5:15 PM - 6:45 PM
[MIS22-P08] Ground-truthing, detailed bathymetry, and transition of bathymetry at a shallow methane hydrate-bearing mound, Umitaka Spur, off Joetsu, Sea of Japan
Keywords:off Joetsu, Umitaka Spur, shallow methane hysrate, bathymetry, acoustic mapping
Summary
The Umitaka Spur in the eastern margin of the Sea of Japan is a characteristic topographic feature. There is a nearly circular shaped pockmark and two mounds within a short distance. Existence of shallow methane hydrate (MH) was confirmed in the mounds. Here we report result from seafloor observation via 3D photo imaging of the seafloor in 2021, and a local bathymetric change between 2013 and 2021, on the western mound in the central part of the Umitaka Spur.
Data acquisition
Acoustic mapping using the autonomous underwater vehicle (AUV) Deep1 (Fukada Salvage Co., Ltd.) was conducted in 2013 with a multi-beam echo sounder and a side-scan sonar and sub-bottom profiler system at ~25 m above the seafloor with ~3 knots of cruising speed.
SeaXerocks1, a high-resolution three-dimensional imaging system, was mounted on an unmanned remotely operated vehicle (ROV) Kaiyo3000 (KAIYO ENGINEERING CO., LTD.) in 2021 for 3D photographic observation. The ROV observed the seafloor at an altitude of 4–5 m with ~0.4 knots, obtaining still images and bathymetric information with laser scanning.
Observations
The high-resolution bathymetric observation revealed detailed characteristics of the mounds. The diameter of the pockmark is ~500 m. Two mounds, the north and west mounds, are drop-shaped and 200–250 m in size on the immediate north margin of the pockmark, and an oval-shaped and 250–300 m in size on the northwest margin of the pockmark, respectively. The top surfaces of both mounds showed some irregularities, but they are relatively flat within a depth difference range of ~20 m. The backscatter strength was low over the entire pockmark but was significantly higher on the two mounds than that of the surrounding seafloor, and it tended to be relatively higher on convex and lower on concave topographies.
The mounds were widely covered with sediments, outcrops, bacterial mats, and various organisms (crabs, squid, fish, sea anemones, starfish, etc.). Pillars of bubbles were observed at several locations on the mounds. The pillars appeared from the edges of the sharp cliff, and we could not confirm obvious bottom characteristics at the base of the columns. Occasionally, the ROV behaved erratically; it may have been due to a sudden change in altitude of the ROV, meaning passed over small-scale irregularities that were not represented on the bathymetry map obtained by the AUV. At the outside of the mounds, outcrops and bacterial mats were no longer visible.
A previous description of the western mound in 2013–2015 revealed that methane hydrates were present on the seafloor. A comparison of topographic data from 2013 and 2021 indicated that a transition of topographic pattern occurred at several points on both mounds. Especially on the western mound, rises observed in 2013 had become a depression surrounded by steep cliffs in 2021, and a valley in 2013 had expanded by 2021. Bubbles were observed in the areas where topographic changes had occurred.
Conclusion
Acoustic mapping by AUV and seafloor observation by ROV and SeaXerocks1 was conducted in 2013 and 2021 respectively, at shallow MH-bearing mounds in the Umitaka Spur, off Joetsu, Sea of Japan. Outcrops, bacterial mats, various animals, and bubbles were observed. A transition of the topographic patterns of the areas over an 8-year period was identified based on two bathymetric datasets obtained at two different times.
Acknowledgments
This study was conducted as part of a national gas hydrate research project in the Sea of Japan funded by the Ministry of Economy, Trade, and Industry (METI), Japan.
The Umitaka Spur in the eastern margin of the Sea of Japan is a characteristic topographic feature. There is a nearly circular shaped pockmark and two mounds within a short distance. Existence of shallow methane hydrate (MH) was confirmed in the mounds. Here we report result from seafloor observation via 3D photo imaging of the seafloor in 2021, and a local bathymetric change between 2013 and 2021, on the western mound in the central part of the Umitaka Spur.
Data acquisition
Acoustic mapping using the autonomous underwater vehicle (AUV) Deep1 (Fukada Salvage Co., Ltd.) was conducted in 2013 with a multi-beam echo sounder and a side-scan sonar and sub-bottom profiler system at ~25 m above the seafloor with ~3 knots of cruising speed.
SeaXerocks1, a high-resolution three-dimensional imaging system, was mounted on an unmanned remotely operated vehicle (ROV) Kaiyo3000 (KAIYO ENGINEERING CO., LTD.) in 2021 for 3D photographic observation. The ROV observed the seafloor at an altitude of 4–5 m with ~0.4 knots, obtaining still images and bathymetric information with laser scanning.
Observations
The high-resolution bathymetric observation revealed detailed characteristics of the mounds. The diameter of the pockmark is ~500 m. Two mounds, the north and west mounds, are drop-shaped and 200–250 m in size on the immediate north margin of the pockmark, and an oval-shaped and 250–300 m in size on the northwest margin of the pockmark, respectively. The top surfaces of both mounds showed some irregularities, but they are relatively flat within a depth difference range of ~20 m. The backscatter strength was low over the entire pockmark but was significantly higher on the two mounds than that of the surrounding seafloor, and it tended to be relatively higher on convex and lower on concave topographies.
The mounds were widely covered with sediments, outcrops, bacterial mats, and various organisms (crabs, squid, fish, sea anemones, starfish, etc.). Pillars of bubbles were observed at several locations on the mounds. The pillars appeared from the edges of the sharp cliff, and we could not confirm obvious bottom characteristics at the base of the columns. Occasionally, the ROV behaved erratically; it may have been due to a sudden change in altitude of the ROV, meaning passed over small-scale irregularities that were not represented on the bathymetry map obtained by the AUV. At the outside of the mounds, outcrops and bacterial mats were no longer visible.
A previous description of the western mound in 2013–2015 revealed that methane hydrates were present on the seafloor. A comparison of topographic data from 2013 and 2021 indicated that a transition of topographic pattern occurred at several points on both mounds. Especially on the western mound, rises observed in 2013 had become a depression surrounded by steep cliffs in 2021, and a valley in 2013 had expanded by 2021. Bubbles were observed in the areas where topographic changes had occurred.
Conclusion
Acoustic mapping by AUV and seafloor observation by ROV and SeaXerocks1 was conducted in 2013 and 2021 respectively, at shallow MH-bearing mounds in the Umitaka Spur, off Joetsu, Sea of Japan. Outcrops, bacterial mats, various animals, and bubbles were observed. A transition of the topographic patterns of the areas over an 8-year period was identified based on two bathymetric datasets obtained at two different times.
Acknowledgments
This study was conducted as part of a national gas hydrate research project in the Sea of Japan funded by the Ministry of Economy, Trade, and Industry (METI), Japan.