The assumption that that effective pressure coefficient (n, ) is equal to unity is usually not valid. A coefficient of one means that elastic properties are only dependent on the differential pressure: the difference between confining and pore pressure. The coefficient n scales the influence of the pore pressure. This effective pressure coefficient has a different value for each elastic property and for different boundary conditions. For the shear modulus this coefficient is close to unity, while the effective pressure coefficient for the bulk modulus can range from unity to 0.3 depending on the porosity, differential and pore pressure. If the effective pressure coefficient is not unity, pore pressure predictions and inversion of pressure and saturation from time lapse seismic must be modified to extract the proper effective pressure.

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