The Blawan-Ijen volcanic complex is located in Bondowoso regencies, East Java province. The complex is expected to have geothermal system which is indicated by the occurrence of Blawan hotspring, acid lake on Ijen Crater and alterations. In 2017, measurements of gravity and magnetic methods have been conducted for the first time through the PITTA 2017 program. In 2018, further measurements are carried out to infill the previous data in order to strengthen the interpretation results. There are 151 stations obtained from each method until 2018. In this study, gravity method is used to detect the contrast density of an anomalous body while magnetic method is applied to discover the location of demagnetization zone. This paper presents the integration of both methods in geothermal exploration to determine the geothermal prospect area. The result of CBA and residual gravity indicated the existence of high gravity anomaly in the center to the southwest of the study area. Moreover, after processing RTP on magnetic data, there is the presence of low magnetic anomaly usually associated with demagnetization zone. Generally, the overall results supported one each other and pointed out the occurrence of the geothermal prospect possibly around the center of the study area.

The possibility of seawater intrusion in geothermal systems is very likely to occur. This is caused by several locations of geothermal systems which are very close to the coast. This paper presents a geochemical analysis of the data manifestations of water and gas, to determine the effect of sea water on the Jailolo geothermal system. From the results of the water analysis, it is clear that the effect of sea water intrusion on the Jailolo geothermal system is shown in the anions and cations diagrams, which is almost all of the plots very close to the seawater points. So that the next analysis is focused on the position of mixing sea water with reservoir water and correction of the parameters that are affected by the mixing of sea water. It is intended that the resulting geochemical model can describe the actual reservoir conditions. If we did not include the correction factor to the calculation this will lead into mislead assessment of the reservoir temperature of geochemistry model which is the final result of the study/assessment. It is also necessary take into account for the next step of the process which is exploitation phase.

Integrated measurements of magnetic and gravity methods were carried out over Blawan-Ijen Geothermal Prospect area to discover the indication of prospect zone location more convincing. The acquisition was first conducted in 2017, followed by recent measurement in 2018 with a total of 151 stations for each method. In this research, Reduce to Pole for magnetic data was applied to identify the distribution of demagnetized rock which associated with reservoir zone. The prospect zone is usually indicated by low magnetic susceptibility value. Furthermore, to support the interpretation of prospect zone which usually located overlying the heat source, then 3D inversion of gravity data was conducted. Thus, by applying 3D inversion of gravity method, the distribution of density variation beneath the earth surface which represents the location of heat source can be obtained. The result of Reduce to Pole of magnetic data shows demagnetized zone with low magnetic susceptibility value located in the centre to the southeast at the study area. Moreover, 3D gravity data inversion delineates the existence of heat source indicated by high gravity anomaly which corresponds with demagnetized zone shown from magnetic data.

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.