pipeline is expected to be completed by December 2007. WHEN EARmftJPPED OVER Long Ago, The Earth Changed Side To Keep Balance. And It Could HAPPENAGAIN Earth might have spun on its side to keep its balance in the distant past, and could do so again, scientists reported today. Alaska was suddenly at the equator, the thinking goes. Scientists already know that the North Pole wanders over time. But a theory known as true polar wander suggest that if a very heavy object, like an oversized volcano forms far from the equator, the force of the planet's rotation would pull the object away from the axis the Earth spins around. Should a mass such as the very heavy volcano become unbalanced, Earth would tilt and rotate itself until the extra weight moves somewhere near the equator. Analysed samples of ancient sediments found in the Norwegian archipelago of Svalbard show that such an event may have indeed happened in the past. "The sediments we have recovered from Norway offer the first good evidence that a true polar wander event happened about 800 million years ago," said Adam Maloof, professor of geosciences at Princeton University. CIHEEVIDENCE When rock particles sink to the ocean floor, small magnetic grains within the particle align with the magnetic lines of the Earth. These rocks then become record of the Earth's magnetic field at the time that they were pointing. If a rock has been spun by an unusal geological event, the orientation of its magnetic field will be out of the ordinary. "We found just such anomalies in the sediments," Maloof said. "We made effort to find another reason, such as a rapid rotation of the crustal plate the islands rest upon, but none of them makes as much sense a a true polar wander event when taken in the context of geohemical and sea level data from the same rocks."

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The polar wander could also be responsible for the unusual changes that happened in ocean chemistry 800 million years ago. "Scientists have found no evidence for an ice age occurring 800 million years ago, and the change in the ocean at this juncture remains one of the great mysteries in the an cient history of our planet," Maloof said. "But if all the continents were suddenly flipped around and ·their rivers began carrying water and nutrients into the tropics instead of the Arctic, it could produce the mysterious geochemical changes science has beentrying to explain." Future work can test the true polar wander hypothesis as this type of event would after every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis.

SAKHALIN OIL FORINDIA BY YEAR-END New Delhi: India will ship its share of crude oil from Russian oilfield Sakhalin-I in October-December this year, minister of state for petroleum and natural gas Dinsha Patel said on Thursday. "ONGC Videsh Ltd, a wholly owned subsidiary of state-run Oil and Natural Gas Corp,is planning to bring the first two cargoes of crude oil each having the capacity of approx 700,000 barrels from Sakhalin-I project in Russia into India in October and December, 2006," he told Lok Sabha in a written reply. ONGC Videsh Ltd has 20% stake in the Exxon Mobiloperated Sakhalin-I project in far east Russia. ONGC Videsh Ltd would auction the crude to Indian refmers. "The details of infrastructure - jetties, SBM and pipelines available with ONGC at lawahardweep and offshore to facility auctioning of crude by ONGC are being assessed." Patel said OVL shipped 256,000 tonnes of crude iul from its Sudan property to india in 2005-06. OVL, which has 25% stake in Greater Nile Oil Project in Sudan, had shipped 333,000 tonnes of crude from Sudan to India in 2004-05. It brought 818,000 tonnes of Sudanese crude in 2003-04. The company is entitled to one-fourth of oil output from GNOP but has shipped only a small quantity of its wntitlement. It sells the remaining of its share from GNOP in international market. Patel said ONGC and its subsidiary MRRL is examining the feasibility of setting up an export oriented refmery at Kakinada in Andhra Pradesh.

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261 NANO 1ECHTO BE NEW IHT IN INDIA

Amrita Nair-GhaswaUa Mumbai: Imagine a shirt which protects you from cold during the chill and cools you when its hot. In summers, it changes to a lighter colour. Come winter and it takes on a darker hue. Science fiction? Not really. That's nanote\.. ')logy at work. Shirts like these will be created by putting something called nano-coatinginto it. Stain and creasefree clothing, .dready available in the market, is the outcome of this technology. According of this technology. According to a report, nanotechnology is set to play a major part in drug delivery systems, especially for tumour therapies. The report cites nanoparticles as being suited to curing specific cindition like cancer, as their sizes are comparable with tissue cells. "When combined with available binding agents, these nanoparticles can turn into targeted drug delivery systems, " said Hrisikesh Bidwe from Forst & Sullivan. The emergence of nanotechnology is likely to affect just about every route of administration from oral to injectable. The payoff is likely to be lower drug toxicity, reduced cost of treatments, improved bio availability and an extension of the economic life of proprietary drugs. OZONELAYERSLOWLYHEALINGITSELF Drop In chlorine Levels In Atmosphere Helping: Researchers London: The ozone layer is showing signs of recovering, thanks to a drop in ozone-depleting chemicals, but it is unlikely to stabilise at pre1980 levels, researchers said on Wednesday. Depletion ofthe earth's protective ozone layer is caused by the chemical action of chlorine and bromine released by man-made chlorofluorocarbons (CFCs), which are used in aerosol sprays and cooling equipment. Ozone-depleting chemicals were banned by the 1987 Montreal Protocol which has now been ratified by 180 nations. "We now have some confidence that the ozone layer isresponding to the decreases in chlorine levels in the atmosphere due to the leveling off and decrease of CFCs," said Betsy Weatherhead, of the University of Colorado in Boulder. ''Not only is the ozone layer getting better, we feel it is due to the Montreal Protocol," she added in an interview.

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The depletion of the ozone layer, which absorbs most of the harmful effects of the sun's ultraviolet radiation, increases the risk of skin cancer and cataracts in humans and may harm crop yields and sea life. Despite the signs of recovery, Weatherhead, who reported the findings in the journal Nature, said people should still protect themselves from harmful ultraviolet rays. Weatherhead and Signe Bech Anderson of the Danish Meteorological Institute in Copenhagen analyzen data from satellites and ground stations and information from 14 modeling studies. They found that ozone levels have stabilised or increased slightly in the past 10 years. But full recovery is still decades away. The researchers said depletion has been most to a lesser extent at mid-latitudes covering bands of North America, South America and Europe. Shifting temperatures, greenhouse gases, nitrous oxide (N20) and atmospheric dynamics, which can infuene ozone levels, are going to change in the future, they added. "Therefore we really don't think ozone is going to stabilise back to its pre-ozone-depleting-substance levels," Weatherhead said. Volcanic activity on Earth also has an uimpact. The 1993 Mount Pinatubo eruption in the Philippines caused ozone levels to bakslide for several years, according to the researchers. PACIFIC CLlMATESYSIEMVICI1M OFGLOBAL WARMING Washington: Climate scientists ientified a likely new victim of global warming on Wednesday: The vast looping system of air currents that fuels Pacific trade winds and climate from S(>uth America to Indonesia. This could mean more El Nino-like weather patterns in the US, more rain in the western Paific and less nourishment for marine life along the Equator and off the South American coast. Known as the Walker Circulation, this system of currents functions as a huge belt stetching across the tropical Pacific, with dry air moving eastward from Asia to South America and moist air flowing westward along the ocean's surface, pushing theprevailing trade winds. When the moist air gets to Asia, it triggers rains in Indonesia. Then it dries out, rises and starts the cycle again, heading east. BACIERIATIlATHELPCREATE GOLD NUGGETS

Sydney: Researchers in Australia have uncovered evidence that a

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tiny microbe may have the Midas touch of Greek legend, capable of turning dust to gold. Findings reported in the July 14 issue of Science suggest a bacterial known as Ralstonia metallidurans may playa key role in creating gold nuggets and grains. Scientists led by German-bom researcher Frank Reith collected gold grains from two Australian mines more than 3,000 kilometeres apart, and discovered that 80% of the brains had the bacteria living on them. "What we found out suggests that bacteria can accumulate this gold and contribute at least to the or mation of this gold," Reith said in an interview on Friday. Reith said Ralstonia metallidurans act as microscopic soil scrubbers soaking up heavy metals in their dissolved form and converting them into less toxic solid forms. "Heavy metals are toxic, not only to us but also to microorganisms, in elevated concentrations," he said "It appears to be that the organism can detoxify its immediate environment of this way gain a metabolic advantage," Reith said his [mdings provide the strongest evidence yet that bacteria could playa key role in creating solid gold, although the exact mechanism was not yet known. "What we just wanted to show is that microorganisms are capale of contributing to the formation of gold nuggets and before that was always doubted," Reith said. THE DEADSEAIS REAlLYDYING Ean Gedi (Israel): It has survived since Biblical times, but now the Dead Sea may really be dying. The bare, sun-baked landscape around the Dead Sea, the lowest point on Earth, has always been fed by the fresh water of the Jordan River. But now the Jordan's waters are systematically diverted for agricultural and hydroelectric projects. An evaporation basin to obtain Dead Sea minerals has whole area, which is bordered by Israel, Jordan and the West Bank, is headed for ecological disaster. ''The ecological situation is catastrophic," says Gideon Bromberg of Friends of the Earth Isreal. In 50 years, the Dead Sea has lost a third of its surface area and 98% of the fresh water it previously had from the Jordan. Every year new cracks appear in the seabed.

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