Modeling of subsurface structure by using magnetic methods in the area of Mt. Pandan, Indonesia

Mount Pandan is an active volcano with geothermal phenomena and is located in tectonically active East Java, Indonesia. We conducted a geomagnetic study around Mount Pandan to obtain further information about fault structures and geothermal prospects. We established 245 geomagnetic observation stations covering an area of 20 km 2 . We calculated the magnetic anomalies, applied upward continuation and reduction to the pole, and calculated the improved normalized horizontal tilt angle to interpret the area. Furthermore, modeling was performed that included previous research. Our study identified NW–SE-, SW-NE-, and W-E-trending structures that might be affected by the Kendeng thrust fault and act as pathways for geothermal fluid flow. This geothermal fluid is estimated to flow from the southern area between Mount Pandan and Mount Wilis toward the north. There are several hot springs around Mount Pandan and Tirtohusodo hot spring near Mount Wilis. Thus, the heat source may be located in the area between Mount Pandan and Mount Wilis and supply heat for the geothermal systems of both volcanoes. The modeling results show the intrusion of andesite-type bodies at the summit of Mount Pandan and in the northern part of the study area. We found low magnetic anomalies that might indicate a heated region as a potential geothermal area. The reservoir of the geothermal system in Mount Pandan is interpreted to be tuffaceous sandstone from the Kalibeng Formation with claystone from the Klitik Formation as a clay caprock.