Implications of thin layers for amplitude variation with offset (AVO) studies

Amplitude variation with offset (AVO), or amplitude variation with angle (AVA), analyses of seismic reflection data are becoming increasingly popular in the exploration industry (Ostrander, 1984; Pichin and Mitchell, 1991; Mazzotti and Mirri, 1991) and also in scientific studies of the earth's crust (Juhlin, 1990). In the exploration industry, AVO analyses are particularly suitable for the detection and mapping of gas zones since reservoirs often consist of shale with high Poisson's ratio (high vP/vS) overlying gas bearing sands with low Poisson's ratio (low vP/vS). If the gas sand has lower impedance than the overlying shale, the magnitude of the reflection coefficient will increase with increasing angle of incidence or offset. Other combinations of rock types will also show a similar increase in magnitude, such as shale over hard limestone, but the sign of the reflection coefficient will be positive in most of these cases. Therefore, if the polarity of the reflection can be determined to be negative and there is an increase in the absolute amplitude of the reflection with offset, then this is highly indicative of a gas bearing zone.