Hideki Numanami

Acoustical surveys of Methane plumes using the quantitative echo sounder in Japan Sea

R&T/V Umitaka-maru(Tokyo Univ. of Marine Science and Technology) and R/V 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 in 2004,2005 and 2006 to survey the ocean floor gas hydrate and related acoustic signatures of methane plumes by using a quantitative echo sounder and a multi beam SONER [1]. Detailed bathymetric profiles have revealed a number of mounds, pockmarks and collapse structures within 3 km times 4 km on the ridge at the water depth of 910 m to 980 m. We mapped minutely 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 bottoms both in every 100 m range and very one minute by the echo integrator. We obtained the following results from the present echo-sounder and SONER surveys. 1) We measured the averaged volume backscattering strength (SV) of each methane plume. The strongest SV, -33dB, of the plumes was stronger than SV of fish school. 2) Averaged SV of each methane plume tend to be related to the water temperature and the water pressure. 3) We recovered several fist-sized chunks of methane hydrate by piston coring at the area where we observed the methane plumes. 4) Using this method, we detected methane bubbles floating up points and revealed that the hydrate bubbles float upward until they reach warm waters at 300 m depth. 5) We revealed the hydrate bubbles in the conic container on the sea bottom. Because of results this acoustical method was effective to know the behaviors of the methane hydrate under water and to monitor the area of the methane seep. As a following up project, we are planning 1) to measure SV of methane hydrate bubbles and methane hydrate floating in water columns in situ, 2) to make a trial calculation of amount of floating methane bubbles and methane hydrates and 3) to study how to sample the acoustical data of methane plumes using the side scanning SONAR, called SeaBat.