Repeat Microgravity Measurements Using Absolute and Relative Gravimeter for Geothermal Reservoir Monitoring in Ogiri Geothermal Power Plant, South Kyushu, Japan

Abstract

Repeat microgravity measurements using absolute and relative gravimeter (hybrid microgravity measurements) have been applied at Ogiri geothermal power plant (30MW) on the western slope of Kirishima volcano, southern Kyushu, Japan. In this study we try to detect the short-term gravity change caused by the temporary shut in the production and reinjection wells for the regular maintenance in 2011 and 2013. Repeat microgravity measurements were conducted using A-10 absolute gravimeter (Micro-g LaCoste) and CG-5 relative gravimeter (Scintrex) in the period before and after the regular maintenance. We carried out the gravity measurements in 27 stations (6 stations for absolute and 26 stations for relative). The accuracy of these instruments is 10 micro gals. After remove some effects (Earth tide, precipitations, shallow groundwater level changes and so on), the residual gravity changes can be subdivided into five types of response. We detected gravity decrease (up to 20 micro gal) in the reinjection area, and gravity increase (up to 30 micro gal) in the production area after 1 month later the temporary shut in. Then almost gravity stations are recovered after the maintenance. We estimated the temporal density change in the geothermal reservoir using these gravity changes.