The resistivity of materials is a good indicator and parameter for the nature and the weathering. Hence electrical prospecting methods have been used for a long time in geological and geotechnical engineering [12] [13] . These methods, both qualitative and quantitative, are based on the Ohm law [12] [13] . They consist in the injection of a direct current in the ground and the measurement of the electrical potential which enables to obtain the true resistivity of encountered formations [12] [13] . In our study area, while considering the geological setting, Schlumberger sounding and profiling methods have been used to determine: the thickness, the lateral extension and the nature of formations encountered along a profile; and to highlight the geometry of geological bodies related to contrasted electrical characteristics [12] [13] .

Field electrical methods (DC) consist in injecting an electrical current in the ground between two electrodes A and B, and then, measuring the induced potential drop between two so-called potential electrodes M and N [12] [13] . For the current intensity is known and the potential drop measured, it is therefore possible to determine the ground apparent resistivity [13] . This apparent resistivity depends on the current and potential electrodes array. The apparent resistivity ρ_a (Rho) can be expressed as a function of the potential drop and the current intensity [14] .

where r_a is in Ohm∙m, and K (in m) is the geometric factor depending on the electrodes’ array which is given by the formulae 2 below.

・ V_MN: potential drop between electrodes M and N, in mV.

・ I_AB: electric current injected between electrodes A and B, in mA.

For the Schlumberger symmetrical configuration (Figure 4), the apparent resistivity ρ_a is given by Equation (3) as follow [13] [15] .

This resistivity value enables to characterize a formation at a given position (O) or station (Figure 4). The resistivity of an earth’s material depends essentially on the humidity and the clay proportion for a given volume of that material [13] [16] . While clay and water fill in any vacuum in a rock, one assumes that the resistivity is function of parameters such as fracturing, fractures and fissures clay filling in, porosity, the clayey clogging of alluvium [4] [12] [13] [16] .

The induced polarization (IP) method ameliorates electrical methods by measuring the apparent chargeability or chargeability M of the earth material [17] [18] . Chargeability enables to assess the earth’s capability to accumulate and return an electric current so do a capacitance when currents are injected then interrupted [15] [18] . The chargeability is measured by a ground injection of electric signal gaps (current (I)) using two injection electrodes A and B while the potential drop is measured using two receiving electrodes M and N (Figure 4). When IP effects occur, a voltage decreasing curve Vs(t) is observed at the receiving electrodes M and N during current stops between each gap [15] [18] .

The chargeability M is defined as the ratio of secondary voltage (V_S) measured at a moment t after the current stoppage, over the primary voltage (V_MN) measured before this break [15] [18] .

The secondary voltage can be measured at a single point after the stoppage of the current, but it is wise to wait a bit for measurement to be reliable [12] [15] ; then the area below the curve can be calculated. The chargeability is given by the formulae below:

It appears that this value is independent of the current injected in the earth. A time dependent transformation is often made in order to enable chargeability values to be comparable from one instrument to another [12] [15] . So, the chargeability expression becomes:

The chargeability unit is generally mV/V.

The flow of the electrical current within ground goes along with electrochemical processes whose intensity and characteristics are function of physical and chemical properties of the earth material. This current’s flow can follow two patterns as stated by many authors [12] [15] [18] .

・ By electrical conductibility for which free electrons’ motion occurs in metallic particles (pyrite, chalcopyrite, ...);

・ By electrical conductibility for which ions’ displacement occurs in the fractures and pores fluid contents of rocks

The induced polarization causes electrochemical processes occurring when the current flows from an ionic conductibility medium (water) to an electronic conductibility medium or, from an ionic conductibility medium to a low conductive medium, or with a contact between two different ionic conductibility media [12] [15] [18] .

Interpolation of electrical dragged data (AB = 500 m, AB = 200 sets AB = 100 m) has produced resistivity and chargeability maps. They represent apparent resistivity and chargeability isobaths for field trenches exposed at apparent depths of 95 m, 38 m and 19 m (Figure 5 and Figure 6). These maps help to discriminate areas with conductive anomalies, favorable to tectonic features (faults, fractures or shear zones) to those of polarizability anomalies, favorable to metalliferous disseminations [12] [13] [24] [25] .

The resistivity maps presented in Figure 5 show conductive discontinuities zones characterized by low resistivity values (Rho < 1500 Ω∙m). At 19 m deep, a conductive zone (dashed blue line) characterized by nearly low resistivity gradients (Rho < 2000 Ω∙m) covers most of the investigated area. Its extension decreases with depth. It is (surrounded by) resistant area characterized by high resistivity values (Rho > 3000 Ω∙m) and tightening of iso-resistivity contours. Also, topographic surveys show that this area is of low altitude (Figure 5). It can thus be interpreted as a fracturing zone and alteration of subsurface formations [26] [27] . East of the iso-value maps at all investigated depths (19 m, 38 m and 95 m), a NE-SW conductive anomaly characterized by high resistivity contrast and tightening of iso-contours is observed parallel to the Kadei tectonic line. It was interpreted as a major discontinuity (fracture or fault) and characterized as a major NE-SW tectonic lineament [26] [28] . The iso-resistivity contours of resistant blocks are concentric and discontinuous. They show a N-S discontinuity zone observable at all depths (Figure 5). Moreover, the high resistivity ranges observed in this study, and the works of several authors [7] [24] [27] show that the study area rests on a very strong or granitic basement.

Figure 6 shows the distribution of chargeability at 19 m, 38 m and 95 m depths, respectively. This distribution characterizes the surface distribution of polarizable disseminated structures for each depth level [18] [29] [30] . The eastside of the three maps shows an anomaly (Figure 6) characterized by high chargeability values and changes in the NE-SW direction. Its extension is assigned to a vein evolution. This anomaly can be interpreted as a high concentration of disseminated metalliferous minerals in a quartz and pegmatite veins [18] [29] - [31] . At all depths, the NW oriented central zone is almost devoid of polarizable structures. It is partly localized in low altitude areas. The low polarization patterns may reveal either the presence of highly fractured

and altered granitic structures or sulfate minerals [31] [32] . At 19 m depth two high chargeability gradient anomalies are clearly visible. Their gradients decrease significantly with depth (Figure 6). They are located in low altitudes areas and may be interpreted as metallic mineral deposits in shallower weathered horizons [12] [30] [31] .

The resistivity and chargeability distribution maps (Figure 5 and Figure 6) at specific depths beneath the study area highlight a broad NE-SW major conductive and highly polarizable discontinuity which aligns itself with the Kadéi tectonic line.

Figure 7 shows the resistivity and chargeability pseudo-inverse sections. They were realized using Res2Dinv software [20] [21] and correspond to the electrical panels along L1 and L4 profiles, respectively (Figure 2 and Figure 5). The depth of investigation for each profile is about 120 m. This depth corresponds to the maximum length of the cable (AB = 600 m) according to the position xl along a profile [19] .

Electric sections in Figure 7(a) and Figure 8(a) show alternating resistant fields (Rho > 2500 Ω∙m) slightly resistant (1000 < Rho < 1500 Ω∙m) at surface (depth < 9 m). These observed resistivity ranges are attributed to the nature of laterites and sedimentary layers that form the cover of the study area (Figure 1). Less than 60 m deep and below the sedimentary cover, eleven (11) conductive zones (CZ) were identified (Figure 7(a)). They are characterized by conductive cores (Rho <1000 Ω∙m) and can be interpreted as conductive discontinuities (fractures, faults or shear zones) [12] [25] [26] . The CZ_3 conductive zone extends more than 120 m (elevations difference) and deep and it is limited between positions xl = 650 m and xl = 1100 m by sub vertical iso-resistivity contours (Figure 7(a)). It can be interpreted as a major discontinuity, either a fracture or a shear zone [10] [12] [26] . Between positions xl = 1100 m and xl = 2100 m along profile L1, most of the crossed subsurface layers are very resistant (with resistivity gradients sometimes greater than 20,000 Ω∙m), especially in depth (Figure 7(a)). They seem to have experienced tectonic stresses which are observed as folded and heterogeneous forms displayed by all the iso-resistivity contours [26] . The high resistivity values recorded and the forms of iso-resistivity contours observed (Figure 7(a)) characterize resistant or granitic rocks which have suffered considerable tectonic stresses [12] [26] .

Moving towards the north (Figure 8(a)), the pseudo-section of profile L4 has almost the same geological and structural setting as profile L1. However, an expansion to the west of the CZ_3 conductive zone is noted (between xl = 50 m and xl = 750 m). It represents the conductive area CZ_1^* along profile L4 (Figure 8(a)). It is also noted (Figure 8(a)) on other areas (CZ_2^* CZ_3^* CZ_4^* CZ_5^* CZ_6^*) but many of them are extensions of the discontinuities observed along the profile L1.

The electrical section shown in Figure 7(b) allows to appreciate the distribution of chargeability gradients along the profile L1. One notes to this effect an unbalanced distribution of chargeability gradient (Figure 7(b)). The area between the positions xl = 600 m and xl = 1100 m has a chargeability anomaly characterized by high values [12] [18] [29] . They are attributed to a strong polarization of structures and can be interpreted as a high concentration of metalliferous minerals disseminations in quartz and pegmatite veins [12] [18] [29] [32] . Out of this area, the chargeability gradient is low reflecting a low concentration of disseminated metalliferous minerals or the presence of sulfate minerals [32] [33] .

Moving towards the north (Figure 8(b)), the above-mentioned anomaly at profile L1 pass through profile L4 between positions xl = 50 and xl = 750. The observed gradient is higher. Additionally, along profile L4, three discontinuity zones characterized by chargeability contrasts are observed. They are located between xl = 1050 m and xl = 1350 between xl = 1550 m and xl =1750 m and between xl = 2950 m and xl = 3550 m. They suggest the presence of relatively polarizable particles concentrations disseminated in the basement [12] [18] [29] [32] .

Figure 9 and Figure 10 present the geological sections along profiles L1 and L4. They were realized through the interpretations of sounding curves and geological reconnaissance of outcrops (Figure 1). These sections quantitatively illustrate the lithology of the study area [16] [34] .

The spacing of geological columns is 100 m along profiles L1 and L4 (Figure 9 and Figure 10). This spacing varies in the presence of marshy areas. Geological columns from the eastside of the study area are dominated by thick conductive horizons. These thicknesses confirm the presence of a major conductive anomaly therein [16] [24] [27] that cut profiles L1 and L4 over a deep and lateral extension 115 m and 600 m respectively (Figure 9 and Figure 10). Moreover, west of the study area, the profiles display less thick (<35 m) conductive horizons (Figure 9 and Figure 10). These horizons are associated to a sandy clay alteration. They show that the western zone is a shallow fracturing zone and deposit of weathered formations. These highlights comfort early results [16] [24] [27] . Conductive or alteration horizons identified by geological sections, can confirm the tectonic accidents (fractures or shear zones) suggested by previous results [12] [25] [27] .