Fluid substitution; estimating change in V p without knowing V s

Two methods are presented for estimating the change of seismic P-wave velocity that accompanies pore fluid changes in a rock in the common situation when S-wave velocity is unknown. In contrast, Gassmann9s relation operates on the rock bulk modulus, which can only be calculated when both V P and V S are measured. The first method operates directly on the P-wave modulus and is equivalent to replacing the bulk moduli of the rock and mineral in Gassmann9s relation with the corresponding P-wave moduli. The second method uses a graphical construction to estimate the decomposition of the measured P-wave modulus into bulk and shear moduli, which then allows the conventional Gassmann9s formula to be used. When applied to a large set of sandstone data, the predictions of both methods, computed with V P only, are within a few percent of the Gassmann9s relation, using both V P and V S .