Joint interpretation of magnetic and magnetotelluric method to identify promising zone in Blawan-Ijen geothermal area

Abstract

Blawan-Ijen geothermal prospect is a typical of caldera-hosted geothermal system which is located in the east side of East Java, Indonesia. Geologically, the prospect area is situated inside the Kendeng Caldera. The only thermal manifestation is Blawan bicarbonate hotspring which is located in the northern part of Kendeng caldera rim. Land magnetic survey and magnetotelluric (MT) survey were carried out inside the Kendeng Caldera. A total of 72 land magnetic stations and 95 MT stations have been acquired to identify promising zone in Blawan-Ijen geothermal area. In geothermal exploration, magnetic method can be used to detect demagnetization zone which may correlate with hydrothermal alteration rocks. Reduce to Pole (RTP) transformation and upward continuation are applied to generate total force magnetic anomaly pattern in Blawan-Ijen. The result shows a wide range of low magnetic anomaly inside the Kendeng Caldera which was then integrated with 3D MT inversion. 3-D Inversion of MT data was performed to delineate distribution of conductive layer that most likely associated with clay alteration in geothermal area. Resistivity structure derived from 3-D MT inversion clearly identify the possible upflow zone which is indicated by updome shape pattern in the the center of Kendeng Caldera. It has good correlation with low magnetic anomaly pattern in Blawan-Ijen. Joint interpretation of magnetic and magnetotelluric method effectively identify the promising zone in Blawan-Ijen geothermal area.