EVALUATION OF THE BACKGROUND NOISE FOR MICROGRAVITY MONITORING IN THE OGUNI FIELD, JAPAN

Abstract

To identify and study the reservoir behavior after exploitation begins, precise gravity monitoring has been planned and the background gravity changes have been monitored repeatedly. So far seasonal changes in gravity were detected, and the change of shallow ground water table is considered to be the primary factor causing background gravity changes. To remove the effects due to the 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 gravity changes estimated from the precipitation data were compared with the actual gravity measurement data. It appeared that fluctuation of measured gravity were larger than the amplitude of estimated value from precipitation.