2018 Volume 76 Issue 2 Pages 61-75
Gas plume in the water column is often observed over the shallow gas hydrate occurrences in the eastern margin of the Japan Sea. It has been concerned that the ground strength will be reduced by the gas-rich fluids rising in the shallow layer of sediments. In this study, mechanical test, physical test, and chemical analysis were conducted on the sediment cores collected from the shallow gas hydrate and gas plume fields in off Joetsu and off Akita/Yamagata. There are no differences in physical properties of the each cores. The mechanical properties were low and dissolved CH4 concentration was high at the gas plume site of off Joetsu area. As the sedimentation rate increased, the sediment strength tended to decrease at gas plume-free site of off Akita/Yamagata area. There was no relationship between dissolved CH4 concentration and sediment strength in either area. These results suggest that the sediment strength at the gas plume site is low in situ.
