3-D travel time tomography of the gas hydrate area offshore Vancouver Island based on OBS data




Zykov, Mikhail Mikhailovich

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This dissertation presents results from a complex seismic study using Ocean Bot¬tom Seismometers (OBS) conducted at a site of deep sea gas hydrate occurrence. The site is located on the accretionary margin of the northern Cascadia subduction zone offshore Vancouver Island. Canada. The major objectives for this study were the construction of a 3-D velocity model around the Bullseye vent zone by the means of travel time inversion tomography and the analysis of the amplitude data for reflections from the water-sediment interface and the bottom simulating reflector (BSR). Secondary objectives included the integration of the results from this study with previous knowledge about the vent zone for further clarification of its structure and evolution. The OBS seismic data set consisted of 22 parallel lines at 200 in spacing with three perpendicular crossing lines recorded on five OBS stations. Multichannel and single channel conventional seismic data along these lines were also acquired. The OBS experiment geometry required corrections for the coordinates of sources and receivers initially obtained in the field. A new comprehensive source and receiver localization technique was developed for the case of stationary hydrophones and multiple seismic lines. The horizontal size of the created 3-D velocity model is 3 km x 2.7 km. The modelled volume is limited by the seafloor at the top and by the BSR at the bottom. The size of a grid cell is 50 m x 50 m x 20 m. The uncertainty for the velocity value of individual cells was as low as 20 m/s. although the resolution of the model was reduced by the sparse receiver geometry. The inversion results indicate a fairly uniform velocity field around and inside the vent zone. Velocities are nearly equal to values expected for sediments containing no hydrate, which supports the idea that the bulk concentrations of gas hydrates are low at the site. The largest velocity anomaly with an amplitude of +25 m/s is spatially associated with the limits of the blank zone. The anomaly suggests greater gas hydrate concentrations inside the vent zone than outside. Low vertical resolution of the model did not provide information on the depth distribution of the hydrate. However, the combination of the information from the velocity inversion with previous studies suggests that the zone of high hydrate concentration (15-20% of the pore space) associated with a hydrate lens, located at the top of the sediment section. The vent site is characterized by a negative anomaly of the seafloor reflection coefficient, outlined by a high amplitude rim. The low reflection coefficient is believed to be the result of the processes taking place above the hydrate lens, methane venting in particular. and the high amplitude rim to be the effect of carbonate formation. The seafloor reflection coefficient zonation appears to be correlated with the distribution of low magnetic susceptibility zone in the first 8 in of the sediment section. Both phenomena can be related to the distribution of upward fluid flow at the vent site. The cause of the blanking phenomena is likely different for different frequencies of the seismic signal. The blanking at high frequencies is an effect of near-surface disturbed sediments due to active venting and, possibly. free gas presence at the top of the vent zone. The blanking for the middle range of seismic frequencies is mostly the effect of reduced impedance contrast between the sediment layers inside the blank zone due to local presence of gas hydrates in small concentrations (2-3%). It is concluded that. the Bullseye vent zone, which shows very low activity presently, was probably much more active in the past (similar to a mud volcano). The past ac¬tivity may have led to the formation of the bathymetric expression of the vent site (a mound), together with the hydrate lens and authigenic carbonates.



natural gas, submerged lands, hydrates, British Columbia, Vancouver Island