Acoustical Surveys Of Methane Plumes Using The Quantitative Echo Sounder In Japan Sea

The research and training vessel Umitaka-maru (Tokyo Univ. of Marine Science and Technology) and the research vessel Natsushima (JAMSTEC) sailed to the methane seep area on a small ridge in the Naoetsu Basin, in the eastern margin of the Sea of Japan on July 2004, July 2005, August-September 2006 and July 2007 to survey the ocean floor gas hydrate and related acoustic signatures of methane plumes by using a quantitative echo sounder. We mapped minute details of active methane plumes by using a quantitative echo sounder with positioning data from GPS. We also measured averaged echo intensity from the methane plumes and sea bottom features both over every 1m range and every 4 seconds employing the echo integrator. We took still pictures and filmed the methane seep area. We obtained the following results from the present echo sounder survey. 1) We measured the size (diameter=5mm) and rising flux (6.4ml/sec/seep point) of methane bubbles. We also clarified the relationship between bubbles and water depth. 2) We calculated and measured Ts of methane bubble (=6.25 × 10-4) and methane hydrate (=1.56 × 10-6) floating in water tanks. 3) We showed the relation that exists between bubbles and volume backscattering strength (=3.16 × ~10-6) on the echogram. Our preliminary calculations indicate 2 methane hydrate bubbles/m3/sec and 40 seep points within a 100m perimeter. 4) On the survey in 2007, we checked on the echogram the migration of several methane plumes from the 2004, 2005 and 2006 survey positions. Based on the results, this acoustical method is an effective approach to record the behaviors of the methane hydrate in the water column and to monitor the course of methane hydrate bubbles floating up to the surface. As a following up project, we are planning to measure SV of the methane hydrate bubbles and the methane hydrate floating in water columns by using the submarine vehicle, called Hyper Dolphin, to observe minute acoustical characteristics of methane plumes and to make a trial calculation of the amount of floating methane bubbles and methane hydrates.