Shallow Structure Fault and Fracture Mapping in Jaboi Volcano, Indonesia, Using VLF–EM and Electrical Resistivity Methods

The area surrounding Jaboi volcano, Indonesia, has been planned for geothermal energy development with capacity of ~ 50 Mwe. Geophysical surveys can contribute to the understanding of the hydrogeology of the area because they can provide information without interferences, which could potentially react to the flow of hot water. In many cases, the occurrence of a hot spring is closely related to its local fault system, which permits the flow of thermal fluid to its surface. This research applied the very low-frequency electromagnetic (VLF–EM) method and then compared it with electrical resistivity data to map fractures, faults, and geological aspects in the Jaboi volcano manifestation area. In tropical areas such as Indonesia, the VLF–EM method has been applied widely in studying subsurface structures. However, it is rarely used in volcanic areas with high terrain conditions even though the mobility of VLF methods is simpler than the electrical resistivity method. At the west side of the volcano, the 2D models from both VLF–EM and electrical resistivity showed the conductive zone (log 0.5–1.5 Ωm) found as a response to the manifestation, which has a similar direction with a local fault known as the Ceuneuhot fault. At the east side, the 2D models from both methods showed a similar conductive zone at depths of 15–25 m, which is caused by the existence of bedrock (> log 2.5 Ωm). The bedrock at depths of 0–10 m inhibits the rise of fluid to the surface so that there is no geothermal manifestation in the other local fault known as the Leumomate fault. In addition, the 2D models from the two methods showed the contrast of some fracture zones as entry and exit points of fluid in the Jaboi volcano. In addition, analysis of Landsat 8 optical images also showed high-temperature values and low vegetation indices in several active craters as indicators of volcanic activity. Based on the processed data, it is concluded that VLF–EM can be used as a fast and accurate method to provide information about near-surface structures in a geothermal field, and it can be used safely to suggest a location for drilling programs or to give contributions to protect geothermal springs.