ACOUSTIC INVESTIGATION OF GAS HYDRATE-BEARING VENTING STRUCTURES OFF NE SAKHALIN ISLAND, THE SEA OF OKHOTSK

Abstract

The Sea of Okhosk is a well-know place in the world where gas hydrate is widespread and thick pure gas hydrate (~50 cm thick) was sampled in 2005 CHAOS cruise. We conducted high-resolution geophysical survey using the equipments with various frequencies. Side-scan sonar with 5 kHz SBP, sparker seismic, hydroacoustic souding data reveal that large-scale marine channel structure froms the northern boundary of gas venting area. Gas hydrate BSR is getting deep with water depth of seafloor. The BSR touches at the seafloor at 300 m of water depth, indicating the top of gas hydrate stability zone in the area is 300 m. In the area shallower than about 1000 m of water, gas venting structure area is defined depth and circular-shape gas venting structures are uniquely developed. In the deeper area, mud-diapir strucures form high bathymetric expressions. Gas flares in the water column occur large range of water depth between 110 m and 1220 m. The maximum height of the flares is up to 750 m, reaching to 150 m of water depth. This implies that some amounts of methane gas released from subbottom could be safely delivered into atmosphere through the water column.