In the past decade, 3D reflection seismology has replaced 2D seismology almost entirely in the seismic industry. Recording 3D surveys has become the norm instead of the exception. The application of 2D seismology is limited mostly to reconnaissance surveys or to locations where recording 3D data is still prohibitively expensive, such as in rough mountains and wild forests. However, academic research and teaching have struggled to keep up with the 3D revolution. As a consequence of this tardiness, no books are available that introduce the theory of seismic imaging from the 3D perspective. This book is aimed at filling the gap.
Seismic processing of 3D data is inherently different from 2D processing. The differences begin with data acquisition: 3D data geometries are considerably more irregular than 2D geometries. Furthermore, 3D acquisition geometries are never complete because sources and receivers are never laid out in dense areal arrays covering the surface above the target. These fundamental differences, along with the increased dimensionality of the problem, strongly influence the methods applied to process, visualize, and interpret the final images. Most 3D imaging methods and algorithms cannot be derived from their 2D equivalent by merely adding a couple of dimensions to the 2D equations. This book introduces seismic imaging from the 3D perspective, starting from a 3D earth model. However, because the reader is likely to be familiar with 2D processing methods, I discuss the connections between 3D algorithms and the corresponding 2D algorithms whenever useful.
The book covers all the important aspects of 3D imaging. It links the migration methods with data acquisition and velocity estimation, because they are inextricably intertwined in practice. Data geometries strongly influence the choice of 3D imaging methods. At the beginning of the book, I present the most common acquisition geometries, and I continue to discuss the relationships between imaging methods and acquisition geometries throughout the text. The imaging algorithms are introduced assuming regular and adequate sampling. However, Chapters 8 and 9 explicitly discuss the problems and solutions related to irregular and inadequate spatial sampling of the data.
Velocity estimation is an integral component of the imaging process. On one hand, we need to provide a good velocity function to the migration process to create a good image. On the other hand, velocity is estimated in complex areas by iterative migration and velocity updating. Migration methods are presented first in the book because they provide the basic understanding necessary to discuss the velocity updating process.
Seismic-imaging algorithms can be divided into two broad categories, integral methods (e.g., Kirchhoff methods) and wavefield-continuation methods. Integral methods can be described by simple geometric objects such as rays and summation surfaces. Thus, they are understood more easily by intuition than wavefield-continuation methods are. My introduction of the basic principles of 3D imaging exploits the didactic advantages of integral methods. However, wavefield-continuation methods can yield more accurate images of complex subsurface structures. This book introduces wavefield-continuation imaging methods by leveraging the intuitive understanding gained during the study of integral methods. Wavefield-continuation methods are the subject of my ongoing research and that of my graduate students. Therefore, the wavefield-continuation methods described are more advanced, although less well established, than the corresponding integral methods.
Seismic-imaging technology is data driven, and the book contains many examples of applications. The examples illustrate the rationale of the methods and expose their strengths and weaknesses. The data examples are drawn both from real data sets and from a realistic synthetic data set, the SEG-EAGE salt data set, which is distributed freely and used widely in the geophysical community. For the reader's convenience, a subset of this data set (known as C3 narrow-azimuth) is contained in the DVD included with this book. Appendix 2 briefly describes this data set.
The software needed to produce many examples also will be distributed freely over the Internet. A reader with the necessary computer equipment (a powerful Unix workstation) and the patience to wait for weeks-long runs could reproduce the images obtained from the SEG-EAGE salt data set. Appendix A describes the foundations of SEPlib3d, the main software package needed to generate most of the results shown in this text.
The book starts from the introduction of the basic concepts and methods in 3D seismic imaging. To follow the first part of the book, the reader is expected to have only an elementary understanding of 2D seismic methods. The book thus can be used for teaching a first-level graduate class as well as a short course for professionals. The second part of the book covers more complex topics and recent research advances. This material can be used in an advanced graduate class in seismic imaging. To facilitate the teaching of the material in this book, the attached DVD includes a document in PDF format that has been formatted specifically to be projected electronically during a lecture. All the figures in this electronic document can be animated by clicking on a button in the figure caption. Several of these figures are movies that provide a more cogent illustration of the concepts described in the text. All figures are included on the attached DVD as GIF files.
