Groundwater Investigation Using Vertical Electrical Sounding and Gis: a Case Study of Five Districts in Mogadishu, Banadir Region, Somalia

Abstract

Groundwater occurrence in the open spaces within unconsolidated sediments and rocks is highly influenced by the geological structures of an area. Five different districts are located in Banadir region in the southeast of Somalia which is herein referred to as study area. The study area is dominated by unconsolidated sedimentary deposits covered by red sand dunes. Unconsolidated sediments have high porosity and have made the study area to have high groundwater potential. The main aim of this research was to delineate the subsurface geo-characteristics with a view to determining groundwater condition and their geometric characteristics in the study area. A total of 18 Vertical Electrical Sounding (VES) were used to determine the subsurface structure through 1D inversion approach to interpret the geo-electrical data. VES curves were achieved by using a preliminary quantitative interpretation. The modeling results from IP12WIN program were used to construct a five geo-electric section. From the research discovers, noted that the area of study is confirmed by four to six geoelectric layers to a depth of about 230m. The first topsoil covers all the study area consists of red sand dune which has resistivity value estimated from (25.5 – 460 Ω.m) and average thickness (0.75 – 3.83 m). The second layer is considered as fluvial sand that has resistivity values ranges from (10.4 - 569 Ω.m) and average thickness (0.72 – 14.9 m) considered as shallow aquifer layer. The third layer considered as a saturated aquifer and consists of fine marine sand has rated low resistivity values from (3 – 1351 Ω.m) and has a thick average thickness (3.88 – 133 m). The fourth layer is limestone has a resistivity value range from (1.54 – 766 Ω.m) considered a fractured layer and has the thickest average thickness from (11– 236 m) which indicates the possibility of groundwater potential is too high. The fifth layer resistivity ranges from (4.78 – 1828 Ω.m) the average thickness is infinity. The low resistivity layer considers as marine sand which is made of saline water. In this study area: there are two main aquifers, the shallow aquifer and deep aquifer. The analysis and outcome of geophysics indicates that the type of aquifers known as shallow aquifers are occurring within from (15-35m) from the surface, contained in unconsolidated fluvial sands material. Deep aquifers occur at (60 m to 90 m) below the surface contained fractured limestone.