Imaging a hydrate-related cold vent offshore Vancouver Island from deep-towed multichannel seismic data

The Bullseye vent, an approximately 500-m-diameter deep-sea, hydrate-related cold vent on the midslope offshore Vancouver Island, was imaged in a high-resolution multichannel survey by the Deep-towed Acoustics and Geophysics System (DTAGS) The structure was drilled by the Integrated Ocean Drilling Program at site U1328. Towed about 300 m above the seafloor, the high-frequency (220-820 Hz) DTAGS system provides a high vertical and lateral resolution image. The major problems in imaging with DTAGS data are nonlinear variations of the source depths and receiver locations. The high-frequency, short-wavelength data require very accurate positioning of source and receivers for stacking and velocity analyses. New routines were developed for optimal processing, including receiver cable geometry estimation from node depths, direct arrivals and sea-surface reflections using a genetic algorithm inversion method, and acoustic image stitching based on relative source positioning by crosscorrelating redundant data between two adjacent shots. Semblance seismic velocity analysis was applied to common-reflection-point bins of the corrected data. The processed images resolve many subvertical zones of low seismic reflectivity and fine details of subseafloor sediment structure. At the Bullseye vent, where a 40-m-thick near-surface massive hydrate layer was drilled at U1328, the images resolve the upper part of the layer as a dipping high-reflectivity zone, likely corresponding to a fracture zone. Velocity analyses were not possible in the vent structure but were obtained 180-270 m to either side. Normal velocities are in the upper 50 m, but over the interval from 50 to 100 m below the seafloor at the northeast side, the velocities are higher than the average normal slope sediment velocity of approximately 1590 m/s. These high velocities are probably related to the high reflectivity zone and to the bottom portion of the massive hydrate detected by resistivity measurements in the upper 40 m at U1328.