HR: 0830h
AB: Between June 19th and July 3rd, 2000 we conducted a high-resolution, 3-D seismic investigation of Hydrate Ridge on the Oregon continental margin to image structures that control the migration of methane-rich fluid and free gas in this high-fluid-flux convergent margin setting. We acquired a volume of 3-D seismic reflection data with the Lamont-Doherty Earth Observatory portable high-resolution seismic acquisition system on board the {\it R/V Thompson}. The 3-D survey comprises 81 11-km-long lines shot 50 m apart, with a shot spacing of 15 m and a record length of 4 seconds. Shots were fired using two synchronized 40 in$^{3}$ GI airguns and recorded on a 600 m, 48-channel streamer towed 3 m below the sea surface. The system recorded reflections between 30 - 200 Hz with vertical resolution approximately 5 m. In addition to the seismic reflection data acquisition, we deployed 21 ocean bottom seismometers (OBSs) across Hydrate Ridge to record airgun shots and obtain wide-aperture refraction data. The survey data volume covers a 4 x 11 km area across the top of southern Hydrate Ridge 10 km south of Ocean Drilling Program (ODP) Site 892 and provides a framework for ODP drilling planned for 2002. Preliminary examination of the seismic reflection data indicates subsurface coverage to a depth of about 1 km, although this will probably be extended downward by incorporation of information from the OBSs. The data reveal a consistently stronger bottom-simulating reflection (BSR) in one of the two distinct sedimentary sequences in which hydrates have accumulated. Where the BSR forms in the accretionary complex it is generally 2-4 times higher in amplitude than where it forms in the 300 - 500 m thick sequences of gently deformed slope basin sediment deposited on the accretionary complex. Numerous "bright" reflections beneath the gas hydrate stability zone terminate at the BSR in both the accretionary wedge and slope basin sediment. Bright reflections are presumably indicators of free gas beneath the hydrate. We infer that free gas is common beneath the BSR throughout Hydrate Ridge, but on the basis of models by Xu and Ruppel (1999) a consistently higher amplitude BSR in the accretionary wedge sediments may be an indication of higher permeability and thus higher methane flux in the accretionary wedge sediment than within the slope cover. The lack of slope cover at the crest of the ridge may enhance the flux of methane. The data also indicate local concentrations of gas and/or hydrate both within and beneath the gas hydrate stability zone underlying the southern summit of Hydrate Ridge, where massive gas hydrates have been collected from the seafloor and where an authigenic carbonate chimney is forming. Full 3-D processing and analysis of the data is planned to image in detail the pathways feeding these structures.
AN: OS61B-22
TI: High-Resolution 3-D Seismic Imaging of an Active Margin Gas Hydrate-Free Gas System Beneath Hydrate Ridge, Offshore Oregon
AU: * Bangs, N L
EM: nathan@utig.ig.utexas.edu
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759 United States
AU: Trehu, A M
AF: College of Ocean and Atmospheric Sciences, Oregon State University, Ocean Admin. Bld. 104, Corvallis, OR 97331 United States
AU: Nakamura, Y
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759 United States
AU: Diebold, J B
AF: Lamont-Doherty Earth Observatory, Rte 9W , Palisades, NY 10964 United States
AU: Potter, S
AF: College of Ocean and Atmospheric Sciences, Oregon State University, Ocean Admin. Bld. 104, Corvallis, OR 97331 United States
AU: Johnson, J
AF: College of Ocean and Atmospheric Sciences, Oregon State University, Ocean Admin. Bld. 104, Corvallis, OR 97331 United States
AU: Zhang, L
AF: College of Ocean and Atmospheric Sciences, Oregon State University, Ocean Admin. Bld. 104, Corvallis, OR 97331 United States
AU: Mukherjee, A
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759 United States
DE: 8005 Folds and folding
DE: 8105 Continental margins and sedimentary basins
DE: 4806 Carbon cycling
DE: 4820 Gases
DE: 5114 Permeability and porosity
SC: OS
MN: Fall Meeting 2000