Figure 3, modified from KRTA (1989), shows the Ulumbu geothermal area and its location relative to Ruteng, Heng, Ponggeok and W Kokor. As indiated earlier a reasonable • amount of surface scientificsurvey work has been completed. However, one of the technical philosophies. underlying this type of rural development in that initial size of development should be small compared with the possible ultimate size of the prospect reducing the risks involved in siting initial production wells. The extent of scientific work completed at Ulumbu and the depth of knowledge of the prospect, exemplifies these comments. Geology Figure 4, taken from KRTA (1989) representing the geological map of Setiawan and Suparto (1984) shows the major geological features of the area. According to the above authors the Ulumbu geothermal field occurs on the flanks of the Paco Leok volcanic complex at an elevation of some 650 metres above sea-level. The Quaternary rocks rise to an elevation of approximately 1,600 metres RSL and rest on a Tertiary basement comprising mainly lavas, breccias and tuffs, and possibly calcareous sediments. It is considered that the Tertiary base• ment rocks have been down-faulted to the south along an east-west trending fault known as the Cancar Fault. Its trace passes Ruteng some kilometres to the north of the town. The resulting Carcar Depression is assumed to have been partially infilled successively by massive decitic tuff from pyroclastic flows and andesite lava • flows erupted during early Quaternary volcanism centred north of Ulumbu probably in the vicinty of the Mandasawu Volcanic Ran• ges. Volcanic activity is believed to have then moved further south where succesive eruptions of andesite lava and brecias formed the large Rii strata-volcano which eventually collapsed to form the Rii Caldera. Renewed volcanism near the centre of the caldera produced andesite lava and tuff breccia partially infilling the caldera. The younger of these formations, the Wara andesite occupies the centre of the crater. There is considerable discussion on the displacement of the tertiary basement within the caldera structure. KRTA assumed a displacement of 500 metres which put the top of the basement witin the caldera at a depth of some 300 metres below sea level which corresponds to the high resistivity basement .. It was suggested that the Tertiary volcanic basement was responsible for the modelled high resistivity 'basement' and that it was largely impermeable espect where it has been cut by faults which serve as channels for circulating geothermal fluids.

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