A Comparison of the ELF-MT Resistivity Structure and the Thermal Structure in Takenoyu Geothermal Area

ELF-MT surveys were carried out for the study of the geothermal structure in the Takenoyu Geothermal Area, Central Kyushu. The ELF-MT method measures natural electromagnetic field at three frequencies in Schumann reasonant band from 8 to 20 Hz, and the VLF band at 17.4 kHz are also measured. Magnetic and electric field intensities are processed with a handheld computer. Scalar apparent resistivity can thus be obtained immediately in the field. Apparent resistivity maps were compiled from 70 sites with about 0.5 km interval. Major low apparent resistivity zone, less than 25 ohm-m, is associated with Takenoyu-Hagenoyu fumarous area where high underground temperatures are found at shallow depth. High underground temperatures at large depth are found at the foot of Mt. Waita and in an area 1 or 2 km north of Takenoyu-Hagenoyu. In these area, apparent resistivity values are not very low. Two dimensional resistivity model analyses were done by the finite element modeling technique. Those models indicate that low resistivity layers are associated with high underground temperature zones in shallow depth. Considering the resolution of the ELF-MT method, it was concluded that detectable depth of the bottom of the low resistivity layer is relatively shallow.