Elastic parameters from constrained AVO inversion: Application to a BSR in the Mahanadi offshore, India

We have estimated P-wave velocities (V-P) and thicknesses from traveltimes data, and S-wave velocities (V-S) and densities (rho) from amplitudes versus offset (AVO) data by constraining the V-P for a set of seismic reflection data including a bottom simulating reflector or BSR (marker for gas hydrates) in the Mahanadi offshore. The elastic parameters (V-P, V-S and.) of the whole-layered model have been derived simultaneously using the genetic algorithm global optimization technique that converges into a global minimum. The result shows a low velocity (1506 m/s) free-gas zone below the BSR that lies at 241 m below the sea floor. The presence of underlying free-gas is further corroborated by the observation of increasing negative reflection coefficients with angles. The V-P and V-S for gas hydrates bearing sediments above the BSR are estimated to be 1655 and 468 m/s respectively, and match reasonably with the sonic log at site NGHP-01-08A passing through the seismic line. Gas-hydrates and free-gas have been qualitatively estimated as 7.0% and 0.9% using three-phase weighted equation of Lee and Gassmann equation respectively. The combined usage of V-P and V-S corresponding to a Poisson ratio of 0.4565 provides a better estimation of gas hydrates, which shows comparable result that has been estimated using the Simandoux equation to the resistivity log data at site NGHP-01-08A.