Estimating the thickness of the free gas zone beneath Hydrate Ridge, Oregon continental margin, from seismic velocities and attenuation

Recent Ocean Drilling Program (ODP) results in the Oregon accretionary pris m and on the Blake Ridge indicate that the zone containing free gas beneath the hydrate-bearing near-surface sediments is considerably thicker than pr eviously thought. In this paper, we present results from travel time invers ion of refracted seismic waves that show very low (<1.85 km/s) velocities e xtending for 500-600 m beneath the base of the gas hydrate stability zone i n Hydrate Ridge on the Oregon continental margin near ODP site 892. The low -velocity near-surface layer extends across Hydrate Ridge and beneath the a djacent continental slope to the east. Because Pliocene sediments are expos ed at the crest of Hydrate Ridge in an erosional setting, we suggest that t hese low velocities indicate the extent of a zone of dispersed free gas rat her than recent sedimentation. Strong frequency-dependent attenuation of am plitudes is observed for P waves crossing this zone. Amplitude spectra, ref erenced to spectra for similar accretionary complex paths that do not cross the interpreted gassy layer, indicate a very low P wave quality factor (Q( P)) within this zone, with Q(P) <similar to> 12 compared to Q(P) > 100 in t he "normal" accretionary complex sediments west of Hydrate Ridge. These res ults suggest that refraction seismic techniques are a powerful way to const rain the depth to which free gas is present in sediments beneath the hydrat e stability zone. Defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating it s impact on slope stability and potential contribution to global climate ch ange.