Subsurface structure and groundwater of the Takamatsu crater (cauldron) by CSAMT method and boring data

摘要

The Takamatsu crater is a buried depression structure identified by a low gravity anomaly in the southern part of the Takamatsu Plain, northeast Shikoku, southwest Japan (Kono et al., 1994). The CSAMT survey suggests that the crater is composed of low resistivity material, while the surrounding bedrocks have high resistivity. The depth of the crater is estimated to be about 700-900 m in depth. The surface geology and boring cores in and around the crater have confirmed that the Takamatsu crater consists mainly of rhyolitic tuff and tuff breccia, and bedrocks are granitic rocks. The resistivity structure and abundant pyroclastic flow deposits strongly support the idea that the Takamatsu carter is a cauldron. In the cauldron, most of the water is confined in the voids of the tuffaceous rocks and cannot be pumped up. The fissure water is the only available water resource in the cauldron. Limited pumping tests in the cauldron have shown that the pumping capacity is 100-150 t/day. Therefore the Takamatsu crater cannot be a huge underground water reservoir.