In Takigami geothermal area, we have continued the geothermal reservoir monitoring by using gravity change from 1991, in order to monitor the mass transfer caused by production and reinjection of geothermal fluid. We, however, had measured only relative gravity measurements by using relative gravimeters (SCINTREX CG-3, CG-3M and CG-5), so we haven't been able to evaluate the gravity change at the reference station of the relative gravity measurements. Therefore, we introduced an A10 absolute gravimeter (Micro-g LaCoste, Inc.) in 2008. We used the A10 gravimeter for not only the assessment of the gravity changes at the reference station, but also the detection of the absolute gravity change caused by the subsurface fluid mass changes at some other measurement stations. As a result of absolute gravity measurements, the gravity change at the reference station of the relative gravity measurements is small enough for this evaluation, within about 10 (mu)gal. So we estimated that this reference station is appropriate for the relative gravity measurements. Because we judged that the gravity change detected by the relative and absolute gravity measurements illustrated the mass transfer in the geothermal reservoir, we divided the Takigami geothermal area into 3 areas from the pattern of the gravity change after the commencement of the Takigami geothermal power plant, and we estimated the 4 stages of geothermal fluid flow pattern from temporal gravity change. Based on these classifications, we led a conceptual reservoir model of the Takigami geothermal area.
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