Geological Structure Around the Ogachi Hot Dry Rock Test Site Using Seismic Reflection and Csamt Surveys

Abstract

We have conducted a basic experiment to utilize Hot Dry Rock energy as an electric power at the Ogachi test site in northeast part of Japan. It was confirmed by 1,100 m drilling data that the depth to the top of pre-Tertiary granitic rocks at the site was about 300 m and the temperature was measured 230Åé at 1,000 m depth. A seismic reflection and Controlled Source Audiofrequency Magneto-telluric (CSAMT) methods were carried out in order to locate a fault exactly and to estimate vertical displacement of the fault in granitic rocks. The fault was inferred to position at the west of the site by the previous gravity surveys. A CSAMT method was performed with 60 measurement points scattered in an area of 2 km x 2 km and the seismic reflection method was carried out along the E-W line with a length of 3.5 km using two vibro-seis sources. The resistivity profiles by a 2-D inversion of the CSAMT method indicates that the low resistivity zone (10-50É?m) corresponds to Neogene volcanic rocks and a high resistivity zone (100-1,000É?m) to pre-Tertiary granitic rocks. The low resistivity zone is found about 500 m southwest of the Ogachi site and the boundary between the low and high resistivity zone is likely to oriented in NW-SE. The geological boundary between the volcanic rocks and the granitic rocks was recognized as reflectors clearly in the seismic reflection profiles. The elevation of a reflector was about GL 300 m near the Ogachi site, however, at the west of the site it was about GL -300 m. Therefore, we estimated the fault position at about 500 m west of the site and the vertical displacement of the fault of 600 m in granitic rocks.