Modern Environmental Science and Engineering (ISSN 2333-2581) September 2018, Volume 4, No. 9, pp. 882-895 Doi: 10.15341/mese(2333-2581)/09.04.2018/012 Academic Star Publishing Company, 2018 www.academicstar.us

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê,

Kouassi Kouamé Auguste1, Kouassi Francis Williams1, Bouadou Rock Armand Michel1, and Coulibal Y. Adama2 1. Nangui Abrogoua University, UFR Science and Environmental Management, Laboratory Geosciences and Environment, Ivory Coast 2. Department of Science and Technology of Water and Environmental Engineering, UFR of Earth Sciences and Mining Resources, Félix Houphouët-Boigny University, Ivory Coast

Abstract: Most of Gbêkê’s groundwater resources are found in crystalline and crystallophyllian basement fractures. For the extraction of these groundwater, geophysical techniques have been implemented for the choice of drilling sites. Thus, the findings of the geophysical surveys and those of the mechanical surveys showed divergent results in some localities of the study area. The objective of this work is to verify the degree of reliability of the information provided by the geophysical prospection by comparing them with the results of the drilling. The methodological approach used is to make a comparative analysis of the results of the electrical soundings and the corresponding drillings (SE, F). The parameters to be analyzed according to the state of the boreholes (positive or negative drilling) are: the fracture directions, the types of sounding, the thicknesses of alterite, the soundings of fractures and the arrivals of water. The analyzes of the results of the electrical soundings and their corresponding drillings showed dissimilarities in the total thicknesses of alteration, the position and the presence or absence of fractures arrivals of water in the subsoil. The failure of drilling in the study area is largely caused by these differences in results between electrical soundings and mechanical soundings.

Key words: geophysical prospecting, drilling, electrical survey, Ivory Coast

groundwater resources [3]. Since the 1970s, many 1. Introduction efforts have been made in Ivory Coast to install Water plays an important role in the evolution of boreholes to supply drinking water to populations with human societies. Thus, all countries must face the the help of national and international partners [4]. water problem and find more rational solutions for the Unfortunately, at present, many boreholes are out of exploitation of all water resources [1]. In fact, order for reasons of maintenance and malfunction. groundwater is the third largest freshwater reservoir in Added to this is a growing population that has led to the world after glaciers and polar ice caps [2]. increased water requirements [4]. Thus, in order to Apart from sedimentary basins, the main meet these growing water needs of the populations groundwater resources are found in fissured basement and maintain it through positive drilling and good rocks and alterities [3]. These aquifers are derived flow, geophysical prospecting campaigns have respectively from the fracturing of the basement and identified favorable foraging sites in the Gbêkê region. the alteration of the rocks. This fracturing of the However, during the mechanical surveys of these sites, geological formations thus directs the prospections of one often attends the realization of negative boreholes. This situation could be the result of the likely Corresponding author: Kouassi Kouamé Auguste, Junior shortcomings during the interpretation of geophysical Lecturer; research areas/interests: hydrogeology. E-mail: [email protected]. survey data [5-7]. It is within this framework that the

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 883 present study proposes to verify the degree of ferralitic or rejuvenated soils, on granite, or remolded reliability of the information provided by the soils, with subsoil-individed, impoverished or geophysical prospection by comparing it with the hydromorphic groups. It belongs to the results of drilling (mechanical prospecting). Paleoproterozoic domain. On the lithological level in the Fig. 1, there are 3 large sets that are: 2. Presentation of the Study Area  the archeen (antebirrimian) consists of accidents, The Gbêkê region is located in central Ivory Coast faults, migmatites and general directions in the between longitudes 4°30' and 5°48' West and latitudes folded zones; 7°15' and 8°10' North. It covers an area of 10429 km2  the lower Proterozoic is represented by the or 3.23% of the Ivorian territory [8]. The region is Baoulé complex (biotite homogeneous characterized by loosely tabular surfaces whose granitoids, heterogeneous biotite granitoids interfluves fray and hills and undulations become and subalcalin granitoid to 2 micas) and the more frequent as one moves south [9]. The region is Abronian complex (granodiorites, diorites); subject to a tropical subhumid climate, with greater  the Birrimian is composed of undifferentiated amplitudes, which oscillate between 14°C and 33°C rocks, schists, andesites, spilites, basalts, with a hygrometry of 60% to 70% [8]. In this region amphibolites, greywackes, formations of more the cumulative annual water deficit varies between intense metamorphism and polygenic 700 and 800 mm of rain and the annual rainfall conglomerates with grauwackeux cement. oscillates between 1000 and 1600 mm [10]. This The Gbêkê region belongs to the crystalline and climate region also experiences four seasons: two dry crystallophyllian basement in which two levels of seasons, from November to March and from July to superimposed aquifers can be distinguished: August and two rainy seasons, from August to superficial aquifers or alterites resulting from the October and from March to June. In addition, the soils physicochemical alteration of the source rock and the of the Gbêkê region belong to the group of ferralitic fissure aquifers created in favor of tectonic events soils moderately and weakly desaturated in bases in [11]. the B horizon or Eutric Ferralsols These are typical

Fig. 1 Geological map of Gbêkê region.

884 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

The applied geophysical method is that of electrical 3. Materials resistivities. Measurement of the resistivity of the 3.1 Data formations of the subsoil was carried out by injecting electric current into them, by means of two injection or 3.1.1 Geophysical Prospecting Data emission electrodes (A and B) and by measuring the The geophysical data used in this work are derived difference of potential created by passing the current from the results of drilling and electrical sounding through two other measuring or receiving electrodes (M conducted in the Gbêkê region and recorded in and N) [12-17]. Its implementation was made thanks to geophysical survey data sheets. These data concerned two devices: the device of the rectangle gradient for the the following parameters: apparent resistivities and electric trainers and that of Schlumberger for the their corresponding distances. installation of the electrical soundings. 3.1.2 Drilling Data The data used come from the technical data sheets 4.1 Identification of Fracture Directions of drilling implanted following the above-mentioned Considering the first fracture in the Fig. 2, it geophysical studies. These data are: flow rate (Q), intercepts profiles L1_0+42 North and L1_0+00 total depth drilled (Pt), alteration thickness (EA), respectively at the abscissa X1 and X2 anomalies. drilled basement depth, and water inflow (AE). This fracture makes an angle “α” with the vertical [18]. 3.1.3 Abacuses Used The tangent of this angle “α” is equivalent to the ratio Charts enabled the interpretation of electrical survey of the abscissa difference between X1 and X2 over the data carried on tracing paper with a bi-logarithmic mark. distance between the two electrical profiles. The These abacuses are of two types: the Abacus CH1 and orientation of the profiles being N 95°, the orientation the auxiliary Abacus of types K, H, Q and A. “θ” of the fracture is therefore equal to: 95° + 90° + α 4. Methods or else 95° - 90° + α [18].

Fig. 2 Identification of fracture directions on electrical profiles [18].

as a bi-logarithmic scale diagram. The lengths AB/2 4.2 Interpretation of Electrical Soundings Using are plotted on the abscissa in logarithmic scale and the Abacuses values of apparent resistivities on the ordinate (in The results of apparent resistivities are represented logarithmic scale). The shape of the curves obtained is

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 885 a function of the resistivity, the thickness of the layers  first step: determination of h1, ρ1, ρ2/ρ1 as and the configuration of the measuring device [13]. illustrated in Fig. 3. The beginning of the field Their interpretation made it possible to determine: the curve is made to coincide with one of the thicknesses of the layers and the true resistivities of theoretical curves of the CH1 abacus, while the latter. respecting the parallelism of the axes. The values Experience has shown that to obtain fast and (ρ1 and h1) read under the cross to the left of the efficient results, the interpretation must be done in the CH 1 abacus are the resistivity and the thickness field according to the lithology according to the of the first field, obtained after a double method of Hummel [19]. The abacuses are a series of translation from the abscissa 1 and the ordinate 1. typical curves calculated for various resistivity and Thus, we determine the ratio ρ2/ρ1 which thickness contrasts for different soil layers. The corresponds to the number of the curve which interpretation consists of finding the curve that best coincides with our survey curve. We therefore fits the measured curve to determine the parameters of deduce ρ2 the resistivity of the second field. the subsoil prospected. According to [18], the  second step: determination of h2 and ρ3 is approach is as follows: presented in Fig. 4.

Fig. 3 Use of the CH1 abacus [13].

Fig. 4 Interpretation by reduction of an S.E with the type H LCD [18].

886 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

Given the shape of the terrain curve, we choose the given pair (SE, F) the result of each element studied. corresponding auxiliary abacus (Example: type H). The The approach is presented in diagrams [11]. The cross on the left marked on the tracing paper is made to elements to be analyzed according to the status of the coincide with the origin of the LCD (or origin of the boreholes (corresponding positive and negative ratio ρ2/ρ1), and the curve corresponding to this ratio is drilling) are: then drawn. We leave on the CH1. One of the curves of  The directions of the fractures on which are the CH1 is made to coincide with the second part of the implanted the corresponding drillings. terrain curve by sliding the cross on the curve In this part, the identified fracture directions are corresponding to the ratio ρ2/ρ1 while respecting the represented as graphs according to the state of the parallelism of the axes. We thus obtain ρ3/ρe. boreholes (positive and negative drilling). Once the coincidence is perfect, we mark a second  The types of sounding on which the cross on the left on the curve corresponding to the corresponding drillings are located. ratio ρ2/ρ1. We start again on the same type H LCD After the interpretation of electrical soundings, by always making coincide the first cross on the left types and families of sounding curves are determined. with the origin of the LCD or by making coincide the The proportions of each type and families are then curves corresponding to the report ρ2/ρ1 of the LCD calculated. and the tracing paper. The second cross is projected on  The thicknesses of alteration the axis of the ratio h2/h1; the other axis The thicknesses of alteration determined from the corresponding to the ratio ρ2/ρ1. We can now abacuses and corresponding drillings are plotted on a determine the thickness h2 of the second field since graph and interpreted to reveal any possible that of the first field is already known. The position of relationships. the second cross on the left makes it possible to For alteration thickness values obtained by deduce the value of the equivalent resistivity ρe. The electrical soundings and corresponding mechanical latter corresponds to that of a single layer electrically surveys, thickness classes are formed and reported on equivalent to the first two layers. We deduce ρ3 which a graph. This graph will present the proportions of the is the resistivity of the third field. alteration thickness classes for each sounding and drill pair corresponding to the drilling condition (positive 4.3 Exploitation of Drilling Results and negative drilling). The drilling results consist of the following  Fractures of electrical soundings, water inflows variables: the state of the boreholes (positive or and flows negative), the flow of the boreholes, the thicknesses of The fracture positions of the electrical soundings alteration of the drillings, the water inflows and the and the water inlets of the corresponding drillings are total depth of the drillings. These variables were represented on a graph. On this graph, the positions of represented as graphs (diagrams) using the Excel the couples of the fractures of the electrical soundings software and then analyzed and interpreted. and the water arrivals of the drillings which coincide or not are noted. Fracture of sounding and water 4.4 Comparison of Geophysical Results and arrivals that have the same positions will justify the Corresponding Boreholes predictions of geophysical surveys. To better understand the concordance or 5. Results discrepancy between the drill results (F) and those of the electrical sounding (SE), it is confronted for a In the Gbêkê region, the electrical resistivity

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 887 method that has been implemented is trailing and (19%), N0-45, N90-135 and N180-225 (12%), N45-90, electrical sounding. Indeed, the localities surveyed N270-315 and N315-360 (8%). benefited from 91 electrical profiles and 90 electrical According to Fig. 6, we note that of the 23 holes soundings which favored the choice of 36 sites likely drilled, positive and negative drilling is present on the to implement drilling. But only 23 holes were drilled. eight (8) classes of fracture directions with the exception of the direction class N315-360 where all 5.1 Drilling Conditions and Fracture Directions drilling is positive. For the most commonly used The fracture directions according to their fracture class N135-180, 83% of the holes are positive orientation, are grouped into 8 orientation classes and 17% are negative. Then, for the secondary corresponding to 45 degree angular croissants. The steering class N225-270, we have 80% of positive angular orientation, relative to the number of drilling 20% of negative drilling. Finally, in Fig. 5, for directions, indicates that the dominant fractures are the classes of the following directions, N0-45, N45-90, oriented N135-180 (SSE-NNW) and N225-270 N90-135, N180-225, N270-315 and N315-360, we (SWW-NEE). These are therefore directions of high have respectively 67%, 50%, 67%, 67%, 50% and fracturing density. In fact, the direction classes that 100% of positive holes and 33%, 50%, 33%, 33%, emerge according to their percentage are organized 50% and 0% of negative holes. and shown in Fig. 5: N135-180 (23%), N225-270

25%

20%

15%

10% direction

centage of fracture centage of fracture Fig. 5 Histogramr of the directions5% of the fractions of the study area.

Fig. 6 Fracture directions according to the state of the boreholes (positive drilling or negative drilling).

888 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

Based on the North-South (N-S) and East-West resulted in 88% of positive drilling and 13% of (E-W) directions, the eight Fractional Leadership negative drilling. As for the sounding curves in the Classes are grouped into two major management form of bell and bottom of the boat (CPFB), we have groups. Classes N0-45, N135-180, N180-225 and 56% of positive drilling and 44% of negative drilling. N315-360 are of the N-S steering group and the For those in the form of bell in the bottom of the boat N45-90, N90-135, N225-270 and N270-315 classes (CDFB) and stairs on the rising branch (EBM), their are part of the E-W steering group. However, there are uses gave 100% of positive drilling. Finally, the use of 42% of the positive borehole fracture directions and single-branch “upwind” (SBM) sounding curves 12% of the negative boreholes that are in the yielded 33% of positive holes and 67% of negative North-South directional plan. While we find that there holes. However, of the total number of drill holes are 31% of the fracture directions of the positive implanted, the most used types of borehole curves are drillings and 15% of the negative drillings that are in the bell-shaped and then bottom-of-boat sounding the East-West directional plan according to Fig. 7. curves (CPFB) with 39.1% and those at the bottom of the boat (FB) with 34.8%. 5.2 Types of Sounding Curve and Drilling Status

The Fig. 8 shows us that the drill installation on the sounding curves at the bottom of the boat (FB)

Fig. 7 Distribution of the fracture directions in two groups according to the state of the drillings.

Fig. 8 Type of sounding according to the condition of the borehole.

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 889

By grouping the types of sounding curves, three of stepped curves on the rising branch (EBM family) large families were obtained which are: gave 100% of positive holes.  family of curves in bottom of boat (family FB) 5.3 Total Thickness of Alteration composed of curves in bell then in bottom of boat and those in bell in the bottom of boat; The total thickness of alteration is very variable in  single-branched curve family (SBM family); the studied region. It ranges from 8.5 to 31.0 m in  family of stepped curves on the rising branch geophysical surveys and from 5.9 to 29.9 m in drilling. (EBM family). Overall, Fig. 10 presented, eleven (11) drillings on the We found that the family of curves at the bottom of 23 drillings yielded results similar to those of the the boat (FB family) resulted in 72% of positive electrical soundings. This proportion equals parity on drilling versus 28% of negative drilling. Then for the predictions. family of single-branched curves (SBM family), their The Table 1 shows the localities where the results use yielded 33% of positive drilling and 67% of of electrical soundings are relatively similar to those negative drilling. Finally, in Fig. 9, it show, the family found in the corresponding drillings.

Fig. 9 Families of types of soundings according to the state of the drilling.

Fig. 10 Comparison of alteration thicknesses for each corresponding borehole and drilling pair.

890 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

Table 1 Similarity between EA sounding and EA drilling in some localities in the study area. Localities EA of the sounding (m) EA drilling (m) AHOKOKRO (F1) 10 9.36 ALLOBO (F1) 8.5 8.89 ASSENGOUKPLI (F1) 10 7.79 KOMABO (F1) 13.8 12.51 LOGBAKRO (F1) 24 21.76 LONGBO N'GATTAKRO (F1) 19.6 17.66 N'GBEDJO-ADJOBLESSOU (F1) 12.2 12.94 N'GBEDJO-ADJOBLESSOU (F2) 10 11.86 TIENDIBO (F1) 23.2 24.66 YOBOUEKRO-LANGAMA (F1) 24.65 23.84 ZEDE N'DREBO (F1) 20 19.96

For alteration thickness values obtained, three 67.08 and 70.73 m. The minimum depths of these AEs classes were established: range between 18.00 and 58.00 m while the maximum  the class of thicknesses 0 < EA < 15 m; depths are between 78.00 and 111.50 m. The analysis  the class of thicknesses < EA < 25 m; in Table 2 above indicates that the first and second  the class of thicknesses greater than 25 m. AEs represent a proportion of 52.44%. The third, For the class of thicknesses of alterities [0-15], fourth and fifth classes make up 47.56% of AE figure 11 shows that the EA of the sounding have a encountered during drilling. As a result, there are at proportion of 50% for the positive drilling and 57% least two (2) water inflows and a maximum of five (5) for the negative boreholes while the EA of the water inflows in the drillings. It is therefore these sounding are 25% for positive drilling and 43% for inflows of water (AE) that contribute to flows in the negative drilling. study area. However, by comparing the water inflow For the class of the thicknesses of alterities [15-25], rate and the depth drilled, the Fig. 12 shows that the EA of the soundings are 44% for positive drilling and most productive water inflows are located in the first 43% for negative drilling. Also, those of EA of the 55 meters of depth. drillings are 63% for positive drilling and 57% for The Fig. 13 shows the presence of fractures on the negative drilling. soundings curves in the localities of Allobo (F1) at 20 Finally, the class of thicknesses of alterites greater m, Adjebonou (F1 and F2) at 32 m each and than 25 m is only found in positive boreholes. We Alloukro-Yakro (F1) at 50 m depth. On the other hand, therefore have 6% for EA of the soundings and 13% they are absent in the boreholes, that is to say that for EA of the drillings. there are no inflows of water in the boreholes at these predicted depths. We also have water inflows in the 5.4 Identified Fractures and Water Inflow boreholes that correspond to the fractures identified on The Table 2 shows the distribution of water arrivals the sounding curves. These are sometimes wet or dry according to the average depth drilled. Water arrivals fractures. This is the case in Allobo (F2), have been identified at various depths. Yebouekro/Lang (F1) and N'gbedjo-Adjoblessou (F1). In the study area, there are mainly five water At Allobo, drilling F2 reveals two dry fractures and a inflows (AE) that correspond to hydraulically active wet fracture. They are respectively 29 and 90 m deep fractures. The water inlets AE1, AE2, AE3, AE4 and and 57 m. In Yebouekro/Lang and N'gbedjo- AE5 have average depths of 38.30; 49.60; 59.16; Adjoblessou, the boreholes each highlight three wet

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 891 fractures. They are at 62.5; 67 and 88 m deep for drillings each show a dry fracture. They are Yebouekro/Lang (F1) and 52.56 and 86 m deep for respectively at 20, 32, 32 and 50 m deep. N'gbedjo-Adjoblessou (F1). However, in Allobo (F1), In this same Fig. 13, we also see that water inflows Adjebonou (F1 and F2) and Alloukro-Yakro (F1), the that contribute more to the flows of the region are generally located between 30 m and 75 m deep.

Fig. 11 Comparison of the alteration thicknesses for each pair of borehole and corresponding drill according to the state of drilling.

Table 2 Proportion of water inflow in boreholes.

Type of AE Number Percentage of AE Average depth (m) Minimum depth (m) Max depth (m) AE1 22 26.83% 38.30 18.00 78.00 AE2 21 25.61% 49.60 23.24 96.00 AE3 16 19.51% 59.16 36.00 107.00 AE4 13 15.85% 67.08 53.00 111.50 AE5 10 12.20% 70.73 58.00 90.00

Fig. 12 Relationship between water inflow and borehole depth in the study area.

892 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

Fig. 13 Comparison of fractures for each drill and drill corresponding to drilling condition and flow.

steep dip, as the anomaly can be determined at the 6. Discussion apex of the fracture. Geophysical methods of electrical resistivity are Based on the soundings types, we identified five (5) techniques that constitute indirect recognition devices types that are divided into three (3) families, including essential to the mapping of underground geological the FB family, the SBM family, and the EBM family. formations. Thus, the choice of drilling site was made The FB family is the most used during geophysical thanks to the geophysical parameters. With regard to surveys. On the curves of this last family, the presence these parameters, the parallel electrical profiles made of fractured horizon is not visible. Therefore, we have it possible to highlight fractures of various directions, no information on the position of the fractures on the the choice of which for drilling execution depends on latter. This corroborates the works of Kouakou et al. their orientation. Eight (8) orientation classes (2016), Savadogo (1984), Biemi (1992), and Savané corresponding to 45 degree angular crescents are (1997) [11, 21-23]. These have shown that the most divided into two (2) major directions including regrettable defect of the “bottom-of-the-boat” curves North-South (N-S) and East-West (E-W). We have lies in the fact that they give no indication of the respectively the presence of 12% and 15% of negative fissured or crushed area at the roof of the underlying drilling on these directions. We can say that these basement. failures can be due to the capture of the superficial As for the total thickness of alteration, 11 drillings parts of the N-S and E-W directions, which often have out of the 23 realized gave results similar to those of steep slope with respect to the vertical. As a result, the the electrical soundings, which equals parity on water flowing through these fractures will be deeper in geophysical predictions. This difference in alteration the direction of the slope. The studies of Kouakou et thicknesses can be explained by the fact that certain al. (2016) [11] and Sombo (2012) [20] have shown anomalies can be due to the presence of a high density that failures can be related to a fractured area with a of clay layer. These clay soils can be filled with water

Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast 893 and identified on the sounding curve with low anisotropy as a function of the distance and the pace resistivity levels alternating with resistant zones. This of this curve. It makes it possible to give precise can influence the interpretation of electrical soundings precisions as to the presence or not of a fractured and lead to drilling failures. This hypothesis has been zone. demonstrated by Kouakou et al. (2016) [11] and Same 7. Conclusion (1999) [12] because they think that these failures can be due to several facts related to the subsoil of which At the end of this study, the comparison of the the clay horizons evoked. results of geophysics and those of drilling gave Fractures of the sounding are at an average depth of appreciable information on the drilling failures 46.65 m and the first most productive water in the encountered in the Gbêkê region. This information is drillings is located within 55 m. The prediction of the important and useful for orienting hydrogeological depths of the fractures identified on the electrical surveys and future drilling campaigns. This sounding curves are different from those of the information relates to fracture direction classes, the drillings with a difference of 8.35 m. This depth most exploited in the region are: N135-180 difference also influences the interpretations of (SES-NWN) and N225-270 (SWW-NEE). They sounding curves for the prediction of water inflow, resulted respectively in 83% and 80% of positive which implies the presence of fracture in the subsoil. drilling and in 17% and 20% of negative boreholes. The disadvantage is that we often observe fractures on However, we also note that drilling failures can be the sounding curves that are absent during drilling. present on all fracture directions with a rate of That is to say, the arrivals of water seen on the curves between 17% and 50%. The electrical soundings of electrical soundings are absent on the subsoil. carried out allowed us to obtain five (5) types of Which leads to negative drilling. According to soundings: the curves in bottom of boat (FB), curves Kouakou et al. (2016) [11], the materialization of in check mark then in bottom of boat (CPFB), curves fracture on the sounding curves are due to significant in bell in the bottom of the boat (CDFB), disturbances in the crystalline basement. They single-branch-up curves (SBM) and stepped-line correspond rather to an altered basement roof weakly curves (EBM). The most common type of sounding is cracked and strongly clogged by the overlying the family of curves at the bottom of the boat (family damaged formations. In this same vision, Sombo FB) a success rate of 72% against 28% of negative (2012) [20] and Kouakou (2012) [24] think that the drilling. In addition, depending on the state of the fracture even if it is highlighted at the level of the drilling (positive and negative), the alteration electrical sounding can correspond to a sterile fracture thicknesses of the soundings are between 0 and 15 m or a mineralized fracture with very low water content. deep, while drilling is between 0 and 25 m deep. At This is confirmed in the localities of Allobo (F2), least two (2) water inflows and a maximum of five (5) Yebouekro/Lang (F1) and N'gbedjo-Adjoblessou (F1). water inflows are observed in the boreholes. It is These localities highlight wet and dry fractures. For therefore these inflows of water (AE) that contribute this it would be necessary, according to Same (1999) to flows in the study area. The depths of these water [12] quoted by Kouakou et al. (2016) [11], perform arrivals are between 18 and 111.50 m. Finally, the some additional electrical soundings or resistivity interpretations of the sounding curves showed the profiles with a square device especially in a crystalline presence of fracture likely to contain water, while the medium. Indeed, the square device makes it possible arrivals of water in the boreholes which coincide with to establish the curve of variation of the coefficient of these fractures, showed that they are either wet, or dry.

894 Comparative Study of Geophysical Prospection and Drilling Results in the Region of Gbêkê, Ivory Coast

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