3-D seismic investigations of northern Cascadia marine gas hydrates

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dc.contributor.author Riedel, Michael
dc.date.accessioned 2018-10-24T00:10:21Z
dc.date.available 2018-10-24T00:10:21Z
dc.date.copyright 2001 en_US
dc.date.issued 2018-10-23
dc.identifier.uri http://hdl.handle.net/1828/10177
dc.description.abstract This dissertation presents results from 3-D (parallel 2-D) high resolution seismic surveys and associated studies over an area with deep sea gas hydrate occurrence. The study area is located on the accretionary prism of the northern Cascadia subduction zone offshore Vancouver Island, Canada. The major objectives of this study were the imaging of a gas/fluid vent field found in the study area and detailed mapping of the tectonic setting and geological controls on fluid/gas venting. Secondary objectives were the characterization of the gas hydrate occurrence and constraints on the seismic nature of the bottom-simulating reflector (BSR) and its spatial distribution. The main grid was 40 lines at 100 m spacing with eight perpendicular crossing lines of multichannel and single channel seismic reflection, and 3.5 kHz subbottom profiler data. In addition to the main 3-D seismic grid, two smaller single channel grids (25 m spacing) were collected over the vent field. The multichannel seismic data acquired with the Canadian Ocean Acoustic Measurement System (COAMS) streamer required correction for irregular towing depth and shot point spacing. A new array element localization (AEL) technique was developed to calculate receiver depth and offset. The individual receiver depths along the COAMS streamer varied between 10-40 m, which resulted in the occurrence of a prominent receiver ghost that could not be completely removed from the seismic data. The ghost resulted in limited vertical resolution and a coarse velocity depth function. The vent field is characterized by several blank zones that are related to near-surface deformation and faulting. These zones are 80-400 m wide and can be traced downward through the upper 100-200 m thick slope sediment section until they are lost in the accreted sediments that lack coherent layered reflectivity. The blank zones are also characterized by high amplitude rims that are concluded to result from the interference effect of diffractions. These diffractions result due to relatively sharp discontinuities in the sediment physical properties at the blank zone boundary. 2-D vertical incidence seismic modeling suggests an increase in P-wave velocity inside of the blank zone with only minor changes in density. Blanking is believed to be mainly the effect of increased hydrate formation within the fault planes. The faults are conduits for upward migrating fluids and methane gas that is converted into hydrate once it reaches the hydrate stability field. Carbonate formations at the seafloor can also contribute to blanking especially at higher frequencies. Free gas may be present in case of full hydrate saturation or strong fluid flow. Geochemical analyses of pore water and water-column samples carried out in cooperation with Scripps Institute of Oceanography indicate relatively low fluid fluxes of less than 1 mm/yr and there is no heat flow anomaly present over the vent field. Methane concentrations of 20 n-moles/L (about 8 times the ocean background concentration) were detected in water-column samples of the first 100-200 m above the main blank zone of the vent field. Venting is also believed to be strongly episodic with a recently more quiet time. However, the observed carbonate crusts indicate a long-term activity of the vents. en_US
dc.language English eng
dc.language.iso en en_US
dc.rights Available to the World Wide Web en_US
dc.subject Chemical oceanography en_US
dc.subject British Columbia en_US
dc.subject Vancouver Island en_US
dc.subject Gas fields en_US
dc.subject Oceanography en_US
dc.subject Pacific Coast en_US
dc.title 3-D seismic investigations of northern Cascadia marine gas hydrates en_US
dc.type Thesis en_US
dc.contributor.supervisor Hyndman, R. D.
dc.degree.department School of Earth and Ocean Sciences en_US
dc.degree.level Doctor of Philosophy Ph.D. en_US
dc.description.scholarlevel Graduate en_US

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