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 KA Kouassi et. al., / International Journal of Engineering & Science Research 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. Copyright © 2017 Published by IJESR. All rights reserved 122 KA Kouassi et. al., / International Journal of Engineering & Science Research 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]). Copyright © 2017 Published by IJESR. All rights reserved 123 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.

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