Subsurface Analysis Using Gravity Data at Lili Sepporaki Geothermal Area

Gravity survey plays an important role in early stage of subsurface imaging in geothermal system. The aim of this study is to understand the subsurface structure and possibly the heat source based on rock density distribution with three dimensional inversion modeling (3D). In 2010, Center of Mineral Resources, Coal and Geothermal of Indonesia was performed gravity survey at Lili Sepporaki geothermal area with 205 stations within a 10 x 10 km coverage area. The complete Bouguer anomaly was obtained with the density of 2.8 g/cm3 and this density was used to do 3D modelling. Bouguer anomalies ranging from 40 mgal to 62 mgal were observed in the study area with low anomalies values located in the northern part and high anomalies located in the southern part of the study area. Based on the 3D inversion modeling results, there are low density rocks with values between 2.0 - 2.3 g/cm3 around the hot springs of the research area with a depth of <1 km. These rocks are interpreted as rocks that associated with fractures forming a fault zone that controls the surface manifestation of the study area. High density rocks with values between 3.0 - 3.3 gr/cm3 are located south of research area with depth >2 km. This rocks are interpreted as intrusive igneous rocks and are thought as the heat source of geothermal systems in the study area.