The single most important rock physics equation is probably the widely used Gassmann equation which estimates the change in bulk modulus of the saturated rock when the original fluid is replaced by a fluid with a different modulus. This is the essence of the fluid replacement problem where we want to estimate the change in P-wave velocity (VP) as a function of fluid properties. The change in density (ρb) is straightforward provided we know porosity and mineral and fluid densities. The S-wave velocity (VS) is only sensitive to the change in density....
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