Imaging Reservoir Structure of Mt. Pancar Geothermal Prospect Using Audio-Frequency Magnetotelluric (AMT) and Gravity Technology

Mt. Pancar geothermal prospect area is located in Bogor district, West Java. The geological condition of the area is dominated by quaternary volcanic rock which probably associated with Mt. Pancar as a parasitic cone of Mt. Gede-Pangrango. Surface thermal manifestations that can be found in this area are altered rocks, warm grounds, and neutral pH hot springs with temperature range of 45 to 68 o C. The manifestations are distributed in the eastern and northern side of Mt. Pancar. However, there is no impressive manifestation found on the top of Mt. Pancar. It indicates that geothermal system in Mt. Pancar is classified as low to moderate temperature system. Based on Na/K geothermometry, the estimation of subsurface temperature in Mt. Pancar geothermal prospect is in the range of 140 to 180 o C. In order to develop conceptual model and understand the subsurface structure in the prospect area, geophysical methods including Audio-frequency Magnetotelluric (AMT) and Gravity were then conducted. To overcome the complex volcanic-tectonic structure, 3-D inversion of AMT data was performed using MT3DINV-X Software with frequency ranging from 1 Hz to 10 kHz. The results of 3-D AMT inversion show the presence of conductive layer. It is interpreted as clay cap distributed between Mt Pancar and Kawah Putih and between Mt Pancar and Kawah Merah. The conductive layer has value of less than 15 ohm-m with its thickness varies from 500 meters to 1500 meters. The up-dome shape was found beneath Mt Pancar and it was interpreted as upflow zone. The resistive layer found underlying clay cap layer with resistivity range of 20-100 ohm-m is indicated as reservoir zone. The highest resistivity layer (>100 ohm-m) is indicated as basement of Mt Pancar. The indication of geothermal reservoir obtained by AMT data is also indicated by gravity data. Complete bouguer anomaly (CBA) and residual anomaly data show that the region of Mt. Pancar is dominated by low gravity anomaly. Two-dimensional forward modeling result of gravity data also shows the indication of clay cap with low density layer. It supports the indication from AMT data. Therefore, integration of resistivity and density model supported by geological and geochemical data was used to develop conceptual model.