Tree elastic parameters necessary for Gassman fluid replacement modeling, namely the velocity of the pressure wave (Vp), velocity of the shear wave (Vs) and bulk density (Rhob). These can be estimated from seismic data through inversion, using direct wireline log measurements or by conducting laboratory measurements on dry cores using ultra sonic frequencies. Whereas the three elastic parameters are measured directly on cores and in wells, seismic is unique in the sense that we can most accurately invert for acoustic impedance (Z-p) and with 4-C seismic also quite accurately estimate gamma, or Vp/Vs ratio, for a sufficiently thick reservoir interval.