HR: 1340h
AN: OS23A-1052    [Abstracts]
TI: Gas Hydrate Accumulations – Where and how Much? Can Seismic Reflection Intensities Provide the Answers?
AU: * Edsall, D W
EM: edsall@usna.edu
AF: United States Naval Academy, Physics Department, Annapolis, MD 21402,
AU: Coffin, R B
EM: richard.coffin@nrl.navy.mil
AF: Naval Research Laboratory, 4555 Overlook Ave., Washington, DC 20375,
AB: Location of gas hydrate accumulations on seismic reflection records provides qualitative overview of distributions. This data analysis is supported with shallow pore water geochemical profiles and vertical fluid migration measured with heatflow probes. These indirect measurements of acoustic signal intensities need further evaluation to provide quantitative estimates of the hydrate loading between sites and within sites. Geophysical and geochemical data are compared for three different continental margin locations -two passive (Atwater Valley and Alaminos Canyon, Gulf of Mexico) and one active (Hikurangi, New Zealand). In order to provide quantitative estimates of subbottom hydrate distributions we have interpreted the following features on records from these three locations. BSR; seismic blanking zones above the BSR which are adjacent to vertical seismic blanking zones; strong reflectors below the BSR and regions tentatively identified as perched BSRs. Seismic interpretations are integrated with pore water geochemical and vertical fluid flux profiles. While the Alaminos Canyon profile is in deeper water than the Hikurangi Margin profile, the same interpretation of seismic features applies. Both areas have variable BSR signal strength along this transect and clearly exhibit well-developed seismic blanking areas, regions of vertical flux and strong reflectors below the BSR. In addition, both records show the existence of perched BSRs. Our recognition of the perched BSR follows the location of faults that cross the lower BSR and terminate at the base of an overlying free-gas pocket indicated by the presence of a very strong acoustic reflector. Beneath the gas accumulation, the seismic blanking indicates that gas hydrate is present. The difference between these two sites is primarily in the strength and continuity of the reflective horizons, which are initially interpreted as a function of the water depth and the presence of avenues for vertical migration. Atwater Valley provides further seismic reflection data calibration for comparing Hikurangi Margin and Alaminos Canyon interpretations. In this region, salt diapirs are present below the hydrate stability zone. Geochemical pore water and vertical fluid migration profiles indicate high vertical gas fluxes from deep hydrate deposits. However, high pore water chloride concentrations in these profiles, lead to the conclusion that salt diapirs cause lower hydrate stability and rapid vertical gas migration. Notably absence on the seismic reflection record for Atwater Valley is a perched BSR. This is explained by the upward migration of the underlying salt dome and the associated tensional faulting which provides the necessary pathways for rapid vertical gas flux.
DE: 0910 Data processing
DE: 1030 Geochemical cycles (0330)
DE: 4820 Gases
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting