Controlled source electromagnetic study on the response of cold vent sites and gas hydrate occurrences on the northern Cascadia margin

The northern Cascadia margin offshore Vancouver Island is an accretionary sedimentary prism with abundant fluid seepage including cold vents. Cold vents represent gas-rich fluid conduits that often reach the seafloor, contain gas hydrates and support large chemo-synthetic communities. Fluids rise pervasively through the sediments and locally through fault and fracture networks from as far down as the subducting plate to the seafloor. Transported gases may result locally in high amounts of gas hydrate within the upper few tens of meters below the seafloor. The controlled source electromagnetic (CSEM) method is sensitive to resistivity contrasts and therefore to free gas and hydrate accumulations. Our data were acquired with the electric dipole-dipole CSEM configuration built at the University of Toronto that is especially designed to detect resistive anomalies close to the seafloor. We utilize 1-D data inversion and model parameter uncertainty analysis, as well as complementary information from seismic, bathymetry and borehole data. Our data were acquired near the prominent vent site Bullseye, as well as close to the shelf at the edge of the gas hydrate stability field. We analyze the 3-D effect of vents on the CSEM response through forward modeling of a variety of vent models and compare the results to our data. We conclude with a geological interpretation for the upper few hundred meters of the sediment section on the middle and upper slope of the northern Cascadia margin.