Efficiency of Inverse Slope Method in The

ISSN 2277-2685 IJESR/Oct. 2017/ Vol-7/Issue-10/121-130 KA Kouassi et. al., / International Journal of Engineering & Science Research

EFFICIENCY OF INVERSE SLOPE METHOD IN THE INTERPRETATION OF ELECTRICAL RESISTIVITY SOUNDINGS DATA OF SCHLUMBERGER TYPE 1 *1 2 1 1 FW Kouassi , KA Kouassi , A Coulibaly , B. Kamagate , I Savane 1Nangui Abrogoua University, UFR Science and Environmental Management, Laboratory Geosciences and Environment, 02 BP 801, Abidjan 02, Ivory Coast. 2Félix Houphouët-Boigny University, Department of Science and Technology of Water and Environmental Engineering, UFR of Earth Sciences and Mining Resources, 22 BP 582 Abidjan 22 (Ivory Coast). ABSTRACT In rural hydraulic, the requested flow is relatively weak, thus the setting up of drillings is often done summarily, without the use of numerical methods for the interpretation of geophysical data. The vertical electrical sounding (SEV) is the most widely spread geophysical exploration technique for subsurface. Data from this technique can be interpreted by different methods to describe the profile of the subsoil. In other words, their interpretation leads to the determination of the number, the thickness and the real resistivity of the layers which compose the subsoil. In Nassian, five vertical electrical soundings have been implemented in three villages. The data were then interpreted with the inverse slope method. This has highlighted the characteristics of the different geological layers which compose the profile of the subsoil in each village. These results were then compared with electrical soundings graphs and drilling data to assess the reliability of that method of interpretation. Thus, we can deduce that the reverse slope method gives good results and can therefore be used in hydrogeological prospecting in base zone.

Keywords: Slopes, inverse, soundings, resistivity, schlumberger. 1. INTRODUCTION The continuous electrical sounding in two and three dimensions, which produces a large number of measurements using automated acquisition systems, is revolutionary in geophysics. Indeed, their interpretation is carried out very quickly and reliably. Nevertheless, the vertical electric sounding (SEV) is still the most widespread technique because it is inexpensive and very useful especially when the place of study is very large and deep [5, 7, 8]. The Schlumberger device used there also offers a good penetration of the electric current, thus a good depth of investigation and a better discrimination of the geological layers. This technique is regularly used to solve various hydrogeological problems such as the determination of depths, thicknesses and boundaries of aquifers [3, 7, 9, 12]. For the interpretation, several techniques are available, each one based on a distinct approach and methodology. However, very little work is available on the limits, relevance, accuracy and reliability of these techniques in relation to the subsoil conditions. In this work, our objective is to study the contribution of one of these techniques i.e. the inverse slope method, in the prospection of the subsoil of the region of Nassian. 2. GENERALITIES ON THE STUDY AREA 2.1 Presentation of the study area The Zanzan District is the result of the implementation of Decree No. 2011-263 of 28 September 2011 on the organization of the territory into districts and regions. This district is divided into two regions: the Bounkani and the Gontougo. The Bounkani region has four departments: Bouna, Doropo, Tehini and Nassian (Figure 1). Two sub-prefectures of the latter department are the subject of this study. These are Nasssian and Sominassé. This department is limited to the north by the Comoé national park, to the South and East by the region of Gontougo (the Department of Bondoukou and Sandégué) and to the West by the Department of Dabakala in the district of Bandama Valley. *Corresponding Author www.ijesr.org 121

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Fig. 1: Location of the study area 2.2 Vegetation, relief and climate The vegetation of the region consists of two types of savannah: the Sudano-Guinean savannah and the wooded savannah. We can also add light forest to these two types of savannas. Because of abusive deforestation, this vegetation gradually gives way to shrubs and grasses. This region is fairly monotonous. It is drained by several rivers of which the most important are the Comoé and the Black Volta. With a Sudanian climate, the Bounkani region has two seasons: one rainy (4 to 5 months) and the other dry (7 to 8 months). 2.3 Geological and hydrogeological contexts The study area, located in the North-East of Côte d'Ivoire is part of the Baoulé-Mossi domain and separated from the Archean domain by the N-S Sassandra fault. A single geological domain stands out namely the lower Proterozoic (Figure 2).

Fig. 2: Geological map of the study area [17] This domain consists of a vast orogenic plutonic complex made of heterogeneous biotite granitoids, two-mica subalcalin granitoids and akeritic granites. Also, the general orientations in the pleated areas are N-S and E-W.

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From the hydrogeological point of view, two types of aquifers stand out: alterite aquifers and groundwater aquifers. Alterites are surface formations resulting from the processes of physicochemical alteration and erosion of the basement. Located above the crystalline and crystallophyllian basement, these alterite reservoirs are composed of clay sands and granite arenas. The alterites from the granular formations consist of clays, sands, sandy clays and / or lateritic clays. They are not very thick and vary from 0 to 30 m ([11]). Basement aquifers develop in the crushed and / or cracked areas of the base. They are immune to seasonal fluctuations and most types of pollution. These reservoirs are exploited by drilling to meet the daily needs of the populations. The possibilities of formation of these reservoirs are linked to the density of the fracture of these reservoirs [4, 15]. 3. MATERIALS AND METHODS 3.1 Data and Hardware The resistivity meter used is a Syscal R1 of Iris instrument, which directly measures the apparent resistivity after entering spacings between electrodes AB and MN. During the surveys, the inter-electrode distances (AB/2) used vary from 2 to 400 m. Thus, five vertical electric soundings (SEV) were carried out in three villages: 02 SEV in Landé, 01 SEV in Sirikibango and 02 SEV in Sominassé. The electrical probing curves were constructed with Excel on a bilogarithmic scale. 3.2 Methods for acquiring resistivity data The use of geophysical methods to detect and characterize aquifers is a widespread practice. In hydrogeology, electrical resistivity method is widely used for identification of aquifer and determination of their thickness. This electrical method consists of the emission of a continuous electric current in the subsoil using electrodes (usually two, A and B) implanted on the surface. This current creates a difference of potential which is then measured by another pair of electrodes (reception electrodes M and N). The value of the measured electrical resistivity, known as apparent electrical resistivity, comes from the contribution of several layers of the ground through which the current emitted from the surface goes. In general, quadrupole devices are used to acquire these data. Vertical electric sounding (SEV) principle employs a symmetrical configuration in which the receiving electrodes (M and N) are closely spaced and implanted in the center of the device while the emission electrodes (A and B) are moved progressively outwardly: it’s Schlumberger device. In this, more the electrode spacing A and B is large, more the depth of investigation is significant ([16]). 3.3 Methods of Interpretation Inverse slope method as suggested by [14] is used to identify underground formations by studying the variations in their electrical characteristics. It has been developed and implemented in India in different geological contexts and has yielded good results, correlating the drilling data quite well. Moreover, due to its easy implantation, its low cost and its ability to discriminate different geological formations (true resistivities and depths), it is of great importance in the field of groundwater exploration. This method was originally proposed for the interpretation of Wenner survey data. According to this approach, the inverse of the resistance graph (1/R) is first constructed according to the inter-electrode distance "a". Then, from the points defining this graph, one identifies segments of straight lines. Each segment represents a layer and the intersections of the segments correspond to the depths to layers. However, this method can also be adapted to the interpretation of Schlumberger survey data with certain modifications. Indeed, from the data of apparent resistivity ρa and measured distances AB, a graph is constructed. It comprises on abscissa the half-distance AB/2 and on ordinate the ratio (AB/2)/ρa; the two axes being each on arithmetic scale. While the inverse slope of segments directly gives the true resistivity of layers, the intersections of these segments must be multiplied by 2/3 to obtain the depths of interfaces. The number of layers revealed on the prospected field corresponds to the number of segments identified on the graph (Figure 3) ([13]).

KA Kouassi et. al., / International Journal of Engineering & Science Research

Fig. 3: Graph AB / 2) / ρa = f (AB / 2) [13] These results were then compared with those of the electric sounding curves and the corresponding drilling data. 4. RESULTS 4.1 Interpretation of electrical resistivity data a- Case of the village of Sominasse Chart 4 below presents two graphs related to the electrical soundings carried out in the village of Sominassé. These graphs highlight several geological layers that are materialized by distinct segments.

Sondage 1 Sondage 2

A (0;0) B (10;0,19) C (50;0,23) C’ (55;0,22) A (0;0) B (12;0,13) C (36;0,21) D (100;0,23) D (70;0,21)

Fig. 4: Interpretive graphs of the surveys (VES1 and VES2) at Sominassé On the graph 1, the first layer, denoted by segment [AB] has a true resistivity (ρ1) of 52.63 Ω.m and a thickness (h1) of 6.6 m. As for the second layer [BC], it displays these characteristics : ρ2 = 1000 Ω.m and h2 = 35 m. Finally, the last layer [C'D] has a true resistivity (ρ3) of 5000 Ω.m and an infinite thickness. As for the graph related to the sounding 2, it made it possible to identify the characteristics of the first two layers which are as follows: Layer 1 [AB]: ρ1 = 100 Ω.m and h=08m. Layer 2 [BC]: ρ2 = 303.03 Ω.m and h = 24 m. The electrical sounding curve (ρa = f (AB/2)) established with these resistivity data is of type "H" or "bell- shaped in the bottom of the boat" (figure 5).

KA Kouassi et. al., / International Journal of Engineering & Science Research

Fig. 5: Electrical sounding curve (VES2) of type "H" at Sominasse This type of curve reveals three layers distinguished by their electrical resistivity. The resistivity of the second layer (ρ2) is inferior to those of the first (ρ1) and third (ρ3) layers (ρ1>ρ2<ρ1). In addition, as a result of the geophysical survey, the drilling performed in this village (SEV 2) indicates an alteration thickness of 6.4 m and a base depth of 53.49 m. This basement is of granitic nature (figure 6).

Fig. 6: Sominasse drilling log b- Case of Sirikibango In the village of Sirikibango, only one electrical survey was carried out. The interpretation of the resulting graph (Chart 7) describes a two-layered terrain. The first layer represented by segment [AB] has a true resistivity of 400 Ω.m and a thickness of 06.6 m. As for the second layer, it has a true resistivity of 1250 Ω.m. This latter is defined by the segment [B'C] or [C'D], both of the same slope.

A(0;0) B (12;0,13) B’ (20 ; 0,02) C (36;0,21) C’ (55 ; 0,04) D (70;0,21) Fig. 7: Interpretive graph for survey 1 at Sirikibango

KA Kouassi et. al., / International Journal of Engineering & Science Research

The electrical sounding curve (ρa = f (AB / 2)) thus established is of type “A” or a curve "with only one rising branch" (figure 8). This type of curve reveals two layers whose characteristic is the following: the resistivity ρ1 of the first layer is inferior to that of the second layer ρ2 (ρ1<ρ2).

Fig. 8: Electrical sounding curve (VES1) at Sirikibango Finally, the drill hole we made parallel to the electrical probing revealed an alteration thickness of 3.7 m and a granitic base depth of 46.2 m (figure 9).

Fig. 9: Sirikibango drilling log c- Case of the village of Lande Finally, at Landé, two graphs (Chart 10) were designed for the interpretation of the data from the two soundings through the inverse slope method. These graphs reveal the existence of three layers. Their interpretation reveled the characteristics which are recorded in table I below.

LANDE 1 LANDE 2

A(0 ; 0) B(20 ; 0,13) C(60 ; 0,21) A (0 ; 0) B(12 ; 0,09) C(44 ; 0,21) D(90 ; 0,23) D(90 ; 0,24)

Fig. 10: Interpretive graphs of the surveys (VES1 and VES2) at Lande

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Table I: Characteristics (number, thickness and resistivity) of geological layers Landé 1 Landé 2 Layer True resistivity (훀.m) Thickness(m) True resistivity (훀.m) Thickness (m) 1 153,84 13,33 133,33 08 2 5000 40 266,66 29,33 3 151,51 - 1538,46 - As a summary, we note that at Landé, three layers have been encountered whose characteristics are as follows: Layer 1: ρ1 = [133.33, 153.84 Ω.m] and h1 = [08 and 13.33 m], Layer 2: ρ2 = [266.66, 5000 Ω.m] and h1 = [29.33, 40 m], Layer 3: ρ3 = [151.51, 1538.46 Ω.m]. Also, the two electric probing curves (ρa = f (AB / 2)) established with the same resistivity data are of type "H" or "bell-shaped in the bottom of the boat" (figure 11).

Figure 11: Survey curves (VES1 and VES2) at Lande This type of curve reveals the existence three layers of which the second layer resistivity (ρ2) is inferior to those of the first (ρ1) and third (ρ3) layers (ρ1> ρ2 <ρ1). Finally, the drilling carried out in this village revealed an alteration thickness of 23.98 m and a granitic base depth of 61.29 m. It was set up at the level of sounding 2 (figure 12).

Fig. 12: Lande drill log 4.2 Correlation between the geoelectric section and the lithology In order to assess the reliability of the inverse slope method, it is necessary to compare and establish a possible correlation between the number, true resistivities and the calculated thicknesses of the identified layers. Thus, the results we obtained from the inverse slope method show a certain similarity with those of the sounding

KA Kouassi et. al., / International Journal of Engineering & Science Research

curves on the one hand and the drilling data on the other hand. Indeed, the inverse slope method and the electric sounding curves we constructed have indicated the same number of layers. The subsoil profile at Sominassé and Landé consists of three layers whereas in Sirikibango it consists of two layers (Figures 5, 8 and 11). These layers correspond, from top to bottom, to: first to the lateritic clays, then to clay-sandy alterites and finally to the granite basement. Concerning drill holes lithology, the geoelectric section of the electric surveys interpreted through the inverse slope method provides more accuracy. Indeed, the latter makes it possible to clearly discriminate the first geological layer. As a whole, the calculated alteration thickness and the measured one are very close, excepted in the village of Sominassé where we noticed a 5 m difference between the measured thickness and the calculated one (Table II). Table II: Calculated and measured alteration thicknesses of the first layer Thickness (m) Villages calculated measured Sirikibango 6,66 5,09 Landé 8 - 13,33 7,88 - 13 Sominassé 1 6,66 11,87 Otherwise, the calculated and the measured basement depths may vary. Indeed, a difference of approximately 10 m was observed between the Calculated and the measured depth. In other words, the basement depths were underestimated by the interpretation method of interpretation, excepted in Sirikibango where it was over- estimated (table III). Table III: Relation between calculated and measured base depths Depth (m) Villages Calculated Measured Sominassé 1 43,32 53,49 Landé 53,33 61,29 Sirikibango 56,65 46,2

5. DISCUSSION The determination of the exact number of layers is one of the interests of the inverse slope method. Indeed, the geological layers are discriminated from clear registered resistivity contrasts observed through successive measurements of electrical resistivities. This resistivity contrast is also illustrated by the slope difference. Thus, on all the prospected sites, the number of segments or slopes corresponds to the number of layers of the subsoil profile. This is in line with the results of the work of Sandjin (2010) and Aditya (2012) on different sites in India. The gaps of approximately 5 m and 10 m respectively estimated at the level of alteration thickness and basement depth are largely due to the operators. According to some authors ( Savadogo, 1984; Dieng et al, 2004) the gaps between calculated and measured base depths are due to the difference in the base conception from the point of view of the driller and that of the geophysicist. In fact, from the geophysical point of view, the cracked fringe of the sound basement is an integral part of the conductive arena level and that the basement is exclusively constituted by the "compact" zone underlying resistance. On the contrary, the driller rather observes the repressed cuttings of the drill holes and the advancing speed of the "bottom hammer" to appreciate the different thicknesses / depths of the geological formations he crosses. Nonetheless, these gaps are not sufficiently important to question the results of the inverse slope method. Conversely, the relative values of resistivity calculated using the inverse slope method do not always match with those of the electric sounding curves. In fact, although the nature of the first and third layers is clearly displayed by these two methods, this is not the case for the second layer. The latter, of clay-sand type according to the section of drill holes, therefore geophysically conductive, is rather characterized by relatively high electrical

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resistivities. Now, the relatively high resistivity values are indicative of very thick layers corresponding to unaltered or not fractured rocks. In the contrary, relatively low resistivity values indicates a thick but altered zone (Karuppannan 2015).This strong resistivity of the second layer (for instance at Sominassé) would indicate the presence of lithological units between the alterites and the healthy base; presumably the crushed base. This means that, in general, the second layer has seldom been identified by the inverse slope method. This is far from being a handicap since in geophysical prospection in basement areas, fractures, among other geological structures, are the main quests. There are likely to contain and drain groundwater. The discrepancies between graphs and sounding curves may be due to measurement step AB / 2. Indeed, in the process of data acquisition with the Schlumberger device, the measuring step is variable and rising contrarily to the Wenner device. This variation of the measurement step influences the slopes of the graph insofar as the ordinate corresponds to the quotient of the measurement step on the apparent resistivity. In addition, in the inverse slope method as well as in all empirical methods, the depth of investigation is either equal or in part to the inter-electrode current distance. Although this consideration is criticized, it is still largely taken into account in geophysical prospections. This is due to the fact that this method is not only simple to implement but, better, it gives estimation of layer depths that are reasonably reliable in the majority of cases. These are the reasons why it is used in the present study for the interpretation of electrical sounding data. 6. CONCLUSION Vertical electric sounding (VES) data were interpreted in comparison with the theoretical curve models and also with the inverse slope method. The interpreted results were then correlated with drill logs. This geophysical interpretation highlighted the number of geoelectric formations encountered globally in the study area. It also discriminated against the first layer of the others and gave an acceptable estimate of the base depth. However, the profile of the subsoil described by this method does not always correspond to that of boreholes log and that of the electric sounding curves. Indeed, if the number of layers is clearly identified, their true resistivity and their thickness may differ from one layer to another. Nevertheless, this method makes it possible to locate the base and therefore the fractures. These are the ones that are likely to store groundwater. Thus, thanks to its simplicity, its easy and very fast implementation, this method is relatively efficient in the interpretation of electrical resistivity data. Therefore, it can be used for the installation of village hydraulic drilling when the discounted flows are relatively modest. REFERENCES [1] Asfahani J. Geoelectrical investigation for characterizing the hydrogeological conditions in semi-arid region in Khanassser valley. Syria. J. Arid Environ 2006; 68: 32-51. [2] Aditya KB. An Assessment of Electrical Resistivity Soundings Data by Different Interpretation Techniques. International Journal of Biological, Ecological and Environmental Sciences (IJBEES) 2012; 1(3): 2277- 4394. [3] Bello AA, Makinde V. Delineation of the aquifer in the South-Western part of the Nupe Basin, Kwara State. Nigeria. J. Am. Sci. 2007 ; 3: 36-44. [4] Biemi J. Contribution à l’étude géologique, hydrogéologique, et par télédétection des bassins versants subsaheliens du socle précambrien d’Afrique de l’Ouest : hydrostructural, hydrodynamique et isotopie des aquifères discontinus de sillons et aires granitiques de la Marahoué (Côte d’Ivoire). Thèse de Doctorat d’Etat ès- Sciences Naturelles, Université de Cocody, Abidjan (Côte d’Ivoire), 1992 ; 479. [5] Chieh-Hou Y, Wei-Feng L. Using direct current resistivity sounding and geostatistics to aid in hydrogeological studies in the Choshuichi Alluvial Fan, Taiwan: Ground Water, 2005; 40; 165-173. [6] Dieng B, Kouassi AH, Bakyono BA. Optimisation de l'implantation géophysique des forages en zone de socle au Nord du Burkina Faso. Sud sciences et technologies, 2004; 12: 21-30. [7] Hamzah U, Malin EP, Samsudin AR. Groundwater investigation in Kuala Selangor using vertical electrical sounding (VES) surveys. Environmental Geology 2007; 51(13): 1349–1359.

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