Aquifers Characterization in Agaie, North-Central Nigeria Using Electrical Resistivity Method and Borehole Lithologs

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Figure 2. 3D Relief map of Agaie and environs

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3. Electrical Resistivity Method

The principle of the resistivity methods is that an electric current is passed into the ground through two electrodes (C₁ and C₂) and the resulting potential difference is measured by another two electrodes (P₁ and P₂) as illustrated in Schlumberger Figure 3 and Wenner arrays (Telford et al., 2001; Zohdy et al., 1974; Virba, 1999; Selemo et al., 1995). The electrode spacing is progressively increased, keeping the center point of the electrode array fixed (Olasehinde and Taiwo, 2000; Oteze, 1989; Schwarz, 1986). At small electrode spacing, the apparent resistivity is nearly the resistivity of the surface material, but as the current electrode spacing increases the current penetrates deeper within the ground and so the apparent resistivity reflects the resistivity of the deeper layers (Olarewaju et al., 1996; Olashinde and Awojobi 2004; Olugboye, 2008).

Figure 3. Schlumberger Array

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Figure 4. Wenner Array

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4. Inventory of Boreholes and Hand Dug Wells

The inventory of hand-dug wells and boreholes were taken in Agaie town and neighboring communities in the study area. A total of 89 hand-dug wells were measured in Agaie town while 19 boreholes were drilled in Agaie and environs by the researcher during the period of investigation and these values where used to generate the groundwater flow direction of the area (Figure 5).

Figure 5. Groundwater flow direction of Agaie and environs

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5. Geophysical Investigation

Geophysical Investigation provides information on the subsurface geology, stratigraphy and structural signatures as well as aquifer properties of the study areas (Singh, 1984; Shekwolo, 1993; Amadi et al., 2010; Olasehinde, 2010). The electrical resistivity method was used in this research because it is the most effective and economical means of groundwater investigation.

6. Data Acquisition

Pre-drilling geophysical investigation involving vertical electrical sounding (VES) and horizontal resistivity profiling (HRP) was carried out in Agaie and environs using schlumberger (Figure 3) and Wenner (Figure 4) arrays respectively. A total of 19 VES and HRP were carried out in the study area. For schlumberger electrode configuration, maximum current electrode separation (AB/2) was 60 m while two current and two potential electrodes having equal intra and inter electrodes separation of 40 m interval moved together at the same time in leap frog method was used for Horizontal Electrical Resistivity Profiling.

7. Data Interpretation

Resistivity VES data was interpreted by plotting apparent resistivity against electrode spacing (Figures 6 and Table 1; Figure 7 and Table 2). The data was interpreted qualitatively by examining the curve shape and quantitatively by using computer modeling. Quantitative analysis involves identifying from the apparent resistivity curve the number of layers, their thickness and resistivity interpretation using Winresist modeled subsurface resistivity interpretation software. This interpretation was compared with the field curve matching in order to ensure high accuracy.

Figure 6. A representative VES curve in Ebugi village, Agaie

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Figure 7. A representative VES curve in Esangi Village, Agaie

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Table 1. Interpreted geoelectric section of Figure 6

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Table 2. Interpreted geoelectric section of Figure 7

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8. Geoelectric Section

One of the outcomes of VES is the generation of geoelectric sections which is a reflection of the subsurface lithologic signatures. The result revealed five geoelectric sections for the area which include: top soil, clay, sandy-clay, clayey-sand and sandstone (Figure 8). It can be deduced that the top soil generally has a thickness of about 3m in all the locations except in Emi-Alhaji Nusa, Emi-Ndanshimu and Emi Gozan where the thickness is about 2m. The thickness of clay in the study area ranged between 40-50m in many locations and also between 8-10m in few locations. The two aquitards which are clayey-sand and sandy-clay varied between 15-35m in most locations while the aquifer in the area (sandstone) ranged between 20-35m in the study area.

Figure 8. Geoelectric section of Agaie and environs

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Figure 9. Borehole litholog of Agaie and environs

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This indicates that the depth to aquifer and aquiclude varies across the sounding stations. The corresponding lithologs obtained from drilled boreholes in the area is shown in Figure 9. A strong correlation exists between the geoelectric section (Figure 8) from the geophysical investigation and the lithologs from the drilled boreholes (Figure 9). This is a good development for characterizing the aquifer system in the area.

9. Groundwater Potential Evaluation in the Area

The groundwater potential of the study area was evaluated by taking the inventory of some hand-dug wells in Agaie and environs. At each location, static water level, total depth of the well, longitude, latitude and elevation were taken. The study revealed the area with shallow and deep wells and conforms to the groundwater flow direction of NE and SW as areas with high groundwater potential (Figure 10) while the southern part of Agaie has medium groundwater potential. The ground water potential of the study was also evaluated using Horizontal Electrical Profiling (HEP) and Vertical Electrical sounding (VES) of the electrical resistivity to investigate the resistivity signatures of the subsurface. The data obtained from the survey was used to generate geoelectric sections were drawn to determine the existing relationship between the lithologic and the geoelectric section so as to delineate areas with high water bearing potential. Litho sample obtained while drilling were also used to evaluate groundwater potential of the study area. The lithologs obtained presents two aquitards (sandy-clay and clayey-sand) and sandstone aquifer which is fine to medium grained. About 85% of the aquiferous unit in the study area is semi-confined and or confined. Pumping test was carried out in each of the drilled boreholes in the area and the result used to evaluate the groundwater potential of the study area. The boreholes pumped 8 litres/sec for more than an hour except boreholes at Ebugi and Essangi Communities where the pumping lasted for less than an hour.

Figure 10. Well depth contour map of the study area

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