Acoustic Surveys of Methane Plumes by Quantitative Echo Sounder in the Eastern Margin of Japan Sea

During methane hydrate exploration and research, remote and on board acoustic surveying and monitoring of methane hydrate can be easily and economically conducted using a quantitative echo sounder.Simultaneously, the structure and the floating up speed of methane plumes can be obtained from an analysis of acoustic data.We conducted a survey of methane plumes from 2004 through 2008 at a spur situated southwest off the coast of Sado Island, tentatively called Umitaka Spur and at the Joetsu Knoll.In 2007 and 2008, we performed experiments by releasing methane hydrate bubbles and methane hydrate, and letting them float upward. Consequently, we demonstrated that acoustical reflection from the methane plumes correlates with water temperature and depth, that the floating up speed is constant but depends on the conditions of methane hydrate, that the discharge of methane hydrate bubbles changes, and that there is a wide scattering of materials below the seafloor where methane plumes are located.The method will be applied not only to basic research on methane hydrate but also to assessments of the environmental impact of methane hydrate exploitation.