Crustal structure and marine gas hydrate studies near Vancouver Island using seismic tomography




Dash, Ranjan Kumar

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This dissertation work applies seismic tomographic inversion methods to two different datasets - one to address the earthquake hazard within the Strait of Georgia and the other to estimate hydrate concentration and distribution in the continental slope off Vancouver Island. In the first part of the study, seismic refraction/wide-angle reflection data from onshore-offshore experiments in 1998 and 2002 were inverted for a smooth three-dimensional (3D) velocity structure down to depths of 6-7 km beneath the Strait of Georgia, a seismically active region where an earthquake swarm (with magnitude up to 5) occurred in 1995-1997. The objectives were to map structures that contribute to seismic hazard evaluation in the Georgia Basin. The main structural features obtained from the inversion are: a northeast-southwest trending hinge line at the location of the earthquake swarm, where the basin deepens rapidly to the southeast; a northwest-southeast trending velocity discontinuity that correlates well with the surface expression of the shallow Outer Island fault; sediment thickening from north to south; and basement uplift at the San Juan Islands, possibly caused by a thrust fault. In the second part of the dissertation, seismic single channel and wide-angle reflection data collected in September 2005 were analyzed for a 2D profile of ocean bottom seis¬mometers (OBSs) on the continental slope region off Vancouver Island, near ODP Site 889 and IODP Site 1327. The objectives were to determine the shallow sediment velocity structure associated with marine gas hydrates and to estimate the hydrate concentration in the sediment pore space. Combined inversion of single channel and OBS data produced a P-wave velocity model down to the depth of the BSR at 230 m below seafloor. Strong attenuation of P-waves below the BSR indicates the presence of free gas. To investigate structures below the BSR, forward modelling of S-waves was carried out using the data. from the OBS horizontal components. Both the P- and S-wave models match very well with the sonic log data from ODP Site 889 and IODP Site 1327. The increase in P-wave velocity of the hydrate bearing sediments relative to the background no-hydrate velocity was utilized to estimate the hydrate concentration by using a simple porosity-reduction equation. An average concentration of 15% was estimated from the P-wave velocity model. Prestack depth migration was applied to the OBS data to image the structure along the 2D profile containing the OBSs. The primary and multiple arrivals were migrated separately. Conventional migration of the primary arrivals produced an image with a very narrow illumination and the shallow subsurface layers including the seabottom were not imaged. However, migration of the OBS multiples, using a mirror imaging technique, pro¬duced a continuous structural image of the subsurface including the shallowest layers. The lateral illumination is much wider with a quality comparable to that of vertical incidence reflection data.



Gas hydrates, Vancouver Island, B.C.