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 prism and on the Blake Ridge indicate that the zone containing free gas beneath the hydrate‐bearing near‐surface sediments is considerably thicker than previously thought. In this paper, we present results from travel time inversion of refracted seismic waves that show very low (<1.85 km/s) velocities extending for 500–600 m beneath the base of the gas hydrate stability zone in Hydrate Ridge on the Oregon continental margin near ODP site 892. The low‐velocity near‐surface layer extends across Hydrate Ridge and beneath the adjacent continental slope to the east. Because Pliocene sediments are exposed at the crest of Hydrate Ridge in an erosional setting, we suggest that these low velocities indicate the extent of a zone of dispersed free gas rather than recent sedimentation. Strong frequency‐dependent attenuation of amplitudes is observed for P waves crossing this zone. Amplitude spectra, referenced to spectra for similar accretionary complex paths that do not cross the interpreted gassy layer, indicate a very low P wave quality factor (Qp) within this zone, with Qp∼12 compared to Qp>100 in the “normal” accretionary complex sediments west of Hydrate Ridge. These results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate 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 its impact on slope stability and potential contribution to global climate change.