COMBINED VERTICAL-INCIDENCE AND WIDE-ANGLE SEISMIC STUDY OF A GAS HYDRATE ZONE, BLAKE RIDGE

Wide-angle ocean bottom seismic data and single-channel seismic reflection data were collected in June 1992 over an area where gas hydrates are thought to be extensive on Blake Ridge, offshore South Carolina. Wide-angle reflections were observed on four Woods Hole Oceanographic Institution ocean bottom hydrophones at offsets up to 15 km. Results from traveltime inversion show that the bottom simulating reflector (BSR) that marks the base of the hydrate stability field lies 400-500 m below the seafloor and is overlain by a 200- to 300-m-thick layer of average velocity 1.9 km s(-1). There is no evidence for significant lateral velocity variation associated with lateral changes in BSR character. Ray-tracing calculations show that the observation of the BSR out to large offsets (7 km) constrains the maximum vertical velocity gradient to about 0.5 s(-1). Amplitude-versus-offset (AVO) analysis was performed for ranges up to about 6 km and incidence angles of more than 70 degrees. The high amplitude of the BSR in the normal incidence data can be explained by two models, both of which include a strong negative impedance contrast: the ''hydrate wedge'' model and the free gas layer model. Synthetic seismogram calculation for the wedge model, using the reflectivity method, shows a strong increase in the BSR amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge. This amplitude increase is clearly not observed in the data, which shows only a relatively modest increase in amplitude with offset. This observation agrees better with the calculated AVO for the gas layer model. The lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and/or that the base of the gas layer is a gradational boundary.