INTEGRATED GEOPHYSICAL-STUDY OF THE GEOTHERMAL SYSTEM IN THE SOUTHERNPART OF NISYROS-ISLAND, GREECE

The study of the high-enthalpy geothermal field of Nisyros Island is o f great importance, because of the planned construction of a geotherma l power station. The purpose of the applied geophysical surveys - grav ity, SP, VLF and audio-magnetotelluric in southernmost Nisyros was to investigate the major and minor faulting zones which are geothermally active, i.e. whether geothermal fluid circulation occurs in these zone s. The survey lines, four parallel traverses of about 1500 m length, w ere chosen to be almost transverse to the main faults of the area. The SP method was the main reconnaissance technique, with the VLF and gra vity measurements correlating with the ''SP model''. Previously propos ed SP data acquisition and reduction techniques were used, followed by a 2-D interpretation of the SP map which apparently locates the posit ion of the fracture zones (geothermally active). The SP and VLF anomal ies are believed to be generated by the same source (subsurface flow o f fluid, heat and ions). Hence, at the place of a vertical geothermal fluid circulation zone, the curve of SP dipole-like anomaly changes it s behaviour and the curve of the VLF anomaly takes maximum values for the in-phase component and minimum values for the out-of-phase compone nt. On the VLF map of the survey area, the zones detected with the SP interpretation coincide with the maximum values of the VLF in-phase co mponent. The geothermal fluid circulation zones, detected by the SP me thod, appear to be well correlated with corresponding features derived from the gravity and the AMT surveys. In particular, the AMT sounding s indicate two zones of geothermal fluid circulation instead of the on e the SP method detected in the central part of the investigated area.