Three - Dimensional Magnetotelluric Investigation in Kakkonda Geothermal Area, Japan

Abstract

The New Energy and Industrial Technology Development Organization (NEDO) began the Deep Geothermal Resources Survey (DSGR) program in 1992. The objective is to attain fundamental knowledge about deep geothermal systems at the Kakkonda geothermal field, Japan. As part of the DSGR project, three-dimensional magnetotelluric (MT) inversion was studied for actual exploration activities. Thus, we have developed a generalized rapid relaxation inversion (GRRI) method to interpret MT data. The GRRI method is extremely fast because it allows separate onedimensional (1-D) inversions for updating the conductivity structure. We have derived this scheme based on a locally 3-D analysis to improve the computational efficiency and stability of inversion. The developed algorithm has been realized in the GRRI-3D code, which has been tested by synthetic 3-D MT data. It clearly demonstrates the utility of the high-dimensional MT inversion, and enhances a complex and realistic 3-D conductivity mapping capability. In 1998, a 3-D MT survey was conducted in Kakkonda geothermal field located in Iwate prefecture, Japan. GRRI-3D algorithm was applied to these MT data and a resistivity structure of Kakkonda geothermal field recovered.