Seismic characteristics of gas hydrate system at the Hydrate Ridge, offshore Oregon

Seismic wave velocities vary in the presence of gas hydrate and free gas in the sediments. Seismic properties (velocities) of the gas-hydrate bearing sediments allow us to identify the presence of gas hydrates, to study their character, formation and distribution, and to estimate the amount of gas hydrate and/or free gas that may be present in the sediments. Accuracy in the estimation of distribution and saturation of gas hydrates and free gas depends on the interval velocities of P- and S-waves. We have carried out an interactive velocity analysis of P- and converted S-waves in the tau-p (intercept time – ray parameters) domain,<br>which directly gives the interval velocities. This requires multicomponent seismic data. A two-ship seismic experiment was carried out (to record multicomponent seismic data) in summer 2002 at the Hydrate Ridge to map the gas hydrate. Our approach to multicomponent velocity analysis comprises three steps: 1) P-wave velocity analysis, 2) PP to PS event correlation, and 3) Swave velocity analysis. PP to PS correlation is performed using synthetic seismograms. Observed velocities are matched with modeled velocities to estimate gas hydrate saturation. P- and S-wave velocities are modeled with a “Modified Wood equation” which is a modification of Wood equation with a rock physics model and an empirical relation, respectively. We present results from the multicomponent ocean bottom seismometer data recorded at the Hydrate Ridge, offshore Oregon. The Pwave velocity is found to be more sensitive to the saturation of gas hydrates and free gas than S-wave velocity. Gas hydrate is estimated to be upto 7% of rock volume (12% of pore space). The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus we conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region.