Resistivity soundings and VLF profiles for siting groundwater wells in a fractured basement aquifer in the Arabian Shield, Saudi Arabia

Seasonal shortages of groundwater are common in parts of the Arabian Shield, where complex basement hydrogeology can make siting of water wells difficult. To identify optimal production well locations, six 200-400 m-long Very Low Frequency (VLF) electromagnetic traverses and ten Vertical Electrical Soundings (VESes) were run at the western edge of the Arabian Shield near At-Taif town, Saudi Arabia. Here wadi sediments overlie fractured Precambrian basement, which in turn overlies unfractured basement. The fractured basement forms the water supply aquifer. Both VLF and VES data indicate significant lateral heterogeneity in the electrical conductivity of both wadi and basement deposits over lengths scales as small as-100 m. VES results correlate closely with data from two wells in the study area. The change in resistivity at the wadi-to-fractured basement contact is relatively subtle, but the transition from low resistivity fractured basement to high resistivity unfractured basement is well resolved. Inferred wadi thicknesses range from 0 to 14 m; the electrically conductive fractured basement extends from wadi down to 12-32 m depth. VES data indicate the fractured basement aquifer thickens progressively to the south in this area. A production well, sited on the basis of the VES analysis, successfully yielded 70m(3)/day. The relationship between VLF and VES data is complex, suggesting that the terrain is heterogeneous on the scale of the different effective sampling volumes of the two methods, and/or that fracture azimuth is locally heterogeneous. Overall resistivities in this study are similar to those observed at other locations in Saudi Arabia, suggesting these methods may be widely applicable for siting of groundwater wells in the complex basement of the Arabian Shield. (C) 2015 Elsevier Ltd. All rights reserved.