Resistivity and Self-Potential InvestigationsOfSubsurface Water Seepage PhenomenonAt Omdurman

Abstract

ABSTRACT The Omdurman subsurface water seepage problem constitutes a serious phenomenon at present because of the high- density population and lavish use of water. The Omdurman region is covered by rather flat Cretaceous Nubian sandstones with gentle centripetal dips that hinder the outflow of the groundwater into the River Nile. The sandstone bands are intercalated with mudstone layers or lenses, and injected by Tertiary basalts along the bedding planes. Both rock types are impervious and retard the inliltration of the surface water, hence over-flooding. Plastic clay soil (Banat east, Al- Abassia and AL-Ahlya square) when present may also hamper vertical water seepage, cause swelling potential and consequent damage. Geophysical investigations applying resistivity and selt potential (SP) methods are conducted to reveal the occurrence and distribution of the impervious layers underneath, and to detect the location and characteristic features of the subsurface streamlines. The results obtained indicate the presence of significant almost horizontal bands and lenticular bodies of mudstone intercalated with the sandstones of the Nubian formation. Some of these bands are situated close to the land surface e.g. at Bait- Almal, Al-Mulazmean, A1-Busta, Banat east and Al-Arda l districts. These layers have been successfully delineated by the electrical resistivity method. Geo-electrical sections and maps were constructed to show their locations, thicknesses, attitudes and inclinations. The mudstone layers being impervious prevent water infiltration from natural and artificial sources and can therefore, create temporary perched aquifers located at the interface between the superficial deposits and the top very layer or lens of the mudstones. From these aquifers water is expected to rise up and seep into the surface land and cause damage. Negative SP anomalies show the occurrence and variable intensities of the subsurface streamlines. The distribution of the SP contour lines indicate the haphazard orientation of water flow, hence, the phenomenon appears to be of local nature. 2