The interpretation of subsurface structures in Semurup geothermal area has been carried out using the geomagnetic method. Data were collected in an area of 1500 m × 1400 m consisting of 160 points. The magnetic anomaly value obtained was derived from the total magnetic induction value that has been corrected by IGRF and diurnal variation, then transformed by reduction to equator and upward continuation to remove noise and separate local and regional anomalies. The results of data processing showed the total magnetic field values in the study area ranged from -1730.4 nT to 1909.0 nT. Magnetic anomalies in this study area are dominated by negative values that may be caused by demagnetised rocks (a result of hydrothermal alteration). The results of 2D modeling, it has 5 rock layers that can be classified into 3 main parts of the geothermal system The first and second layers are caprock with a depth of up to 850 meters consisting of sedimentary rock, clay, and sandstone. The third layer is indicated as a reservoir with a depth from 850 to 1450 m and is dominated by sandstone and clay alteration Hot rock in the fourth and fifth layers is dominated by basalt igneous rock and the presence of dacitic lava intrusion from the northeast of the study area at depths below 1450 m, and the Siulak fault as a outflow zone for geothermal fluid. The presence of the caprock, reservoir, hot rock, and fault zones indicates that the Semurup area has geothermal potential and is suitable for further exploration.

Geological structures in the Bukit Gadang geothermal area have been identified using the geomagnetic method to determine the type of fault. Data was measured using a magnetometer in the area with dimensions of 1200 m × 1200 m consisting of 144 points at 12 tracks, and the spacing between points was 100 m. Magnetic anomaly data performed diurnal and IGRF (International Geomagnetic Reference Field) corrections. Furthermore, reduced to poles and continuous upwards processes were carried out to remove noise and separate local and regional anomalies. The magnetic field anomaly ​​in the study area ranges from -1771.8 nT to 1089.9 nT, dominated by negative values, indicating the presence of heat sources and the influence of demagnetization of subsurface rocks. The 2D modeling results show that two primary rocks dominate the study area; pyroclastic flow units and andesite lava rock, which come from the Jantan and Batino volcanic formations. The caprock rock layer was identified in the upper layer with a depth of 850 meters. The reservoir rock layer with low susceptibility values ​​was below the caprock layer. The 3D modeling results show a normal fault with a depth of 300-800 meters or at the border of the Jantan volcanic formation with the Batino volcanic formation. The fault line leads to the southeast-northwest (N160°E). The faults obtained from the 2D and 3D models are suspected to be the outflow of geothermal fluid from the reservoir rock layer and form a manifestation in the form of a hot spring at Bukik Gadang.

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