Elastic properties of hydrate-bearing sediments using effective medium theory

Accurate and detailed models of the seismic velocity structure of gas hydra te-bearing sediments may be determined by careful analysis of controlled so urce seismic data. However, interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical f ormulae and/or simple effective medium theory. We develop a rigorous theore tical scheme to relate the seismic properties of a clay-rich hydrate-bearin g sediment to its porosity, mineralogy, microstructural features and hydrat e saturation. We consider separately the two possible end-members for the d istribution of hydrate in the pore space: (1) hydrates are unconnected and located in the pore voids without appreciable grain contact and (2) connect ed hydrates are forming cement binding around the grains. The scheme is tra nsversely isotropic, to allow far anisotropy due to alignment of clay plate lets, and is based on a combination of a self-consistent approximation, a d ifferential effective medium theory, and a method of smoothing for crystall ine aggregates. We have applied the scheme to lithological and seismic velo city data from Ocean Drilling Program Site 995 on the Blake Ridge (southeas tern U.S. continental margin) to make estimates of the hydrate saturation. It was found that the hydrates are probably unconnected, and their volume c oncentration varies between similar to 0% at 100 m below the seabed and sim ilar to 9% at 400 m depth, just above the "bottom simulating reflector", if the clay platelet orientation distribution resembles the function we have used.