Geoelectrical Parameters for the Estimation of Groundwater Potential in Fracture Aquifer at Sub-Urban Area of Abakaliki, SE Nigeria

There is a limited availability of groundwater potential within the study area, as there is steady increase in population due to movement of various faculties to main campus of Ebonyi State University (Perm Site). Therefore new water supplies are required to meet the demands for water. A combination of resistivity and borehole data were used to identify different water bearing horizons and their protective capacity. Twenty-three Vertical Electrical Soundings (VES) using the Schlumberger configuration with maximum current electrode separations of 200 m was used to map the subsurface lithology. Distribution of the resistivity values at different depths were presented in maps. Correlation of the obtained resistivity values with the geology of the area showed three to six layers with varying hydrological properties exist in the study area. The fracture shale is identified as good water bearing formation and this formation possess low to high longitudinal conductance, high transverse resistance, and anisotropy values.


Introduction
Water is regarded as one of the most important natural resource that sustains life on earth, it exist as surface water (lakes, streams, river and sea) and beneath the earth surface as groundwater. Anomohanran [1], stated that fresh surface water can be assessed easily in most places except for areas in the desert regions. Despite this advantage of easy accessibility, surface water is often polluted by anthropogenic activities making groundwater to be a viable option in satisfying our demand for qual-resistivity method is generally, considered to be the most promising and most suitable method for ground water prospecting [2][3][4]. The vertical electrical sounding (VES) which essentially determines the vertical variation of subsurface resistivity with depth has been very successful in delineating subsurface layering, overburden thickness and depth to fractured aquifer. The geoelectrical resistivity surveys for groundwater exploration has also been used in determining aquifer protective capacity [5]. The selection of geophysical methods in groundwater exploration depends on the on the contrast between the physical properties of the target and the surrounding medium. For the purpose of this study emphasis will be place fractured shale aquifer, because groundwater within the study area exist in fracture shale [2,6]. MacDonald and Davies [7], stated that the predominant lithology in the study area is shale which ordinarily lacks the required permeability to supply water at economic quantity. However, previous research suggested that only where the shales are significantly fractured that you have water of economic quantity [6]. The study was aim at determining groundwater potential and protective capacity of water bearing unit.

Location and accessibility
The area is located between latitude 6°17 / N -7°01 / N and longitude 8°06 / E -8°56 / E ( Figure 1). It located close to Abakaliki the state capital in Ebonyi State. The area is accessible through Enugu-Abakaliki express way and network of other roads.

Geology of the study area
The area falls within the Lower Benue Trough (SBT). Murat [8] and Hoque [9] gave detailed information on lower Benue Trough using three tectonic sedimentary cycles, namely: Marine transgressions and regressions which started from the Albian to the Coniacian in age [6]. Marine transgression was the first that occurred in Benue Trough within the middle Albian period, with the deposition of the Asu River Group [8] Reyment [10]. The Asu River Group sediments are predominantly underlain by shales as stated by Reyment [10]. (Figure 2), siltstone, sandstone and limestone facies as well as extrusive and intrusive. Benkhelil [11] stated that Asu River Group has an average thickness of about 2000 m and uncomfortably overlies the Precambrian Basement. The Santonian tectonic phase give rise to series of fracturing and folding of these   (23) sounding were completed within the study area using the Schlumberger configuration with electrode spacing (AB) ranging from 2 to 200 m and the potential electrode spacing (MN) ranged from 0.5 m to 10 m. Global Positioning System (GPS) for taking accurate coordinate of the VES point and elevations, metal electrodes, measuring tape, labelled tag (used in locating station position), hammer (used in driving the electrodes into the ground). The Schlumberger array was adopted. The electrode spread of AB/2 was varied from 1 to a maximum of 100 m. The electrical resistances obtained were multiplied by the corresponding geometric factor (k) for each electrode separation to obtain the apparent resistivity. Qualitative interpretation of the smoothened curves was performed using master curves and standard charts [16] after which they were subjected to computer modeling using the IX1D software. The software makes provision for sounding data to be entered as apparent resistivities versus half current electrode spacing (AB/2) for Schlumberger array. The input parameters, depths and resistivities used for the starting model in the inversion process were estimated from the manually interpreted field curves. This software automatically interprets the Schlumberger sounding curves. The plotted curves reveal the number of layers, thickness, depth and rocks, giving rise to chains of anticlines and syncline known as the Abakaliki Anticlinorium [10]. The dominant fracture trend that hosts the leadzinc forming minerals is in NW-SE and NNW-SE [12]. In addition Agumanu [13], demarcated the Asu River Group sediment into Abakaliki and Ebonyi Formations. The Abakaliki formation is predominantly dark grey colored shale (weathered to brownish material in the greater part of the formation), blocky, and indurated in most locations, yet fissile in some locations.

Hydrogeology of the study area
Groundwater movement and storage within the study area is primarily influenced by thickness, lithology, and structure of rock formation [14]. Shale is an aquiclude and does not permit reasonable passage of water, especially when fresh and unweathered. But when fractured in most case, it serves as water bearing formation. As water bearing formation within the study area exists in fractured shale and limestone [15]. The major sources of water are stream, hand-dug well, manual borehole and motorized borehole.

Materials and Methods
The resistance of the earth was measured using Abem Terrameter SAS 1000 using Schlumberger electrode configuration, Twenty three  The total transverse unit resistance (T) of a VES can be observed by using the equation.
The total transverse unit resistance is The average longitudinal resistivity for a given VES curve is given by using the equation.
The longitudinal resistivity is The average transverse resistivity for a given VES curve is given by using the equation.
The transverse resistivity is The coefficient of anisotropy (λ) of a formation could be observed by using the formation. respectively.

Well data acquisition
A total of 3 wells and two productive and one abortive boreholes (BH1, BH2 and BH3) were visited where well information were taken ( Figure 4a and Figure 4b).
1. The total longitudinal unit conductance (S) is calculated by using the formula.
For 'n' layers, the total longitudinal conductance is This parameter is used to quantitatively assess the properties of a thin conducting layer. Studies have shown that hydraulic conductance has an inverse relationship with electrical resistivity values, thus high groundwater potential aquifers are usually characterized by high conductance values [17]. TR of a geoelectric layer is defined as the product of h and its corresponding ρ [18,19].  tivity and they will act as single geo-electric layer [17]. The secondary geo-electrical parameters are calculated to provide the results more realistic and are described as Dar-Zarrouk parameters [20]. The parameters of the area generated by this study are summarized in Table 1. Resistivity and thickness was used to map out the nature of materials that overlain the mapped aquifer, the longitudinal unit conductance (S), the transverse unit resistance (T) and the coefficient of anisotropy (λ) were used to determine its capacity to prevent infiltration of unwanted fluids into the aquifer. Olorunfemi, et al. [21] further stated that earth materials act as a natural filter to percolating fluids; therefore its ability to retard and The coefficient of anisotropy is The parameters T and S (defined as transverse resistance and longitudinal conductance respectively) play an important role in the interpretation of sounding data. These are called the Dar-zarrouk parameters as shown in Eqn 1 to 5.

Secondary geo-electrical parameters
The true resistivity obtained as a result of interpretation may also subject to variations, since different lithological layers possess same resis- values are normally utilized in evaluating aquifer protective capacity. The aquifer protective capacity essentially assesses the natural ability of overburden units to filter or retard percolating fluid pollutants [21]. The significance of filter percolating ground surface polluting fluids is a measure of its protective capacity. In other words, geologic materials overlying an aquifer could act as seal in preventing the fluid from percolating into it. The longitudinal conductance Based on these estimates it was found that the coefficient of anisotropy λ ranges from 0.06 to 5.62 which depicts the true variation of the anisotropic character of rock formations. The area with high values of λ suggests that the fracture system must have extended in all the directions with different degrees of fracturing, which had greater water-holding capacity from different directions of the fracture(s) within the rock resulting in higher porosity, this is in line with [17]. At the same time, unidirectional fracture may not produce good yield of water and such areas show low values of λ. Consequently, it indicates the presence of macro-anisotropy in the present geoelectric strata in the area, which is clear to distinguish the individual layers for a given VES earth model [23] (Figure 5b and Table  2). Longitudinal resistivity (LR) calculated from VES ranges from 390.9 to 3929.04 Ωm, The longitudinal conductance is inversely proportional to average longitudinal resistivity. The higher average longitu-this aquifer parameter is that low S value shows poor aquifer protection zones, while high S value shows vice versa. The estimation shows that the total longitudinal conductance varies from 0.03 to 13.81 Ω -1 in the area (Figure 5a and Table 2). A comparison of the S results with the protective capacity rating model of Oladapo and Akintorinwa [22] indicates that the aquifer protective ca-    Table 4a. Has HK curve, with first layer having resis-dinal resistivity value aquifers will be of freshwater zone where as lower average longitudinal resistivity value aquifers are identified with fractured shale which is of groundwater is moderate in these zones (Figure 5c and Table 2). Transverse resistivity (TR) varies from 268.64 to 3758.98 Ω.m 2 . Henriet [24], stated that on a pure empirical basics, it can be admitted that TR is directly proportional to transmissivity. Braga, et al. [25] and Toto, et al. [26] further pointed out that TR is used to determine zone of high groundwater potentials and areas suitable for drilling (Figure 5d and Table 2).

Vertical electrical sounding (VES) curves analysis
Four curve types exist in the study area, namely: HK, Q, HKA, Q, H and QH. The HK and Q VES curves are the predominant curve ( Figure 6). The high degree of variation in curve types confirms the heterogeneity of the geology of the study area.

Geoelectric survey
For better understanding of the subsurface geol-

HK
HKA Q H QH respectively, its interpreted as top soil, layer two has depth of 11.84 m and thickness of 8.16 m with tivity value of 260.23 Ωm, with thickness and depth of 2.54 m respectively and its interpreted as topsoil, the second layer has resistivity value of 46.00 Ωm, with thickness of 6.07 m at depth of 8.62 m and its interpreted as shale/clay, while the third layer has resistivity value of 109.21 Ωm with thickness of 4.91 m at depth of 1.53 m and it is interpreted as shale, the fourth layer have resistivity value of      m was interpreted as shale/clay, lastly, layer fire has resistivity value of 5872.8 Ωm, was interpreted as siltstone, it was observed that VES 37 has HKH curve as shown Figure 2d and Table 4d.

Geo-electric and lithologic logs correlation
A correlation of the geo-electric data of the layers and lithologic (borehole) logs is presented in resistivity value of 285.88 Ωm, was interpreted as fractured shale and lastly, the fourth layer has resistivity value of 17.70 Ωm and was interpreted as shale/clay, it was observed that its VES 28 has HK curve as shown Figure 2c and Table 4c. First layer has resistivity value of 352.66 Ωm with thickness and depth of 2.54 m respectively and was thickness and depth of 2.54 m respectively and was interpreted as topsoil. Layer two has resistivity value of 72.078 Ωm with thickness and depth of 7.15 m and 9.70 m respectively and was interpreted as shale/ clay, while the third layer has depth value of 25.12 data in the study area has contributed to a better understanding of the groundwater prospect and aquifer protective capacity in this part of sedimentary terrain of Southeastern Nigeria. The study area is underlain by three (3) to six (6) lithology these include the top soil (mostly laterite), clay/shale, siltstone, shale, consolidated shale and fractured shale. The resistivity, thickness of subsurface formations, longitudinal conductance (S), transverse resistance (R) and anisotropy (λ) were used to characterize the lithological formations and also groundwater potential and recharge zones were delineated. The study also revealed that most parts of the area are underlain by materials of poor to excellent protective capacity. From study it was observed that the middle part of the study area has high aquifer protective capacity compared to other part of the study area this could be attributed to the fact the area is underlain by shale and clay which are aquiclude and are known to have negligible permeability, hence it would difficult for contaminant to flow through such rock unit. The major curve type that cut across the study area is curve HK. Figure 4a and Figure 4b. Correlation analysis suggests that the first geo-electric layer corresponds to the top lateritic soil, the second correspond to clay/shale, third and fifth corresponds to fractured shale, fourth corresponds to clay/shale layer. This third most probably marks the upper level of the aquifer, while the fifth layer is considered to be the aquiferous unit (Figure 4a). This information shows good agreement with earlier data within the Asu River Group aquifer given by Ebong, et al. [2] although there tend to slightly change in resistivity value (Figure 4b). Showed failed borehole log correlation analysis suggests that the first geo-electric layer corresponds to the top lateritic soil, the second correspond to clay/shale, third corresponds to clay/shale, fourth and fifth layer corresponds to clay/shale layer Table 5.

Description of geo-correlation
The profile line that cuts across the study area are; profiles A-A / B-B / , C-C / and D-D / . Profile A-A ̕ runs NW to NE direction, hence it can be called NW-SE direction. This profile line cut across the following VES points namely: VES 4, 13, 12, 1 and 2. It's depth of top soil ranges from 0.00 to 6.28 m the top soil formation is characteristics of laterite, with layers ranging from three to six and resistivity values ranges from 1.57 to 3596.9 Ωm. The inferred lithology of this profile are mainly top soil, clay, clay/ shale, fractured shale siltstone and shale ( Figure 7a and Table 2). Profile B-B ̕ cut across VES 17, 22 and 14. It's depth of top soil ranges from 0.95 to 3.43 m represents the top soil formation is characteristics of laterite, with layers ranging from four to five and resistivity values ranges from 7.43 to 721.81 Ωm. The inferred lithology of this profile are mainly; top soil, clay, clay/shale, fractured shale siltstone and shale (Figure 7b and Table 2). Profile C-C ̕ cut across VES 11, 2, 18, 10, 9 and 12. It's depth of top soil ranges from 0.93 to 3.22 m represents the top soil formation is characteristics of laterite, with layers ranging from three to five and resistivity values ranges from 1.57 to 1016.7 Ωm. The inferred lithology of this profile consists of top soil, clay, clay/ shale, fractured shale siltstone and shale ( Figure 7c and Table 2).
Note: Resistivity value of rock unit was tied to borehole data and Telford, et al. [27].