Geoelectrical Constraints for Ground Water Models Around Legedadi, North East of Addis Ababa
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Date
2008-08
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Addis Ababa Universty
Abstract
Geophysical surveys were carried out around Addis Ababa namely on the Ada’a plain, the Becho plain and different traverses with different orientations in the Abay plateau by Water Works Design and Supervision Enterprise (WWDSE). The methods employed were electrical resistivity sounding, electrical resistivity imaging and magnetic mapping. The study mainly focuses on the evaluation of the groundwater potential of Ada’a and Becho plains and was designed for the purpose of pressurized irrigation development.
In this thesis work, only electrical resistivity sounding method is employed and the study area is restricted along a transverse of Gohatsion-Chancho-Sululta-Legedadi-Adaa which is located between 9004’78’’& 9056’58’’N and 38027’29’’ & 38054’67’’E. The study aims to map the possible paths of fluids and to develop geoelectrical models of a given area interms of ground water studies. This includes detail reprocessing and analysis of the existing data. And to acquire practical experience in the interpretation of resistivity data.
From the pseudosection along profiles and the sliced depth section at different depth levels show the northern part is relatively resistive than southern part. This may indicate the thickness of the hard rock pledges southward or an extended conductive response of the overburden.
The interpreted result of the individual VES points and the two geoelectric sections shows, in the northern flank there is a favorable condition to aquifer shallower than the southern flank. But thick aquifer is observed on the southern flanks as compared to the northern flank. The graphs obtained from hydrodynamic data also confirm this result. And the ground water flows from north to the south direction.
The depth of investigation attained by the resistivity data was not deep enough to penetrate the lower volcanic aquifer. Therefore the use of deep probing, natural source EM geophysical methods such as controlled source or conventional Audio Magnetotelluric (CS/AMT) is recommended. Besides, the potential methods (gravity and magnetic) integrated with other techniques such as remote sensing in GIS framework are recommended to come up with full picture of the subsurface map of the survey area Generally results from vertical electrical soundings as obtained from the stacked graphic plot, pseudosections and geoelectric section analysis show that northern flank is relatively resistive than the southern part. Therefore, from the hydrogeological point of view the southern part deserve more interest for ground water exploration.
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Around Legedadi, North East of Addis Ababa