Magnetotelluric Sounding in a Geothermal Area

Abstract

The Takigami geothermal area is located in the Hohi geothermal region, northeastern Kyushu Island, Japan. This paper demonstrates the application of magnetotelluric (MT) method for geothermal investigation in the Takigami area to determine the subsurface resistivity structure, which is correlated to the potential geothermal reservoir and the distribution of fault systems and geological trends. Advanced interpretation techniques of one-dimensional (1-D) and two-dimensional (2-D) inversions were used to derive a new insight of the geothermal system in the Takigami area. The resistivity section obtained by 1-D inversion of a survey line (Line-1:southwest-northeast direction) was compared with electrical resistivity logs of several wells drilled in the same direction. The result shows a good correlation. The 2-D inversion result of the same line (Line-1) was also compared with the temperature and lost circulation data obtained during the course of drilling. The result shows a good agreement. It is concluded from the MT survey that the geothermal reservoir of the Takigami area is separated by the Noine fault at different depths. The geothermal reservoir in the east of the Noine fault is shallower than the reservoir in the west. The results of MT survey are in a good agreement with a three-dimensional (3-D) geological model constructed by the use of drilling data and well logging data in the Takigami area.