3-D seismic investigations of northern Cascadia marine gas hydrates

This dissertation presents results from 3-D (parallel 2-D) high resolution
\nseismic surveys and associated studies over an area with deep sea
\ngas hydrate occurrence. The study area is located on the accretionary
\nprism of the northern Cascadia subduction zone offshore Vancouver Island,
\nCanada.
\nThe major objectives of this study were the imaging of a gas/fluid vent
\nfield found in the study area and detailed mapping of the tectonic setting
\nand geological controls on fluid/gas venting. Secondary objectives were
\nthe characterization of the gas hydrate occurrence and constraints on the
\nseismic nature of the bottom-simulating reflector (BSR) and its spatial
\ndistribution.
\nThe main grid was 40 lines at 100 m spacing with eight perpendicular
\ncrossing lines of multichannel and single channel seismic reflection, and
\n3.5 kHz subbottom profiler data. In addition to the main 3-D seismic grid,
\ntwo smaller single channel grids (25 m spacing) were collected over the vent
\nfield. The multichannel seismic data acquired with the Canadian Ocean
\nAcoustic Measurement System (COAMS) streamer required correction
\nfor irregular towing depth and shot point spacing. A new array element
\nlocalization (AEL) technique was developed to calculate receiver depth
\nand offset. The individual receiver depths along the COAMS streamer
\nvaried between 10-40 m, which resulted in the occurrence of a prominent
\nreceiver ghost that could not be completely removed from the seismic data.
\nThe ghost resulted in limited vertical resolution and a coarse velocity depth
\nfunction.
\nThe vent field is characterized by several blank zones that are related to
\nnear-surface deformation and faulting. These zones are 80-400 m wide
\nand can be traced downward through the upper 100-200 m thick slope
\nsediment section until they are lost in the accreted sediments that lack coherent
\nlayered reflectivity. The blank zones are also characterized by high
\namplitude rims that are concluded to result from the interference effect
\nof diffractions. These diffractions result due to relatively sharp discontinuities
\nin the sediment physical properties at the blank zone boundary.
\n2-D vertical incidence seismic modeling suggests an increase in P-wave
\nvelocity inside of the blank zone with only minor changes in density.
\nBlanking is believed to be mainly the effect of increased hydrate formation
\nwithin the fault planes. The faults are conduits for upward migrating
\nfluids and methane gas that is converted into hydrate once it reaches the
\nhydrate stability field. Carbonate formations at the seafloor can also
\ncontribute to blanking especially at higher frequencies. Free gas may be
\npresent in case of full hydrate saturation or strong fluid flow. Geochemical
\nanalyses of pore water and water-column samples carried out in cooperation
\nwith Scripps Institute of Oceanography indicate relatively low fluid
\nfluxes of less than 1 mm/yr and there is no heat flow anomaly present over
\nthe vent field. Methane concentrations of 20 n-moles/L (about 8 times the
\nocean background concentration) were detected in water-column samples
\nof the first 100-200 m above the main blank zone of the vent field. Venting
\nis also believed to be strongly episodic with a recently more quiet time.
\nHowever, the observed carbonate crusts indicate a long-term activity of
\nthe vents.

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