Application of Integrated Geophysical Techniques to Map Groundwater Potential Zones and Geological Structures at Gelchet Area, Borena Zone, South Ethiopia
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Date
2011-02
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Addis Ababa Universty
Abstract
Integrated geophysical survey using vertical electrical sounding (VES) and gravity (with
differential GPS) methods were conducted to investigate groundwater potential zones and
geological structures at Gelchet area, Borena zone, South Ethiopia. The main objectives of this
study were to map groundwater potential zones and determine depth to the basement.
Geologically, the area is located within the Cenozoic volcanic plain that followed by
Precambrian crystalline basement rocks to the southeast of main Ethiopian rift.
The data acquired from twenty eight (28) VES points using Schlumberger electrode array with
maximum half current electrode spacing (AB/2=1000m) and 115 gravity points (supported by
high accuracy height control from GPS method) were interpreted both qualitatively and
quantitatively in order to understand the local 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 gravity profile plots and different gravity anomaly maps. Finally, the
overall qualitative interpretation was done by integrating all the above results together with the
geologic , topographic maps and borehole information. As a result of this, geologic structures
and groundwater potential zones are identified and drilling sites are recommended.
The quantitative interpretations of the VES data were conducted by modeling the VES data
using Resix-Ip and WinResist modeling softwares and constructing geoelectric sections along
selected survey lines, using the result from individual VES point interpretations. The depth and
lithologic units from the boreholes were used to fix parameters during the modeling of VES
data. The information from the VES data and the boreholes were used to constrain the depth
information in the gravity modeling which were conducted on two survey lines. The VES
results of the data revealed four main geoelectric layers which differ in degree of weathering,
fracturing, composition and depth of burial. The geoelectric sections enabled to identify the
overburden thickness and the depth to the aquifer along survey lines whereas the gravity model
helped to identify basement undulation in places where there is no VES data. Moreover, the
vertical electrical sounding survey helped to identify groundwater potential areas and determine
the depth to the aquifer whereas the gravity survey helped to map the basement topography and
identify places having high groundwater reservoir potential
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Gelchet Area, Borena Zone, South Ethiopia