Abstract

: Seismic investigation in different continental margins successfully imaged the bottom simulating reflectors (BSRs), which signify the base of the methane hydrate stability field. Though in many cases the presence of free gas beneath the BSR have been reported, the free gas may or may not be present depending upon the geological situation. The predicted seismic velocities above the BSR in hydrate bearing sediments are very high compared to that of the free gas-bearing zone. Different empirical relationships have been proposed to fit the estimated velocity from the seismic data. But none of the existing techniques can unambiguously describe the nature of variation of P and S-wave velocities with hydrate saturation as well as free gas saturation because of complex nature (origin and occurrence) of gas hydrate sediments. A weighted equation based on the three-phase time average and Wood equations has been applied by Lee et. al. (1996) to derive a relationship between the compressional wave (P wave) velocity and the amount of hydrates filling the pore space. Behaviour of the weighted equation in the porosity range of 40- 80% is satisfactory but the different constants involved in the method are confusing in their physical significances. To explain the high seismic velocity in hydrate bearing sediments, Dvorkin and Nur (1993, 1996) have proposed contact cement model concept. The effective medium theory is being attempted to explain this issue also. Based on these velocity models, AVO responses using original Zoepritz equations are generated to estimate the hydrate saturation above BSR as well as the existence of free gas beneath it. The relative AVO responses of these velocity models are presented here. Also the merits and demerits are discussed from the rock physics concept.