DC, EM & NMR technologies

[DE-02] Relationship between detailed three-dimensional resistivity structure and fault distributions in a geothermal area: a case study in Japan.

*Nagi Yamashita¹, Tada-nori Goto¹ (1. Graduate School of Science, University of Hyogo (Japan))

It is well known that a geothermal reservoir is kept underground by a hydrothermal altered layer (called caprock) and upcoming hydrothermal fluid along fracture zones (faults) in a geothermal area. The understanding is indispensable since the reservoir have a high potential as a renewable energy resource (i.e., for geothermal powerplant). The magnetotelluric (MT) method is used as one of the standard geophysical methods to understand the subsurface structure of geothermal areas. The distribution of hydrothermal altered layer can be clealy imaged estimated by the MT method, since the layer is extremely low resistive due to clay. The upcoming hydrothermal fluid along conduits (e.g., active faults) follows hydrothermal alternation, resulting in vertically elongated low resistive zone along the faults. Therefore, the resistivity structure can prove the relationship between the caprock and faults; however, such detailed relationship has not been clarified. In this study, we carried out high-density audiofrequency-MT (AMT) explorations in a geothermal area, and estimated the detailed three-dimensional(3D) resistivity structure near the surface. As a result, we found that a bell-shaped low resistivity zone exists underground, whose center coincides with the surface geothermal fumaroles and hot spring area on the surface. It is also recognized that a resistivity contrast along a known active fault in the survey area. The former low resistivity zone possibly corresponds to a hydrothermal altered layer (i.e., the cap above the reservoir). The resistivity contrast is due to the fault. The fault is a geological boundary, and is considered to be related to hydrothermal circulation since the bell-shaped low resistivity zone seems to be obvious at the southern area of the fault. Thus, the dense AMT survey has a potential for revealing the production and reduction processes of geothermal fluid.