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| | Abstract | | The Nakano-shima Island, Oki-Dozen, located in Shimane Prefecture, Japan, is a basaltic Island on a continental shelf. The seafloor around the Island was most widely exposed to above the sea surface during the last glacial period. According to the drilling report of a hot spring well in the Island, the existence of the high salinity groundwater in the shallower basalt and the fresh water in deeper sedimentary rocks was suggested. In order to investigate the distribution of salt/fresh water beneath the Island, the CSAMT method was applied along the survey line of 2.5 km length. The analyzed 2D resistivity section showed the high resistivity zone (H1) from the surface to about 100 m in depth, a horizontally consecutive low resistivity zone (L1) at a depth of 100-200m below sea level, higher resistivity zone (H2) at a depth of 200-500m, and lower resistivity zone (L2) further below. We can interpret that the low resistivity zone (L1) is saturated by high salinity groundwater, and the high resistivity zone (H2) by fresh water. Furthermore, the resistivity section might suggest the effect of sea level change, i.e., fresh water recharged during the last glacial period still remains and sea water intruded after transgression. |
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