AVO inversion of BSRs in marine gas hydrate studies

We examine the usefulness of amplitude versus offset (AVO) analysis of bottom-simulating reflections (BSRs) for estimating associated marine gas hydrate and free-gas concentrations. A nonlinear Bayesian inversion is applied to estimate marginal probability distributions (MPDs) of physical parameters at a BSR interface, which are related to overlying gas hydrate and underlying free-gas concentrations via rock physics modeling. The problem is constrained further by prior information and re-parameterization of inversion results. Inversion of BSR AVO data from offshore Vancouver Island, Canada, shows that gas hydrate and free-gas concentrations are, respectively, 0%-23% and 0%-2% of the pore volume, at a 90% credibility level. This result indicates that the data do not provide sufficient information to independently resolve gas hydrate and free-gas concentrations to useful accuracy. The Study is directed primarily at AVO for gas-hydrate-related BSRs, but may have important applicability in testing the degree of constraint of formation characteristics in other AVO studies. The inversion method is applied also to synthetic AVO data generated from Ostrander's gas-sand model. In this case, MPDs sufficiently constrain the relationship between P- and S-wave velocities in the sandstone unit to determine if it is gas-charged. The variable degree of model constraint obtained in this AVO study highlights the need to include rigorous quantitative uncertainty analysis in all AVO studies.