Integrated geophysical investigations in the Hisar geothermal field, Demirci, western Turkey

The shallow, low-temperature geothermal field of Hisar, in western Turkey, was studied by means of vertical electrical sounding (VES), Wenner, self-potential (SP) and very low-frequency electromagnetic (VLF-EM) profiling surveys. The VES survey, conducted along the valley in an E–W direction, provided lithological and structural information that is in good agreement with the well data, and suggests that the field is characterized by low-resistivity values (≤30 Ω m). Because the resistivity structure in a N–S direction could not be investigated due to the rough topography towards the north, SP, Wenner and VLF-EM measurements were made to identify and characterize a possible hot-fluid carrying fault/fracture zone that forms one of the boundaries of the valley. There is a good correlation between the results of the SP, VLF-EM and Wenner surveys, which confirm the existence of the NE–SW striking fault zone that had been inferred from geologic information. Tensional fractures that developed perpendicular to the fault zone were also identified.Consistent with the results of the SP and Wenner profilings, the two-dimensional model derived from VLF-EM data and a Karous–Hjelt current density pseudo-section detected the conductive fault zones bearing the geothermal fluids.