A Tutorial on Gassmann Fluid Substitution: Formulation, Algorithm and Matlab Code

Fluid substitution is an important part of the seismic rock physics analysis (e.g., AVO, 4D analysis), which provides a tool for fluid identification and quantification in reservoir. This is commonly performed using Gassmann’s equation (Gassmann, 1951). Many authors (Batzle and Wang, 1992; Berryman, 1999; Wang, 2001; Smith et al., 2003; Russell et al., 2003; Han and Batzle, 2004) have discussed the formulations, strength and limitations of the Gassmann fluid substitution. However comprehensive discussions on this subject have been rather scant. In this paper I present an explicit formulation, algorithm and a matlab code (computer program) for the Gassmann fluid substitution.