Gravity and Electrical Methods for Groundwater Resoures Potential Assesement and Mapping of Aqufiers, Wobok-Dilo Area, Borena, South Ethiopia
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Date
2011-10
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Addis Ababa Universty
Abstract
A combined geophysical survey using vertical electrical sounding (VES) and gravity methods
was conducted to investigate groundwater potential zones and geological structures at Wobok-
Dilo area, Borena zone, in South Ethiopia. The area being dominantly of basement rock lithology
with interspersed volcanics and with scarcely known groundwater potential, the main objectives
of this study were to map groundwater potential zones and determine depth to the basement.
Geologically, the Wobok-Dilo area lies on dominantly volcanic terrain. The exposed basaltic
formations over the area are highly vesicular and scoracious basalts. The geophysical surveys
included twenty one (21) VES points, using Schlumberger electrode array ,with maximum half
current electrode spacing AB/2=1000m and 103 gravity measuring points. These were
interpreted both qualitatively and quantitatively in order to infer the subsurface geology and
identify aquifer bearing horizons.
The qualitative analysis of VES data were performed by using pseudodepth sections and
different apparent resistivity maps. Similarly, the qualitative interpretations of gravity data were
performed by using different gravity anomaly maps. The quantitative interpretations of the VES
data were conducted by modeling the VES data using ResixIP and WinResist modeling software
and constructing geoelectric sections along selected survey lines, using the result from individual
VES point interpretations. The depth and lithologic units from the borehole was used to constrain
the parameters during the modeling of VES data. The information from the VES data and the
borehole were also used to constrain the depth information in the gravity modeling which were
conducted on one survey line. The VES results revealed six to seven main geoelectric layers
which differ in degree of weathering, fracturing, composition and depth of burial. The
geoelectric sections enabled to identify the depth to the aquifer along survey lines whereas the
gravity model helped to identify basement undulation and map structures. Finally, the overall
interpretation was done by integrating all the above results together with the topographic maps
and borehole information. As a result of these, geologic structures and groundwater potential
zones are identified and drilling sites for potentially productive boreholes are recommended
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Keywords
Aqufiers, Wobok-Dilo