The Use of DC-Resistivity to Outline the Subsurface Hydrogeological and Structural Setting Beneath a Proposed Site for Subsurface Dam Building, Makkah Al-Mukarramah, Saudi Arabia

The present study is conducted in the down stream area of Wadi No'man, Makkah Al Mukarramah, Saudi Arabia. Wadi No'man is bounded by Gabal Kabkab from the north and Gabal No'man from the south; it discharges its water to the Red Sea. The area under inves- tigation forms a part of the Arabian shield of the western Saudi Arabia. Accordingly, igneous and metamorphic rocks cover the study area. In the upstream of Wadi No'man, the escarpment is a conspicu- ous structural feature, besides; major and minor faults are dominant in Wadi No'man and adjacent areas. Historically, Ain Zubida used to be the main groundwater source in the area. Intensive and uncontrolled discharge led to lowering the groundwater level. The aim of this study is to determine the main hydrological and structural setting beneath a proposed site of subsur- face dam. The purpose of building the subsurface dam is to control the groundwater flow in this area to rise the water table in the area surrounding Ain Zubida galleries. DC resistivity using profiling and vertical electrical sounding surveys was conducted for horizontal and vertical investigation of the hydrological and structural parameters beneath the study area. This method lead to the classification of alluvium, which covers the base- ment rocks but, according to the DC resistivity values, it is divided into two distinctive alluvial layers. The upper layer has relatively high resistivity values (dry alluvium) and the lower alluvial layer has rela- tively low values since it consists of moist alluvium. The middle layer