火山

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We conducted AMT, VLF-MT and high resolution electrical resistivity survey in the northeastern foot of the Unzen Volcano in order to compare shallow resistivity structure with temperature profile of the borehole (USDP-1). These surveys revealed that a low resistivity layer (40Ω・m) lies at 40-50m depth and a high resistivity layer (200Ω・m) lies below 60m depth. The low and high resistivity layers correspond to the high temperature zone (37℃, 42m depth) and temperature-decreasing zone, respectively. To examine the factor which decreases the resistivity and increases temperature at around 40m depth, we observed the feature of the drillcore samples of USDP-1 precisely. As a result, argillaceous impervious layer was found at 67-100m depth, just beneath the low resistivity and high temperature zone. This suggests that groundwater is maintained shallower than 67m depth and that geothermal water decreases the resistivity of this zone.

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