Caprock and Reservoir Interpretatio of Arjuno-Welirang Geothermal System from 2D MT and 3D Gravity Model

Abstract

This study was conducted to model the geothermal system Arjuno-Welirang northern part from the MT and gravity data. MT modeling done in 2D and gravity in 3D. From 2D MT model of North-South trending shows model 2 layer resistivity, ie resistivity low ( less than 10 Ohm.m) associated with basalt lava rock with clay mineral alteration and an alleged caprock. And layer with moderate resistivity (60-100 Ohm.m) associated with the Lower Quaternary andesite-old who is the reservoir system. Distribution of Bouguer anomaly reflects the lithology of rock on the surface. Relatively low anomalies in the North (25-28 mGal) allegedly associated with pyroclastic flows. High anomaly in the South ( more than 43 mGal) associated with Mount Anjasmoro products spread to the South West section with the composition of andesitic-basaltic lava. 28-40 mGal anomalies are thought to relate to Lava Welirang I spread toward the center of the North with the composition of basalt lava. From 3D models Bouguer anomaly inversion results show a layer with a density of 1.49 gr/cm3 - 2.4 gr/cm3 with clay type material which is suspected as caprock, located at a depth of 900 m - 2100 m DBMTS. There is a low density anomaly associated with Coban Hotspring. And layer with a density of 2.4 gr/cm3 - 3.0 gr/cm3 an alleged andesite reservoir, is at a depth of about ¡Ý 1.5 km DBMT.