Delineation of a fractured granite aquifer in the Alwadeen area, Southwest Saudi Arabia using a geoelectrical resistivity survey

In most areas of the Arabian Shield, the local people meet their water needs through shallow dug wells in fractured basement rocks where groundwater is commonly less polluted and economic; this means that groundwater exploration has therefore become a major concern. The present study uses integrated 1D and 2D geoelectrical resistivity surveys to delineate and detect the groundwater potential of the granitic rocks in the Alwadeen area in Khamis Musheet city, southwest Saudi Arabia. The 1D resistivity survey was conducted through 17 vertical electrical soundings (VES) distributed in the study area, using Schlumberger array and current electrode separations up to 500m. In addition, eight 2D resistivity tomography traverses were acquired using Iris Syscal Pro resistivity imaging system with 5m spaced 72 electrodes and dipole-dipole configuration. The inspection of the VES curves revealed the presence of four distinct geoelectrical resistivity layers characterizing the area. These layers start with the topmost layer of unconsolidated alluvium deposits that is characterized by a wide range of resistivities (3 to 340 Omega.m) and a thickness of up to 5m. This wide range in resistivity is associated with variations in the lithology and humidity of the layer. The resistivity of the second and third geoelectrical resistivity layers is changed spatially and is characterized by elongated zones of low resistivities (around 100 Omega.m), indicating fractured and weathered granite freshwater aquifer. The values and distribution of the resistivities along the geoelectrical sections indicate that the fractured granitic rocks are separated by highly resistive massive granitic blocks. The vertical and horizontal variations in the resistivity can be related to the degree of weathering and fracturing of the granitic rocks and hence to its water saturation.