Seismic velocity studies of a gas hydrate bottom-simulating reflector on the northern Cascadia continental margin: Amplitude modeling and full waveform inversion

On the northern Cascadia subduction margin, the multichannel seismic amplit ude-versus-offset (AVO) behavior of a bottom-simulating reflector (BSR) sug gests a P wave velocity change from high-velocity hydrate-bearing sediment to lower velocity sediment containing a small amount of free gas. The obser ved nonlinear. AVO behavior, constant or slightly decreasing amplitudes at near-to-mid offsets and a large-amplitude increase at far offsets, can be r eproduced in the models if an S wave velocity enhancement is assumed as exp ected from hydrate cementation. The AVO behavior of the hydrate BSR is foun d not to be as useful as was earlier thought for determining the amount of free gas below the BSR. This is because Poisson's ratio change below the BS R due to gas is likely very small in high-porosity unconsolidated sediments . The uncertainty in the models is large, as there is no reliable S wave ve locity information for the sediments containing hydrate or gas. AVO modelin g alone is not sufficient to distinguish different velocity models across t he BSR. Our interpretation of the BSR amplitude behavior is that a P wave v elocity increase above the BSR is the main cause of the BSR reflection ampl itude increase at large incidence angles. Caution must be taken in applying AVO analysis, as little is known about S wave velocities. However, subtle differences in BSR amplitude behavior and reflection waveform can provide c onstraints through very careful full waveform inversion. A well-defined ref erence velocity-depth profile is also required to represent water-saturated sediment unaffected by either hydrate or free gas. Using full waveform vel ocity inversion, a high-resolution velocity model for the hydrate BSR has b een derived. The best fit model for the seismic data near the Ocean Drillin g Program (ODP) sites 889/890 consists of a high-velocity zone above the BS R and a thin low-velocity layer below, in agreement with the ODP downhole v elocity data.