Evaluation of Groundwater Potentiality and Subsurface Structural Setting by Using Geophysical Data in Wadi Hubuna, Southwest Najran, Saudi Arabia

Abstract

A geophysical study was conducted to explore the shallow groundwater aquifer and to identify the subsurface structure on the basis of resistivity and geomagnetic surveys. The study area is situated in the southwestern portion of Saudi Arabia, southwest of the Najran region. The total intensity magnetic map is reduced to the northern magnetic pole. The estimated mean depths of the regional and residual magnetic sources are 3 km and 1.4 km respectively. Eighty vertical electrical soundings were measured using the Schlumberger configuration with AB/2 spacing of up to 700 m. Four geoelectrical layers were identified, and could be described from ground surface to deeper layers as the following: The first layer encompasses wadi deposits with true resistivity values ranging from 452 Ω.m to 1353 Ω.m and thicknesses of 2-4 m. The second layer is comprised of dry fine to medium grain-sized deposits with true resistivity values ranging from 104 Ω.m to 451 Ω.m, and thickness varying between 2 and 16 m. The third layer consists of water-bearing friable deposits beside the topmost part of the underlying weathered (fractured) basement. The true resistivity of this layer ranges from 22 Ω.m to 188 Ω.m, whereas the thickness of this layer is highly variable and ranges from 5 to 35 m. Generally, the resistivity values and the thickness of these three layers exhibit an increasing trend toward the main wadi stream. A fourth layer was also detected and is represented by a thin clay layer directly overlying the hard rocks of the basement complex. The dominant tectonic trends controlling the shallower parts are N-S and NNW-SSE. Meanwhile, the structural trend controlling the deeper parts is NE-SW, E-W, and. NNE-SSW. The ideal areas for drilling wells, according to the interpreted cross-sections, are along the main stream of Wadi hubuna and other small wadis, as well as the lower portions of the wadi in the extreme east direction.