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

He, Tao, Spence, George D., Wood, Warren T., Riedel, Michael and Hyndman, Roy D. (2009) Imaging a hydrate-related cold vent offshore Vancouver Island from deep-towed multichannel seismic data. Geophysics, 74 (2). B23-B36. DOI 10.1190/1.3072620.

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The Bullseye vent, an approximately 500-m-diameter deep-
sea, hydrate-related cold vent on the midslope offshore Vancou-
ver Island, was imaged in a high-resolution multichannel survey
by the Deep-towedAcoustics and Geophysics System
DTAGS. The structure was drilled by the Integrated Ocean Drilling Pro-
gram at site U1328. Towed about 300 m above the seafloor, thehigh-frequency
220–820 Hz DTAGS system provides a high
vertical and lateral resolution image. The major problems in im-
aging with DTAGS data are nonlinear variations of the source
depths and receiver locations. The high-frequency, short-wave-
length data require very accurate positioning of source and re-
ceivers for stacking and velocity analyses. New routines were de-
veloped for optimal processing, including receiver cable geome-
try 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-re-
flection-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 frac-
ture zone. Velocity analyses were not possible in the vent struc-
ture but were obtained 180–270 m to either side. Normal veloci-
ties 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 approximate-
ly 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.

Document Type: Article
Keywords: hydrate-related cold vent; seismic data; Bullseye vent; Vancouver Island
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1190/1.3072620
ISSN: 0016-8033
Date Deposited: 18 Nov 2015 08:37
Last Modified: 18 Nov 2015 08:37

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