Understanding Seismic Anisotropy in Exploration and Exploitation

Edited by Thomsen Leon


“All rock masses are seismically anisotropic, but we generally ignore this in seismic acquisition, processing, and interpretation. The anisotropy nonetheless does affect data, in ways that limit the effectiveness with which we can use it, as long as we ignore it. This book, produced for use with the 2002 SEG/EAGE Distinguished Instructor Short Course, helps us to understand why this inconsistency between reality and practice has been so successful in the past and why it will be less successful in the future as we acquire better seismic data (especially including vector seismic data) and correspondingly higher expectations of it. This book helps us to understand how we can modify our practice to more fully realize the potential inherent in data through algorithms which recognize the fact of seismic anisotropy. Sections include 1: Physical Principles, 2: P-waves (Subsurface Imaging), 3: P-waves (Subsurface Physical Characterization), 4: S-waves, and 5: C-waves. (DISC on DVD, 751A, is also available.)”

  1. Page 1

    The subject of seismic anisotropy has a long history, but only recently has it come to be seen as a central feature of geophysics as applied to the exploration for hydrocarbons, and to their exploitation. The reason for the long neglect of anisotropy is, of course, that isotropy is simpler. The equations are simpler, and the application of one’s intuition is more direct. And, perhaps, because of their simplicity, these basic, isotropic ideas have enabled the discovery of most of the world’s known hydrocarbons.

    I am told, even, that hydrocarbons were found at one time by using the ideas of acoustic wave propagation! Improbable as it sounds, this myth does explain how very useful simple ideas can be, and what a strong hold they have upon the imagination.

    However, in those ancient days, exploration geophysics was much less effective than it is today. Wildcat success-rates were as low as 10% in the 1950s, growing slowly to 25% by the 1970s, before exceeding 50% today. There are three basic reasons for our increased success-rates in the modern era:

  2. Page 1-1

    We begin by discussing the physical principles that underlie the phenomena of seismic anisotropy. The first task is to define the term; various experts have provided their own take on this definition (see, e.g., Crampin, 1988 and Winterstein, 1990). Not surprisingly there are certain differences among these definitions. Since I have the floor here, I get to make my own definitions.

  3. Page 2-1

    Now that we have established the fundamental ideas of seismic anisotropy, it is time to consider how the effects of anisotropy show up in our data. We will consider the implications for subsurface imaging in this lecture, and for subsurface characterization in the following Lecture.

  4. Page 3-1

    Only a few years ago, the ultimate goal of seismic data processing was to form an image of the subsurface, i.e., to delineate the geometry of subsurface reflectors with optimal resolution and precision. However, in more recent times, more has been demanded of seismic data. We now expect that the data should yield, not just processing parameters, but parameters that tell us something specific about the physical properties of the subsurface rocks. This process is a significant extension of subsurface imaging; we call it physical characterization of the subsurface

    We do this using concepts and equations that are admittedly a gross simplification of the real world. We routinely ignore many features of rocks that affect our data, features like small-scale structure, anelasticity, finite stress, and certain aspects of poroelasticity. Most of these are beyond the limits of the present short course.

    But, we will consider here how seismic anisotropy affects our data, and our conclusions from it that concern the physical properties of the subsurface. In fact, this short course was motivated in the Introduction by considerations of rock properties, so this is a natural line of inquiry.

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    Of course, most of us, most of the time, are not concerned with shear waves. There is a good reason for this: they are more complicated, hence, we have to work harder to get useful information out of them. Also, they attenuate more rapidly than P-waves do, so they usually image the earth with less temporal and spatial resolution, and with shallower penetration.

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    Converted waves are generated at every discontinuity within an elastic body, as an incident P-wave converts part of its energy to S-waves (and vice-versa!), Figure 5-3. In fact, elastic conversionof energy is one of the principal ways that primary transmitted waves lose energy, and is often more effective in this regard than is attenuation. In P-waves, this loss of energy is responsible for the appearance of ∆Vs in the AVO gradient expression (Figure 3-10), since the energy for the converted S-waves comes at the expense of the P-waves.

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