SEA FLOOR TOPOGRAPHY OF SHALLOW GAS HYDRATE AREA——DATA FROM OKHOTSK SEA

Shallow gas hydrates have already been sampled by gravity corer, fishing net, and submersible machines on the sea floor and in the sediment near the sea floor. However the formation mechanism of shallow gas hydrate, its distribution and relationship with sea floor topography remain unclear. Based on the side scan sonar and sub-bottom profile data from the 2006 gas hydrate cruise in the Okhotsk Sea, we found that the occurrence of shallow gas hydrates are closely related to some domelike structures. Domelike structures are common on the slope, especially along the Middle and Lower Sakhalin Slope. They are about several hundred meters wide and several ten meters high. Different from sea floor sand waves and sand ridges, the domelike structures are isolated structures with a slightly longer lower wing and a shorter upper wing. Sub-bottom profile recorder showed that buried dome structures are also well developed on the slope beneath a 30 cm modern sediment layer. A joint compress stress field from the Deryugin basin to the Sakhalin Slope and from the north Sakhalin Slope to the South Sakhalin Slope along the strike slip fault on the Sakhalin Slope was the main cause of the formation of the dome structures, and the diapir structures among the dome structures. The acoustic turbidity beneath the diapir structures clearly show the migration of free gases from deep to the gas hydrate stability zone and to the sea water body from the mini crater on the top of the dome structures. Gas hydrates formed due to the availability of free gas within the diapir structures from the bottom of gas hydrate stability zone up to the sea floor.