5:15 PM - 6:30 PM
[MIS21-P02] Subsurface structures and fluid transportation visualization by acoustic mapping in a shallow methane hydrate area off Sakata, eastern margin of the Sea of Japan
Keywords:shallow methane hydrate, acoustic mapping, 3D pictorial mapping system, different scale observation matchup
Shallow methane hydrates (MH) have been reported atop a knoll off Sakata (Sakata knoll, a tentative name), along a reverse fault located at the southwest foot of the knoll, along the eastern margin of the Sea of Japan (Temma and Goto, 2018). Fluid seepage not only helps in generating shallow MH, but also assists in the cultivation of chemosynthetic animal communities, and generates carbonate rocks from a mixture of gas-containing fluid and seawater. The distribution of chemosynthetic animal communities and rocks on and below the seafloor in this area is a slight indicator of fluid emission, including gas, in turn suggesting the distribution of buried shallow MH. However, the relationship between the distribution of the communities and rocks, and the route of gas emission is unclear.
Prior observation cruises in this area acquired data from: acoustic mapping using research vessels and autonomous underwater vehicles (AUVs), optical observations using remotely operated vehicles (ROVs), logging-while-drilling (LWD) measurements, and sediment sampling (Morita, 2015; Temma and Goto, 2018; Yokota, 2020). These results reveal a depression bordered by steep slopes atop the knoll (~1 km×~300–400 m in diameter, ~12 m deep), two terraces on its northern and southern sides, a single hill near its center, and swells on the top around the depression (~15 km×~5–10 km in diameter, ~120 m of relative height from the southwestern foot of the knoll). The distribution of backscatter strength was the largest on the steep slopes, relatively large on the terraces within the depression and the surrounding swells, and continued southwestwards along the knoll. The shallow subseafloor structure showed an anticline along the topography of the knoll. Faults developed were more in number on the southwestern slope of the knoll, compared to a few found in the shallowest layer. A large acoustic blanking which developed directly beneath the depression prevented the visibility of shallow subsurface structures. Relatively common smaller acoustic blankings on the northeast side of the knoll, seemed to cut the layers.
To determine the detailed distribution of chemosynthetic communities and carbonate rocks on the seafloor, optical seafloor mapping was performed with a 1K20 cruise using Kaiyomaru no.1 (Kaiyo Engineering Co. Ltd.), and ROV-Kaiyo3000 with SeaXerocks 1 (high-resolution 3D mapping system, Kaiyo Engineering Co. Ltd.) in June 2020, in and around the depression. The seawater was turbid with increased biomass in the depression. Methane hydrates remained unconfirmed on the seafloor. The two terraces and swells were extensively covered with sediments, but rare rock outcrops and small biological mats were observed.
Since shallow subseafloor structures beneath the depression were unrecognizable because of acoustic blanking, it was difficult to extract information corresponding to the distribution of communities, rocks, and subseafloor structures. Nevertheless, we recognized larger visible faults just outside the depression, most of which stopped below the top layer of the sediment without reaching the seafloor. No communities or rocks were observed directly above these faults, suggesting the likelihood of less fluid and gas discharge through surface sediment. More communities and rocks on the steep slope surrounding depression than in the surrounding areas should be assisted by larger amount of fluid seepage through the exposure of the layers.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry (METI).
References:
Temma and Goto (2018) 33rd investigative commission for methane hydrate development, METI (in Japanese)
Morita (2015) FY2014 AUV survey in shallow methane hydrate fields in Japan Sea, Japan Geoscience Union meeting 2015, MIS24-02, abstract (May 27, Chiba)
Yokota (2020) Results of high-resolution 3D seismic reflection survey, Research and development of shallow methane hydrate, FY2020 Achievements Report Meeting, Research and development project for shallow methane hydrate, AIST (in Japanese)
Prior observation cruises in this area acquired data from: acoustic mapping using research vessels and autonomous underwater vehicles (AUVs), optical observations using remotely operated vehicles (ROVs), logging-while-drilling (LWD) measurements, and sediment sampling (Morita, 2015; Temma and Goto, 2018; Yokota, 2020). These results reveal a depression bordered by steep slopes atop the knoll (~1 km×~300–400 m in diameter, ~12 m deep), two terraces on its northern and southern sides, a single hill near its center, and swells on the top around the depression (~15 km×~5–10 km in diameter, ~120 m of relative height from the southwestern foot of the knoll). The distribution of backscatter strength was the largest on the steep slopes, relatively large on the terraces within the depression and the surrounding swells, and continued southwestwards along the knoll. The shallow subseafloor structure showed an anticline along the topography of the knoll. Faults developed were more in number on the southwestern slope of the knoll, compared to a few found in the shallowest layer. A large acoustic blanking which developed directly beneath the depression prevented the visibility of shallow subsurface structures. Relatively common smaller acoustic blankings on the northeast side of the knoll, seemed to cut the layers.
To determine the detailed distribution of chemosynthetic communities and carbonate rocks on the seafloor, optical seafloor mapping was performed with a 1K20 cruise using Kaiyomaru no.1 (Kaiyo Engineering Co. Ltd.), and ROV-Kaiyo3000 with SeaXerocks 1 (high-resolution 3D mapping system, Kaiyo Engineering Co. Ltd.) in June 2020, in and around the depression. The seawater was turbid with increased biomass in the depression. Methane hydrates remained unconfirmed on the seafloor. The two terraces and swells were extensively covered with sediments, but rare rock outcrops and small biological mats were observed.
Since shallow subseafloor structures beneath the depression were unrecognizable because of acoustic blanking, it was difficult to extract information corresponding to the distribution of communities, rocks, and subseafloor structures. Nevertheless, we recognized larger visible faults just outside the depression, most of which stopped below the top layer of the sediment without reaching the seafloor. No communities or rocks were observed directly above these faults, suggesting the likelihood of less fluid and gas discharge through surface sediment. More communities and rocks on the steep slope surrounding depression than in the surrounding areas should be assisted by larger amount of fluid seepage through the exposure of the layers.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry (METI).
References:
Temma and Goto (2018) 33rd investigative commission for methane hydrate development, METI (in Japanese)
Morita (2015) FY2014 AUV survey in shallow methane hydrate fields in Japan Sea, Japan Geoscience Union meeting 2015, MIS24-02, abstract (May 27, Chiba)
Yokota (2020) Results of high-resolution 3D seismic reflection survey, Research and development of shallow methane hydrate, FY2020 Achievements Report Meeting, Research and development project for shallow methane hydrate, AIST (in Japanese)