An audiofrequency magnetotelluric (AMT) survey was conducted for detailed investigation of the reservoir at the Ogiri geothermal field, southwestern Japan. The AMT stations were arranged with an interval of about 150 m on a grid-like array whose size was approximately 1.5 km x 1.5 km. This survey was targeting an expansion of the currently producing geothermal reservoir to the east. There had been conducted several stages of MT surveys in the same area before. This report presents three-dimensional (3D) inversion of the AMT data, together with comparison with 3D models of the MT data. The resultant 3D AMT models are consistent with the MT models reported in the past. The boundary between the low-resistivity cap layer and the high-resistivity reservoir zone is more complicated in the AMT model than the MT models. This suggests that high-resolution imaging is possible for delineating the top surface of geothermal reservoir by the AMT method.

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