196 Geophysics in Geothermal Exploration at the Olkaria geothermal field in Kenya. The main production fields (OWF and NEF-DOMES) are associated with a highly elevated brittle/ductile transition, indicating highly elevated heat sources. Figure 6.6 Illustration of highly elevated brittle/ductile transition zone which correlate with geothermal production fields (OWF and NEF-DOMES). Modified after Georgsson (2009). Focal mechanisms In addition to locating microseismic events, understanding their focal mechanism provides additional understanding about the tectonic processes that drive the geothermal context of an area. It helps understanding the regional and/or local stress field and provides information about the type of faults in the exploration area and the orientation of the fault planes and/or fractured zones. Analyzing jointly the stress field and the faults and fracture configuration is useful to understand the local thermal fluid flow process (i.e. identifying the most permeable zones and their potential to serve as thermal fluids pathways). Figure 6.7 illustrates the kind of results that can be obtained through focal mechanisms analysis. Antayhua-Vera (2017) produced this map after a microseismic analysis conducted at the geothermal prospect of Las Tres Virgenes, in Mexico. Their findings demonstrated the correlation of the stress field at that site with the regional tectonic regime of the Gulf of California, and provided local understanding of fault location, orientation and type, and hence of how hydrothermal fluids are expected to flow within the geothermal system.
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