A Study of Gas Hydrates With Ocean-Bottom-Seismometer Data on the East Coast of Canada
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A number of BSR locations have been identified along the Scotian Slope from geophysical evidence, but as yet none have been confirmed to contain gas hydrate through direct sampling. Studies near the Mohican Channel (200 km offshore Halifax) show the BSR is around 450 ms below sea floor with a possible underlying low- velocity zone (indicative of free gas in the sediments). Models with hydrate as part of the sediment frame give hydrate concentrations of 2--6% and free gas concentrations of less than 1% (LeBlanc et al 2007). In a joint project between the Geological Survey of Canada and the Dalhousie University, 19 ocean-bottom- seismometers (OBS) were deployed in 2006 to study the geophysical structure of the Mohican Channel BSR. In this area, a clear BSR beneath the channel and in its levee disappears in a direction away from the channel and parallel to slope. This observation may be related to fining of sediments distal to the channel levee, but there are no direct samples in this zone to confirm this hypothesis. Fining of sediment may restrict vertical fluid flow by reducing permeability, as well as reducing pore spaces available for hydrate formation. Wide-angle reflection and refraction data are used to ascertain the thickness of the sub-BSR low-velocity zone as a function of position on the seismic transect away from the Mohican Channel, and to use these thicknesses to calculate the variation in upward fluid flow along slope. Preliminary results show refractions with apparent velocities of 1850 to 1900 m/s for a depth range of 350 to 600 mbsf. However, initial traveltime inversions using refractions and wide-angle reflections indicate no significant low-velocity zone below the BSR. Furthermore, there appears to be no strong lateral velocity contrast between regions with and without BSR observations. Further use of S-wave arrivals from the geophone components may provide additional constraints on hydrate and gas distribution and help to characterize fault patterns.
Schlesinger, A., D. Mosher, J. Cullen, K. Louden, et al. "A Study of Gas Hydrates With Ocean-Bottom-Seismometer Data on the East Coast of Canada." Proceedings of the 2009 American Geophysical Union Joint Assemby, Toronto, Ontario (Canada), 24-27 May 2009.