AVO and Anisotropy
The AVO studies that we discussed in the preceding chapters assume that subsurface rock formations are isotropic. Often that assumption is not strictly true, because subsurface formations can exhibit various kinds of anisotropy. In such cases, serious errors can creep into AVO analysis and lead to erroneous interpretations. On the other hand, anisotropic behavior can be used beneficially to characterize fractured reservoirs. Thus, it is important to study the different types of anisotropy encountered in subsurface formations, along with their effect on seismic data, and then to determine their ultimate effect on AVO analysis. We will explore all of these considerations in this chapter.
Figures & Tables
We begin this book with a brief discussion on the basics of seismic-wave propagation as it relates to AVO, and we follow that with the rock-physics foundation for AVO analysis — including the use of Gassmann’s equations and fluid substitution. Then, as food for the inquisitive mind, we present briefly the early seismic observations and how they led to the birth of AVO analysis. Next, we examine the various approximations for the Zoeppritz equations and identify clearly the assumptions and limitations of each approximation. We follow that with a section on the factors that affect seismic amplitudes and a discussion of the processing considerations that are important for AVO analysis. A subsequent section explores the various techniques used in AVO interpretation. Finally, we discuss topics such as the influence of anisotropy in AVO analysis, the use of AVO inversion, estimation of uncertainty in AVO analysis, converted-wave AVO, and the future of the AVO method.