MAGNETOTELLURIC INTERPRETATION OF THE KARAHA BODAS GEOTHERMAL FIELD INDONESIA

The Karaha Bodas Field is a vapor-type geothermal system having a temperature of up to 350°C. The reservoir is hosted in a volcanic environment comprises of tuff breccias with minor andesite lavas, and quartz diorite intrusions. Based on the surface thermal distribution the system might occupy an area of 5x13 km 2 . One hundred and eighty magnetotelluric (MT) stations recorded in 1996 and 1997 were used to assess the clay cap extension. One -dimensional MT interpretations were performed. A north-south MT profile crossing the entire field was chosen to show the extent of the possible resources. The profile shows a very thick conductive layer (1-10Ω m) extending 7 km northward from the Bodas Crater. The thickness of the conductive layer is about 10001200 m having a base at about 200-300 meters above sea level. Between 7-12 km north of the crater the layer thins (to 700 m), and the resistivity increases slightly to 14Ω m. The interface between the conductive layer and the underlying layer also coincides with fluid loss zones during drilling operation. This observation suggests that the conductive layer is still the clay cap of the reservoir. Other, and more complex, resistivity structures caused by multiple hydrothermal processes are not imaged by the MT data. At a distance of 12 km from the crater the conductive layer vanishes. Some thermal springs and steaming ground also occur, suggesting the northern boundary of the system.