HR: 09:15h
AN: OS21A-06    [Abstracts]
TI: Seismic Reflection Blank Zones in Ulleung Basin, Offshore Korea, Associated With High Concentrations of Gas Hydrate
AU: * Stoian, I
EM: istoian@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada
AU: Park, K
AF: Korea Institute of Geoscience and Mineral Resources, 30 Gajeongdong, Yuseong-gu, Daejeon, 305-350, Korea, Republic of
AU: Yoo, D
AF: Korea Institute of Geoscience and Mineral Resources, 30 Gajeongdong, Yuseong-gu, Daejeon, 305-350, Korea, Republic of
AU: Haacke, R R
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada
AU: Haacke, R R
AF: Natural Resources Canada, Geological Survey of Canada-Pacific, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Riedel, M
AF: Earth and Planetary Sciences, McGill University, 3450 University St., Montreal, QC H3A 2A7, Canada
AU: Hyndman, R D
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada
AU: Hyndman, R D
AF: Natural Resources Canada, Geological Survey of Canada-Pacific, 9860 West Saanich Road, Sidney, BC V8L 4B2, Canada
AU: Spence, G D
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada
AB: It has recently been recognized that high concentrations of gas hydrate occur in localized zones of upwelling fluids. We report a study of multi-channel seismic reflection data across such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few bottom-simulating reflectors. The structures are commonly up to several km across and a few hundred metres tall, and are characterized by seismic blanking (reduction in reflectivity) and upbowing (in time sections) of the sediment reflectors. We interpret the seismic pull-up to be mainly a velocity effect, consistent with velocities derived from stacking analysis, although physical deformation due to faults imaged on time-migrated sections, is not ruled out. Structures associated with geological indicators of a high upward flux of gases show the largest amount of pull-up for reflectors close to the seafloor, and a decrease in the rate of change of seismic pull-up with increasing depth. This implies the presence of concentrated gas hydrate in the near-surface layers. Other structures, associated with a moderate upward flux of gases, show the largest increase of seismic pull-up close to the base of the gas hydrate stability zone. The spectral character of the rays travelling through the body of the blanked zones is not significantly different to rays travelling outside these zones, which indicates that the blanking results from loss of reflectivity of the hydrate-filled sediments, rather than an intrinsic attenuation caused by hydrate. The correlation between low velocities and reduced reflection amplitudes point to the accumulation of free gas several kilometres beneath the base of gas hydrate stability zone. This gas migrates upwards to feed the vents through near-vertical conduits, in addition to the regional, upward fluid flow caused by tectonic compression of the basin.
DE: 0714 Clathrate
DE: 0910 Data processing
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting