ORGANIZATION OF RESISTIVITY EXPLORATION DATA FOR DEVELOPMENT OF DEEP GEOTHERMAL SYSTEMS – REPROCESSING MAGNETOTELLURIC DATA FROM THE KAKKONDA GEOTHERMAL FIELD, NORTHEAST JAPAN

Abstract

To determine a drilling target for deep geothermal systems, one should know the distribution of subsurface temperature and fluids. The resistivity structure estimated by electromagnetic explorations represented by magnetotellurics (MT) includes such information. However, there are non-unique relationships between rock resistivity and temperature or fluid content. We started to organize pre-existing data including resistivity structures and borehole logs to estimate temperature and fluid content at the target depth. In 1995, the deep geothermal drilling project at the Kakkonda geothermal field, northeast Japan, completed the WD-1 well, which reached a Quaternary granite intrusion with a temperature of 500°C at a depth of 3700 m;. MT surveys were conducted in this field in 1994, 1996, and 1998 to estimate the 3-D resistivity structure. The central part of the granite was found to be a strong conductor, which was interpreted as a saline-fluid-rich zone. However, in the resistivity structure, we did not find clear boundaries at high temperatures of the fluid-rich zone. The technical limitations in analysis at the time can be considered one of the reasons for such broad resistivity boundaries. To more accurately resolve the resistivity structure, we applied the latest analyzing technique to the previous MT data and obtained effective results. In this paper, we report these results and integrated them with other geophysical and geological data to further interpret the subsurface environment.