It is important to understand the geothermal reservoir behavior in order to produce geothermal fluid for a long time. The mass changes at subsurface with production and reinjection of geothermal fluid cause the gravity changes on the surface. Repeat gravity measurements have been applied at the Takigami geothermal field in Central Kyushu, Japan, where the Takigami power plant has been generating since November 1996. We used a Scintrex CG-3, CG-3M and CG-5 relative gravimeters in order to measure gravity change caused by production and reinjection of geothermal fluid, but we could not estimate the gravity change at the reference station. To solve this problem, we introduced an A10 absolute gravimeter (Micro-g LaCoste, Inc.). In addition, the A10 was used for not only the assessment of the gravity changes at the reference station, but also the detection of the gravity change caused by the subsurface fluid mass changes at some other measurement stations. However, it was impossible that the A10 absolute gravimeter was applied at all of the stations, because the condition of the measurement was strict. Therefore we have applied the relative gravimeters in such strict situations. Thus both absolute and relative gravimeters can complement each other.

We have measured the gravity repeatedly since the commencement of power generation at the Takigami geothermal power plant, and we have detected the gravity changes which were consistent with the changes in mass balance in the geothermal reservoir. As a result, we inferred that the current fluid mass in the Takigami geothermal field has recovered to as much as that before production and reinjection had started. The absolute gravity measurement was able to evaluate the gravity change at the reference station, but the application of the hybrid measurement was effective because the condition of the absolute gravity measurement was strict.

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