Currie, Claire (1999): Deep sea gas hydrate; a seismic study over the Cascadia accretionary prism, offshore Vancouver Island

Leg/Site/Hole:
ODP 146
ODP 146 889
ODP 146 890
Identifier:
2002-039499
georefid

Creator:
Currie, Claire
author

Identification:
Deep sea gas hydrate; a seismic study over the Cascadia accretionary prism, offshore Vancouver Island
1999
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.
English
Thesis or Dissertation
Coverage:Geographic coordinates:
North:51.0000
West:-128.3000East: -123.3000
South:48.3000

Oceanography; Applied geophysics; accretionary wedges; aliphatic hydrocarbons; alkanes; bottom-simulating reflectors; British Columbia; Canada; Cascadia subduction zone; continental slope; East Pacific; fault zones; faults; gas hydrates; geophysical methods; geophysical profiles; geophysical surveys; hydrocarbons; Leg 146; marine methods; methane; North Pacific; Northeast Pacific; Ocean Drilling Program; ODP Site 889; ODP Site 890; organic compounds; Pacific Ocean; seismic methods; seismic profiles; surveys; Vancouver Island; Western Canada;

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