A vlf survey in Vinadio geothermal area

In this study, after a brief introduction concerning geothermal energy and the usual geologic environment, in which geothermal exploration is usually conducted, it is described a vlf (very low frequencies) survey carried out across the geothermal field of Bagni di Vinadio (Cuneo, Italy). The main goal is to confirm the effectiveness of the geophysical vlf method in delineating subsurface conductive structures across an area with sub-vertical contacts between formations having strong resistivity contrasts. The survey occurred on 28th november 2011 across a profile about 2500 m long, using a combined magnetometer/vlf system called omni-vlf. This system has been designed and manufactured by eda instruments inc. Located in Toronto (Canada). Vlf data have been analysed at three different frequencies (16 khz, 19, 6 khz and 22, 1 khz) and then filtered with fraser and karous-hjelt filters. This data processing led for each frequency to modeled current density pseudo-sections of the profile, at two different hypothesized site resistivities. Comparing these data with previous geological maps, the presence of many sub-vertical contacts, zones of conductive geologic material and probable local buried sources of potentially exploitable hot water was confirmed. In this way, vlf surveys prove to be one of the most effective geophysical method in qualitatively detecting new geothermal fields.