Study of saltwater intrusion using 1D, 2D and 3D resistivity surveys in the coastal depressions at the eastern part of Matruh area, Egypt

Saltwater intrusions into shallow aquifers near the coast are an important environmental problem along the whole coastal strip of the western desert of Egypt. This phenomenon was studied in two selected coastal depressions on the eastern side of the Matruh area by a geoelectrical survey which comprised several ID sounding points as well as 2D and 3D resistivity-imaging surveys. For the 2D and 3D field surveys, pole-dipole (both forward and reverse) and pole-pole arrays were used, respectively. The spacing between adjacent electrodes was 10 m for the 2D resistivity surveys. A 5 x 5 grid of electrodes was used for the 3D survey. The spacing between electrodes was 10 or 20 in, depending oil the expected depth of freshwater in each respective area. The choice of electrode spacing was based oil the results of interpretation of the depth of the freshwater-saltwater boundaries, as derived from 1D resistivity soundings using a Schlumberger array. The sounding points were distributed over the study areas and the soundings were conducted prior to the 2D and 3D survey. The data sets were inverted using the smoothness-constrained least-squares method. The inversion results indicate that the subsurface resistivity distributions in the whole area are highly inhomogeneous and change rapidly within a short distance. However, from the resulting models, it was possible to correlate the ranges of resistivity with subsurface geological information available from shallow boreholes. The horizontal and vertical sections presented illustrate the configuration of the subsurface conditions that would have been, most probably, insufficiently accurate in the case of 1D modelling.