A remarkable reflection record of a deeper part (> 200 m) appeared on the fish finder. Gas bubbling was confirmed directly by the ROV (Hyper-Dolphin/Natsushima) diving off Joetsu in the eastern part of the Sea of Japan. Visualization, using geographical feature mapping and side-scan sonar images of the topography, was useful for investigating methane hydrate and associated carbonates at the sea bottom. Besides, a sub-bottom profiling study produced useful information for detecting buried methane hydrate and carbonates, because some of them had been identified previously by piston core sampling studies.
DAI-PACK (Deep sea Acoustic Imaging Package) was set on the ROV (Hyper-Dolphin) and the survey extended to a length of approximately 20 km and covered a total area of 900,000. A detailed distribution of patch-shaped rough geographical features, some fault-like lineaments and sub-bottom profile records (about a maximum of 15 m deep), were obtained by five dives during the NT07-20 and the NT08-09 cruises. The round-shaped patches of rough geographical features are thought to have been formed by gas supplied to the surface through fault systems.