Seismic Wave Propagation
The subsurface of the earth comprises rock layers that have different physical properties and are in contact with one another (i.e., are stratified). The boundaries separating the individual layers are referred to as interfaces. Those interfaces may represent contacts between, for example, a sand layer and a shale layer (i.e., a sand-shale interface), or between a shale layer and a limestone layer (a shale-limestone interface), or they may be an interface between gas-filled and water-bearing layers of sandstone. It is helpful to understand the propagation of sound waves through such subsurface interfaces. A simple way to model the partitioning of energy at an interface is to consider a basic model of the subsurface, in which the interface is perfectly planar and separates two infinitely homogeneous, isotropic, elastic media. We refer to such an interface as an ideal reflector.
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.