Abstract

The phenomenon of AVO response is based on the theory of reflection magnitude as a function of incidence angle. In general, reflection magnitude decreases as offset increases. Zoeppritz first introduced AVO concepts in 1919, based on reflection coefficient. Further development with the algorithms for simplified computation has been applied by many authors. The three most significant parameters that control the AVO response are offset, acoustic impedance and Poisson's Ratio. This paper describes a specific phenomenon of the AVO response considering these three parameters.

Amplitude reversal is defined as a local maximum or minimum value of amplitude, either in positive or negative reflections. This phenomenon occurs only when the directions of increase of acoustic impedance and Poisson's Ratio are of opposite sense.

Normally the changes of these two parameters (acoustic impedance and Poisson's Ratio) are in the same direction. Both might be greater or smaller when a seismic ray impinges an interface. This concordant relationship of acoustic impedance and Poisson's Ratio can be distorted by the presence of gas. Gas in a sandstone reservoir tends to increase Poisson's Ratio but tends to decrease the value of acoustic impedance; this discordant relationship might result in amplitude reversal as the angle of incidence increases.