High‐resolution, deep‐towed, multichannel seismic survey of deep‐sea gas hydrates off western Canada

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Abstract
A multichannel seismic survey was carried out using the high‐resolution deep‐towed acoustics/geophysics system (DTAGS) to image the structure of deep‐sea gas hydrates on the continental slope off Vancouver Island and to determine the velocity profile of the hydrated sediments. The high‐frequency DTAGS data provide the means to estimate the frequency response of the bottom simulating reflector (BSR) that defines the base of the hydrate stability field in these sediments, over a broad frequency band from 15 to 650 Hz. The DTAGS sections resolved fine‐scale layering as thin as a few meters within the hydrated zone and below the BSR, and they revealed small‐scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas. Interval velocities determined from the DTAGS data indicate uniformly low values of about 1500 m/s to depths of 100 m below sea floor (mbsf), increasing to about 1850 m/s at the BSR (250 mbsf). The reflection from the BSR that is normally well defined in conventional low‐frequency seismic surveys is at least twenty times weaker at the high DTAGS frequencies. The reflection coefficient‐versus‐frequency data support a new model for the velocity profile at the BSR that consists of a thin, 4–8‐m layer at the BSR in which the velocity decreases by 250 m/s. The thin transition layer at the BSR implies relatively high methane flux rates of at least 1.5 mm/year.