Imaging a Hydrate-Related Cold Vent Offshore Vancouver Island From Deep- Towed Multichannel Seismic Data

摘要

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 imcrosscorrelating 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 sub seafloor 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.