Resistivity soundings and VLF profiles for siting groundwater wells in a fractured basement aquifer in the Arabian Shield, Saudi Arabia A.I. Ammar a, b, * , S.E. Kruse b a Research Institute for Groundwater, National Water Research Center, Cairo, Egypt b School of Geosciences, University of South Florida, Tampa, FL, USA article info Article history: Received 20 May 2014 Received in revised form 26 October 2015 Accepted 17 December 2015 Available online 20 December 2015 Keywords: VLF DC resistivity Vertical electrical sounding Fractured basement Arabian shield Hydrogeophysics abstract 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 12e32 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, suc- cessfully 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. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Water supply is limited on the Arabian Shield; an outcrop of hard rocks that extends from the western part of Saudi Arabia to the Gulf of Aqaba to the Gulf of Aden (Fig. 1a and b). Water availability in the Arabian Peninsula is generally governed by rainfall distri- bution, which is related in turn to topographic and geologic fea- tures. The western, southwestern and southeastern mountain ridges, as well as the central plateau of the peninsula bring orographic rain. On the rest of the peninsula surface runoff is less abundant. The Shield in particular has limited groundwater stored in the alluvial deposits of wadi channels, and/or geological joints and fractures zones (Al Alawi and Abdulrazzak, 1996). Wadi systems collect water from flash floods and store it within the wadi deposits and the fractured basement rocks. Water becomes scarce in the dry season. The At-Taif area on the Arabian Shield (Fig. 1b and c) is charac- terized by shallow groundwater aquifers, with low productivity relative to the other deep aquifers of Arabian troughs. As elsewhere on the Shield, water supply from hand-dug surficial aquifer wells is often seasonally limited. Drilled wells are required for reliable water supply in this area. However, basement aquifers are variable and discontinuous, making siting of water supply wells difficult. Wells drilled without geophysical and hydrogeological study often fail to produce groundwater (Sharma and Baranwal, 2005). This paper describes a case study in which two geophysical methods, very low frequency electromagnetic profiling (VLF) and resistivity soundings (VESes), were used to identify an optimal site for a drilled well. These methods were selected because they are sensitive to terrain conductivity. (Resistivity ¼ 1/conductivity; both terms will be used here.) Zones of fractured or weathered basement * Corresponding author. Research Institute for Groundwater, National Water Research Center, Cairo, Egypt. E-mail address: abdallah_ammar_708@hotmail.com (A.I. Ammar). Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci http://dx.doi.org/10.1016/j.jafrearsci.2015.12.020 1464-343X/© 2015 Elsevier Ltd. All rights reserved. Journal of African Earth Sciences 116 (2016) 56e67