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

Abstract

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 occurred in this area include solfataric-fumaroles with temperature of 137oC found in the summit of MT Welirang, and bicarbonate hotsprings with temperature of 39-55oC found in the western and northwestern vicinities of Mt Welirang. The occurrence of solfataric-fumaroles with magmatic gases indicates an existence of volcanic geothermal system in the subsurface. The surface manifestations are geologically controlled by northeast-southwest fault structures. Gas (CO2) geothermometer indicates high temperature reservoir of about 260oC. To delineate the subsurface resistivity structure associated with the geothermal system, a 3-D inversion was performed using Occam Data Space algorithm. The 3-D inversion was applied for 35 MT soundings with frequency range from 320 to 0.01 Hz. The quality of the MT data used is overall good to very good. The subsurface resistivity structure revealed by 3-D inversion shows low resistivity (1-15 ohm-m) altered rock at the upper part overlying slightly higher resistivity (20-60 ohm-m) reservoir zone. The lower part of the resistivity structure shows highest resistivity value ( more than 80 ohm-m) indicating a hot rock region. Geometry of the resistivity structure shows updome structure of the base of conductive layer (BOC) centered below Mt Welirang summit indicating an upflow zone as supported by the occurrence of solfataric-fumarole on the summit of Mt Welirang. The depth of BOC at this position is at about 1 km while those at the surroundings are deeper following the elevation of vicinity of Mt Welirang. The 3-D resistivity inversion can also reveal 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.