Abstract. Ijen geothermal area is high-temperature geothermal system located in Bondowoso regency, East Java. It is categorized as caldera-hosted geothermal system which is covered by quaternary andesitic volcanic rocks with steep topography at the surrounding. Several surface thermal manifestations are found, such as altered rocks near Mt. Kukusan and a group of Blawan hotsprings in the northern part of the caldera. Geomagnetic survey was conducted at 72 stations which is distributed inside the caldera to delineate the existence of hydrothermal activity. Magnetic anomaly was obtained by reducing total magnetic measured on the field by IGRF and diurnal variation. Reduction to pole (RTP) method was applied with geomagnetic inclination of about -32 0 . In general, the result shows that high magnetic anomaly is distributed at the boundary of study area, while low magnetic anomaly is observed in the centre. The low anomaly indicates demagnetized rock that probably caused by hydrothermal activity. It has a good correlation with surface alteration observed close to Mt. Kukusan as well as high temperature reservoir drilled in the centre of caldera. Accordingly, the low magnetic anomaly also presents the possibility of geothermal reservoir in Ijen geothermal area.

There has been studied of the rock physics modelling in sand reservoir based on Voigt-Reuss and cement contact theory method case study at FRR field Kutai Basin. The objective of this study to modelling rock physics based on Voigt-Reuss and cement contact theory from geological information. Voigt-Reuss bounds model and cement contact was applied. This method related link between geological information and seismic properties. The data used in FRR field Kutai basin has the sandstone majority marked by 20 % P wave velocity. The results obtained by rock physics modelling shown modulus bulk to porosity has pattern close to lower bounds, that is existence the pore type has soft characterize and decrease bulk modulus with increase clay content and fluid saturation. The results show correlation between bulk modulus estimation and log data have good correlation. This suggests that Voigt-Reuss and cement contact theory model useful for bulk modulus estimation in sand reservoir

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