Reservoir Monitoring by Repeat Gravity Measurements at the Takigami Geothermal Field, Central Kyushu, Japan

Abstract

Repeat gravity measurements have been conducted since May 1991 at the Takigami geothermal field in Kyushu, Japan in order to monitor the underground geothermal fluid flow system. The observed gravity changes include gravity changed due to shallow groundwater level change. Therefore, we tried to remove the influence of shallow groundwater by applying a statistical technique in order 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 1996. Although an obvious correlation between measured shallow groundwater level change and gravity change wasn't found out, the multivariate regression analysis was applied in order to establish the relationship between gravity and precipitation, and a good correlation was obtained. Therefore, it is now possible to estimate the gravity change due to groundwater level change at each observation station by using the precipitation data collected prior to the gravity measurements. We thereby define the residual gravity, which is the observed gravity minus this estimated gravity. The results of repeat gravity measurements show that repeat gravity surveys is an effective method to monitor the underground hydrological systems.