Visualization of shallow subseafloor fluid migration in a shallow gas hydrate field using high-resolution acoustic mapping and ground-truthing and their implications on the formation process: a case study of the Sakata Knoll on the eastern margin of the Sea of Japan

Acoustic mapping enables an understanding of the surface distribution of shallow gas hydrate (GH) and related products. Acoustically characteristic materials such as fluid-seepage-related methane-derived authigenic carbonate and/or shallow GHs, may be widely distributed beneath the shallow seafloor of the Sakata Knoll. High-amplitude reflectors over the knoll are the top of gas-bearing permeable layers and connect to the reverse fault at the foot of the knoll. Shallow GH and bacterial mats were observed at the high-amplitude layer cut by depression and/or the locally disturbed seafloor. Acoustic blanking zones observed on the sub-bottom profiler sections are current gas migration routes from the depth to the seafloor. Optical observations indicate that fluid seepage is not active in the current seafloor, and it is not necessarily observed above the acoustic blanking zones or shallow faults reaching the seafloor. In the Sakata Knoll, the tectonically formed reverse fault and gas-bearing permeable layers play more important roles in fluid migration from depth to the summit area of the knoll compared to acoustic blanking and shallow faults. The depression at the summit area of the Sakata Knoll was formed by the dissociation of a shallow GH at around the last glacial maximum. Limited fluid seepage is currently witnessed within and around the depression and it is less extensive than that in the past. Such knolls, with tectonically formed large faults and an anticline are abundant in the area and they can be good reservoirs for shallow GH along the eastern margin of the Sea of Japan.