3-Dimensional Inversion of MT Data over the Arjuno-Welirang Volcanic Geothermal System , East Java ( Indonesia )

The Arjuno Welirang geothermal area is located about 60 km southwest of Surabaya, the capital city of East Java. Geologically, the prospect area is dominated by quaternary volcanic rocks, both lava and pyroclastic. Impressive surface thermal manifestations can be found in this area, which include solfataric-fumaroles with temperature of 137C found in the summit of Mount (Mt) Welirang and bicarbonate hotsprings with a temperature range of 39 to 55C found in the western and northwestern vicinities of Mt Welirang. The occurrence of solfataric-fumaroles with magmatic gases indicates the existence of a volcanic geothermal system in the subsurface. The surface manifestations are geologically controlled by the northeast-southwest fault structures. The CO2 gas geothermometer indicates a high temperature reservoir of about 260C. To delineate the subsurface resistivity structure associated with the geothermal system, a 3-D inversion was performed using the Occam Data Space algorithm. The 3-D inversion was applied to 34 MT soundings with a frequency range of 320 to 0.01 Hz. Overall, the quality of the MT data used varied from good to very good. The subsurface resistivity structure revealed by 3-D inversion showed a low resistivity (1-15 ohm-m) altered rock at the upper part overlying a reservoir zone with slightly higher resistivity (20-60 ohm-m). The lower part of the resistivity structure showed the highest resistivity value (> 80 ohm-m), which indicated a hot rock region. The geometry of the resistivity structure showed the updome structure of the base of the conductive layer (BOC) centered below the Mt Welirang summit. The updome structure indicated an upflow zone that was supported by the occurrence of solfataric-fumaroles on the summit of Mt Welirang. The depth of BOC at this position was about 1 km, while those at the surroundings were deeper, following the elevation of the vicinity of Mt Welirang. The 3-D resistivity inversion also revealed the subsurface fault structures as indicated by remote sensing data at the surface. The 3-D MT inversion result was then incorporated with geological and geochemical data to construct a conceptual model of the Arjuno-Welirang geothermal system.