[MIS17-P09] Surveys Using an ROV in Areas with Gas Seeps in the Sea of Okhotsk Off the Coast of Abashiri, and the Estimated Amount of Gas Seeps
Keywords:methane hydrate, Sea of Okhotsk, remotely operated vehicle, gas seep
Thus, in July 2017, submersible surveys were conducted at two sites in the Sea of Okhotsk off the coast of Abashiri. One of these sites is the top of a seamount about 550m below the sea surface, and the other was at a submarine canyon about 750m below the sea surface. Kaiyo 3000, a remotely operated vehicle (ROV) carried by the Daiichi Kaiyo-maru, a vessel owned by Kaiyo Engineering Co., Ltd., was used for the submersible surveys.
The surveys confirmed the concentrated presence of many vents that were leaking gas in relatively small areas, and the emission of large quantities of gas was successfully filmed. Gas was trapped by using a funnel-shaped container. The trapped gas was collected in a pressure vessel attached to the top of the container. When the seafloor surface near the gas vents was excavated by using the ROV’s manipulator, small pieces of methane hydrate and gas ascending through the sea were observed. This indicated that methane hydrate exists in the surface layer of the seafloor. In the areas that were surveyed by submersible, there were many bacterial mats and carbonate aggregates. Many crabs of various species were observed near the carbonate aggregates and gas vents. Additionally, deep-sea cold-seep clams (Calyptogena soyoae) were collected. This clam species lives in areas of the ocean floor where methane seepage occurs.
Images recorded during the submersible surveys were used for estimating the amount of gas seeping in a 200m by 100m area on the top of a seamount 550m below the sea surface. In the survey areas, gas was found to be seeping from about 20 sites. Some sites had only one vent, and others had clusters of multiple vents. The amount of gas seeping from an area 5m in diameter where vents were concentrated was estimated at 170,000m3 a year. The amount of gas is valued at approximately 4 million yen. The amount and value of gas seeping from the entire survey area are estimated at 1,000,000m3 and 25 million yen, respectively.