SEDIMENTS WITH GAS HYDRATES - INTERNAL STRUCTURE FROM SEISMIC AVO

We interpret amplitude variation with offset (AVO) data from a bottom simulating reflector (BSR) offshore Florida by using rock-physics-base d synthetic seismic models. A previously conducted velocity and AVO an alysis of the in-situ seismic data showed that the BSR separates hydra te-bearing sediments from sediments containing free methane. The ampli tudes at the BSR are increasingly negative with increasing offset. Thi s behavior was explained by P-wave velocity above the BSR being larger than that below the BSR, and S-wave velocity above the BSR being smal ler than that below the BSR. We use these AVO and velocity results to infer the internal structure of the hydrated sediment. To do so, we ex amine two micromechanical models that correspond to the two extreme ca ses of hydrate deposition in the pore space: (1) the hydrate cements g rain contacts and strongly reinforces the sediment, and (2) the hydrat e is located away from grain contacts and does not affect the stiffnes s of the sediment frame. Only the second model can qualitatively repro duce the observed AVO response. Thus inferred internal structure of th e hydrate-bearing sediment means that (1) the sediment above the BSR i s uncemented and, thereby, mechanically weak, and (2) its permeability is very low because the hydrate clogs large pore-space conduits. The latter explains why free gas is trapped underneath the BSR. The seismi c data also indicate the absence of strong reflections at the top of t he hydrate layer. This fact suggests that the high concentration of hy drates in the sediment just above the BSR gradually decreases with dec reasing depth. This effect is consistent with the fact that the low-pe rmeability hydrated sediments above the BSR prevent free methane from migrating upwards.