Employing 3-D inversion of geomagnetic data to identify demagnetized rock associated with reservoir zone of Blawan-Ijen geothermal prospect area

Blawan-Ijen geothermal prospect area is placed at Bondowoso, East Java. Geologically, the presence of geothermal system in this field is controlled by the huge ancient Kendeng caldera complex structure. The heat source of the geothermal system is predicted to be situated at the base of Kendeng caldera. Geomagnetic method was used for detecting the presence of the heat source. The high temperature zone as the main target of geothermal exploration mostly situated around the heat source location. The geomagnetic method was applied because of its ability to identify the magnetic variation of rocks such as the demagnetized zone which is caused by the high temperature condition in geothermal area especially in the center of geothermal reservoir. Geomagnetic survey was conducted in 2017 and 2018 with the total of 151 stations. The 3-D inversion of geomagnetic data was then carried out to image the subsurface magnetic variation which is representing the complex structure of geothermal system in this area. The subsurface magnetic variation structure revealed by 3-D inversion showed the distribution of demagnetization reservoir rock inside the Kendeng caldera which has low magnetic susceptibility. In the upper part of this layer that is a higher magnetic susceptibility layer which may associated with the clay alteration that is overlaying the geothermal reservoir in this area. Moreover, the very low magnetic susceptibility anomaly can be observed start from -1000 m elevation, which is interpreted as the heat source of the geothermal system in this area.