Hybrid Gravimetry for Optimization Time Lapse Monitoring Data: A Case Study in Kamojang Geothermal Field

Abstract

Time-lapse gravity monitoring is a way to assist in building up knowledge of the changes of subsurface condition. Gravity monitoring techniques have been applied to the investigation of dynamic processes in various types of geothermal and volcanic fields. Gravity changes enable the characterization of subsurface processes: i.e., the mass of the intrusion or hydrothermal flow. A key assumption behind gravity monitoring is that changes in earth’s gravity reflect mass-transport processes at depth (Battaglia et al., 2008). Combined absolute and relative gravimeter is an advance method that is newly used for monitoring. Use of an absolute gravimeter as a reference for other relative measurements, a technique called hybrid gravimetry (Okubo et al., 2002), was applied successfully to geothermal studies (Sugihara et al., 2008; Nishijima et al., 2010). We use relative gravimeters (LaCoste-Romberg or Scintrex) and were combined by absolute gravimeter (A-10 Micro-g) to monitor the subsurface condition. A10 absolute gravimeter is a new generation of portable absolute instrument and has accuracy 10 µGal. This method has many advantages for monitoring method. Some of the advantages are the ability to reduce uncertainties caused by regional gravity variations and reduce the drift correction factor. The repeated absolute gravity measurement was conducted in the Kamojang geothermal field between 2011. On the other hand, the repeated relative gravity measurement also was conducted in the same field. Some gravity benchmarks were measured using both absolute and relative gravimeter and is used as the reference benchmarks. The combined absolute and relative gravity measurement can reduce drift correction of relative measurement. In the long time period, the hybrid gravimetry method will improve the result of gravity change data for monitoring in Kamojang geothermal field. This present study introduces a time lapse monitoring method that will enhance reservoir monitoring using repeated precisely-gravity measurements.