It is important to understand the faults and basement rock structure in hot spring area. Precise gravity survey has been conducted around the Minou fault system, Fukuoka prefecture. We set 913 measurement points in Kurume, Asakura and Ukiha cities. We carried out the frequency analysis in order to remove the regional trend and noises. A three-dimensional gravimetric modeling was conducted under the assumption of two homogeneous layers condition. The survey result indicates that there are three graben structures on the north side of the Minou fault system. The maximum depth of the grabens was estimated at about 2000 m. A steep slope with north dipping exists on the southern side of graben. We compared the locations of hot springs with the basement rock structure, which is estimated from our precise gravity survey. The result of survey indicates that almost all of the hot springs are located above the steep slope portion of the basement rock, while some of them are above the deepest or gentle slope portions of the basement.

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The Otake Geothermal Power Station (12.5MW) is located in Otake-Hatchobaru geothermal field, one of the most active geothermal fields in Japan. This power station has been operating since 1967 and maintaining a stable operation since 1988 using 4 production wells. However, the electric power output has declined since 2006 due to the decreasing mass flow of the main production well (O-22). In 2010, a magnetotelluric (MT) survey was conducted to detect a drilling target for a make-up production well. In 2012, make-up well O-21 was drilled to the target detected in the MT survey and it successfully discharged steam and water. As a result of the successful drilling of the make-up well, the power output of the Otake Geothermal Power Station recovered to its 2006 level of output.

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Soil thermal properties are significant parameters affecting heat transfer between ground and buildings. This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. We apply a time-spatial dependent boundary condition at far field of soil region. Finally, we perform the modeling of a Borehole Heat Exchanger (BHE) for in in-situ soils in Alsace region in France. Generally, we can observe that a thermal equilibrium stage can be reached after several heating and cooling cycles.

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