Deep sea gas hydrate is found within the sediments beneath the continental slope of western Canada. It exists as an ice-like structure, composed of methane and water. The base of the hydrated sediments can be identified by the bottom-simulating reflector (BSR) on seismic reflection records, a high-amplitude reflector parallels the seafloor. In August 1998, a marine multichannel seismic reflection survey was carried out west of Vancouver Island. This well-studied area contains a 30 km wide band of gas hydrate, which occurs approximately 300 m below the sea floor. Data collected along a survey line passing over Ocean Drilling Program (ODP) wells 889/890 forms the basis of this study. Before an analysis of the data was performed, the seismic traces were processed. The first step was to determine the survey geometry. Since the array was not horizontal in the water, accurate receiver depths were calculated using the arrival times of key events. Each trace was time-shifted in order to move the receivers to a horizontal datum. Conventional seismic processing was then completed. These steps resulted in a seismic section that shows the BSR approximately 0.3 seconds below the seafloor reflection. Reflection coefficients were determined for both the seafloor and BSR. In calculating the seafloor reflection coefficients, the reflection coefficient of the air/water interface had to be considered due to scattering of seismic energy at this boundary. The spatial variation in the BSR reflection coefficients suggest that there is a greater concentration of hydrate/free gas beneath the topographic high. This can be explained by upward migrating methane being focussed up a fault zone and accumulating in this region. The frequency dependence of the BSR was also studied. It appears that the BSR has larger reflection amplitudes at lower frequencies, indicating that the hydrate/free gas boundary may be gradational.

West: -128.3000 East: -123.3000 North: 51.0000 South: 48.3000

West: NaN East: NaN North: NaN South: NaN

Expedition: 146

Site: 146-889

Site: 146-890

Provider: SEDIS Publication Catalogue

Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=2002-039499 (c.f. for more detailed metadata)

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