Acoustical survey of methane plumes using the quantitative echo sounder in the eastern margin of the Sea of Japan

Research and training vehicle Umitaka-maru sailed to the methane seep area on a small ridge in the eastern margin of the Sea of Japan on July to August 2004 to survey the ocean floor methane hydrate and related acoustic signatures of methane plumes using a quantitative echo sounder [C. Aoyama et al., (2004)]. We carried out high-resolution mapping of methane plumes using a quantitative echo sounder with positioning data from GPS and also measured averaged echo intensity from the methane plumes both in every 100 m range and every one minute by the echo integrator. We obtained a number of interesting results from the present echo-sounder survey. We registered 36 plumes on echogram, ranging about 100m in diameter and 200 m to 700 m in height, reaching up to 300 m to 600 m below sea level and measured the integrated volume backscattering strength (SV) of each methane plume. The strongest SV, -33 dB, of the plumes was stronger than SV of fish school. Averaged SV tend to show the highest values around the bottom and the middle of plumes, whereas the SVs are relatively low at the top of plumes. We recovered several fist-sized chunks of methane hydrate by piston coring at the area where we observed methane plumes. Methane hydrate was recovered throughout two meters-long piston core interval, indicating thick hydrate deposits in shallow sediments near the methane plumes. A follow-up project, we are planning to measure SV of methane bubbles and methane hydrate floating in water columns through an experimental study in a large water tanks.