The exploration of groundwater in the western part of the Tolon Kumbungu District using electromagnetic (EM) and resistivity methods

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Electromagnetic (EM) and resistivity methods have been used to study and identify suitable sites for borehole locations for potable water in some communities in the Tolon Kumbungu district within the Voltain Formation. The communities lack potable water due to perennial drying of rivers and streams in the district. The field methodology involved the study of air photos with special emphasis on lineaments, vegetation, and drainage and bedrock characteristics indicative of underlying transmissive fractured zones. This was followed by the use of the electromagnetic (EM) profiling method to investigate the lateral variation of the apparent conductivity and to delineate points on the profiles that had apparent conductivities within the target conductivity range of 15- 70 mS/m for the study area. The EM profiling was carried out using the EM 34-3 equipment which was operated in both the horizontal dipole (HD) mode and the vertical dipole (VD) mode using the 20 m intercoil spacing, therefore probing depths of 15 and 30 m respectively. Preliminary interpretation of the EM profiles enabled the delineation of appropriate zones on the profile lines which showed significant anomalies associated with water-bearing structures i.e. faults and fractures. These zones were further investigated using the vertical electrical sounding (VES) technique to determine the variation of the vertical resistivity with depth and therefore, to find out the depth at which the aquifers occur and their thicknesses. At Zoolanyili, five traverses were run covering a total length of 1060 m with conductivity ranging between 28 and 70 mS/m. Four points were selected for vertical electrical sounding (VES). When the VES points were modelled, it gave three layers of subsurface rocks. Two sites were selected for test drilling in this community. The points were BA 150 and AC 190. The first layer of the point BA had a resistivity of 51 Ωm and thickness of 11.4 m. The second layer had a resistivity 19 Ωm and thickness 43.3 m. The third layer had a resistivity of 331 Ωm. At Wayamba, the total length of profiles that were run was 700 m. Five traverses were run in this community with a conductivity range of 11 to 37 mS/m. Four points were sounded and two points earmarked for drilling. The points selected for drilling were AB 70 and DB 60. Both points had three layer models. The first layer of AB 70 extended from the ground surface to a depth of 4.4 m with a resistivity of 586 Ωm. The second layer had a resistivity of 26 Ωm and a thickness of 23.4 m. The third layer had a resistivity of 2378 Ωm. DB 60 had a first layer of depth 4.9 m and resistivity 407 Ωm. The second layer had a resistivity of 14 Ωm and a depth of 21.4 m. The third layer had a resistivity of 948 Ωm. At Chirifoyili, four traverses were run and five points were selected for VES. The conductivity range on the traverses was between 18 and 66 mS/m. After the sounding, the points AB 80 and CC 70 showed three layers when they were modelled. The first layer at point AB 80 had a resistivity of 150 Ωm and a thickness of 4.5 m. The second and third layers had resistivities of 11 and 16 Ωm respectively with the second layer having a thickness of 36.6 m. The point CC 70 also had three layers. The first layer of this point had a resistivity of 279 Ωm and thickness of 3.8 m. The second layer had a resistivity of 14Ω m and a thickness of 21.4 m. The third layer had a resistivity of 222 Ωm. At Zagua, four profiles were run with a total traverse length of 900 m. Four points were selected for VES on the profiles and two points chosen for test drilling. The points were BA 280 and CA 40. Three layers were shown on the model curve of BA 280. The first layer was quite resistive with a resistivity of 333 Ωm and a thickness of 4.5 m. The second layer had a resistivity of 10 m and a thickness of 38.3 m. The third layer had a resistivity of 182 2m. Point CA also had three layers of the subsurface rocks with the first, second and third layers having resistivities 86, 8 and 124 Ωm respectively. The second layer had a thickness of 32.9 m. At Tolon S.S.S; the total length of the traverses that were run was 730 m. Four sounding points were selected on the three traverses. Points BA 60 and BA 100 were selected for test drilling. At BA 60, a three layer subsurface was shown. The first layer had a depth of 10 m and a resistivity of 25 Ωm. The second layer had a resistivity 33 Ωm and a thickness of 30.4 m. The third layer had a resistivity of 117 Ωm. Point BA 100, also showed three layers with first, second ‘and third layers having resistivities of 24, 16 and 55 Ωm. The first layer had a thickness of 14.5 m and the second layer of thickness of 18 m. In general; the EM profiles have a conductivity range of 11.5 to 88 mS/m. The apparent resistivity’s ranged between 3-2738 Ωm and potable water is expected to in the resistivity range of 15-100 2m. Groundwater is expected to be intercepted at the weathered and fractured zones in the rocks at an average depth of 15 to 50 m. In all, ten sites were demarcated for test drilling in the communities. This geophysical investigation would provide groundwater for the communities in the district. It is however, recommended that 40 m intercoil should be used in addition to 20 m intercoil spacing. In addition to this, other EM methods e.g. Very Low Frequency (VLF) technique which is an effective tool for mapping faults and fractures should also be tried in the survey area so as to assess the success of the combined used of any of these EM methods and the vertical electrical sounding.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of Master of Science in Geophysics, 2000