Factors Controlling the AVO Response of a BSR at a Gas Hydrate–Free Gas Interface

The bottom-simulating reflector (BSR) is an important seismic marker of the base of the gas hydrate stability zone and underlying free gas. Its amplitude variation with offset (AVO), expressed here as angle-dependent P–P reflection amplitude, depends on fluid composition, pore structure, clay content, and hydrate occurrence mode. Using a horizontal two-layer model, we compare the effects of hydrate saturation, free-gas saturation, hydrate reservoir porosity, clay content, and the hydrate frame participation fraction fms—the fraction of the total hydrate volume incorporated into the load-bearing solid phase—on BSR AVO responses. Elastic properties are calculated by solid averaging, a reference-constrained frame relation, fluid mixing, and Gassmann substitution. Response trends are checked using the exact Zoeppritz solution and two-dimensional staggered-grid finite-difference time-domain (FDTD) modeling, and the fms responses are compared between two dry-frame models. Increasing hydrate saturation from 0 to 80% changes the numerical intercept from −0.0971 to −0.2361; under uniform Wood mixing, the nonlinear free-gas response is concentrated at low saturation; porosity primarily changes the impedance contrast, whereas clay content primarily changes the amplitude variation with incidence angle. Increasing fms raises velocity, makes the intercept more negative, and increases the gradient, but whether the gradient changes sign depends on the frame model.