| Abstract |
In geothermal systems, faults, fractures, or contacts between intrusive and surrounding rocks may become conduits for thermal water. Fault and Fracture Density (FFD) is a simple method which is applied in geothermal exploration to locate highly fractured areas. These areas are assumed to be associated with thermal water reservoirs at the depth. The method has been applied in some low terrain volcanic geothermal systems in New Zealand such as Mokai, TeKopia and Rotokawa geothermal fields. Zones with high FFD values usually correlate well to thermal manifestations and associated subsurface reservoirs. In contrast, the FFD method gives a different result when it is applied to non-volcanic high terrain geothermal systems. In this paper, the FFD is applied to three non-volcanic high terrain geothermal systems in Sulawesi. The results show that the high FFD zones have no correlation with the occurrence of surface manifestation, low resistivity anomalies, or demagnetized bodies, which are the main common indicators of the occurrence of geothermal reservoirs at depth. Instead, thermal manifestations appear at the transitions from high to low density FFD contours, which reflects the flank or the slope of the terrain. High FFD areas are mainly associated with the top of high elevation topography where the terrain is very rugged. The fractures here are the result of tectonic deformation and erosion. The result of the study suggests that the occurrence of the thermal springs is dominantly controlled not only by the structure, but also by the regional hydrology, such as formation dips, faults, fractures, or contact. Thermal water from distant reservoirs may be transported by regional hydrological flow to the manifestation area. Consequently, the sources of the thermal water in geothermal reservoirs are not indicated by high FFD zones, but at the transition of moderate to low FFD zones. These zones appear to correlate well with horizontal derivative images from gravity data, which suggest a deep highly fractured zone that may be a permeable zone with geothermal potential. |