Estimation of hydrogeological structure using airborne electromagnetic survey in the Aso western piedmont geothermal areas

Yusuke SAKAI, Katsushi KAWATO, Koichiro SADO and Yasuyuki HIRAKAWA

In order to detect high risk slopes with possible deep landslide hazards in volcanic areas, it is necessary to understand the hydrogeological structure at large scales. We consider Airborne Electromagnetic (AEM) survey to be very effective for large]scale hydrogeological survey. In Aso volcanic area, we used AEM system as well as borehole and microtremor array exploration to study the hydrogeological structure. The results show:1) An observed unique contour pattern that is the lowest resistivity point at each sample stretches horizontally. The connected low resistivity contours match the groundwater layer detected in the borehole and the location of the spring water nearby. 2) The pattern of low resistivity contour line matches the low]velocity zone of the S]wave of the PS logging in the borehole and microtremor array exploration. Therefore the low resistivity contour indicates a groundwater layer and soft layer, which can be an important factor to cause deep landslide in the studied survey area. This suggests that finding the same low contour pattern could lead to detecting possible deep landslide hazards in other high risk areas.