1. Results and Discussion
Groundwater Investigations in the Granite Formation of Ghana Using Integrated Geophysical Methods
Hafiz MOHAMMED NAZIFI and Levent GÜLEN
Sakarya Univesity, Dept. of Geophysical Engineering, Serdivan, Sakarya, Turkey
Abstract
Methods and Materials
Electromagnetic method using Slingram technique and vertical electrical sounding (VES) were used for investigation of groundwater within the Achiase Community, Ghana. In both geophysical methods conventional and innovative methods of interpretations of the obtained data were used and the results were in agreement with one another.
The Slingram technique was used to collect Electromagnetic data with Geonics EM 34-3 in both Vertical Dipole (VD) and Horizontal Dipole (VD) using a coil separation of 20m. The EM data were interpreted both qualitatively and quantitatively using terrain conductivity graphs and AIEMIV program, respectively.
Electrical sounding were implemented in the study area according to the Slingram technique survey’s results. The Schlumberger array was used with a maximum AB/2 spacing of 83 m. Both qualitative and quantitative interpretations of the sounding data were done. The quantitative interpretations were done using inversion (RES1D program). Two qualitative methods of interpretation were used. First is the classic curve type method and the second is the isoapparent resistivity map method as used by Nejad et al In this work the resistivity values from the inversion results were used in creating the isoapparent resistivity maps. This gives us more accurate results that using the raw field data.
Introduction
Water shortage issue is a chronic problem for people in most of the Sub – Saharan African countries and Ghana is not an exception. Although Africa has an immense supply of rainfall, with an annual average of 744 mm (Obuobie & Barry 2010) however most African countries face water shortage problems especially during the dry seasons.
The aim of this work was to use Slingram technique and Vertical Electrical Sounding to determine suitable sites of borehole locations for public water supply and to understand the electrical characteristics of the Granite formation of Ghana which underlie the study area.
Figure 4. The isoapparent resisitivity map of Achiase Community.
Results and Discussion
Three electromagnetic traverses were run in the study area. One of the traverses has a length of 350 m and the other two have lengths of 250 m figure 2.  
The VD inversion results(figure 3) revealed two layers beneath the study area with the first layer having a thickness range of 1 – 10 m and the second layer’s thickness is half space. The resistivity values is in log 10 and the resistivity value of the first layer ranges from 2.5 – 3.5 Ωm. The resistivity value of the second layer is between 3.0 – 5.0 Ωm. There is a good agreement between the inversion results and the terrain conductivity graphs.
The isoapparent resistivity map (figure 4) revealed the lateral variation of apparent resistivity over horizontal plane at a certain depth (Nejah et al. 2011). According to Frohlich et al. 1996, the maximum depth of penetration of ANMB method is to of the maximum distance of the current electrodes (AB).
In this work, of AB were used to construct the isoapparent resistivity maps at distances AB/2 = 1.5, 4.4, 19.0 and 58.0 m, which reflect the lateral variation of resistivity at depth 0.75, 2.2, 9.5 and 29.0 m. As it could be seen from Figure 4, the isoapparent resistivity maps help us to get a resistivity value at a depth and in making drilling decisions. The results of the 1D VES results and the ranking of the VES points based on both the 1D resistivity curve results and the isoapparent resistivity map are shown in Table 1.
Study Area
The study area is Achiase Community, which is located within the Twifo – Hemang Lower Denkyira District of the Central Region of Ghana. The district is located within Latitudes 5 ͦ 50’ N and 5 ͦ 51’N and Longitudes 1 ͦ 50’ W and 1 ͦ 10’ W.
The study area is underlain mostly by granitic intrusives and granites (Gyamera & Kuma 2014). See figure 1. Most rock types comprise of well – foliated, medium – grained, potash – rich, muscovite –biotite granites, granodiorite and pegmatites (Kesse 1985; Gyamera & Kuma 2014). The area falls in the Cape Coast Granite Complex which is one of the three granite groups in Ghana.
Within the granite formation (crystalline basement rock region), there are two types of aquifer system occurrences which are; the fractured basement rocks and the weathered basement rocks (Ehinola et al. 2006). This is because granite is not inherently permeable but secondary permeability and porosity are developed as a result of fracturing and/or weathering.
Figure 2. Electromagnetic terrain conductivity results along Profile B in Achiase Community .
Figure 3. Electromagnetic Vertical Dipole (VD) inversion result of the Profile B in Achiase Community
Table 1. Summary of VES results in Achiase Community
VES Point
Layer
ρ(Ω-m)
Depth (m)
Thickness (m)
Rank
Location (GPS)
Remarks
B164 1
59.55
1.1
1st A 2
89.84
20.75
19.65 N 3 - W
A214
756.05
2.91
3rd H
289.28
13.63
10.72 N W
C208
148.91
1.14
1.45
2nd
33.45
4.36
3.22 N W
B58
0.83
4th N
240.06
22.29 W
A152
752
5.82
5th
180.71
13.33
7.51 N
8310.8 W
Conclusions
Both EM and VES geophysical methods and their interpretation were successful in helping to identify zones beneath the subsurface that have groundwater potential. Both the qualitative and quantitative interpretations of the resistivity measurements revealed a promising indication of weathering within the study area and that several investigated points are recommended for drilling.
Figure 1. The map of the study area (Mainoo et al. 2007) and the simplified geological map of Ghana (Wymana et al.2008).
Contact
References
Hafiz Mohammed Nazifi
Dept. of Geophysical Engineering,
Sakarya University, Serdivan, sakarya, Turkey
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Gyamera, E.A. & Kuma, J.S., Hydrogeological studies on soils developed over granitic deposits. , 2, pp.29–40.
Kesse, G.O., The mineral and rock resources of Ghana, Rotterdam, Netherlands: A. A. Balkema. p.610.
Nejad H. T. et al Vertical electrical sounding (VES) resistivity survey technique to explore groundwater in an arid region, Southeast Iran. Journal of Applied Science, 11(23), pp
Obuobie, E. & Barry, B., Groundwater in sub-Saharan Africa : Implications for food security and livelihoods., Accra, Ghana.