Mapping the uprising of highly mineralized waters occurring along a fault zone in the “Oltrepò Pavese” plain upper aquifer (Northern Italy) using VLF-EM survey

The Oltrepò Pavese plain (Northern Italy) is characterized by the uprising of highly mineralised waters from the marine substrate. These Na-Cl rich waters mix with the shallow groundwater and pollute them, thus preventing their use. This phenomenon is connected to the presence of the “Vogherese Fault”, a buried tectonic discontinuity along which the saline waters are distributed. A VLFEM survey, carried out along the “Vogherese Fault”, indicates that the uprise of highly mineralised waters produces high electrical conductivity anomalies where the marine substrate is shallower. Different algorithms, some of which are already mentioned in litterature, were used for the asymmetry inherent in dip-angle data remotion, noise reduction and low pass filtering. Current-density pseudosections and a contour filtered dip-angle map of the area obtained by the in-phase and out of phase quadrature fields, show that strong anomalies are localised along the fault surface trace. Furthermore, there is a clear correspondence between the spatial distribution of conductivity anomalies, the monitored wells and the low resistivity anomalies shown by the results of SEV investigations. The use of VLF-EM data integrated with electrical resistivity sounding, permitted surface and subsurface mapping of the complex geometry with which the high mineralised waters uprise along the fault zone.