Gradient resistivity trends modeling of the Quaternary aquifer and its overlaying and underlying layers in the semi-arid Khanasser valley region, northern Syria

Vertical electrical sounding (VES) technique is applied in this paper to assess and follow the trends of layered gradient resistivity (GR) variations and aquifer vulnerability to contamination in the Khanasser Valley region, Northern Syria. GR parameter, termed also as longitudinal resistance (LR), having the Ohm unit (Ω) is therefore developed in this paper. GR is by definition the inverse of longitudinal conductance (LC) (one of the second order Dar-Zarrouk geo-electric indices). A model composed of three hydrological layers is used to test and characterize the GR parameter, which are the Quaternary aquifer and its overlaying and underlying layers. Thirty four VES points already carried out in the study area with Schlumberger array, and a maximum half current spacing AB/2 of 500 m are used in this research. The one dimensional (1D) quantitative interpretations of those VES show that the first overlaying layer resistivity and thickness ranges are of 0.6–212 Ω.m with an average value of 56 Ω.m, and 1.1–28 m with an average value of 4.64 m respectively. The second Quaternary aquifer layer resistivity and thickness ranges are of 4.3–43 Ω.m with an average value of 15 Ω.m, and 4.5–59.2 m with an average value of 27.5 m respectively. The third underlying layer resistivity and thickness ranges are of 1.7–79.5 Ω.m with an average value of 11.77 Ω.m, and 14.3–283 m with an average value of 82.1 m respectively. The (GR) parameter shows also that the three modeled hydrological units are averagely vulnerable to contaminations.

Two resistivity reflection coefficients K1 and K2 are also developed and used to follow the resistivity distribution trends of the three mentioned modeled hydrological units, where an alternating sequence of clay, alluvial and sandy components of ramel aswad, and marly clay is decided. GR parameter can be therefore employed worldwide to follow the electrical resistivity variations with depth, for several useful applications, such as assessing aquifer vulnerability to contamination, mapping the geologic sequence with depth for evaluating the subsurface for mineral explorations and for locating the engineering sites.