| Title | Three-Dimensional Resistivity Structure of the Yanaizu-Nishiyama Geothermal Reservoir, Northern Japan |
|---|---|
| Authors | Toshihiro UCHIDA, Shinichi TAKAKURA, Takumi UEDA, Tatsuya SATO, Yasuyuki ABE |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | magnetotelluric survey, 3D inversion, resistivity, geothermal reservoir, Yanaizu-Nishiyama |
| Abstract | We conducted three-dimensional (3D) magnetotelluric (MT) survey in the Yanaizu-Nishiyama geothermal field, Fukushima Prefecture, northern Japan, where a 65 MWe geothermal power plant has been in operation since 1995. The purpose of the survey was to obtain a detailed electrical resistivity structure of the geothermal reservoir in three dimensions in order to provide information for selecting appropriate location for drilling of additional production wells. The 3D MT measurement was carried out at 30 locations in 2010. In addition to them, we utilized existing MT data obtained for a 2D MT survey by the Geological Survey of Japan, AIST, in 2000 and 2001 for a regional geophysical study. Two subsets of MT data were prepared for 3D interpretation. The first subset (Area-1), consisting of 51 MT stations with impedance rotation of N35°W, densely covers the main geothermal field. For the second subset (Area-2), eight outer stations were added to the first subset, with changing the rotation direction to 0 degree. This is to examine how the inversion result differs when we set different survey coverage over the target area. The resistivity models obtained by 3D inversion indicate clear upper and lower boundaries of the low-resistivity cap layer over the geothermal reservoir. Distribution of feed zones recognized in production boreholes is confined below the lower boundary of the low-resistivity clay cap. The shape of the low-resistivity layer also shows a good correlation with a low-temperature zone in the distribution of underground temperature. The 3D model of Area-1 shows high-resistivity anomalies along the northern and southern edges of the interpretation zone at depth from 1 km to 3 km. On the other hand, Area-2 does not show such anomalies at the locations in Area-1. This indicates that the survey area should be sufficiently wider than the zone of interpretation in order not to produce artifacts caused by an anomalous structure at the edge of the target area. |