3D Seismic Survey Design Second Edition
Gijs J. O. Vermeer Geophysical References Series No. 12 Craig J. Beasley, volume editor Lianjie Huang, managing editor
Tulsa, Oklahoma, USA
Cover figure courtesy of Mustafa Al-Ali. This figure appears in the book as Figure 10.17 and compares time slices of the same diffraction-flattened reflection sorted in cross-spreads and in offset-vector tile gathers.
ISBN 978-0-931830-47-1 (Series) ISBN 978-1-56080-303-4 (Volume) © 2012 by the Society of Exploration Geophysicists All rights reserved. This book or parts hereof may not be reproduced in any form without permission in writing from the publisher. First edition published 2002 Second edition published 2012 Printed in the United States of America
Library of Congress Cataloging-in-Publication Data Vermeer, Gijs J. O. [3-D seismic survey design] 3D seismic survey design / Gijs J.O. Vermeer ; Craig J. Beasley, volume editor. -- Second edition. pages cm. -- (Geophysical references series ; no. 12) ISBN 978-1-56080-303-4 (volume : alk. paper) -- ISBN 978-0-931830-47-1 (series : alk. paper) 1. Seismic prospecting. I. Beasley, Craig J. II. Title. TN269.8.C69 2012 6229.1592--dc23 2012040211
To Tini
Contents About the Author xi Foreword to the Second Edition xiii Foreword to the First Edition xv Acknowledgments to the Second Edition xvii Acknowledgments to the First Edition xix Introduction xxi
Chapter 1 2D symmetric sampling 1
1.1 Introduction 1 1.2 Shot/receiver and midpoint/offset coordinate systems in two dimensions 1 1.3 Symmetric sampling 4 1.4 Symmetric sampling versus asymmetric sampling 9 1.5 The total stack response 11 1.6 Concluding remarks 13
Chapter 2 3D acquisition geometries, their properties, and their sampling 15 2.1 2.2 2.3 2.4 2.5
Introduction 15 Classes of 3D geometries 16 2.2.1 Examples of various geometries 16 The continuous wavefield 19 2.3.1 The shot/receiver and midpoint/offset coordinate systems 19 2.3.2 3D subsets of the 5D wavefield 19 2.3.3 3D subsets and acquisition geometry 23 2.3.4 Fold in 3D seismic data 28 Sampling the continuous wavefield 31 2.4.1 Binning 31 2.4.2 Sampling and the unit cell 32 2.4.3 Aspect ratios 33 2.4.4 Spatial continuity 34 2.4.5 3D symmetric sampling 34 2.4.6 Line geometries 35 2.4.7 Areal geometry 40 Pseudo-COV gathers 41 2.5.1 Introduction 41 2.5.2 Offset-vector tiles 41 2.5.3 Construction of pseudo-COV gathers 42 v
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2.6 2.7
2.5.4 Example of OVTs in 64-fold regular geometry 44 2.5.5 Reciprocal OVTs 45 2.5.6 A measure of spatial discontinuity 46 2.5.7 A plethora of OVT gathers 47 Application to prestack processing 47 2.6.1 Introduction 47 2.6.2 Noise removal 48 2.6.3 Interpolation and regularization 50 2.6.4 Muting 51 2.6.5 First-break picking 51 2.6.6 Nearest-neighbor correlations 52 2.6.7 Residual statics 52 2.6.8 Velocity analysis and DMO 52 2.6.9 Multiple suppression 54 2.6.10 Amplitude variation with offset 55 2.6.11 Amplitude variation with azimuth 57 Conclusions 57
Chapter 3 Noise suppression 59 3.1 3.2 3.3 3.4
Introduction 59 Properties of low-velocity noise 59 3.2.1 Direct waves 59 3.2.2 Scattered waves 59 Shot and receiver arrays in 3D data acquisition 65 3.3.1 Introduction 65 3.3.2 Effect of linear array on noise 65 3.3.3 Effect of linear array on signal 68 3.3.4 Direct wave noise suppression 70 3.3.5 Scattered-wave noise suppression 71 3.3.6 Analysis of various array combinations 72 3.3.7 Discussion 76 Stack responses 77 3.4.1 Introduction 77 3.4.2 The 2D stack response 77 3.4.3 Multiple suppression by stacking 77 3.4.4 3D stack responses 80 3.4.5 Discussion 84
Chapter 4 Guidelines for design of 3D geometry on land 87 4.1
Introduction 87 4.1.1 Spatial continuity 87 4.1.2 Regular geometry 88 4.1.3 Geophysical requirements/constraints 88 4.1.4 General recipe for 3D survey design 88
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4.2 4.3 4.4 4.5 4.6 4.7 4.8
Preparations 89 4.2.1 Objective of survey 89 4.2.2 Know your problem 89 The choice of geometry 90 4.3.1 Parallel geometry versus orthogonal geometry 90 4.3.2 Zigzag geometry versus orthogonal geometry 91 4.3.3 Slanted geometry versus orthogonal geometry 92 4.3.4 Comparison of sampled minimal data sets of crossed-array geometries 92 4.3.5 Areal geometry 92 4.3.6 Hybrid geometry 94 4.3.7 Target-oriented geometries 95 Selecting the main geometry parameters 96 4.4.1 Establish representative velocity function 96 4.4.2 Identify main targets 96 4.4.3 Resolution requirements and maximum frequency 96 4.4.4 Establish maximum dip angle in each target 99 4.4.5 Establish spatial sampling intervals 99 4.4.6 Establish maximum stretch factor and mute function 104 4.4.7 Establish required fold in each target 107 4.4.8 Determine line intervals 110 4.4.9 Determine maximum offset 111 4.4.10 Other geometry parameters 112 4.4.11 Harmonizing all requirements 112 Implementing the nominal geometry 113 4.5.1 Zippers 114 4.5.2 Templates 114 4.5.3 The survey grid and the survey area 121 4.5.4 Dealing with obstacles 123 4.5.5 Source selection 127 Testing 133 Discussion 134 4.7.1 Attribute analysis 134 4.7.2 Model-based survey design 134 4.7.3 Acquisition footprint 140 4.7.4 Random sampling 140 What to do and not to do in 3D survey design 141
Chapter 5 Marine seismic data acquisition 143 5.1 5.2 5.3
Introduction 143 5.1.1 Air-gun arrays 144 Geometry imprint 144 Streamer acquisition 146 5.3.1 Introduction 146 5.3.2 Multisource multistreamer configurations 146
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5.4 5.5 5.6 5.7
5.3.3 Crossline-offset variation 147 5.3.4 Source interval, number of sources, and fold 149 5.3.5 Inline bin size and crossline bin size 149 5.3.6 Offset-vector-tile gathers in streamer acquisition 150 5.3.7 Effects of spatial discontinuity in marine streamer acquisition 151 5.3.8 Maximum offset 160 5.3.9 Depth of sources and streamers 161 5.3.10 Over/under technique 164 5.3.11 Multicomponent streamer 164 5.3.12 Variable-depth streamer 167 5.3.13 Shooting direction 167 5.3.14 Multiazimuth acquisition 169 5.3.15 Wide-azimuth acquisition 172 5.3.16 Operational aspects 186 Stationary-receiver techniques 187 5.4.1 Introduction 187 5.4.2 Some history 188 5.4.3 Source array 189 5.4.4 Coupling issues 189 5.4.5 Noise at the sea bottom 189 5.4.6 Dual-sensor application 190 5.4.7 OBC 191 5.4.8 Ocean-bottom seismometers 195 5.4.9 Transition-zone acquisition 197 Time interval between shots and source strength 198 Time-lapse acquisition 198 5.6.1 Introduction and objective 198 5.6.2 Measures of nonrepeatability 199 5.6.3 Causes of nonrepeatability 201 5.6.4 Ways to improve repeatability 202 Discussion 204
Chapter 6 Converted waves: Properties and 3D survey design 207 6.1 6.2 6.3
Introduction 207 Properties of the PS-wavefield 208 6.2.1 Traveltime surfaces and apparent velocity 208 6.2.2 Illumination 209 6.2.3 Imaging 210 6.2.4 Resolution 212 6.2.5 NMO stretch in PS-waves 218 6.2.6 Shear-wave splitting 218 3D survey design for PS-waves 219 6.3.1 Choice of geometry 219 6.3.2 Sampling 225
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6.4 6.5
Processing considerations 225 Conclusions and recommendations 226
Chapter 7 Some lowfold data examples 229 7.1 7.2 7.3 7.4
Introduction 229 3D microspread 229 7.2.1 Introduction 229 7.2.2 Acquisition parameters of 3D microspread 229 7.2.3 Cross sections and time slices 229 7.2.4 f-k filtering results 231 7.2.5 Discussion 232 Nigeria 3D test geometry results 234 7.3.1 Introduction 234 7.3.2 Acquisition geometry 235 7.3.3 Some processing results 238 7.3.4 Interpretation results 238 7.3.5 Discussion 238 Prestack migration of lowfold data 241 7.4.1 Introduction 241 7.4.2 Migration of a single cross-spread 242 7.4.3 Lowfold prestack migration 243 7.4.4 More lowfold migration results 244 7.4.5 Discussion 246
Chapter 8 Factors affecting spatial resolution 247 8.1 8.2 8.3 8.4 8.5
Introduction 247 Spatial resolution formulas 248 8.2.1 Spatial resolution — The link with migration/inversion 248 8.2.2 Spatial resolution formulas for constant velocity 249 Spatial resolution measurements 251 8.3.1 Procedure for resolution analysis 251 8.3.2 2D resolution in the zero-offset model 252 8.3.3 2D resolution in the offset model 253 8.3.4 Asymmetric aperture 254 8.3.5 3D spatial resolution 255 8.3.6 Sampling and spatial resolution 257 8.3.7 Sampling and migration noise 257 8.3.8 Bin fractionation 258 8.3.9 Fold and spatial resolution 259 Discussion 260 Conclusions 261
Chapter 9 DMO 263
9.1 Introduction 263 9.2 DMO in arbitrary 3D acquisition geometries 264
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9.3 9.4 9.5 9.6
9.2.1 The time of a DMO-corrected event 264 9.2.2 Contributing traces in cross-spread 265 9.2.3 DMO-corrected time in the cross-spread 266 9.2.4 Extension to other geometries 266 Results with synthetic data 267 New DMO programs 268 DMO in OVT gathers 269 Conclusion 269
Chapter 10 Prestack migration 271 10.1 10.2 10.3 10.4 10.5
Introduction 271 Migration basics 271 10.2.1 The purpose of migration 271 10.2.2 Migration as a two-step process 273 Fresnel zone and zone of influence 275 10.3.1 Modeling 275 10.3.2 Migration 275 Experiments with synthetic data 278 10.4.1 Description of model experiments 278 10.4.2 Prestack migration with minimal data sets 278 10.4.3 Prestack migration with pseudo-COV gathers 280 Discussion 290
Appendix A Sampling and aliasing 293 Appendix B Aperture for sampled wavefields 297
B.1 B.2 B.3 B.4
Introduction 297 Continuous aperture function 297 Discrete aperture function 297 Conclusions 299
Appendix C Useful formulas in 3D seismic survey design 301
C.1 C.2
Introduction 301 Algorithm for average_fold 302
Appendix D Fold requirement of 3D survey 303 Appendix E Nomenclature 307 References 311 Index 333
About the Author Gijs J. O. Vermeer received an M.S. degree in applied mathematics (1965) and a Ph.D. in geophysics (2001) from the Technological University of Delft. He spent nearly 30 years with Shell, where he worked in research as well as in operations on seismic processing, seismic interpretation, and 3D seismic data acquisition techniques. In 1997, he founded 3DSymSam Geophysical Advice, focusing on 3D seismic survey design and analysis. In addition to authoring 3-D Seismic Survey Design (2002, 2012), Vermeer has written Seismic Wavefield Sampling (1990), both published by SEG. He has authored numerous papers on 3D symmetric sampling and related subjects. Vermeer’s main interests are seismic data acquisition and seismic data processing, including imaging and their interrelationships. He has presented courses on 3D seismic survey design at numerous locations throughout the geophysical world and has completed seismic survey designs for various companies. Vermeer is a member of SEG, EAGE, and CSEG.
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Foreword to the Second Edition problems, and eventually I agreed to sit on his Ph.D. committee (more for my benefit than his) and eventually became volume editor for the first, and now, the second edition. As the subject of survey design moved on and the theme of connecting imaging and data acquisition evolved, the need for a second edition became obvious. The wealth of new acquisition technologies introduced since the first edition is impressive, if not astounding. Wide azimuth, full azimuth, single sensor, simultaneous source, and broadband acquisition, to name a few, were not in practice at the time of the first edition but now are used commercially in seismic acquisition. As in any era, it is tempting to marvel at our sophisticated technology and wonder how we could do better, and yet we always push forward. Once again, this splendid second edition serves as a definitive reference, but at the same time it recognizes that seismic survey design is a moving target. As a result, every effort has been made to incorporate the latest thinking on the subject, albeit perhaps incomplete and certainly evolving. Although acknowledgments properly come from the author and can be found herein, I would like to express my gratitude to all of those who aided in producing this volume. Particular thanks are due to the reviewers who graciously helped in reviewing the volume. Finally, I would like to thank Gijs for the invitation to serve as the volume editor for both the first and, now, this second edition. It has been a great pleasure.
It now has been 10 years since 3-D Seismic Survey Design was published — enough time for me to forget the rigors of being the volume editor and thus gladly accept that duty again. Now that it is done, I couldn’t be more pleased to have been associated with this fine work. At the time of its original publication, probably no one contemplated a second edition. The original Foreword, which I highly recommend as it eloquently sets the book in historical context, saw it as a crystallization of decades of work by many researchers and practitioners of survey design, resulting in a “definitive work” on the subject. It was and still is, in the context of traditional 3D survey design. However, it was also clear that the subject of seismic survey design was undergoing a fundamental change as the last two chapters, which involved prestack imaging, began to address the connections between survey design and imaging. Indeed, the initial foray into understanding these connections was the force that brought the author and me into contact and resulted in my involvement as the volume editor. It began more than 10 years prior to the first edition as several researchers working in prestack imaging and survey design began to understand that, despite the fact that we now had 3D surveys to work with, illumination of the subsurface could be nonuniform (and sometimes absent) even when traditional measures of survey design quality indicated good coverage. Further, we realized that our processing algorithms — particularly the imaging algorithms — did not comprehend or account for such irregularities. Needless to say, this was disruptive. I was previously a wave equation/imaging researcher; but, by necessity, I was inexorably drawn into survey design considerations. And so I met Gijs Vermeer who was working on the same
Craig J. Beasley Chief Geophysicist, WesternGeco and Schlumberger Fellow
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Foreword to the First Edition Any developing technology is heralded by a proliferation of papers of variable quality, ultimately superseded by the definitive work. It is just such a definitive work, in the field of 3-D seismic survey design, that I am proud to introduce here. In the early days of the seismic method and prior to the implementation of 3-D techniques, the literature is not short of papers and discussions on seismic survey design. Almost every individual operations expert seemed to have his or her own favorite source array or detector array or both, and wished to go into print about their various advantages. Survey design was clearly a popular subject, fueled by the fact that our wavefield sampling was necessarily woefully inadequate–not only was it “2-D,” but it was further compromised by constraints such as instrumentation limitations. Eventually the number of these papers dwindled, because, for most of us, adequate wisdom on 2-D sampling had been expressed by authors such as Nigel Anstey, Leo Ongkiehong, and Henry Askin, in papers with titles such as “Towards the universal seismic acquisition technique,” and “Whatever happened to groundroll?” Then, by the mid-1980s, the use of the 3-D method was growing fast. Again, we experienced a proliferation of experts’ favorite designs and opinions, both for marine and for land exploration, especially the latter. For some, the only viable configuration was the “brick,” and for others it was the narrow swath. Designs were seen in which the statics solutions for adjacent subsurface lines floated independently of one another. Some designs made unfair demands of field crews, requiring huge overlapping receiver arrays that crossed and recrossed each other in the search for perfection. In some cases, the acquisition staff were so out of touch with the data processors that the latter eventually had to plead with the former not to use their current favorite patented designs!
What was sorely needed was the 3-D equivalent of the older 2-D Anstey, Ongkiehong, and Askin papers–a clear and correct explanation of the issues and their solutions. This is what you now have, as into the breach stepped Gijs Vermeer, who is particularly well qualified to author a book on this subject. His academic background is in physics and mathematics and he has earned his living not just as a theorist, but also with real data as a processor and an interpreter. Most of his earlier publications have been internal Shell documents, but we are indeed fortunate that from the early 90s we have seen a growing quantity of Gijs’s work in the literature. His first major publication was the book entitled “Seismic Wavefield Sampling,” published in 1990, which quickly became a necessity on our shelves. Several of his papers since then—with titles such as “3-D symmetric sampling”—showed the directions of his research. Luckily for us, he has recently focused on this subject, and he embarked on a PhD at the University of Delft. This book is the result, and I commend it to you. Within its covers you will find a full insight into the fundamentals of 3-D survey design as well as the common practical issues such as the “footprints” that often plague 3-D data volumes and are especially important as the emphasis on reservoir geophysics increases. Comprehensively, the book covers not just land and marine surveys, but also ocean-bottom, vertical cable, and converted wave data. The survey design issues and criteria are developed and taken all the way from acquisition through to prestack depth processing. It will become both a textbook and a general reference and will benefit survey designers, data processors, interpreters, and earth scientists both in industry and academia.
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Ian Jack, BP
Acknowledgments to the Second Edition This second edition has more than twice as much text and many more figures than the first edition. Many of the new figures originate from the geophysical literature and were made by others. There are also a few figures contributed by others which at the time of printing had not been published anywhere else. Shell International had already contributed several figures to the first edition, and they agreed with publishing the additional Figures 7.3 and 10.3 in this second edition. There are more than 30 new figures, published in Geophysics, The Leading Edge, or SEG’s Expanded Abstracts, copyrighted by SEG and for which permission to use them is almost automatic. Yet, each figure corresponds to a paper written by one or more authors, who are duly acknowledged in the caption of each figure. I am grateful to these authors for their useful papers and trust they are happy to see their work reproduced in this book. I also thank the European Association of Geoscientists and Engineers (EAGE), which granted permission to use 15 copyrighted figures (Figures 2.39, 2.50, 4.6, 4.8, 4.16, 4.25, 4.39, 4.43, 5.22, 5.23, 5.26, 5.45, 5.52, 6.64, and 6.20) in this book. Figure 4.43 first appeared in Geophysical Prospecting, and all other EAGE figures appeared in First Break. The figures published in the EAGE Extended Abstracts remain the copyright of the authors or their organizations. BHPBilliton is acknowledged for permission to use Figures 5.15 and 5.16; Barry Hung and Mark Stanley provided the high-quality figures, and without the efforts of Stephen Whitney, permission would not have been given. CGGVeritas gave permission to use Figures 2.43 and 3.22, and Patrice Guillaume and Thomas Bianchi provided high-quality originals. Figure 2.49 was the product of a joint research effort by CGGVeritas and Total; both companies gave permission to use this figure, and Didier Lecerf provided the high-quality copy. Josef Paffenholz and FairfieldNodal allowed the use of Figure 5.51. Total (or Elf Gabon) also agreed with the use of Figure 5.31, and Henri Houllevigue provided better originals. There are also those figures that were published in other journals or were not published before at all. Schlumberger is acknowledged for permission to use
Figure 7.3 from their journal, Oilfield Review. Mustafa Al-Ali provided Figure 10.17, a highly instructive illustration that compares time slices of the same diffractionflattened reflection sorted in cross-spreads and in OVT gathers. Al-Ali’s figure is adopted as the cover of this book. As a reviewer of Chapter 4, Ian Jack provided two extra figures on charges, 4.37 and 4.38, and BP (Tim Summers) gave permission to use them. Edison S.p.a. (Stefano Carbonara) provided the data for Figure 4.28, a great test to determine the required migration distance. Petroleum Geo-Services (Gregg Parkes) provided the data used to construct Figure 5.27. As a reviewer of Chapter 8, Gary Hampson (Chevron) provided two new figures (8.4 and 8.9) that are very helpful to convey the message of that chapter. Total (Jean-Luc Boelle) provided Figure 5.50, which illustrates the occurrence of (seemingly) incoherent events on shot gathers that are coherent in receiver gathers. To illustrate various aspects of seismic survey design, I have made extensive use of software from Gedco (now part of Schlumberger) and Mathematica. Figures 2.29, 2.31, 4.24, 4.26, 4.27, 4.44–4.47, 5.13, 5.18, 5.4, 5.46, 5.55, 6.23, 10.9, 10.12, 10.13, 10.22, and part of 10.18, 10.19, and 10.21 were made with Gedco’s Omni, and Figure 10.23 was made with their Vista package. Figures 2.27, 2.44, 3.1–3.9, 3.12, 3.15, 3.18–3.21, 3.23, 3.24, 3.28, 3.29, 4.1, 4.29, 5.9, 5.10, 6.2–6.11, 6.13–6.15, 6.21, 6.22, 6.25, 6.26, 7.13, 8.11, 8.12, 9.4, 9.6, 10.2, 10.10, 10.11, 10.15, 10.16, and part of 10.18, 10.19, and 10.21 were made with Mathematica. This book also has a guest author, Gary Hampson, who wrote Appendix B on the true size of a sampled aperture. The manuscript was peer-reviewed by 13 geophysicists, each one having agreed to review one chapter. This meant that some reviewers had much more work to do than others, but they were all instrumental in making this a better book by pointing out ways to improve the contents. The illustrious group of reviewers consisted of Craig Beasley, Malcolm Lansley, Kees Hornman, Ian Jack, Bill Dragoset, Guy Drijkoningen, Dave Monk, Gary Hampson, Denes Vigh, Andrew Long, Guido Baeten, Mohamed Hadidi, and an anonymous reviewer. xvii
xviii Acknowledgments to the 2nd Edition Malcolm Lansley especially helped by adding and correcting some historical facts, Guido Baeten made me rewrite Appendix D, and Mohamed Hadidi encouraged me to remove an appendix. Craig Beasley graciously agreed to be the volume editor, a task he took care of for the first edition as well. Lianjie Huang was managing editor, taking care mostly of starting up and rounding off the review process. I am grateful to all of these individuals for their time and effort spent on the manuscript. With their work, the number of mistakes and omissions was greatly reduced. In discussions at conferences, during courses, and by e-mail, many geophysicists helped me to refine and expand my ideas on what is important in 3D seismic survey design: Adel El-Emam, Alexander Calvert, Andreas Cordsen, Andrew Aouad, Aziz Khan, Bee Bednar, Bill Pramik, Bob Heath, Boff Anderson, Bryan DeVault, Carl Regone, Chris Walker, Daniel Trad, David le Meur, Denis Mougenot, Edith Miller, Eivind Berg, Eivind Fromyr, Eric Verschuur, Federico Martin, Gareth Williams, Gerrit Blacquière, Ghassan Rached, Gordon Brown, Heloise Lynn, Ian Jones, Jack Bouska, Joachim Mispel, Joel Starr, Juan Cantillo, Julien Meunier, Kees Faber, Kees Hornman, Keith Wilkinson, Les Denham, Marc Girard, Marco Schinelli, Mario Sigismondi, Martin Landrø, Mike Galbraith, Mike Hall, Nick Moldoveanu, Paul Brettwood, Peter Cary, Peter Pecholcs, Peter Vermeer, Peter van Baaren, Phil Fontana, Pradyumna
Dutta, Ralph Weiss, Richard Stocker, Robert Ross, Robert Simpson, Roice Nelson, Sandra Tegtmeier, Sharon Dickie, Shuki Ronen, Stephane Gesbert, Steve McIntosh, William Goodway, Xiao-Gui Miao, and Xinxiang Li. Writing acknowledgments with so much input from others is a bit hazardous. Undoubtedly, I have forgotten to mention some geophysicists who also helped shape this book in one way or another. So, many thanks go to all you unnamed contributors as well. A few more persons need be mentioned for their contributions. First, Jennifer Cobb of the SEG office, who has done a marvelous job in running the not-soeasy Editorial Manager software and by looking after all phases of development of the manuscript. A particularly onerous task she accomplished with a lot of patience was the conversion of my RGB figures to CMYK. Next, Kathryne Pile, the copyeditor, who took her job very seriously. She implemented numerous improvements in my English, came up with lots of useful suggestions, removed dangling sentences, and made me to rewrite Chapter 9 into a coherent story. Finally, I would like to thank my wife Tini for her patience and understanding during all those years in which this book was taking shape. Gijs Vermeer, Fall 2012
Acknowledgments to the First Edition To a great extent this book is the product of many years of research in a stimulating Shell environment. In 1991 I joined the project “Fundamentals of 3D seismic data acquisition” and together with Justus Rozemond I developed the theory of 3-D symmetric sampling, based on earlier work on 2-D symmetric sampling. The main insights were developed during the first year, but thereafter we continued to expand and refine the ideas and we had the opportunity to test the ideas in practice. It is not really possible to mention all colleagues who have contributed in one way or another to the work described in this book, but I do appreciate their help. I have to make an exception for a few of them. Kees Hornman was my boss during a large part of this period until 1996. He has been a major discussion partner ever since. Jerry Davis, as an adviser to Shell Operating Companies, showed that theory could be put into practice. Whenever there was a technical question, Peter van der Sman found the time to answer it. Since early 1999, Rick Stocker keeps showing me that it is not always easy to put theory into practice. I am very grateful to Bill Kiel, who made it possible for me to take early retirement from Shell in 1997 and to start 3DSymSam—Geophysical Advice. He allowed me to maintain a good relationship with Shell, while I could
devote more of my time to geophysics and to this book than would otherwise have been possible. The good relationship with Shell manifested itself in Shell’s permission to show many of their data examples in this publication. The horizon slices in this publication have been made with a prototype version of Omni Workshop, which was kindly made available by Seismic Image Software Ltd. TNO’s permission to include the prototype version of the Acquisition Design Wizard on the CD-ROM of this book is gratefully acknowledged. I would also like to express my gratitude to Jacob Fokkema and Kees Wapenaar who graciously agreed to be my supervisors for the PhD version of this book. After all the work done for the thesis, I was happy that Craig Beasley, the Editor, came up with some useful suggestions and mild criticism only. It was a great pleasure to work with Jerry Henry, Judy Hastings, and Ted Bakamjian to convert the PhD version into a new tome in SEG’s Geophysical Reference Series. My gratitude also extends to Ian Jack who was so kind to write a positively phrased foreword. Finally, I would like to thank my wife Tini for her kind editing of English grammar in the original thesis, but above all for being there.
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Introduction The first edition of this book was a slightly modified version of my dissertation (defended in February 2001). This second edition has been extended considerably. Many technological developments of the past 10 years have been included. Feedback from students attending my course on 3D survey design has helped clarify various not-so-clear discussions in the book. Another major difference is the inclusion of many new figures copied from the literature. Most of the existing figures have been redrawn to comply with the high standards used for figures in Geophysics, and all references are now compiled in a single list. Although the main text for this edition was ready by the end of 2010, some developments in the field of seismic data acquisition that occurred in 2011 and 2012 have still been included.1 Three-dimensional seismic surveys have become a major tool in the exploration and exploitation of hydrocarbons. In 1999, Exxon received the SEG Distinguished Achievement Award for inventing the 3D seismic method in the 1960s and acquiring many singlefold cross-spreads before 1970 (Walton, 1972). The first multifold 3D seismic survey was acquired on land in 1971 (Hardman, 1999), soon followed by more 3D surveys on land (van der Kallen and Pion, 2010) and marine (Watson, 2009), although it took until the early 1990s before they gained general acceptance throughout the industry. Until then, the subsurface was mapped using 2D seismic surveys. Theories on the best way to sample 2D seismic lines were not published until the late 1980s, notably by Anstey (1986), Ongkiehong and Askin (1988), and Vermeer (1990). These theories were all based on the insight that offset forms a third dimension, for which sampling rules must be given. The design of the first 3D surveys was severely limited by what technology could offer. Gradually, the number of channels that could be used increased, providing more options on the choice of acquisition parameters, which naturally led to discussions on what choice constitutes a good 3D acquisition geometry. The general philosophy was to expand lessons learned from 2D acquisition to 3D
survey design. This approach led to much emphasis on the properties of the common-midpoint (CMP) gather (or bin) because good sampling of offsets in a CMP gather was the main criterion in 2D design. Then 3D design programs were developed, which mainly concentrated on analysis of bin attributes and, in particular, on offset sampling (regularity, effective fold, azimuth distribution, etc.). This conventional approach to 3D survey design does not acknowledge the differing properties of the many geometries that can be used in 3D seismic surveys. In particular, the sampling requirements for optimal prestack imaging were not taken into account properly. My book addresses these problems and provides a new methodology for the design of 3D seismic surveys. The approach used in this book is the same as in my Seismic Wavefield Sampling, a book on 2D seismic survey design published in 1990. Before the sampling problem can be addressed, it is essential to develop a good understanding of the continuous wavefield to be sampled. In 2D acquisition, only a 3D wavefield has to be studied, consisting of temporal coordinate t and two spatial coordinates: shot coordinate xs and receiver coordinate xr. In 3D acquisition, the prestack wavefield is five-dimensional, with two extra spatial coordinates: shot coordinate ys and receiver coordinate yr. In practice, not all four spatial coordinates of the prestack wavefield can be sampled properly (proper sampling is defined as a technique that allows the faithful reconstruction of the underlying continuous wavefield). Instead, it is possible to define 3D subsets of the 5D prestack wavefield that can be sampled properly. In fact, the 2D seismic line is but one example of such 3D subsets. The 2D seismic line is a multifold data set with midpoints on a single line only. However, in 3D acquisition, many possible 3D subsets are singlefold, whose midpoints extend across a certain area. These subsets are called minimal data sets (MDSs), a term coined by Trilochan Padhi in 1989 in an internal Shell report. An MDS represents a volume of data (sometimes called a 3D cube) that has illuminated part of the subsurface. If there were no noise, a single MDS would be sufficient to create an image of the illuminated subsurface volume. Most acquisition geometries used in practice generate data that can be considered as a collection of sampled
1
A concise version of many changes in this second edition can be found in Vermeer (2010).
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xxii Introduction MDSs. Therefore, the properties of the MDSs need to be studied for a better understanding of acquisition geometries as a whole. This allows an optimal choice of acquisition geometry (if there is a choice; often, the geometry type is dictated by economic or environmental constraints) and of the geometry parameters. The continuous wavefield to be sampled can be reduced to the wavefield of the characteristic MDS of the chosen geometry. Proper sampling of that wavefield means that at least two of the four spatial coordinates of the 5D prestack wavefield will be sampled properly. It is also recommended to maximize the useful extent of each minimal data set. Together, these two recommendations ensure minimal spatial discontinuities in the total data set. Spatial continuity is maximized, and the migrated minimal data sets contain a minimum of artifacts. Sampling of the other two spatial coordinates will be coarse in general. The coarsely sampled coordinates determine the sparsity of the geometry. Reducing the sparsity of the chosen geometry further reduces migration artifacts. Cost considerations determine in general how far sparsity of the geometry may be reduced. The ideas and results discussed in this book should help one to achieve a better understanding of the structure of 3D acquisition geometries. With this understanding, geophysical requirements can be satisfied with an optimal choice of acquisition geometry and its parameters. Processing techniques can be adapted to honor and exploit the specific requirements of each geometry, especially orthogonal and areal geometries, leading to a more interpretable end product. Based on the principles outlined above, this book addresses a variety of issues except for hardware (instruments, sources, receivers), which is not discussed in detail. Following is a summary of each chapter.
Chapter 1. 2D symmetric sampling This chapter starts with a short summary of 2D symmetric sampling, which is a recipe for optimal sampling of the 2D seismic line. Two-dimensional symmetric sampling is based on a corollary of the reciprocity theorem, which affirms that the properties of the common-receiver gather are the same as the properties of the common-shot gather. As a consequence, sampling requirements of shots and receivers are identical.
Chapter 2. 3D acquisition geometries, their properties, and their sampling Three-dimensional seismic surveys can be acquired using a number of different acquisition geometries. The
most important geometries are areal, parallel, and orthogonal. Each has its characteristic 3D basic subset. If the basic subset is singlefold, it is also a minimal data set. In areal geometry, either shots or receivers are acquired in a dense areal grid. If shots are dense, receivers are sparse, or vice versa. In the first case, 3D common-receiver gathers are acquired. These gathers form the basic subset or MDS of this particular areal geometry. Parallel and orthogonal geometries are examples of line geometries, in which sources and receivers are arranged along straight acquisition lines that are more-orless widely separated. In parallel geometry, the (parallel) shot lines are parallel to the (parallel) receiver lines; in orthogonal geometry, shot and receiver lines are orthogonal. The basic subset of parallel geometry is the midpoint line, which runs halfway between the shot line and each active receiver line. The basic subset of orthogonal geometry is the cross-spread, which encompasses all receivers in a single receiver line that are listening to a range of shots in a single shot line. The cross-spread is an MDS with limited extent. The difference in properties of the various acquisition geometries is illustrated by the difference in diffraction traveltime surfaces of the same diffractor for the basic subsets of those geometries. Fold of coverage can be determined by counting the number of traces per bin; but in crossed-array geometries and areal geometry, fold of coverage can also be counted as the number of overlapping basic subsets of the acquisition geometry. The definition of fold in the latter way can be extended to a definition of illumination fold and image fold. The periodicity of virtually all acquisition geometries is determined by the two spatial coordinates that are sampled most coarsely in x and y, respectively. The area defined by these two largest sampling intervals is called the unit cell of the geometry. The smaller the unit cell, the smaller the sparsity of the geometry. Each acquisition geometry is also characterized by three aspect ratios: for bin size, unit cell, and maximum offset inline versus crossline. The 3D symmetric sampling of orthogonal and areal geometry is characterized by their aspect ratios, all of which should be equal to one in the ideal case. For imaging, it would be ideal to have singlefold data sets that extend across the whole survey area but possess a minimum of spatial discontinuities so that they would produce a minimum amount of migration artifacts. These data sets are called pseudo-common-offset-vector (COV) gathers and can be constructed from so-called offset- vector tiles (OVTs). In most geometries, the size and shape of the OVT is equal to the unit cell. In sparse geometries, subsurface illumination is enhanced by reciprocal OVTs (tiles with the same average absolute offset but located in opposite azimuth quadrants).
Introduction xxiii Depending on the type of process, the basic subsets of each geometry and/or the pseudo-COV gathers may be most suitable as input gathers to various prestack processing steps.
Chapter 3. Noise suppression Sampling in 3D acquisition is usually not dense enough to record low-velocity noise without aliasing. To reduce aliasing effects, shot and receiver arrays may be used. The arrays can be linear or areal. For a proper choice of arrays, the properties of the noise need to be known. An analysis of the energy distribution of low-velocity scatterers shows that in the cross-spread, most energy is concentrated on the flanks of the traveltime surface and there is less energy around the apex. Linear arrays are sufficient to suppress the energy in the flanks. If there is much undesirable energy coming from all directions, circular arrays can be constructed with a circular response. It is often argued that arrays are a threat to the signal. However, in most situations, arrays with length equal to the station interval — chosen to allow proper sampling of the desired wavefield — have only mildly negative effects on the signal yet allow successful noise removal by combining array effect and prestack processing. Yet in situations with strong elevation changes or rapidly varying statics, arrays are to be avoided; the acquisition of very high frequencies also requires single-point acquisition. In a well-designed acquisition geometry, most of the noise can be removed by prestack processing. Nevertheless, it is interesting to see that the stack response of wide acquisition geometries is better than that of narrow geometries with the same fold of coverage.
Chapter 4. Guidelines for designing 3D geometries used on land The theoretical considerations and observations in the first chapters are translated into practical guidelines for choice of geometry and selection of parameters specifically for orthogonal geometry. The first part of the recipe for 3D survey design deals with the choice of geometry and its parameters. The objectives of the survey must be established, and all available information on the survey area — complexity of geology, quality of existing data, etc. — must be analyzed. This analysis may lead to various geophysical constraints that have to be satisfied in the course of the design process. The choice between orthogonal, parallel, and areal geometry is largely determined by acquisition cost,
orthogonal geometry being most efficient on land and parallel geometry in marine data acquisition. Yet geophysical requirements play a role as well, particularly the requirement — especially in complex geology — to illuminate the subsurface from all directions. In that case, orthogonal geometry will be selected not only for land but also for marine with ocean-bottom-cable (OBC) acquisition, whereas areal geometry is most suitable in deepwater situations using sparse nodes (4C receiver units with 3C geophone and hydrophone) and dense shots. The remainder of the chapter focuses on orthogonal geometry. Knowledge of the area provides a representative velocity distribution to be used at various stages of the design process. The interpreter specifies the main targets, resolution requirements, and maximum dip of the targets so the designer can focus on the requirements of those targets and determine the maximum required frequency. A 3D symmetric sampling is taken as a starting point to decide on the six main parameters of orthogonal geometry (two station intervals, two line intervals, and two maximum offsets). This simplifies the choice of parameters considerably: the aspect ratio of the bins should always be one (consequently, the station intervals should be equal), whereas the aspect ratios of the unit cell and the cross-spread should be close to one. The choice of station interval is determined by the maximum wavenumber expected in the wavefield. The preference is for the whole wavefield, but an alternative is the maximum wavenumber of the desired wavefield without noise. In the latter case, it may be necessary to use arrays to compensate for coarse sampling of the noise. The choice of station interval can also be influenced by serious noise or by rapidly varying statics. The mute function (offset versus time) determines the average fold for all levels where mute offset is smaller than maximum offset. This function may be computed from the maximum acceptable normal moveout (NMO) stretch, or it may be determined from the mute function used in earlier processing. The mute offset for the deepest target determines the choice of maximum offset. Fold of coverage can be considered a dependent parameter (area of cross-spread/area of unit cell), but it can also be an independent parameter that must be met by a proper choice of the line intervals. Especially in areas with no earlier 3D surveys, it tends to be difficult to make a reasoned choice. The second part of the recipe deals with the implementation of the designed geometry. The same nominal geometry may be implemented by a large variety of templates or swaths: the one-line roll template (roll = crossline roll), sources outside the template, full-swath roll, multiline roll with extra receiver
xxiv Introduction lines, and superspread can all be used. The technique chosen may depend on availability of equipment, efficiency of acquisition, and risk of theft or vandalism. The required extent of the survey area depends on the fold-taper zone (a given for the selected nominal geometry, assuming a closed grid of shot lines and receiver lines) and the fullfold area of the geometry. The fullfold area depends on the area to be interpretable and the migration radius. This radius and the fold-taper zone may overlap partially, depending on data quality. Obstacles often prevent laying out straight acquisition lines. Spatial continuity then requires the acquisition lines to be smooth. This ensures that common-receiver gathers have similar quality or continuity as commonshot gathers. The choice of source type depends largely on the type of survey terrain. Sometimes it is necessary to use more than one type of source due to terrain variations. The source strength may have to be tested; in cases where the source strength has to be reduced, repeat sources should be considered for sufficient penetration.
Chapter 5. Marine seismic data acquisition In marine seismic data acquisition, multisource multistreamer (MC/MC) configurations are usually preferred over stationary receiver systems due to cost considera tions. Therefore, streamer acquisition gets the most attention in this chapter. Many issues that play a role in the design and choice of streamer acquisition are covered, such as influence of boat speed on acquisition parameters, ghost effect, and feathering. The main weaknesses of streamer acquisition tend to be crossline bin size, crossline roll, ghost effect, and feathering. Other weaknesses are the absence of reciprocal OVTs due to end-on acquisition and, for complex geology, the narrow-azimuth (NAZ) distribution. In the first decade of this century, various techniques have been reintroduced to broaden bandwidth by reducing the ghost effect. All techniques (over/under, dual-sensor streamer, and variable-depth streamer) were proposed or tried earlier but needed technological improvement to become successful. The NAZ limitation of streamer acquisition can be overcome by using multiazimuth acquisition or wideazimuth (WAZ) acquisition. Virtually always, WAZ towed-streamer acquisition tends to forsake desirable geophysical requirements because of the very high cost of achieving such requirements. A very interesting WAZ technique is coil geometry. Simultaneous shooting is a way of reducing the cost of marine streamer acquisition. An interesting application is
to simulate center-spread acquisition with a source boat directly behind the streamers. For WAZ acquisition, simultaneous shooting would improve inline sampling, but it would not reduce the large crossline rolls. Stationary receiver techniques have the advantage that orthogonal and areal geometry are also feasible geometries, next to parallel. These techniques do not suffer from feathering. The main techniques are ocean-bottom cable and ocean-bottom node. The latter is more suitable for large water depths. Time-lapse techniques for seismic reservoir monitoring can be used with any marine acquisition technique, although stationary receiver techniques tend to show considerably better repeatability, which is a much-needed feature of such acquisition.
Chapter 6. Converted waves: Properties and 3D survey design Survey design for PS- (or C-) waves is different from P-wave acquisition, owing to the asymmetry of the PSwave raypath. Differences in PS-wave illumination by minimal data sets of different geometries are much larger than P-wave illumination differences. For instance, a cross-spread with a square midpoint area produces an illumination area with a rectangular shape even for a horizontal reflector. The raypath asymmetry leads to asymmetric sampling requirements for shots and receivers. Shot-sampling interval is determined by P-wave velocity; receiver-sampling interval, by S-wave velocity. A NAZ parallel geometry tends to suffer least from asymmetry effects, whereas orthogonal geometry tends to suffer most. For analysis of azimuth-dependent effects, areal geometry might be the best choice. Processing of orthogonal and areal geometry data has to take into account the spatial discontinuities of the edges of their basic subsets because these edge effects cannot be mitigated by OVT gathers (as for P-wave acquisition).
Chapter 7. Some lowfold data examples Noise spreads or microspreads are acquired with very dense spatial sampling for an analysis of low-velocity events. A cross-spread with very dense spatial sampling was acquired in The Netherlands. Time slices and cross sections illustrate the 3D behavior of the ground-roll cone and the scatterers inside the cone. In 1992, the theory of 3D symmetric sampling was tested in Nigeria, where a cross-spread geometry was compared with the standard brick-wall geometry. The test geometry produced better results (higher resolution and better continuity) at target level than the standard
Introduction xxv g eometry. The improvement can be attributed to larger width (maximum crossline offset) of the test geometry and to its better spatial continuity. Various lowfold migration tests illustrate that under favorable circumstances, very low fold can be sufficient to get acceptable (exploration) 3D prestack migration results.
Chapter 8. Factors affecting spatial resolution
The theory of dip-moveout (DMO) correction was developed for 2D common-offset gathers. Initially, the success of application of DMO correction to 3D data was not really understood. The theory of DMO application to MDSs in general and to cross-spreads in particular dispelled the mystery. The application of existing DMO software to a singlefold data set (cross-spread) revealed serious amplitude and phase artifacts. This prompted improvements in contractor software.
The minimal data sets of the various acquisition geome tries also have different resolution properties. The main factor influencing the theoretically best resolution is the stretch effect, caused by NMO. Therefore, zero-offset data potentially have the best resolution. Resolution is not improved by reducing the midpoint sampling intervals while keeping the shot and receiver sampling intervals the same (bin-fractionation technique). Carefully selected “random” coarse sampling may produce fewer migration artifacts than regular coarse sampling; but to eliminate all artifacts, regular dense sampling is best.
Chapter 10 explores the link between data-acquisition parameters and imaging requirements. The basics of prestack migration are covered, with an eye on sampling requirements of the input data. The zone of influence (ZOI) is used to establish migration-apron requirements. Most MDSs have limited extent, leading to edge effects in migration. However, using pseudo-COV gathers constructed from OVTs tends to produce better singlefold images than other singlefold subsets of the geometry.
Chapter 9. DMO
Appendixes
This chapter is of historical interest; it demonstrates the importance of a clear understanding of the properties of the cross-spread, one of the minimal data sets.
A few appendixes describe some theory that could not be included satisfactorily in the main text. Appendix C lists some useful formulas that are spread out in the main text.
Chapter 10. Prestack migration
For a list of updates and revisions to this text, refer to seg.org/errata. For survey-design software discussed in chapter 4, see http://dx.doi.org/10.1190/1.9781560803041.supp.
