GROUNDWATER EXPLORATION AND WELL SITE SELECTION AT DECHA CHABIR WATERSHED USING VERTICAL ELECTRICAL SOUNDING (VES) METHOD FOR SEKELA TOWN, IN HORO GUDURU WOLLEGA ZONE, AT WESTERN ETHIOPIA

Abstract

Resistivity profiling and vertical electrical sounding (VES) using terrameter instrument were carried out at Decha Chabir watershed for rural water supply for Sekela town in Horo Guduru Wollega zone, in western Ethiopia to alleviate the problem of potable water source in the study area. This study aimed to locate and delineate potential groundwater sites to determine the location of preferred borehole sites. Eight profile lines and two transverse lines were selected and fixed on the ground surface. Each profile line had six VES points. All profile lines were parallel to each other and the distance between two adjacent profile lines was 100 meters. The distance between two adjacent VES points on a profile line was 100 meters that used for check the continuity of the aquifers spatially. The two transverse lines diagonally crossed against each other and profiles. On each transverse line, there were six VES points. The distance between two adjacent VES points on a transverse line was 100 meters. Forty-eight resistivity soundings along profile lines and twelve resistivities sounding along transverse lines using Schlumberger array with a minimum and maximum current electrode and potential electrode distance of AB/2 1.5m to 750 m and MN/2 of 0.5m to 90 m were carried out. Based on the results of the model interpretations of the sounding curves and from measured apparent resistivity value, the respective pseudo depth and Geoelectric sections were constructed using Win Resist and IPI2win software. The results obtained from these sections indicated on average five layers. Weathered basalt, sandstone, fractured basalt, clay, siltstone, and massive basalt. Clay and siltstone have very low resistivity, sand-stone low resistivity, weathered basalt and fractured basalt moderate resistivity, and massive basalt very high resistivity. Low and moderate resistivity layers can be potential formations for groundwater storage. Therefore, these low and moderate resistivity areas were chosen for water well drilling. Accordingly, results indicated that shallow and/or deep boreholes can be drilled to obtain a potable adequate amount of groundwater from aquifers with a depth of 150-450 m on average.