Study on Sustainable Development of Geothermal Energy from Geophysical Monitoring of Geothermal Reservoir - A Case Study of Repeat Gravity Measurement -

Sustainable development of geothermal energy was discussed on the basis of the results of repeat gravity measurement. The repeat gravity measurements clarified the mass changes in the geothermal reservoirs during exploitation of the Hatchobaru and the Takigami geothermal fields and that in the volcanic geothermal reservoir of Kuju volcano after the 1995 phreatic eruption in central Kyushu, Japan. Each case showed clear decrease of gravity after the commencement of power generation or phreatic eruption. The gravity recovered after the period of gravity decrease in the initial stage of power generation or phreatic eruption, which shows recharge of fluid to the reservoir. The mass balance in a geothermal reservoir was estimated based on change in gravity. Sufficient recharge of fluid to the geothermal reservoir was estimated both in the Hatchobaru and the Takigami geothermal fields. A large amount of groundwater recharge to the central part of Kuju volcano after the 1995 phreatic eruption was detected in the volcanic geothermal reservoir. The repeat gravity measurement is an effective technique to detect the recharge of fluid to reservoir and therefore it can contribute to the discussion of sustainable development of geothermal energy.