Strategy for optimizing geophysical deployment: Case study of an HVA drilling project in the Kong bedrock environment

 Corresponding author: Rock Armand Michel BOUADOU Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Strategy for optimizing geophysical deployment: Case study of an HVA drilling project in the Kong bedrock environment Bouadou 1, *, Rock Armand Michel, Coulibaly 2, Adama, Kouassi 1, Kouamé Auguste and Gnagne Théophile 1 1 UFR of Sciences and Management of the Environment, University of Nangui Abrogoua, Abidjan, Côte d'Ivoire 2 UFR of Earth Sciences and Mineral Resources, University of Félix Houphouët-Boigny Abidjan, Côte d'Ivoire World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 Publication history: Received on 21 August 2025; revised on 01 October 2025; accepted on 04 October 2025 Article DOI: https://doi.org/10.30574/wjarr.2025.28.1.3425 Abstract This study aims to develop a strategy for optimizing the selection of geophysical sites for HVA drilling in the bedrock area of Kong. To achieve this objective, the methodology first consisted of extracting the fracture network from satellite image processing. Then, surveys and electrical drags were used to characterize the geometry of the fractured aquifers (the depths of the aquifer zones and the fracturing indices of the conductive anomalies). The conductive anomalies K, H, U, and V were identified by electrical drags and intersected the fractures determined by processing Landsat 7 ETM+ satellite images. In addition, the fracturing index ranges from 1.96 to 2.57. The corrected cumulative lengths of the fractures range from 362.92 m to 835.46 m. Interpretation of the borehole curves showed 4 to 5 aquifer zones in the subsoil. Finally, combining the fracturing index, the depth of the aquifer zones, and the length of the fractures resulted in the following drilling priority order: K (F1), U (F2), V (F3), and H (F4) (K > U > V > H). Keywords: Geophysical Survey; Electrical Resistivity; Fracture Length; Bedrock Aquifer; Kong; Côte d'Ivoire 1. Introduction To manage and prevent crises related to access to water, the theme for World Water Day (WWD) 2022 is “Groundwater: Making the Invisible Visible.” This is because almost all of the planet's freshwater reserves are underground, serving to supply drinking water, sanitation systems, agriculture, industry, and ecosystems. Therefore, to make groundwater (the invisible) visible, indirect methods of exploring the subsoil are used to identify this natural resource before drilling. However, the question is how much is needed to supply populations, industries, and other economic and social activities in the long term. In this context, geophysics [1], remote sensing [2] and geomorphology [3] are methods used to indirectly locate groundwater aquifers. However, the choice of location remains crucial, as the chosen site must be able to produce the amount of water needed to meet expected long-term needs. With this in mind, this study seeks to develop a strategy aimed at optimizing the choice of geophysical sites for HVA-type drilling in order to meet the water needs required to satisfy certain human requirements. 2. General information about the study area 2.1. Presentation of the study environment The project site is located approximately 4 km southwest of the city of Kong. Located at longitudes 4°38'01” and 4°37'38.6” West and latitudes 9°07'04.4” and 9°07'26.9” North, the study area is part of the Tchologo region, whose regional capital is the town of Ferkessedougou. The vegetation is of the tree and shrub savanna type and is severely degraded. The area is drained by the Comoé River. Annual rainfall over the past three decades has varied between 800 and 1,500 mm, with an average of 1,300 mm [4]. World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 187 Figure 1 Geographic location of the study area 2.2. Hydrogeological and geological context Geologically, the site under study is located in an area of crystalline and crystallophyllian rocks. The geological formations in the region rarely outcrop because they are covered by a thick layer of weathered rock. However, the geological map indicates that the project site is located on heterogeneous biotite granitoid formations [3] (Figure 2). From a hydrogeological point of view, the aquifer system in the study area therefore consists of a (superficial) weathered rock aquifer, beneath which lies the fissure and fracture aquifer. Figure 2 Geological map of the study area World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 188 3. Material and methods 3.1. Geophysical prospecting equipment The measuring equipment used is a Syscal R2 resistivity meter from Iris Instruments (Figure 3). It can be used to perform both electrical dragging and electrical sounding. However, its use for geophysical prospecting requires a number of useful devices and accessories, such as: • a 12-volt external battery to supply electrical power; • an amplifier to increase the voltage of the electrical signal from the 12-volt battery to 200 volts, or even 400 to 800 volts; • stainless steel electrodes are implanted in the ground to inject the amplified electrical current; • four (4) reels of electrical cable, including two (2) 200-meter reels and two (2) 400-meter reels. These connect the resistivity meter to the electrodes; • a compass for orienting the electrical drags; • A GPS device is used to record the coordinates (longitude, latitude, and altitude) of each cable and survey carried out. • Clamps are used to connect the electrical cables to the stainless steel electrodes driven into the ground. • Two 100 m measuring tapes are used to position the electrodes at the selected measurement interval. • Hammers are used to drive the electrodes into the ground. Figure 3 Electrical resistivity measurement equipment: a) a Syscal R2 and an electrical current amplifier, and b) four electrical cable coils, a hammer, and electrodes 3.2. Methods 3.2.1. Lineament extraction study The lineament extraction method is presented below. Landsat 7 image contour enhancement to improve image contrast [5]. Band ratios were used to improve the perception of the Landsat 7 image. The band ratios used are: ETM+4/ETM+5; ETM+4/ETM+6; ETM+7/ETM+6; ETM+5/ETM+4; (ETM+7-ETM+4) / (ETM+7+ETM+4); (ETM+6–ETM+4) / (ETM+6 + ETM+4). Finally, the Sobel filter, the Prewitt gradient filter, and the gradient filter developed by [6] were used to highlight the boundaries between two landscapes or specific features of the image such as lineaments, roads, etc. (Table 1). World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 189 Table 1 7 × 7 matrices of Sobel and gradient filters N-S direction Sobel filter E-W direction Sobel filter 1 2 3 4 3 2 1 1 2 3 0 -3 -2 -1 2 3 4 5 4 3 2 2 3 4 0 -4 -3 -2 3 4 5 6 5 4 3 3 4 5 0 -5 -4 -3 0 0 0 0 0 0 0 4 5 6 0 -6 -5 -4 -3 -4 -5 -6 -5 -4 -3 3 4 5 0 -5 -4 -3 -2 -3 -4 -5 -4 -3 -2 2 3 4 0 -4 -3 -2 -1 -2 -3 -4 -3 -2 -1 1 2 3 0 -3 -2 -1 NE-SO direction Sobel filter NO-SE direction Sobel filter 0 1 2 2 3 3 4 4 3 3 2 2 1 0 -1 0 3 4 4 5 3 3 5 4 4 3 0 -1 -2 -3 0 5 6 4 3 3 4 6 5 0 -3 -2 -2 -4 -5 0 5 4 2 2 4 5 0 -5 -4 -2 -3 -4 -6 -5 0 3 2 2 3 0 -5 -6 -4 -3 -3 -5 -4 -4 -3 0 1 1 0 -3 -4 -4 -5 -3 -4 -3 -3 -2 -2 -1 0 0 -1 -2 -2 -3 -3 -4 Prewitt gradient filter [6] gradient filter 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -1 -2 -3 -7 1 1 1 0 0 0 0 0 0 -1 -1 -2 -3 -3 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 -2 -2 -2 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 -1 -1 -1 -1 -1 -1 -1 Using images generated from the processing and criteria defining image discontinuities, a map of visible linear structures was produced. Image discontinuities corresponding to presumed structural lineaments were identified manually following visual analysis on screen. 3.2.2. Geophysical study: Implementation of the electrical resistivity method The electrical resistivity method, which is well suited to groundwater prospecting in crystalline bedrock areas, was used to explore the selected sites. The method was implemented using lateral investigation devices (electrical drags) and vertical investigation devices (electrical soundings). The main drag and its parallels make it possible to highlight the fracture axes [7]. At points along these axes that show interesting signs, the vertical investigation method makes it possible to determine, vertically from these points, the succession of soil layers crossed by the electric current injected into the subsoil. The Gradient Rectangle device is the lateral investigation device chosen for all surveys with a 600 to 800 m current injection line. For electrical surveys, the Schlumberger device is used with a 300 m current injection line (AB). 3.2.3. Study of the fracturing index Based on an electrical profile, conductive anomaly shapes are well defined. They are essential in choosing electrical sounding points in the basement zone [8]. Depending on the shapes of these anomalies, attention is focused on two World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 190 parameters: the width (L) and amplitude (H) of the observed conductive anomalies (Figure 4). These two (2) parameters were used to define the fracturing index (IF) using the following equation: 𝐼𝐹 =𝐻 𝐿×𝜌𝑎𝑚𝑖𝑛 ×100 Figure 4 Calculation model for the fracturing index (IF) The fracturing index is a parameter that provides information on the degree of fracturing in a rock. 3.2.4. Correction of cumulative fracture lengths Considering an electrical sounding point located in a fracture network, it will intersect fractures of various lengths and directions that will have varying contributions to the productivity of future drilling. Fractures in the same direction as the main fracture will contribute more to the productivity of the capture structure than fractures in opposite directions. This is because the change in direction induces singular and linear pressure losses related to the lengths and intersections of these fractures. This is why, in order to determine the cumulative length of fractures directly above or as close as possible to the electrical survey points, correction coefficients have been applied to each gross length to give a corrected length. This correction coefficient is the cosine of the angle between the intersecting fracture or the closest possible fracture and the neighboring fracture (Figure 5). Figure 5 Diagram showing the fracture intersecting or as close as possible to the electrical sounding point and the neighboring fracture The correction was performed using the following formulas: World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 191 𝐿𝑐=𝐿×cos⁡(𝜑) 𝐿𝑐𝑐 =∑𝐿𝑐𝑛 With − 𝐿𝑐 : the corrected length (m); − 𝐿 : fracture length (m); − 𝜑 : difference between the neighboring direction (d) and the direction of the intersecting fracture or the closest possible direction (D) (φ=|D-d|); − 𝐿𝑐𝑐: the cumulative corrected length of the fractures (m); − 𝑛 : the number of fractures. 4. Results 4.1. Map of lineaments The figure shows a detailed map of 65 lineaments identified on Landsat 7 ETM+ images of the study area. The lineaments range in length from 1.31 km to 9.97 km, with an average length of 3.67 km and a standard deviation of 2.04 km. Figure 6 Detailed map of the lineaments in the study area 4.2. Validation of the lineament map Drilling carried out by geophysicists in the study area has enabled the lineament map to be validated. The flow rate of these boreholes is between 7 and 15 m3/h. These boreholes, located on or near the lineaments, confirm that these catchment structures intersect fractures detected by geophysicists (Figure 6). Consequently, the relationship between productive boreholes and lineaments confirms the existence of fractures that are likely to constitute groundwater aquifers. World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 192 Figure 7 Superimposition of existing boreholes on the lineament map 4.3. Directional fracture rosette and choice of orientation for electrical logging The directional rosette shows 18 direction classes with an angular increment of ten degrees (10°). The directional rosette shows an uneven distribution across the 18 classes, with direction family frequencies ranging from 0% (N170180°) to 12.31% (N110-120°) (Figure 8). This diagram shows that the N110-120° (NW-SE) azimuth classes are the most common with 12.31%, followed by the N10-20° (N-S), N40-50° (NE-SW), N100-110° (NW-SE), and N150-160° (NWWSEE). These secondary fracture direction classes have respective proportions of 10.77%, 9.23%, 9.23%, and 9.23%. Figure 8 Directional rosette of fractures In order to intersect fractures from the main NW-SE, N-S, NE-SW, and NWW-SEE direction classes, the electrical survey lines were laid out in an east-west (E-W) or west-east (W-E) orientation. World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 193 4.4. Selection of conductive anomalies based on the fracture map After identifying the direction of the electrical profiles, one main electrical trench L0_0+00 and two (2) others L0_0+54 South and L0_1+00 North parallel to the main one were dug in the N°90 direction, each covering a distance of 390 m. Four conductive anomalies coincided with fractures identified from satellite image processing (Figure 9). These conductive anomalies were therefore selected for vertical prospecting of the subsoil in the study area. Figure 9 Superimposition of electrical profiles on the fracture map World Journal of Advanced Research and Reviews, 2025, 28(01), 186-200 194 4.5. Shape of conductive anomalies and their fracture indices Four different shapes of conductive anomalies were identified (Figure 10). On profile L0_0+00, “V”-type and “U”-type conductive anomalies were identified at measuring stations -20 m and +40 m, respectively. K-type and H-type conductive anomaly shapes were also observed at the +140 m measuring station on the L0_0+54 South profile and at the +180 m measuring station on the L0_1+00 North profile, respectively. Figure 10 Graph showing conductive anomalies identified on electrical profiles Table 2 shows that fracture indices vary from one type of conductive anomaly to another, with values ranging from IF = 1.96 (type “H” anomaly) to IF = 2.57 (type “K” anomaly). Table 2 Fracturing index for types of conductive anomalies Conductive anomaly Electric drag line Width L (m) Amplitude H (Ohm.m) Minimum apparent resistivity (Ohm.m) Fracture index (IF) V L0_0+00 50 862 677 2.55 U L0_0+00 60 916 623 2.45 K L0_0+54 South 30 365 473 2.57 H L0_1+00 North 30 326 554 1.96 The “V” type anomaly has a width of 50 m, an amplitude of 862 Ohm.m, and a fracturing index of 2.55. Next, the “U” type conductive anomaly has a width of 60 m, an amplitude of 916 Ohm.m, and a fracturing index of 2.45. As for the “K” anomaly, a width of 30 m, an amplitude of 365 Ohm.m and a fracturing index of 2.57 were determined. Finally, the “H” anomaly had a fracture index of 1.96, a width of 30 m, and an amplitude of 326 Ohm.m. Conductive anomaly V