Marine electromagnetic methods for gas hydrate characterization

Gas hydrate is a type of clathrate consisting of a gas molecule (usually methane) encased in a water lattice, and is found worldwide in marine and permafrost regions. Hydrate is important because it is a geo-hazard, has potential as an energy resource, and is a possible contributor to climate change. There are large uncertainties about the global amount of hydrate present, partly because the characterization of hydrate with seismic methods is unreliable. Marine electromagnetic (EM) methods can be used to image the bulk resistivity structure of the subsurface and are able to augment seismic data to provide valuable information about gas hydrate distribution in the marine environment. Marine controlled source electromagnetic (CSEM) sounding data from a pilot survey at Hydrate Ridge, located on the Cascadia subduction zone, show that regions with higher concentrations of hydrate are resistive. The apparent resistivities computed from the CSEM data are consistent for both apparent resistivity pseudosections and two- dimensional regularized inversion results. The 2D inversion results provide evidence of a strong resistor near the seismic bottom simulating reflector (BSR), and geologic structures are imaged to about a kilometer depth. Comparisons with electrical resistivity logging while drilling (LWD) data from Ocean Drilling Program Leg 204 show a general agreement except for one of three sites where the CSEM inversion shows a large resistor at depth as compared to the LWD. An overlay of the CSEM inversion with a collocated seismic line 230 from Tr'ehu et al. (2001) exhibits remarkable similarities with the sedimentary layering, geologic structures, and the seismic BSR. Magnetotelluric (MT) sounding data collected simultaneously during the CSEM survey provide an electrical image of the oceanic crust and mantle (50̃km depth) and the folding associated with the accretionary complex (top 2̃km depth). In addition, the MT model provides a complementary low-resolution image of the CSEM inversion results. The CSEM data characterize the gas hydrate stability zone and both CSEM and MT map the geologic structures that allow methane to migrate to the gas hydrate stability zon