Correction of Background Gravity Changes Due to Precipitation: Oguni Geothermal Field, Japan

Abstract

The Oguni geothermal power station will be constructed in central Kyushu, Japan. To identify and study reservoir behavior after exploitation begins, microgravity monitoring has been planned and the background gravity changes have been monitored frequently since 1996. So far, only the background level measurements have been made, in which seasonal changes in gravity were detected. The change of shallow groundwater table is thought to be the primary factor causing background gravity changes because substantial changes in water table depth were observed in this field. To remove the effects due to shallow ground water table changes from measured gravity changes, we tried at first to calibrate gravity change from precipitation through water table at a benchmark where water table was known. To extend the calibration method to benchmarks which do not have nearby observation wells, the empirical equation was applied to precipitation data to estimate the water level changes that might occur near the benchmark. Porosity was determined from the relationship between precipitation and gravity changes associated with water table changes. The microgravity changes estimated from the precipitation data were compared with the actual gravity measurement data, however, no obvious relationship between precipitation and gravity change could be obtained. Repeat gravity measurement should be continued for further examination of procedure for noise reduction.