Reservoir monitoring by observation of gravity changes at the Takigami geothermal field, central Kyushu, Japan

Repeat gravity measurement have been conducted at the Takigami geothermal field in Kyushu, Japan in order to monitor the underground geothermal fluid flow system. Gravity changes at 26 stations have been monitored since May 1991 to study reservoir behavior. The observed gravity changes included gravity changes due to shallow ground water level change. We removed the effect of shallow ground water level change on gravity by applying a statistical technique to isolate the gravity change due to the mass change associated with the commencement of the production and reinjection of geothermal fluids at the Takigami geothermal power station in November 1996. Multivariate regression analysis was applied in order to establish the relationship between gravity and precipitation; a good correlation was obtained. Therefore, it is now possible to estimate the gravity change due to ground water level change at each observation point by using the precipitation data collected prior to the gravity measurement. We thereby define the residual gravity, which is the observed gravity minus this estimated gravity. The results of repeat gravity measurement show that repeat gravity surveys is an effective method to monitor the underground hydrological systems.