HR: 0800h
AN: OS41C-0492    [Abstracts]
TI: Estimation of Free Gas Saturation Using AVO Analysis on 3D Seismic Data at South Hydrate Ridge, Cascadia Accretionary Complex
AU: * Xun, H
EM: hao@ig.utexas.edu
AF: The University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd. Building 600, Austin, TX 78759 United States
AU: Bangs, N L
EM: nathan@ig.utexas.edu
AF: The University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd. Building 600, Austin, TX 78759 United States
AU: Stoffa, P L
EM: pauls@ig.utexas.edu
AF: The University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd. Building 600, Austin, TX 78759 United States
AB: In 2000 we conducted a high-resolution 3D seismic survey of a 4$\times$10 km$^{2}$ region on south Hydrate Ridge on the Oregon continental margin. The objective of the survey is to characterize the regional pattern of fluid and gas migration and its relationship to hydrate accumulations on Hydrate Ridge. These data were acquired with a high-resolution seismic source with source-receiver offsets of up to 644 m, which results in incidence angles of up to 20 degrees. In 2002 new 2D seismic data with source-receiver offsets of up to 1500 m, producing incidence angles of up to 40 degrees, were collected during R/V Ewing Cruise EW0208, which was coordinated with Ocean Drilling Program (ODP) Leg 204. Prior to AVO analysis, we conducted true amplitude recovery by using the seismic range equation, and true amplitude processing through prestack time migration. We also calibrated the 3D data by using the 2D data to remove unfavorable acquisition effects in the 3D survey. On prestack-migrated gathers, we measured the seismic amplitude from the three surfaces associated with free gas accumulations: 1) the bottom simulating reflection (BSR), 2) Horizon A, and 3) Horizon B'. Horizon A is a $\sim$4-m-thick turbidite interval where it was drilled during ODP Leg 204 and it is identified as a primary conduit along which free gas migrates from deep sources to the summit of Hydrate Ridge. Horizon A focuses gas hydrate formation and feeds gas vents near the southern summit of Hydrate Ridge. Horizon B' is a volcanic glass-rich horizon that is also a stratigraphically defined gas conduit. Intercept and slope are obtained from AVO fitting, and the Poisson's ratio is obtained by using intercept and slope with the constraints of well-log data. Gas saturation for the three surfaces is estimated by fluid substitution technique. The distribution of free gas inferred from AVO analysis shows that the highest free gas concentrations lie directly beneath the highest gas hydrate concentrations estimated from Leg 204 core data. For Horizon A, the gas saturation increases toward the summit as Horizon A shallows. Free gas within Horizon A is distributed in isolated pockets rather than in continuously connected veins.
DE: 8010 Fractures and faults
DE: 5102 Acoustic properties
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3025 Marine seismics (0935)
DE: 0910 Data processing
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting