Groundwater prospecting in Northern Sekyere West District using the electromagnetic method

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The lack of potable water in the Northern Sekyere West District, like many other rural communities in Ghana, has made it imperative for the inhabitants to walk several kilornetres from their villages daily in search of water. To overcome this perennial problem, this project was carried out in conjunction with World Vision International Ghana, to use geophysical techniques to map out potential sites for boreholes to be drilled to provide water for these communities. Electromagnetic profiling using the Geonics EM34-3 conductivity meter was carried out in twelve communities. The intercoil spacing used for the EM profiles is the 20 m coil and a station separation of 10 m, with both coils align first in the horizontal dipole HD mode, which probe to a depth of 15 m, and then in the vertical dipole VD mode, which probe to a depth of3O m. Interpretation of the results revealed that potential aquifers within the area could be located with respect to two distinct features namely, weathering and fracturing zones. The method and criteria for selecting a site for a borehole as well as the scientific basis are preserved. Three lithologic logs obtained for points recommended for drilling have been presented and compared with profiling results in order to provide a scientific basis for the sitting of similar boreholes in virgin areas. The results also show a response, which is significantly different from that of the weathering and fracturing zones, it indicates a nearly symmetrical negative response, with values as low as 1 m S/m in some communities. This was interpreted to be due to dike-like structures or a highly conductive material that has been masked, which could contain water. The logs revealed that the topmost part of the area consists of lateritic and clayey soils while below about 7 m; fractured or weathered arkosic sandstones predominate. The yield (volume of water) of the boreholes drilled was between 18 and 290 litres/mm and the aquifer horizon averagely between 18 -45 m. The geophysical technique has demonstrated its capability to identi1,’ suitable sites for sitting of borehole to provide groundwater to needy communities. Some areas with conductivity highs had little or no water when drilled so using the 40 m coil before selecting points for VES is recommended.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of Master of Science Degree in Geophysics, 2000