The study area is Gbêkê region, located in the center of Côte d’Ivoire. It covers the area between longitudes 4˚24' and 5˚43'N and latitudes 7˚12' and 8˚12'W (Figure 1). The population is estimated at 1,200,000 inhabitants. This area is under the influence of the wet tropical climate with two distinct seasons: a long dry season (November-March) and a long rainy season (April-October). The

study area covers 9136 km². The geological bedrock consists of the volcano-sedimentary and the granitoids, which are essentially constituted by granites (Figure 1). On the one hand, the volcano-sedimentary includes meta-sediments mostly constituted of sandstone and schists intruded by several generations of granitoids. On the other hand, the volcano-sedimentary is covered by metavulcanites which consist of amphibolites, meta-andesite, rhyolites, meta-basaltes, metagabbro and metadolerite.

Two aquifers exist in the study area for the groundwater extraction. The most important aquifers are the fractured aquifers of crystalline and schist rocks. Their permeability is conditioned by the presence of discontinuities such as faults and joints and, in some cases, by lithlogic contacts [14] . Over the fractured rocks, the weathered layer may constitute a porous aquifer.

The dataset for the study consisted of 43 boreholes from the rural and urban water supply programs in Gbêkê region (Figure 1). The parameters taken into account are borehole yield Q (m³∙h⁻¹), borehole depth (m), weathering depth (m), resistivity data of the geological structures and specific capacity Qs (m³∙h⁻¹∙m⁻¹). The borehole yield is the air lift flow measured at the end of the drilling by blowing air under pressure at the bottom of the borehole and providing a good estimate of the aquifer’s transmissivity. The specific capacity is defined as the ratio between the outflow from a borehole (Q) and the drawdown (s).

We also used resistivity data from fourty three horizontal profiling and vertical electrical sounding using the Schlumberger array. The electrical resistivity methods are used as described by [12] and [15] . These methods consist in setting a direct current in the soil using electrodes A and B and measuring the potential difference between the two other electrodes M and N, including between A and B. The electrical profiling is the preliminary method to any geoelectric study and is the basis for the activation of other electrical implementations. In this study, the electrical profiling was used to monitor the lateral continuity of layers for a given position, and enabled to confirm the effectiveness or not of conductive anomalies [16] . The electrical profiling was conducted according to the Schlumberger mechanism with the following geometric features: AB = 300 m, MN = 20 m with a 10 m measurement step. Vertical electrical sounding (VES) is performed at the location where the conductive anomaly was detected. That is to quantify the thickness and the resistivity of both the saprolitic and the stratiform fractured layers. The apparent resistivity (ρ_a) values obtained from the survey are estimated as follows [12] :

ρ a = π [ ( A B / 2 ) 2 − ( M N / 2 ) 2 ] M N Δ V I (1)

where ρ_a is the apparent resistivity, ΔV and I are the potential difference measured between the potential electrodes (volts) and the applied current strength (milliampere), respectively. AB represents the distance between the current electrodes (meters), MN is the distance between the potential electrodes (meters).

The apparent resistivity values obtained from the survey are plotted against the half electrode spacing on a log-log plot. The initial interpretation of VES data is made using curve matching techniques utilizing master curves [17] and the corresponding auxiliary curves [18] from which the resistivity values and thicknesses of the layers are obtained. Further, interpretation of sounding data is made using IPI2W in software.

Bivariate analysis was used to determine the relationships between the productivity of boreholes considered as specific capacity values and the depth of drilling and the thickness of alteration as described by [19] . Correlation studies were carried out using the Spearman correlation test. It is a nonparametric technique for measuring the statistical dependence between two variables. The method assesses how well the relationship between two variables can be described using a monotonic function. The advantages of this test are that variables do not need to follow a normal distribution, the method is not very sensitive to outliers, and it is used for data collected on ordinal, interval or ratio scales. In addition to the correlation coefficient (r), standard hypothesis testing was conducted. They tested the null hypothesis that the ranks of one variable do not covary with ranks of the other variable. A significance level (p-value) of 0.05 was used throughout the study. Bivariate analyses were conducted within semi-log space with specific capacity on a logarithmic scale and hydrogeological parameters on an arithmetic scale.

Moreover, the Kruskal-Wallis test was performed to compare specific capacity values between the resistivity sounding curve types in studied cities. The Kruskal-Wallis test is a nonparametric test for comparing more than two independent groups. It assesses a null hypothesis that the data sets originate from the same population. If p-value is below 0.05, then, there is a statistically significant difference between the groups.

Specific capacity values ranged from 0.0088 to 2.20 m³∙h⁻¹∙m⁻¹ (Table 1). The study area was generally characterized by deeper boreholes that the depth varied from 40 to 117.55 m (Table 1). Eight resistivity sounding curve types were delineated namely H curve types for 16 stations (37.21%), KH curve types for 11 stations (25.58%), A curve types for 4 stations (9.30%), QH curve types for 4 stations (9.30%), KHKH curve types for 4 stations (9.30%), KHA curve types for 2 stations (4.65%), QHK curve types for 1 station (2.33%) and HKH curve types for 1 station (2.33%) (Table 2). H and A curve types are the three layers earth models. QH and KH curve types are the four layers earth models. QHK, KHA and HKH curve types are the five layers earth models while KHKH curve types are the six layers earth model.

Figure 2 shows specific capacity evolution with borehole depth. Borehole productivity had a low and negative correlation (r = −0.30; p < 0.05) with borehole depths. A low trend of decreasing borehole productivity with increasing depth is noticeable. The influence of the depth boreholes productivity from this study is consistent with [7] . These authors found that in the Jundiaí River Catchment, a tendency of decreasing well productivity with increasing depth exists, but the correlation coefficient among the variables was considerably low. For [9] , the

^aVertical electrical sounding.

reduction of the borehole productivity is due to the closure of discontinuities by lithostatic pressure to lower the density and connectivity of fractures with depth. However, in this study it is remarkable that aligned points, indicating boreholes of identical depth (50, 60 and 80 m for instance) often present distinctly specific capacity values. In agreement with [7] , this leads to the thinking that borehole depth is defined according to contractual issues, user’s necessity and construction profits, which sometimes prevail over possible productivity gains by increasing the depth. Thus, it was not possible to define a best-yielding depth interval. Although productivity tends to diminish with depth, a deeper borehole can be more productive if it reaches in subsurface a geological structure that is favorable to groundwater flow. Some authors [8] have defined the best depth interval or the maximum depth that a well must reach in order to obtain satisfactory productivity in crystalline rocks. They have shown that, because of the closure of discontinuities by lithostatic pressure in depth, the deeper the well, the lower the productivity.

Figure 3 shows the productivity of boreholes from the weathering depth. There was no correlation between the depth of weathering and the specific capacity (r

= 0.23; p < 0.05). From this study, neither the nature of the regolith, nor the rock in which it was formed has been taken into account. As a result, in agreement with [20] and [11] , a correlation between specific capacity and weathering depth can be verified or identified, as highlighted by the experimental data. Some authors have shown that the productivity of boreholes increases with weathering depth in the crystalline rocks [21] . In this study, we note that high specific capacity values (≥1 m³∙h⁻¹∙m⁻¹) were concentrated where the weathered layer was 15 - 35 m thick. Similar results were obtained by [20] and [22] in Côte d’Ivoire, and [11] in Cameroon. These authors observed generally medium and high yield between 5 and 45 m of regolith thickness.

Figure 4 shows the distribution of Qs values from the resistivity sounding curve types. Qs values varied from 0.009 to 1.017 m³∙h⁻¹∙m⁻¹ in H curve types and from 20.7 to 24,376 in A curve types. The resistivities of H curve types ranged from 11.4 to 60,089 Ω∙m (Table 2). The geoelectrical layers here are generally interpreted as a three subsurface layers that are topsoil-lateritic layer, weathered-fractured layer and the fresh basement or bedrock [13] . The topsoil-lateritic layer is composed of clayey sand and sand. Its thickness ranged between 0.60 and 9.26 m (Table 2). The weathered-fractured layer constitutes the main aquifer unit. It had a thickness ranging between 1.40 and 39.7 m. In the hard rock identified at the H curve types, the resistivity values were often low. It was the case at Yebouekro (ρ3 = 24.6 Ω∙m), Ahougnaou (ρ3 = 41.2 Ω∙m) and TakraMangouakro (ρ3 = 45.6 Ω∙m) (Table 2). These zones with low resistivity could be the fractured formations, vein zones or faults. The resistivity values ranged from 20.7 to 24,376 Ω∙m in A curves. In agreement with to [23] ., the A curve types give a best-fitted three-layered model with resistivity values of topsoil ranging from 1.6 to 4.72 m. The topsoil could be mostly composed of sand with low resistivity values. The second layer could be the weathered basement with resistivity and thickness values varying between 401 and 2342 Ω∙m, and 2.7

- 21.8 m respectively. The third layer could be presumably fresh basement whose resistivity values reached 14,104 Ω∙m.

The KH curve types gave resistivity values ranging from 14.7 to 31,103 Ω∙m. The geoelectrical layer could be interpreted as a four subsurface layered that is topsoil, alluvial deposits, granitic sand and hard rock.

QH curve types gave a four-layered model. The near-surface layer has variable resistivity values ranging from about 651 to 1573 Ω∙m. The difference in the resistivity values is due to the variation in grain size [23] . The thickness of the top soil layer varies was very thin. It was estimated as less than 2 m. The third layer could be a water saturated sand horizon with a low resistivity ranging between 29 and 92 Ω∙m. The thickness was less than 27 m. The fourth layer with high resistivity (475 - 9229 Ω∙m) was associated to bedrock.

The five layers HKH, QHK and KHA curve types were observed at Kouakoubakakro (VES 22), Alloukrouyakro (VES 20), Tiendeho (VES 24) and Badiokouamekro (VES 39). The first has a resistivity range between 197 and 13,308 Ω∙m. The resistivity values of the second and the third ranged from 124 to 47,639 Ω∙m and from 99.5 to 19142 Ω∙m. These layers could be interpreted as topsoil/sand/clay or clayey sand/sand or clayed sand/hard rock or altered bedrock.

Spatial differences of Qs values were clearly found. The highest values of Qs were recorded in KH, QH and H curve types, while their low levels were found in QHK and HKH curve types. The results of the distribution of Qs values following the resistivity sounding curve types are consistent to the Kruskal-Wallis test (Table 3). The test result indicated a significant difference (p = 0.001) between Qs values following the resistivity sounding curve types. The sounding anomalies of K, QH and H types may reflect the discontinuities and geological structures effectiveness such as vein zones, fractures, faults and geological contacts for the fractured aquifer permeability in the crystalline bedrock. However, in

agreement with [7] , the location close to these stuctures does not indicate that a borehole built there should be highly productive. It is necessary to locate the boreholes at a favorable position with the structural dipping with a depth that can reach the geological structure in the subsurface. Furthermore, the direction of the structure should be favorable to opening via tectonical stresses.