4 D seismic measurements are used to monitor changes during the production and stimulation of a reservoir. Parameters such as the type of fluid fill, porosity and pressure distribution are obtained from the results. Interpretation is difficult and is complicated by stress sensitive matrix properties, and phase sensitive fluid properties. Interpretation depends on some theoretical scheme such as Gassmann's equation. Such formulations require a detailed knowledge of the velocity of sound in the fluids present and compression and shear wave velocities in the matrix. These velocity values are obtained from a combination of field and laboratory measurements.