[S-CG48] Ocean Floor Geoscience

[SCG48-15] Bathymetric and geological observation using AUV and other instruments on the East China Sea slope: wavy bedforms and channel-like structures near Oki-Danjo Hills

*Ryoichi Horinouchi ¹, Keita Saito¹, Ryosuke Nagasawa¹, Manabu Ushijima¹, Tetsuhiro Etou¹, Takeshi Tomohisa¹, Tomoki Kawakami¹, Tomo Aoki¹, Yuka Tatera¹ (1.Japan Coast Guard )

Keywords:Bedform, Channel, AUV, MBES, MCS

As reported by Aoki et al. (2023, JpGU), there are distinct bathymetric and geological structures such as faults and lineaments near to the Oki-Danjo Hills in the East China Sea slope. But there is little research on bathymetry. Therefore, in this report, we introduce the observational results of wavy bedforms and channel-like structures distributed in the vicinity of the Oki-Danjo Hills, using AUV and other instruments. The observational data include bathymetric data acquired by Japan Coast Guard survey vessels Shoyo, Takuyo, Kaiyo, and Meiyo in 2014, sub bottom profiles and seismic reflection profiles gathered by Shoyo in 2022, sedimentary samples collected by Shoyo in 2021, and bathymetric data, side scan mosaic images, and sub bottom profiles acquired by AUV, Gondo 4 (manufactured by IHI Corp.) mounted on Heiyo in 2023. We conducted regular data processing, such as noise reduction and gain adjustment, with the bathymetric data, sub bottom profiles, seismic reflection profiles, and AUV survey data (bathymetric data, side scan mosaic images, and sub bottom profiles), respectively. For the sedimentary samples, we conducted grain size analysis on the sedimentary cores collected for each sampling site. The bathymetric data showed that wavy bedforms and channel-like structures were prominently distributed near the Oki-Danjo Hills. The average wavelength of the wavy bedforms was about 200 m, the average height was about several meters, and the average distributed depth was in the range of 220 - 300 m. The average height of the channel-like structures was approximately 30 m, and the average depth range was about 160 - 220 m. A mound-like structure distributed between channel-like structures and wavy bedforms. We counted the wavelength and height of wavy bed forms for each group, and plotted them as a scatter diagram. This showed the same trend as in the previous studies, e.g. Ma et al. (2016), which said that wavy bedforms were developed by the oceanographic field, especially internal gravity waves. The sub bottom profile and seismic reflection profiles showed there is no specific geological structures around these structures. This implies that these structures were not formed by elasto-plastic deformation referring to faults and folds, etc.. We also plotted the grain size of sedimentary samples vs the wavelength and height of the wavy bedform that locates closest to the sampling points as two scatter diagrams. These distributions were consistent within the ranges of the previous study by Flemming (2000), showing the scale of bedforms formed by bottom currents. And we can see microscale wavy bedforms in the photos taken by a camera mounted on the sampler. These structures are too tiny to resolve in bathymetric data, but it implies that the seabed is strongly influenced by bottom currents. The bathymetric data, side scan mosaic images and sub bottom profiles acquired by the AUV also showed smaller-scale wavy bedforms. Survey vessels bathymetric data cannot resolve these smaller-scale wavy bedforms. Also, these smaller-scale wavy bedforms seemed like the superposition of several wavy bedforms with different wavelength. According to the above bathymetric and geological observational results, these wavy bedforms and channel-like structures observed prominently near the Oki-Danjo Hills can have developed due to the fluid motion.