Resistivity characterization of the Arjuno-Welirang volcanic geothermal system (Indonesia) through 3-D Magnetotelluric inverse modeling

The Arjuno-Welirang geothermal area is located in East Java, Indonesia. In the Arjuno-Welirang geothermal area, the Arjuno, Welirang and Anjasmoro volcanic complex is dominated by quaternary volcanic rocks and a complex geological structure. Several impressive surface thermal manifestations are found in the study area, including a high temperature solfataric-fumarole (about 137 °C) on the top of Mt. Welirang and also bicarbonate hot springs with a temperature range of 39–55 °C in the northern and western parts of the study area. The occurrences of the surface manifestations are probably controlled by the NW-SE and NNE-SSW fault structures. The occurrence of the high temperature solfataric-fumarole with high magmatic gas contents indicates that the Arjuno-Welirang geothermal area can be classified as a volcanic geothermal system. To investigate the subsurface condition of the geothermal system, Magnetotelluric (MT) survey was conducted within 34 MT stations distributed in the northern and western parts of the study area. Considering the complexity of the subsurface volcanic rocks and the geological structures of the study area, one and two-dimensional MT data inversion approaches could not be applied properly. Accordingly, a 3-D inversion of the MT data will be the most representative approach in such conditions in order to investigate the reservoir structure of the volcanic geothermal system. The 3-D inversion was then applied to all MT data with a frequency range of 100–0.01 Hz. Generally, the quality of the MT data varies from good to excellent. A clear subsurface resistivity structure revealed by the 3-D inversion showed a good correlation with geological and geochemical evidences. Overall, the resistivity structure consists of a conductive (1–10 ohm m) altered rock at the upper part overlying a reservoir zone with a slightly higher resistivity range (20–60 ohm m). The lower part of the resistivity structure showed the highest resistivity value (>80 ohm m), indicating a hot rock region. The main upflow is represented by an updome structure of the base of the conductive layer (BOC) beneath the Mt Welirang summit. It is supported by the occurrence of solfataric-fumaroles on the summit of Mt Welirang. Moreover, the outflow zones of the Arjuno-Welirang geothermal system are indicated by a thickening of the low resistivity layer toward the northern and western parts of Mt. Welirang supported with the occurrence of bicarbonate hotsprings. The existence of magmatic gas components (i.e. H ₂ S and SO ₂ ) detected in the fumarole, gives an early warning regarding the possible existence of acidic fluid. Further exploration drilling should consider this issue. A conceptual model is then developed to visualize the geothermal system of the Arjuno-Welirang prospect area by integrating the geological and geochemical data.