Wood, Warren T.¹, George D. Spence², John Pohlman³, Dennis A. Lindwall¹, Charles H. Megnin¹, Richard B. Coffin³, Joseph F. Gettrust¹ 
(1) Naval Research Laboratory, Stennis Space Center, MS 
(2) University of Victoria, Victoria, B.C 
(3) Naval Reseach Laboratory, Washington, 

ABSTRACT: High Resolution Deep-Towed Seismic Investigation of Fluid Expulsion Sites in the Gas Hydrate Province off Vancouver Island, Canada

High resolution seismic data was acquired with the Naval Research Laboratory’s DTAGS, (deep towed acoustics/geophysics system), at several sites of known fluid expulsion on the Cascadia margin off Vancouver Island, Canada in October of 2002. At the “Bullseye” vent an acoustic bottom transponder constellation was deployed so that the position of the deep-towed source and receiver could be monitored at all times with an accuracy of 1-2 meters. Similar accuracy was also achieved for a profile of co-located piston cores, two of which encountered massive hydrate in the near surface of the vent. The seismic images exhibit several vertical zones of significant attenuation or seismic wipeout especially intense below the Bullseye vent, whose surface expression is a 6-m-high discontinuous near-circular rim. Throughout the approximately 10 lines that pass through the Bullseye area, no surface expression is observed for the many other wipeouts in the area, that exhibit a range of widths, depths from the surface, penetration depths, and intensities. The presumed existence of gas near the seafloor within what is expected to be the gas hydrate stability zone suggests either a strong perturbation to the gas hydrate stability zone or a dynamic, or a non-equilibrium system in which gas and hydrate coexist. Each scenario has different implications for gas hydrate dissociation in the event of a sea level or bottom water temperature change.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.