A Towed EM Survey Across Seafloor Hydrate Mounds
|EM data collected in Atwater Valley across the JIP drill site (large blue dot). The yellow-brown symbols sjow color coded meaures of resistivity along the tracks of the EM system as measured by the deepest looking 40m receiver. Darker colors indicate lower resistivity (note color scales are slightly different on each line). The lines are plotted over recently acquired sidescan collected by workers at the Naval Research Lab. The small blue dots show locations of cores.|
A seafloor electromagnetic (EM) survey was conducted across the Atwater Valley sites of JIP drilling, in 1300m of water depth. The towed EM system used for the survey was pulled along the seafloor and measured electrical conductivity profiles to a sub-bottom depth along ~17 km of tow line. The survey included data across 2 mounds in the region, both of which have a relief of around 10m and are thought to contain gas hydrate. The EM data collected are used to map the physical properties of the sediments in the area, and responds to changes in porosity, temperature and pore-water salinity. Hydrate bearing sediment is expected to reduce bulk electrical conductivity. Contrary to expectations, the EM data showed raised electrical conductivity across both mounds compared to the surrounding area. The response of the hydrate mounds is indicative of raised temperatures and/or raised chlorinity in the subsurface. This result is confirmed by JIP drilling and other data from the mounds.