Poisson's ratio contrast and AVO responses: model study

Abstract

A critical element in the use of amplitude versus offset (AVO) for gas detection or lithological discrimination is to define a parameter that represents anomalous behaviour. Shuey's approximation to the Zoeppritz equations for the compressional wave reflection coefficient has been widely used as a basis for defining AVO anomalies. Shuey's approximation states the AVO response as a linear equation. The intercept and gradient from this equation have frequently been used as key parameters to quantify AVO response. Generally, large gradients are normally assumed to be related to gas saturation. In order to better understand the gradient term, 25 model pairs of shale over gas sand, and shale over brine sand, were used to analyse the gradient term as the sum of two functions. The first function is defined as the non-Poisson's ratio contrast term, and the second as the Poisson's ratio contrast term. Three conclusions have been reached from this analysis. 1. The gradient may be significant even for a small Poisson's ratio contrast. This effect may be one of the reasons that AVO analysis is misleading in some geological settings; 2. Similar values of Poisson's ratio contrast may produce quite different gradients in different rocks; 3. In most cases the non-Poisson's ratio contrast term is constructive to the magnitude of gradient when the normal incidence reflectivity and Poisson's ratio contrast have opposite signs, while it is destructive to the magnitude of gradient when both have the same signs.