High-resolution shallow structures of gas hydrate-bearing seepages off NE Sakhalin Island, the Sea of Okhotsk

Abstract

Recently many scientists pay attention to gas-hydrate bearing seepage structures on the seafloor as important pathways to deliver methane from deep depths to the water column and economic deposits of gas hydrates. During CHAOS cruises in 2003, 2005, 2006, large hydrate-bearing seepage structures were newly found on the continental slope of the Sakhalin Island at water depths of 400-1,000 m by multi-frequency and high-resolution geophysical surveys including Side-scan Sonar (SSS) mapping, 3.5 kHz sub-bottom profiling, sparker seismic profiling and echo-sounding. In an area of 200 km2, SSS survey identified more than 40 seepage structures. Using short gravity core (<5 m), massive and vein-type gas hydrates were recovered within these seepage structures at very shallow subbottom depths. The total gas accumulations within all seepage structures (4.36 km2) in the area may be as much as 3.5×109 km3. Here, we will present detailed shallow structures of the seepages to clarify the distribution and the nature of near-seafloor hydrate deposits, and to investigate the role of the gas/fluid discharge for the hydrate formation.