In order to monitor the mass variations in the reservoir that might occur due to the pressure changes that would be induced by the changes in the production and reinjection rates when the output doubled from 55 MW to 110 MW, in the Hatchobaru geothermal field, gravity changes have been monitored at 46 benchmarks since May 1990. According to the gravity and reservoir pressure changes monitored using capillary tubes set into several observation wells, a close correlation between these two changes was recognized. Therefore, the gravity changes are considered to indicate changes in reservoir mass behavior. Moreover, when the Hatchobaru No. 1 unit temporarily suspended operation for 47 days, from March 16 to May 1, 1996, because of a lightning accident, gravity changes were observed about two months after the forced outage. On the basis of a two-dimensional analysis, the gravity changes in, the Hatchobaru geothermal field are considered to reflect the changes in reservoir mass caused by variations in the production and reinjection rates.
