Applying audio-frequency magnetotelluric (AMT) and gravity technology for discovering reservoir beneath Mt. Pancar geothermal prospect

Abstract

Geothermal resource occurrence beneath Mt. Pancar volcanic product is indicated by three neutral hotsprings called Kawah Merah, Kawah Hitam and Kawah Putih. The surface temperature looks promising with the range of 45 to 68°C. There are also warm grounds and altered rock found at the surrounding. Unfortunately, there is no impressive manifestation found on the top of Mt. Pancar. Geologically, the area is dominated by quaternary volcanic rock which probably associated with Mt. Pancar as a parasitic cone of Mt. Gede-Pangrango. It is also surrounded by tertiary sedimentary rock that was probably intruded by the volcanic product. A pair of NW-SE geological structures located at the western and eastern side of Mt Pancar could be act as a boundary of the reservoir. The reservoir temperature calculated using Na/K geothermometry is estimated about 180 to 190°C. It indicates that geothermal system in Mt. Pancar is classified as low to moderate temperature system. To discover the geometry as well as the top of reservoir, Audio-frequency Magnetotelluric (AMT) was then applied in the prospect area. In 2014, at least 18 Audio-frequency Magnetotelluric (AMT) stations have been successfully measured. Careful data processing from time-series inspection through static shift correction have been conducted. Furthermore, to overcome the complex volcanic-tectonic structure, 3-D inversion of AMT data was performed using MT3DINV-X Software applied for frequency of 1 Hz to 10 kHz. The results of 3-D AMT inversion show the resistivity pattern that mostly observed in geothermal area. The conductive layer interpreted as clay cap is distributed between Mt Pancar and Kawah Putih and between Mt Pancar and Kawah Merah. It has value of less than 15 ohm-m with its thickness varies from 500 meters to 1500 meters. The upflow of the system indicated by up-dome shape of resistivity was found beneath Mt Pancar. The slightly higher resistivity value, which has range of 20-100 ohm-m, is found underlying the clay cap layer. It is indicated as reservoir zone. The highest resistivity layer (>100 ohm-m) is inferred as basement of Mt Pancar. Moreover, the indication of geothermal reservoir obtained by AMT data is also supported by gravity data. Complete bouguer anomaly (CBA) and residual anomaly derived from 236 gravity stations 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. That is also strengthening each other. Therefore, a conceptual model was then developed by integration of resistivity and density model supported by geological and geochemical data. It explains that the indication of upflow is probably beneath the Mt Pancar, while the outflow towards the northern side and the eastern side. It probably follows the geological structure and reaches the surface as Kawah Merah, Kawah Hitam and Kawah Putih.