Methane hydrate detection with marine electromagnetic surveys: Case studies off Japan coast

Methane hydrate (MH) is expected to be a new energy resource because a large amount of methane gas may be contained in the MH layer. MH has high resistive feature, so that the marine EM surveys will be a useful tool for detecting subseafloor MH. We introduce two case studies of marine electromagnetic surveys off Japan coast to detect MH below the seafloor. The first case is carried out in the Sea of Japan, 2005. Our marine deep towed EM streamer cable could image the subseafloor resistivity distribution to the depth of 100 m below the seafloor, and successfully detected MH zones as high resistivity. The second case, a marine CSEM experiment, with a deep-towed cable and an ocean bottom electromagnetometer (OBEM), is done off the Tokai area, along the Pacific side of Japan, 2006. A thin and deep MH zone (with thickness of about 30 m and depth of about 200 m below the seafloor) recognized in the borehole is not clearly imaged by our survey. The reason is mainly due to fluctuation of the track and direction of deeptowed source dipole. Further and careful analysis of CSEM data with acoustic navigation data will allow us to image the deep MH zone..