[M-IS22] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS22-P02] Exploration of natural gas seepage using acoustic techniques in the Kujukuri offshore region, Chiba, Japan

*HIROSHI ISHIDA¹, Masahiro Suzumura², Aumi Tsukasaki², Masayuki Nagao³, Atsushi Suzuki³, Takeshi Yoshida⁴, Mitsuhiro Ishii⁵, Kota Suzuki⁵, Takao Shima¹, Toshio Ogura⁶ (1.The Marine Ecology Research Institute , 2.Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology, 3.Research Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 4.Chiba Prefectural Environmental Research Center, 5.Chiba Prefectural Fisheries Research Center, 6.Fisheries Cooperative Association of Kujukuri)

Keywords:Natural gas seepage, Methane, Multibeam sonar, Acoustic techniques

Natural gas seepage (NGS) from the seafloor is known to occur in the offshore region of Kujukuri, Chiba, Japan (Yoshida et al., 2007, 2012), and the status of the NGS and its impact on the marine environment have been evaluated. Methane has been reported to account for the majority of the NGS (Suzumura et al., 2021; Kitamaki et al., 2023). Because methane is a greenhouse gas that is 25 times more potent than carbon dioxide, it is desirable to understand the current situation from the perspective of global warming countermeasures.
In this study, we attempted to understand the status of NGS in the Kujukuri offshore by using acoustic techniques (multibeam sonar, sub-bottom profiler, etc.), referring to the Natural Gas Seep Map (GSMap) by Yoshida et al. (in preparation). In addition, we artificially generated a known amount of gas (air) from the seafloor and acquired water column backscatter intensity (WCBI) using multibeam sonar and then quantitatively evaluated the natural gas emission in the Kujukuri offshore region based on the results of these measurements.
The locations of the NGS points identified by the multibeam sonar were almost the same as those shown in the GSMap. A positive correlation was found between the amount of air released from the seafloor in multiple stages and the WCBI, and it was considered possible to estimate the amount of gas being released from the WCBI.
This relationship was then applied to the WCBI obtained from several NGS points and further applied to the NGS points of the entire ocean area to obtain the amount of release. As a result, it was estimated that less than 2 tons of methane per day was being emitted from the entire sea area. When converted to carbon dioxide emissions (GWP: 25), the emission was equivalent to a several hundredth part of the emissions from a 1-million-kW-class LNG power plant per day.
This study is a product of the Research Laboratory on Environmentally Conscious Developments and Technologies (E-code), AIST.