The HydroGeosITe for AEM mapping: characterization through joint inversion of AEM, ground EM and DCIP data

The HydroGeosITe project aims at the establishment of the first Italian calibration and reference site for airborne electromagnetic (AEM), ground EM and electric geophysical methods within the largest AEM campaign carried out in Italy for groundwater mapping and management. The geophysical characterization of the HydroGeosITe combines AEM, ground EM and galvanic direct current and induced polarization (DCIP) surveys, for the retrieval of a unique 3D distribution of conduction and polarization electrical properties, able to describe all geophysical data. This is achieved through a joint inversion of all inductive and galvanic data in terms of dispersive resistivity, with data misfit comparable to the independent inversions and significantly improved resolution. The need for calibration sites of EM systems derives by the sensitivity of EM data to system characteristics, such as receiver transfer function, transmitter current waveform, and transmitter-receiver synchronization and geometry, which if neglected lead to significant bias in the retrieval of the electrical properties (Christiansen et al., 2011). For instance, the Lyngby Danish reference site has been established for ensuring the calibration of both airborne and ground-based EM systems (Foged et al., 2013), while the Menindee Australian test range (Brodie and Cooper, 2018) focuses only on airborne systems, but covers a much longer stretch (more than 35 km). In both cases, calibrated systems are expected to retrieve satisfactory resistivity models, the eventual calibration consisting in adjusting the system characteristics until the inversion model compares well enough with the reference model. The HydroGeosITe will serve as calibration site for future AEM campaigns, as well as for ground-based EM and galvanic surveys. Furthermore, borehole drillings down to several hundreds of meters are being carried out, with lithological description and geophysical logging, for establishing a reference in the interpretations of the resistivity models retrieved by the AEM campaign.