Geological and structural investigation based on regional gravity and vertical electrical sounding data of the East Nile Rift Basin – Sudan
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AuthorOSMAN ADAM ALI, ABDU ELAZEEM
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AbstractThe analysis of geology and structure of the study area (East Nile - North Khartoum) is attempted by utilizing regional gravity and vertical electrical sounding (VES) data to investigate the source of the high gravity encountered over the basin features in this area as well as to reveal the structures, sedimentary sequences and groundwater condition in the area. The regional gravity measurements are achieved along two profiles in the area using Scintrex CG-3 gravimeter. All corrections common in gravity method are applied to the field data to produce Bouguer anomalies. The Bouguer anomalies are combined together with those produced by Sun Oil Company in 1984 and the residual anomaly separation is made using the least-squares method. The residual anomalies are interpreted using a 2D gravmodeller program (computer program) with the aid of geologic information in the area to produce geologic sections of subsurface of the area. As a result, the most convenient explanation of the high anomalies is the presence of granulites with density of 3.1 g/cm3. They are suggested to be uplifted during the period of the Pan-African movement to a depth of less than 3000 m below gneissic rocks with a density of 2.7 g/cm3 beneath the Nubian Sandstone with a density of 2.3 g/cm3, and its tip is thrusted to the surface at Sabaloka. Additional results are obtained from the interpretation of the gravity data. Several sedimentary basins, which were previously discovered, are delineated here again. The most important one is Atbara Basin in which the total thickness of sediments is about 3000 m. A depth map of the Basement surface of the study area is prepared. A dextral strike slip fault parallel to that appears at Sabaloka area is discovered in the area extending beyond River Atbara. The vertical electrical sounding (VES) measurements are conducted in the area using SAS 1000 meter to reveal the sedimentary sequence and further to investigate the groundwater condition in the area. The measurements are concentrated at Musawarat, El Awatib, Es Salama and Wad Musa areas. The present resistivity data are combined together with the old available data for more details. The VES data are interpreted using IPI2win software. A number of 8 geoelectric/geologic cross sections are prepared. As a result, the subsurface of the area consists of six geologic layers: - The first layer is the surface layer consists of undifferentiated sedimentary facies (gravels, sands, clayey sands, sandy clays and clay). - The second layer consists sandy-clay, clayey sands and sands. - The third layer is saturated sandstones. - The fourth layer is silicified sandstones/claystones (aquifuge). - The fifth layer is saturated sandstones. - The sixth layer is Basement Complex. The third layer and the fifth one are upper free aquifer and lower confined aquifer, respectively. The lower aquifer is thicker than the upper one. So, its water is the most abundant and convenient for drinking and irrigation purposes. Five subsurface geologic maps are prepared. Two of them are depth maps of the top surface of the upper and lower aquifers. The other two are thickness map of the upper and lower aquifers. The last one is a depth map of the Basement surface.
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