| Summary: | At the frontal ridge near the base of the slope off Vancouver Island, wide-angle ocean bottom seismometer (OBS) data were acquired in summer 2005, in support of the Integrated Ocean Drilling Program (IODP) Expedition 311. Marine gas hydrate is present beneath the ridge based on the observation of the 'Bottom Simulating Reflector' (BSR) that is interpreted to coincide with the base of the methane hydrate stability zone. Hydrate was also observed in downhole logs and drilling by IODP. The BSR has been identified on single-channel seismic data at -250-260 m depth beneath the ridge crest and on its seaward slope. The OBS data have been analyzed with the objective of determining the velocity structure in the upper portion of the accretionary wedge especially the hydrate stability zone and underlying free gas. As identified by a clear refracted phase, the velocity structure above the BSR shows anomalous high velocities of about 1.95 (±0.5) km/s at shallow depths of 80 - 110 m. On vertical incidence data, high amplitude reflectors are observed near this depth. Below the BSR, the velocities increase to -2.4 km/s at sub-seafloor depths of about 600 m. A strong refracted phase with a velocity of 4.0 km/s is generated at a depth of about 1700 mbsf. Velocities from traveltime inversion of OBS data are in general agreement with the Integrated Ocean Drilling Program (IODP) X311 downhole sonic velocities. In particular, on the log data, a layer with low porosity and high velocities of 2.4 - 2.8 km/s was observed at depths of 50 - 75 m. This probably corresponds with the 1.95 km/s layer at depths of 80-110 m interpreted from the OBS data. The refraction data thus suggest that this high-velocity layer varies laterally through the frontal ridge region, out to distances of at least 4 km from the drillhole. BSR depths (250-280 m) estimated in the present work also agree with the IODP X311 depths. From the velocity structure, we can make estimates of hydrate concentration in a region close to the deformation front, where fluid flow ...
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