Determination of Negative Density Changes in the Kamojang Geothermal Field Using Time-Lapse Microgravity Analysis

Abstract

Ground deformations and gravity changes were mesured in order to study density distribution changes caused by the production and re-injection into the Kamojang geothermal reservoir. In the last two years (2006-2007) we conducted two elevation measurements, in July 2006 and July 2007. In addition, we carried out three microgravity surveys, in June 2006, November 2006 and July 2007. The gravity stations were located at 88 benchmarks to cover the survey area. From those three maps we have already made two time-lapse microgravity anomaly change maps, covering the periods of June-November 2006 and June 2006-July 2007.
Gravity effects due to density change in the reservoir (caused by production/re-injection) were obtained by correcting the measured gravity anomalies of the gravitational effect to vertical ground movements (subsidence) and ground water level changes. Gravity anomaly measured on the surface due to elevation change has a positive value for elevation lowering (subsidence) and the gravity change is approximately 3 μGal for 1 cm elevation change. Gravity anomaly related to dynamic groundwater was corrected by a stripping filter. By using inversion methods, a map of density distribution changes has been produced during this period.