HR: 13:40h
AN: OS52C-01    [PDF]
TI: Gas hydrate distribution and its relationship to free-gas sources within southern Hydrate Ridge: Results from recent seismic experiments
AU: * Bangs, N L
EM: nathan@utig.ig.utexas.edu
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, TX 78759
AU: Pecher, I
EM: i.pecher@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, 000 New Zealand
AU: Trehu, A M
EM: trehu@coas.oregonstate.edu
AF: College of Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
AB: In August and September, 2002, we conducted a two-ship seismic experiment with the R/V Ewing and the D/V Resolution in coordination with Ocean Drilling Program Leg 204 on Hydrate Ridge, offshore Oregon. We acquired three types of vertical seismic profiles, (zero-offset, constant offset, and walkaway VSP) at four drill sites, and ocean bottom seismometer refraction and seismic reflection data across southern Hydrate Ridge using a high-resolution seismic acquisition system. At Site 1247, which lies near the summit of southern Hydrate Ridge, the constant offset VSP records have two arrivals interpreted as primary-to-shear wave conversions from horizons 46 and 109 m below seafloor, 79 and 16 m above the bottom simulation reflection (BSR) respectively. These events correlate with the top of two hydrate intervals observed in cores and inferred from pore fluid geochemistry. The deeper event also correlates with a seismic reflection, which is discordant with local stratigraphy, and is interpreted as a reflection from the top of the hydrate layer. Both VSP and seismic reflection observations are evidence for a regional hydrate layer that extends at least 250 m around the borehole and directly above the intersection of an inferred free-gas source horizon and the BSR. Seismic p-wave velocities were also measured from the high-resolution seismic reflection data to infer hydrate concentrations within the hydrate stability zone. Preliminary results show that north of the summit of southern Hydrate Ridge, near Sites 1244, 1245, and 1246, seismic velocities are notably higher on the flanks of the ridge than at the ridge crest (1700 m/s between the seafloor and BSR on the flanks vs. 1580 m/s in the equivalent interval on the crest). At the summit of southern Hydrate Ridge, near Sites 1249 and 1250, velocities are locally high (1680 m/s in the interval between the seafloor and BSR at the summit vs. 1580 m/s in adjacent, equivalent intervals). VSP and seismic interval velocity data are consistent with hydrate accumulations above the intersection of the inferred free gas source horizons and the BSR.
DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5104 Fracture and flow
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting