The continuous growth of population and development projects in the area has contributed to the indiscriminate existence of failed boreholes and dry wells which pose great concern to the people of the region. Some wells are, most often, susceptible to drought especially in dry weather conditions (dry season); many others go dry soon after well completion, while some do not show any sign of water presence even after drilling at greater depths. This challenge of groundwater supply could be attributed to the difficulty in visualizing the occurrence and movement of groundwater by water borehole drillers in the area, since groundwater is normally hidden from view. As a consequence, this complexity adversely affects the ability to understand and to deal effectively with groundwater related problems. To remedy this situation, a precise and detailed geophysical study becomes critical in order to elucidate the behavior of groundwater aquifer in the area. Aquifer Transmissivity (m²・d⁻¹), hydraulic conductivity (md⁻¹), aquifer thickness (m), Transverse Resistance (Ωm²), Longitudinal Conductance (Ω⁻¹), and Storage Coefficient are the common fundamental properties usually studied [1] . Thus, the aim of this study is to use Dar Zarrouk parameters; Transverse Resistance (Ωm²), and Longitudinal Conductance (Ω⁻¹) to estimate optimal well locations.

Geophysical methods are methods that use the physical properties of earth materials to interpret subsurface structure. The electrical resistivity value of geological formations depends on the lithological properties (density, porosity, pore size and shape), and water content and its quality and temperature [2] . DC-resistivity measurements can contribute significantly to enhance the correctness of the groundwater model by delineating aquiferous zones, using the established relationship between geoelectrical and hydrogeological parameters. [3] estimated the distribution of aquifer hydraulic parameters in the Southern part of Akwa Ibom State, Southern Nigeria using surfacial geophysical measurements. Based on the transmissivity and transverse resistance maps, they were able to identify zones where the aquifer was productive. [4] exploited Dar Zarrouk (D-Z) parameters to differentiate between fresh and saline groundwater aquifers of Sinjar Plain area and the results showed that the distribution of the D-Z parameters and other geoelectric parameters gave insight into the occurrence and distribution of saline and fresh water aquifers in the study area. In an attempt to estimate hydrogeophysical parameters for aquifer characterization in hard rock environment, Yaounde, Cameroon, [5] , carried out a detailed local geological and hydrogeophysical investigations for the aquifer in Yaounde, Cameroon to delineate the architecture of different subsurface geological horizons using lithologs and generated vertical electrical sounding (VES) data. The results showed a direct relation between aquifer transmissivity and transverse resistance. Surface direct measurements have been used by many other workers to estimate the hydraulic properties of aquifers [6] - [11] . Their results indicate that the DC resistivity method does provide a fast, economic and non destructive way to study aquifers. Therefore, the main objectives of this paper are:

1) Field measurements of VES data and interpretation

2) Definition of Dar Zarrouk parameters and their relationships with aquifer transmissivity

3) Correlation between Interpreted layer resistivity and Borehole Litho-log

4) Preparation of contour maps of T (Ωm²), S (Ω⁻¹), aquifer thickness h (m), aquifer resistivity ρ (Ωm), Water Table Depth (WTD) (m), and coefficients of anisotropy

5) Delineation of prolific aquiferous zones on the basis of T (Ωm²), h (m), and ρ (Ωm) values and zones of saline aquifer on the basis of S (Ω⁻¹) values in the study area.

The study area falls within Southeastern Nigeria in the Afikpo Basin, bounded by Long. 7˚45'E to 8˚00'E and Lat. 5˚43'N to 5˚57'N (in Nigerian local datum) and traverses two regions; the Ohaozara area and Afikpo province-consisting of Amasiri, Ozziza, and Unwana, covering an area of about 607.75 Km² (Figure 1). The area lies mostly in the (Eboine) River Basin and the Cross River plains on the South-Eastern flank of the Abakaliki- Benue anticlinorium (Figure 2(a)). The area has an undulating terrain and an elevation of about 170 m above mean sea level. Remarkably, sandstone forms its ridges and the shale forms the valley. The shale unit underlies the bioturbated sandstone. The bioturbated sandstone has very high altitude, this is possibly because they have less period of exposure to erosion.

Geologically, the area falls within the Cretaceous Abakaliki-Benue Rift (Figure 2(a)), and is underlain by the Asu River Group (Albian), the Turonian Eze-Aku Formation and the (Late Campanian-Early Maastrichtian) Nkporo Formation. The Eze-Aku Group is highlighted yellow in Figure 2(b). [12] described the sediments of the Eze-Aku as consisting largely of hard flaggy calcareous shale and siltstones, usually dark grey or black in colour and contain frequent impression of inoceramus and minor sandstone beds. Occasionally, thick sandstones

or limestone units alternate with the calcareous shales. The Turonian Amasiri Sandstone of the Eze-Aku Group is a highly consolidated, well cemented, subangular to subrounded, poorly sorted, very fine to medium grained feldspathic arenite with poor reservoir quality [13] . This unit dominates the northern parts of the study area. Local geology of the study area indicates that extensive marine transgression and regression occurred in the Turonian followed by the period of Orogeny during the Coniancian, especially the Santonian. Contemporaneous with the Orogeny was the intrusion of the igneous bodies described as dolerite sills, into the Eze-Aku Shale [14] [15] .

The Nkporo Formation is the basal lithostratigraphic unit of the Afikpo Sub-basin and comprises dominantly of dark grey to black shales, sandstone, minor limestone and oolitic ironstone beds. In the Campanian-Maas- trichtian, there was subsidence and a marine regression resulted in the deposition of a lateral equivalent to Nkporo Formation named Afikpo Sandstone. The Afikpo Sandstone and the Nkporo shale in alternating sequence within the Afikpo Sub-basin, are Members of the Nkporo Formation. This sandstone unit underlies the hilly terrain of Afikpo town, trending eastwards across the Cross River at Ozizza and Ndibe Beach areas. The sandstones, highlighted yellow in Figure 2(b), occur as two sandstone bodies [16] [17] representing the Afikpo Sandstone. [16] [17] and [18] , reported the presence of an angular unconformity between the Campanian-Maas- trichtian Afikpo Sandstone and the Turonian Eze-Aku Group. The Afikpo Sandstone is the youngest sedimentary unit in the study area. Its relatively loose structure, coarse grains, sorting enables it to be porous and permeable. The formation covers most parts of Afikpo town hence the name, Afikpo Sandstone.

Shale and sandstone are the two major lithologic units in the area. The sandstone units constitute the permeable and saturated aquifers. Afikpo sandstone varies from coarse grained to fine texture, pebbly, poorly sorted, friable, except on places where weathered iron products become secondary cementing materials and increases the strength. The dominant lithology in the Southern zone of the study is the Afikpo Sandstone. Almost the entire Northern area is underlain by well compacted, hard Amasiri Sandstone of the Santonian Eze-Aku Group. The nature of this unit obviously reduces infiltration unless in parts that are fractured or deeply weathered. It is possible that the regional Cretaceous tectonic activity shattered the rocks of the area after the deposition of the sediments. Hence, places that exhibit intense fracturing yield water at shallow depth [19] .

Geoelectrical data was obtained using the Schlumberger array with maximum current electrode spacing of AB = 1362 m. The ABEM Terrameter SAS 1000 was used, which transmits a well defined and regulated square wave that minimizes induction effects and attenuation. The VES points were located using Hand-held GPS-Garmin 76cxs. The positions of the sounding points are shown in Figure 1, with 13 Parametric Soundings [20] [21] , symbolized by double circles and triangles. The method is based on measuring the potentials between a pair of electrodes, while transmitting direct current (DC) between another electrode pair. The depth of penetration is proportional to the separation between the current electrodes. By varying the electrical electrode separation, information about stratification of the ground is provided. The apparent resistivity (ρ_app) of the medium for Schlumberger array is computed from the measurement of potential difference (ΔV_MN) and injected current (I) as:

where, , denotes the geometric factor (K) of the electrode configuration, ΔV_MN represents the potential difference between the potential electrodes M and N, and I is the current injected into the ground. The geometric design for this array is shown in Figure 3. The VES curves (resistivity field curves) were obtained by plotting apparent resistivity, ρ (Ωm) as the ordinate, and half electrode spacing, AB/2 (m)―as the abscissa, in a log-log graph. The log-log graph is preferred because the sampling was logarithmically spaced. Inversion software, Interpex IX 1D, Ver. 3 (S/No. 1226), was used to reduce the VES curves into values of thickness and resistivity of individual layers. Sounding data were interpreted using Zohdy’s method for the automatic interpretation of sounding curves [22] , which leads to a geologically reasonable layer structured models that may be accepted or modified in order to agree with available geological information. Thus, the software generated the number, thickness, and resistivity of the various layers. A total of 50 VES stations were sampled, and the detailed interpretation of sounding curves is contained in [23] .

A geoelectric layer is described by two fundamental parameters: its resistivity ρ_i and its thickness, h_i, where the subscript i indicates the position of the layer in the section (i = 1 for the uppermost layer). Dar Zarrouk parameters; Total Transverse unit Resistance, T (Ωm²) and Total Longitudinal Unit Conductance, S (Ω⁻¹) [24] , including coefficient of anisotropy λ, are derived from the exact resistivity distribution of the given medium by considering a column of unit square cross-section (1 × 1 metre) area cut out of a group of n layers of infinite lateral extent.

For n layers, the total longitudinal unit conductance is

The total transverse unit resistance is

and in the case of a stratified or a fractured anisotropic medium [9] [25] , the coefficient of anisotropy is

where ρ_T and ρ_L are the average transverse resistivity and average longitudinal resistivity respectively. For homogeneous isotropic media, λ = 1, whereas it ranges from 1 to 2 in homogeneous anisotropic media in which ρ_T is greater than ρ_L [9] [26] .

But,

and

where the quantities, and are the secondary geoelectric parameters relevant to the description of geoelectric section consisting of several layers, and are referred to as longitudinal unit resistance (S_i) and transverse unit resistance (T_i) respectively.

Substituting Equations (5) and (6) in Equation (4),

The quantity can be written as, where σ_i is the conductivity (inverse of resistivity), and is analogous to transmissivity, used in groundwater hydrology, where K_i is the hydraulic conductivity of the i^th layer and b_i is its thickness. The mathematical analogy between electrical flow and groundwater flow has long been recognized and widely accepted [27] [28] . The analogy of Darcy’s law and Ohm’s law are the basis for many electrical analog methods for groundwater systems [29] . In general terms, since larger connected pores make for better flow characteristics for both water and electric currents it is expected that at the very least there should be some relationship between electrical and hydraulic parameters.

On the basis of the analogy between Darcy’s law of groundwater flow and Ohm’s law of electric current flow in a horizontal, homogeneous, and isotropic medium, [7] showed that aquifer Transmissivity, T_r is:

or

They assumed that the quantities (Kσ) or (K/σ) remained fairly constant within the investigated area. In further analysis of these two equations, [30] effectively explained the contradiction between direct and inverse relationship of electrical resistivity and hydraulic conductivity. They concluded that Equation (8b) exists in case of highly resistive basement (S-dominant aquifer where electrical current flows parallel to the bedding plane) and Equation (8a) exists in case of highly conducting basement (T-dominant aquifer when the electrical current tends to flow perpendicular to the bedding plane). The geology of the study area corresponds to Equation (8a); the target aquifer (Afikpo Sandstone) is underlain by shale material.

The results and discussions are presented in segments as shown in the subsections under this heading.

The good agreement between the iso-thickness, iso-resistivity and the transverse resistance maps, including the role of the longitudinal conductance values in discerning zones of high saline or high clay aquifers, emphasizes the contribution of Dar Zarrouk parameters to the delineation of aquiferous zones.

Our recognition and gratitude go to Groundscan Services Nigeria Limited for the immeasurable support in providing the Resistivity Meter (ABEM SAS-1000) used for depth probes and some personnel for field assistance in the data collection stage of this work. Several other persons merit our gratefulness. The efforts of Oyim Oko; a practicing geologist of Koyim Consults Nigeria Limited who helped in preparing the borehole lithologic data deserve our appreciation. We are also deeply indebted to all authors whose works we consulted in course of this work.

Robert Egwu Otu Iduma,Tamunoene Kingdom Simeon Abam,Etim Daniel Uko, (2016) Dar Zarrouk Parameter as a Tool for Evaluation of Well Locations in Afikpo and Ohaozara, Southeastern Nigeria. Journal of Water Resource and Protection,08,505-521. doi: 10.4236/jwarp.2016.84043