抄録
Takigami geothermal field, central Kyushu, Japan, is a hot water dominated geothermal system. A new 25MW geothermal power plant is expected to commence the operation in 1996 at this field. A short period production and reinjection test was conducted from November 1991 to February 1992. We conducted repeated gravity surveys at specified intervals (one to three months) for two years before and after the test period to detect gravity changes caused by production and reinjection. Gravity changes of up to 150 microgals have been observed during the two years. Most of such large changes are found to be due to seasonal variations of shallow water level. After correction of such a shallow water effect, we detected gravity increases of up to 30 microgals in the reinjection area. This change is reasonable if we assume that the reinjected water stays for a while beneath the reinjection area. Gravity decreases of up to 40 microgals were also detected in and eastward of the production area. The gravity decreases have not become apparent by the end of March, that is, one month and a half after the test finished.
