3-D seismic investigations of northern Cascadia marine gas hydrates
Date
2018-10-23
Authors
Riedel, Michael
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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.
Description
Keywords
Chemical oceanography, British Columbia, Vancouver Island, Gas fields, Oceanography, Pacific Coast