Reservoir Monitoring by Observation of Gravity Changes at Some Geothermal Fields in Kyushu, Japan

Abstract

Repeat gravity measurements have been conducted at four geothermal fields (Takigami, Oguni, Hatchobaru and Yamakawa) in Kyushu, Japan to monitor movement of geothermal fluid. For example, gravity changes at 26 stations have been monitored since May 1991 to study reservoir behavior in Takigami geothermal field. At some fields, the observed gravity changes include the effect of shallow ground water level change. These are removed using the relationship between gravity and precipitation derived from multivariate regression analysis, and precipitation data collected prior to the gravity measurements. An application of Gaussís Potential Theorem to gravity changes in Hatchobaru geothermal field show that most of the mass discharged to the atmosphere has been replaced by natural inflow. These estimation and the results of repeat gravity measurements show that repeat gravity measurements is an effective method to monitor the underground hydrological systems.