Identification of Subsurface Geological Structures of the Arjuno-Welirang Geothermal Potential Area Using the Gravity Method

Java Island has several geothermal prospects, one of which is located in the Arjuno–Welirang area in East Java. The objective of this study is to determine the subsurface geological structures that have the potential to generate geothermal energy in the region. The study utilizes a gravity-based method that is based on the Global Gravity Model Plus. Complete Bouguer anomalies, first horizontal derivative, and second vertical derivative were analyzed to identify fault structures. Additionally, 2D modeling was performed using inversion modeling implemented in the ZondGM2D software. A total of six major faults were identified four normal faults and two reverse faults primarily distributed near active geothermal manifestations such as Padusan hot springs, Coban Hot Springs and Mount Welirang. Furthermore, 2D gravity inversion modeling was performed along two main profiles (A–A′ and B–B′). The inversion results revealed three key subsurface layers: (1) a low-density zone (<2.3 g/cm³) interpreted as a clay cap formed by hydrothermal alteration, (2) an intermediate-density reservoir zone (2.3–2.5 g/cm³) composed of fractured volcanic rocks, and (3) a high-density magmatic intrusion zone (>2.6 g/cm³) serving as the heat source.