A numerical assessment of electrical imaging for seawater intrusion monitoring in heterogeneous coastal aquifers

Electrical resistivity, very sensitive to pore water salinity, is widely used for the identification of seawater intrusion in coastal aquifers. In many hydrogeological studies, it is common practice to interpret electrical resistivity tomograms assuming that resistivity isocontours are directly related with isovalues of salinity concentration, but this may not be correct in some applications, especially in heterogeneous aquifers. In practice, both the electrode array used in the field for data acquisition, the chosen petrophysical model and the inversion characteristics might lead, for the same hydrogeological scenario, to different salinity estimates including the actual position of the saltwater interface. Consequently, the geophysical methodology from data collection to processing and inversion strongly influences the final tomogram used for interpretation by the geophysicists or the hydrogeologist. In this study we use a forward coupled hydrogeophysical model to generate multiple electrical resistivity datasets for the most common arrays and various seawater intrusion scenarios. Then, we use different options in the resistivity inversion to obtain alternative, but acceptable, tomograms that are compared with the true distribution of resistivities. The study and the results provide guidelines for better use of electrical resistivity and hydrogeophysical interpretation for monitoring and mapping of seawater intrusion.