Indonesia lies in volcanic arc area that is related to geothermal such as in Ungaran which is located 40 km Southwest of Semarang, Central Java. Ungaran is formed by volcano tectonic depressions which form fault that shorten geothermal manifestations in Ungaran that first formed.

In Ungaran, there are several geothermal manifestations such as fumaroles, hot springs, hot pools, diluted bicarbonate waters, silica sinter teraces, and altered grounds. The manifestations which are related to rock permeability is dominated by fracture permeability like in Gedongsongo, Nglimut, and Kaliulo. Gedongsongo is the main geothermal manifestations. The Ungaran geothermal system is water dominated. Temperature manifestation on surface up to 91°C which can be interpretated that the reservoir is high temperature system with minimum reservoir temperature of approximately 280oC, pH between 7-9, which is potential for geothermal energy utilizations.

The geothermal study of Ungaran can be used for knowing the geothermal potency, there for it can be expoited.


The Ungaran geothermal prospect area lies on the North Serayu Range, which is resulted uplift into a geoanticline during the Miocene. Gedongsongo is the main geothermal resource, associated with the upper young stratovolcanic system of Ungaran volcano. The stratovolcano consist of series of andesitic to basaltic lava and breccia with occasional interbedded tuff. This formation is overlying marine sediment formation.

Chemical data of thermal manifestation such as fumarole, hot springs, and acid surface hydrothermal alteration grounds indicate that Ungaran is typical of hot water dominated system with minimum reservoir temperature of approximately 280oC. The pre-caldera volcanic rocks and the tertiary marine sedimentary rocks are inferred to be the main reservoir rocks. This system is mainly controlled by Northwest-Southeast, Northeast-Southwest and the Ungaran collapse structure that runs from West to Southeast (Budiardjo et al., 1997).

Geothermal Manifestations

Most of geothermal manifestation can be found in upstream part of Panjang river in Gedongsongo such as hot springs, fumaroles, warm pools, and altered grounds. The geothermal manifestation in Gedongsongo is the main object of the research. The geothermal manifestation which comes up in the south side of Ungaran (Banaran) is diluted bicarbonate water, the north side of Ungaran (Gonoharjo) is hot springs, Kendalisodo Mountain (hot springs), Diwak (warm-hot springs) and Kaliulo (hot springs). The surface geothermal manifestation temperature in Ungaran is between 34oC (Kaliulo with 364 m msl) to 91oC (Gedongsongo with 1300 m msl).

Geophysics Studies

Geophysics exploration which have been done consists of gravity, geomagnetic, and Magnetotelluric. Geomagnetic anomaly shows that the north side of Gedongsongo, characterized with very low negative anomaly lower than - 44500 nT, while in Darum and Ngipik the anomaly is quite low that is less than -400 nT with 400 m width (Gaffar et al., 2006). The value of susceptibility sediment in Gedongsongo is very low, it is 0.0020 emu (Nuridyanto et al., 2004).

The gravity anomaly around Gedongsongo shows lower value, less than 10 mGal and tends to be lower in the north peak of Ungaran (Gaffar et al., 2006). The highest heat anomaly in Kali Panjang, the upstream of Gedongsongo, is 54oC, while its surrounding area is 20-30oC.

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