Marine gas hydrates : seismic analyses and thermal regime
Date
1998
Authors
Ganguly, Nilanjan
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Abstract
In September 1996, a collaborative marine seismic experiment between the University of Victoria and the Geological Survey of Canada was carried out to investigate the marine gas hydrate layer found on the continental slope off Vancouver Island. The methane hydrate bottom simulating reflector (BSR), which is at the base of the hydrate stability zone, was examined using high resolution short-offset multichannel seismic reflection data. The BSR was studied in two regions: (1) near the top of the continental slope (water depth as shallow as 500 m), where the BSR was expected to approach the seafloor, and (2) in areas of topographic highs where the amplitudes of the BSR were expected to be the highest. The study included: (1) mapping the heat flow from the depth of the BSR , and (2) calculation of the seafloor and BSR reflection coefficients.
Detailed analysis of the seismic data revealed the heat flow patterns of the region, estimated from the depth of the BSR. The average heat flow of the region was found to be between 70 and 80 mW/m2. Although there are many second order variations, there is a clear regional trend of heat flow decreasing landward across the continental margin. This regional trend reflects the combined effects of the processes of sedimentation in the Cascadia basin, tectonic sediment thickening in the accretionary prism and subduction of the Juan de Fuca plate. In addition, a consistent pattern of high heat flow over the flanks of topographic highs and low heat flow over the topographic highs themselves was observed. This correlation between heat flow and seafloor topography was attributed mainly to the effects of topography on superficial thermal gradients. No strong evidence of channeled migration of fluids along faults and other hydrologic conduits was observed.
In the second part of the thesis, reflection coefficients of the seafloor and the BSR were examined. No correlation between seafloor reflection coefficients and seafloor topography was detected. However, a certain correlation between the BSR reflection coefficients and the seafloor topography was observed. High reflection coefficients for the BSR were also observed over a channel between two topographic highs.