HR: 0830h
AN: OS51B-0856    [PDF]
TI: Natural Gas and Hydrate Accumulations in the Aleutian Basin of the Bering Sea
AU: * Barth, G A
EM: gbarth@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AU: Scholl, D W
EM: dscholl@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AU: Childs, J R
EM: jchilds@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 999, Menlo Park, CA 94025 United States
AB: The deep water Aleutian Basin of the Bering Sea is a major storehouse of trapped greenhouse gases. Preliminary volume estimates based on seismic reflection observations easily approach 1000 Tcf of natural gas within the basin. USGS seismic reflection data from the deep water ($>$3500 m) Bering Sea region include over 20,000 km of single channel profiles coincident with GLORIA sonar tracks acquired during the 1986-7 EEZscan program, plus several older multichannel lines also crossing the deep water Aleutian and Bowers Basins. These airgun-source data all provide images from seafloor to basement, over 3 km of penetration. The basin fill includes generally horizontal and uniform sedimentary reflection sequences, comprising predominantly mudstones and distal turbidites, upon oceanic crust of probable Cretaceous age. In seismic reflection images of these flat-lying sediments, methane chimneys overlain by hydrate caps stand out as distinctive velocity pseudostructures. These velocity-amplitude anomaly structures ("VAMPs") characteristically include a zone of velocity pull-up (attributed to high-seismic-velocity hydrate within the sediment) directly overlying a zone of velocity push-down (attributed to low-seismic-velocity gas in the pore spaces). A prominent hydrate bottom simulating reflection (BSR) is also present throughout the basin. Within the VAMPs, the BSR roughly separates the pull-up from the push-down. Hundreds of VAMPs have been imaged, and thousands must exist within the deep water basin. Individual examples vary widely in lateral extent, focused appearance, and amplitude effects. Some also present a seafloor manifestation of slight doming. Five example case studies are presented, focusing particularly on interval travel time anomalies and quantitative interpretation in terms of presence of hydrate and free gas. Individual large VAMPs (1-3 km across, $\sim$30 ms pull-up, $\sim$80 ms push-down) are estimated to contain gas volumes (including hydrate) similar to those of conventional economic gas fields (e.g. $>$10$^{9}$ m$^{3}$). It is intriguing to note that the VAMPs are predominantly located within the "donut hole" of international waters in mid-Bering Sea, beyond the present territorial limits of any nation. However, the majority of the basin could be claimed by the United States under Article 76 of the United Nations Convention on the Law of the Sea, were it ratified.
DE: 0900 EXPLORATION GEOPHYSICS
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting