Gravity Monitoring of Geothermal Reservoirs in the Takigami Geothermal Field, Central Kyushu, Japan - Gravity Changes Caused by Production and Reinjection of Geothermal Fluids -

Gravity changes at 26 stations have been monitored since May 1991 to study reservoir behavior in the Takigami geothermal field, central Kyushu, Japan. The apparent gravity changes were observed and caused by the variation of the shallow ground water level. We removed the effect of shallow ground water level change on gravity by applying a statistical technique to obtain the gravity change due to the mass change associated with the production and reinjection of geothermal fluids. The multivariate regression analysis was used with the gravity and the precipitation data. As a result, we obtained a good correlation between them. It became possible to remove the effects caused by the ground water level change at each observation point by using the precipitation data and calculate the residual gravity variation. The increase of the residual gravity up to 20 microgals in the reinjection zone, and the decreases, up to 40 microgals in the production zone, were obtained after the commencement of the geothermal power plant. The rate of decrease was very high at the beginning of the production but it became smaller, implying that underground fluid flow reached to a new equilibrium state. These residual gravity changes reflect very well the changes in mass balance in the geothermal reservoir.