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.