There are many fumaroles and hot springs in the Takenoyu geothermal area. Heat discharge and 1m depth temperature survey were carried out for identifying the thermal structure of this area. The high temperature ground extends along the Takenoyu fault which is assumed geologically. The total heat discharge by conduction, hot springs, and fumaroles was estimated to be 1.5×106 cal/sec. The underground temperature distribution to depths up to 1000 m was obtained from logging data of 14 wells. The temperature beneath the shallow hot area is higher than the beneath its surroundings. The maximum temperature exceeds 200°C at a depth of 600 m. As no cap rock is assumed beneath the Takenoyu geothermal area, we tried to explain the surface heat discharge and the underground temperature pattern by the two-dimensional thermal convection model for the opened geothermal reservoir. As a result, the following model was suggested; the meteoric water infiltrating into the underground encounters the heat source of 250°C and 900m wide which is stretching horizontally at a depth of 600 m. After heated there, it rises up to the surface and makes thermal manifestations within a width of 600m. The heat discharge and underground temperature distribution calculated from such a model agree well with the observed ones.
Hydraulic fracturing developed in oil reservoir engineering has been used to improve the injection rates of reinjection wells in geothermal areas. Electrical resistivity method was designed to elucidate the fracture initiation, and then the field survey was carried out in 1980 at the Kamitakara-mura, Gifu Prefecture, Japan. Schlumberger VES station was fixed near the drill site of hydraulic fracturing well referred to as the HY well. Waveforms of induced electric current and the resulting electric potential were simultaneously monitored by 2-pen type recorder, before, during and after the stage of hydraulic fracturing operations. From an extensive series of measurements of the hydraulic fracturing processes, the following results were presented:(1) The irregular waveform of electric potentials was occurred only during the period of the hydraulic fracturing.(2) The steady linear relationship between electric currents and electric potentials was comparatively found to fluctuate directly after fracturing process.(3) Apparent resistivity of the ground by Schlumberger array decreases associated with the fracturing operation. The mechanism of the electric potential variation was considered that the streaming potentials increases because of the formation pressure by the water injected from the pump. Electrical resistivity method proved to be efficient for detecting and mapping the fluid flow in the case that the depth penetration of electrical resistivity soundings was larger than the fracturing depth.
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