Seismic Observations and Historical Developments Leading to AVO
Amplitude-variation-with-offset (AVO) analysis attempts to use the offset-dependent variation of P-wave reflection coefficients to detect and/or estimate anomalous contrasts in shear-wave velocities and densities across an interface. Although the conventional P-wave reflection coefficient at normal incidence is, in itself, a hydrocarbon indicator, AVO goes beyond the P-wave normal incidence by producing a second attribute (of one kind or another) that is related to the contrast in Poisson's ratio. A crossplot of those two attributes allows interpreters to look for anomalous fluids. Thus, hydrocarbon detection using AVO is based fundamentally on the anomalous relationships between compressional-wave and shear-wave velocities and densities for hydrocarbon-bearing rocks, relative to those relationships for equivalent fully brine-saturated rocks or other background references.
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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.