Use of time-domain electromagnetic (TDEM) method to investigate seawater intrusion in the Lebna coastal aquifer of eastern Cap Bon, Tunisia

A multi-method geophysical survey, using time-domain electromagnetic (TDEM) fields, is conducted on the north-eastern coastal aquifer of Cap Bon, Tunisia. The aim of the study is to investigate the inland extent saltwater intrusion, to estimate the fresh-saltwater transition zone, and to characterize the lithologic spatial variability of the underground layers. Moreover, the TDEM is used to determine both fluid resistivity and bulk resistivity which are required to calculate the porosity by applying the Archie’s equation. The survey of TDEM profiles is carried out along the left and right overbanks of the Lebna river from the coast until the Lebna dam. The interpretation and the analysis of these profiles allow us to better identify the flux exchanges between the river and the underlying aquifer. On the basis of the 1D and 2D longitudinal and transversal resistivity profiles as well as the existing boreholes data and groundwater quality, it seems that the freshwater-saltwater interface is moving forward inland. Furthermore, electromagnetic results combined to the Archie’s law show that the porosity values are at the ranges of 12–36 and 5–31% for the Quaternary outcropping and Pliocene formation, respectively.