Magnetotelluric exploration of geothermal resources at Takigami area in Japan

A magnetotelluric (MT) survey has been carried out in Takigami geothermal area to determine resistivity structures and the locations of electrical discontinuities that may reflect a possible fault or fracture correlating with promising geothermal reservoir. The MT measurements have been conducted with irregular grid stations covering the Takigami area. The two-dimensional (2-D) inversion results of the MT data show that the resistivity structures in Takigami area are composed mainly of three layers, that is, high resistivity in the first layer overlying low resistivity in the second layer and resistive electrical basement in the third layer. The results are also in a good agreement with electrical resistivity logs, temperature distribution and lost circulation zone during the course of drilling. It is revealed from the interpretation of MT data that the geothermal reservoir of Takigami field is located at two different depths separated by Noine fault zone, which divides the subsurface of the area into eastern and western parts according to the characteristics of resistivity, permeability, temperature and depth of reservoir. The reservoir in the east of Noine fault zone is shallower than that in the west.