Seismic AVO analysis of methane hydrate structures

CONCLUSIONS

A detailed AVO analysis was performed on data from the Blake Outer Ridge to evaluate the origin of the bottom simulating reflector. Reflectivity clearly discriminates the effects of different models and shows that the observed BSR best fits a model of sediments containing substantial amounts of hydrate overlaying sediments containing free gas. This modeling result was supported by a prestack impedance inversion of the seismic data. A transition from hydrate to brine sediments is not consistent with the AVO amplitude responses and the impedance contrasts.

Based on the synthetic modeling, the thickness of the hydrate layer was determined to be approximately 190 meters. It is characterized by a P-wave velocity of around 2.5 km/s and an anomalously low S-wave velocity of 0.5 km/s. The thickness of the gas layer was calculated to be approximately 250 meters. It has a P-wave velocity of 1.6 km/s and an S-wave velocity of 1.1 km/s, yielding a Poisson's ratio of 0.1 which is reasonable for gas. The considerable thickness of the gas layer might suggest the possibility of it being a source rock for the overlaying methane hydrate.

It has to be considered, however, that the synthetic modeling excluded possible tuning effects and thus might represent a simplification of the actual conditions. Nevertheless, the velocity behavior predicted by the model was reinforced by the prestack impedance inversion, thus indicating that a transition from high P-wave velocity and anomalously low S-wave velocity in the hydrate to low P-wave velocity and high S-wave velocity in the gas sediments is required. A detailed investigation of this unusual behavior is performed by Ecker (1994).

Seismic AVO analysis of methane hydrate structures