Repeat Absolute and Relative Gravity Measurements for Geothermal Reservoir Monitoring in the Ogiri Geothermal Field, Southern Kyushu, Japan

5th ITB International Geothermal Workshop

Nishijima, J.^1 ; Umeda, C.^2 ; Fujimitsu, Y.^1 ; Takayama, J.^3 ; Hiraga, N.^3 ; Higuchi, S.^3

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japan^1

Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, Fukuoka, Japan^2

Nittetsu Mining Co. Ltd., Tokyo, Japan^3

关键词: Absolute gravimeter;    Absolute measurements;    Geothermal reservoir;    Gravity measurement;    Microgravity measurement;    Relative measurement;    Residual gravity;    Shallow groundwater levels;   

Others  :  https://iopscience.iop.org/article/10.1088/1755-1315/42/1/012004/pdf
DOI  :  10.1088/1755-1315/42/1/012004

来源: IOP

【 摘 要 】

Repeat hybrid microgravity measurements were conducted around the Ogiri Geothermal Field on the western slope of Kirishima volcano, southern Kyushu, Japan. This study was undertaken to detect the short-term gravity change caused by the temporary shutdown of production and reinjection wells for regular maintenance in 2011 and 2013. Repeat microgravity measurements were taken using an A-10 absolute gravimeter (Micro-g LaCoste) and CG-5 gravimeter (Scintrex) before and after regular maintenance. Both instruments had an accuracy of 10 μgal. The gravity stations were established at 27 stations (two stations for absolute measurements and 25 stations for relative measurements). After removal of noise effects (e.g., tidal movement, precipitation, shallow groundwater level changes), the residual gravity changes were subdivided into five types of response. We detected a gravity decrease (up to 20 μgal) in the reinjection area and a gravity increase (up to 30 μgal) in the production area 1 month after the temporary shutdown. Most of the gravity stations recovered after the maintenance. The temporal density changes in the geothermal reservoir were estimated based on these gravity changes.