AVO inversion of BSRs in marine gas hydrate studies

Abstract

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% and0%–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 syn-thetic 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 quantita-tive uncertainty analysis in all AVO studies.