Acoustic studies of submarine gas hydrates on the Cascadia Margin.

Submarine gas hydrates were detected over large areas in the Cascadia margin off the west coast of British Columbia by the presence of bottom simulating reflectors in seismic surveys. Initial estimates of gas quantity based on these data and results from an ocean drilling program leg were highly optimistic, and subsequently many different experiments were carried out to quantify the estimates. This paper presents results from an air gun survey with ocean bottom seismometers deployed around a hydrated cold vent to determine the hydrate concentration in the vicinity of the vent. Reflection travel time data from the survey were used in a linearized inversion to determine the velocity structure within the hydrate stability zone. Reflection amplitude data were analyzed to generate maps of the reflectivity at the sea floor and at the base of the stability zone. The travel time inversion revealed that the sediment velocities were very low, indicating low concentration of hydrate (less than 2% of sediment volume) in the stability zone. The sea floor reflection amplitude data provided a map showing an elliptical extent of the sea floor vent. Inversion of amplitude data from the BSR indicated a small decrease in velocity, less than 7%.