Subsurface Modeling of Mt. Ciremai Geothermal System Based On Gravity and Magnetotelluric Data

Mt. Ciremai geothermal area is located in West Java Province, Indonesia. Geophysical surveys had been conducted since 1986, which focused on area near Sangkanhurip hot spring. In 2018, Center for Mineral, Coal and Geothermal Resources (CMCGR) enlarged the interest area of magnetotelluric survey to Pejambon, later to be mentioned as Cilengkrang, hot spring. Following the last survey result, which stated that the prospect area wass located near Cilengkrang hot spring and still open to north direction, in 2021, CMCGR conducted gravity and MT surveys to delineate the reservoir boundary. This survey was also part of government drilling program to accelerate the geothermal development in Indonesia. Gravity survey result showed that the Mt. Ciremai was surrounded by low density anomaly (<0 mGal) which was interpreted as huge graben beneath Mt. Ciremai. The low density anomaly was surrounded by high density anomaly (>25 mGal) elongated from the northern, eastern, to the southern part of the mountain. This high anomaly was interpreted as the old eruption product of Mt. Ciremai. The low density anomaly also appeared on the east part of the survey area, near Sangkanhurip and Ciniru hot spring. This low anomaly was interpreted as the respond of near surface sedimentary rock. The existence of Cilengkrang hot spring was controlled by the structure in northwest-southeast direction, while Ciniru and Sangkanhurip hot springs were controlled by north-south direction structure. MT survey result showed the existence of conductive layer (<10 Ohm.m) near Cilengkrang hot spring, interpreted as the clay cap of the Mt. Ciremai geothermal system. Below the conductive layer lied medium resistivity layer (20 – 100 Ohm.m) interpreted as the reservoir of the system in the depth of 1000 – 1500 m.