Water exists in three states (liquid, solid, and gas); the form suitable for consumption is liquid, known as freshwater, and it is a scarce resource. The term “scarce resource,” which is often associated with it, may seem paradoxical given that the total volume of water in our planet’s seas and oceans is approximately 1.33 billion km³ [1]. The fact is that the water needs commonly cited refer to freshwater (non-saltwater), which accounts for only 3% of the volume mentioned earlier. Moreover, from this small percentage, we must subtract the share of permanent snow and ice, which amounts to 1.7%, leaving only 1.3% available [2]. According to projections by the Intergovernmental Panel on Climate Change (IPCC), beyond a 2˚C increase in temperature compared to 1990 levels, each additional degree could result in a 20% reduction in renewable water resources for at least 7% of the world’s population [3]. A situation that could therefore explain the growing scarcity of water resources in many countries [4]. According to studies, it is estimated that each resident will be able to consume 20 liters of fresh water per day [5]. The scarcity of surface water, especially in intertropical countries such as Burkina Faso, is driving the use of groundwater, which accounts for 30% of the planet’s freshwater. Consequently, to meet water needs, people are turning to groundwater. The exploitation of deep bedrock aquifers is considered a promising solution because they offer a water resource that is potentially more sustainable over time and is generally of good quality. [6]. To exploit these resources, it is necessary to construct water collection structures similar to water drilling, since the resources are trapped by rock formations or impermeable layers.

In West Africa, particularly in Benin, Burkina Faso, and Ivory Coast, the failure rate of water drilling in crystalline bedrock areas is estimated to be between 30 and 40 percent, according to [7]-[9]. The study area for this project is located on a granitic aquifer bedrock in the Oubri region. A distinctive feature here is the environment in which geological formations are at or near the surface. Numerous drilling surveys have shown that the city of Ziniaré, northern Ouagadougou, is known to be a very “hard” area. However, during operations at a granite aggregate quarry in 2005, an aquifer was discovered in this outcrop setting, creating an inexhaustible water reservoir. The reasons for the presence of a water body in an area known to be hard have prompted research aimed at finding a scientific explanation, with the goal of unraveling the mystery, understanding the circulation phenomenon, and enabling the exploitation of this groundwater resource. The objective of this study is to use electrical resistivity tomography (ERT) measurements in variable-flow drilling to understand and identify key characteristics of aquifers. This method was chosen because it allows for more precise determination of the exact location of geological discontinuities and for the identification of areas of major hydrogeological interest [7][10][11]. As an extremely interesting approach for characterizing the various compartments of bedrock aquifers while providing accurate imaging of the weathering profile [12]. It also makes it possible to characterize the target while avoiding structures that are highly unfavorable from a hydrogeological standpoint (such as clay-rich zones) [13].

ERT provides an integrated and consistent view of how bedrock aquifers function, thereby reducing the uncertainties inherent in conventional two-dimensional interpretations [14][15]. The originality of this study lies in the use of tomography on boreholes in outcrop and sub-outcrop settings in Burkina Faso to characterize aquifers based on resistivity, lithology, and yield.

The study area is located between Loumbila and Ziniaré and covers an area of approximately 102 km². According to the WGS 1984/UTM Zone 30 North coordinate reference system, it is bounded by the following coordinates: Xmax: 688711.182 m; Ymax: 1392460.625 m and Xmin: 677172.517 m; Ymin: 1383588.916 m in Bassitenga Province, Oubri Region, in central-western Burkina Faso (West Africa) (Figure 1). The climate is of the Sudano-Sahelian type, characterized by a dry season from November through May/June and a rainy season from June/July through October. The vegetation consists of open shrubby savanna, dominated by thorny shrubs and sea-grape trees. In addition, vegetable farming takes place along the edges of the reservoirs, creating a verdant view from satellite imagery during the growing season. The elevation of the area ranges from 280 to 360 meters, with an average of 304 meters.

Geologically, the formations found include granite, porphyroid granite, leucogranite, and basalt. The most dominant formation is granite. (Figure 1). The Ziniaré granite is a massif that extends from Loumbila to Ziniaré in a N60˚E direction. It is a leucocratic biotite granite, with surrounding rocks consisting of porphyroid granodioritic and monzogranitic facies [16].

Figure 1. Location and geology of the study area (modified from [17]).

Aquifers are classified primarily according to their confinement regimes (unconfined, confined, and semi-confined) or their types of porosity. Their geological composition, in turn, determines their ability to store and transmit groundwater. In the bedrock, generally speaking, the water reservoir is related to the degree of fracturing within it.

In the crystalline bedrock zone of Burkina Faso, there are three major hydrogeological units (granitoids, granites, and weathered volcano-sedimentary formations) capable of containing water (Figure 1). These formations are distributed throughout the country by region and can serve as good water reservoirs [17]. The types of aquifers encountered are primarily shallow alterites contained within the weathered layer of the granitic bedrock and deep fracture zones located within the fissures and faults of the granite. In the study area, granitoids are the most prevalent and are characterized by a thin layer of alterites and an abundance of fractures.

Tomography measurements were conducted at two sites, Loumbila and Ziniaré. Resistivity values may vary depending on the degree of saturation of the formations (Table 2).

Table 2. Example of a resistivity value [20].

Tomographic surveys of the various sites show a range of resistivity varying from 10 to over 2810 Ω∙m. This range can be divided into subranges (<150 Ω∙m; 150 Ω∙m to 400 Ω∙m; 400 Ω∙m to 2000 Ω∙m; >2000 Ω∙m) corresponding to three distinct major geological layers in the subsurface.

The study conducted at the Loumbila and Ziniaré sites in the Oubri region identified the key geophysical parameters for optimal drilling placement. Geophysical methods based on electrical tomography using the Wenner-Schlumberger array serve as a tool for characterizing subsurface geological formations. Consequently, the selection of locations for tomographic measurements was based on information from the boreholes. In light of our overall results, it is crucial to examine them carefully to ensure an accurate characterization of the aquifers in both outcrop and sub-outcrop contexts within the bedrock zone.

The tomography watch on Ziniaré’s Site 1 (Figure 4(a)) a water pocket at the 240 meter station, which corresponds to the borehole containing a probable layer of weathered granite with a resistivity between 600 and 800 Ω∙m. This value falls within the range typical of granite (Table 2), superimposed at the top of the water pocket. In the imagery (Figure 4(b)) conducted on the fracture detected during satellite image processing, resulting in a productive drilling yielding 16 m³/h. The imagery (Figure 4(c)) shows, at station 90 m, a zone of alteration ranging in thickness from 3 to 5 m with resistivities between 150 and 400 Ω∙m, which serves as a source of infiltration creating aquifers. Figure 4(d) shows aquifers with layers that may consist of fractured granite, ranging in thickness from 5 to 40 m and oriented vertically. At the Ziniaré 2 site, the imagery reveals areas of water and rock outcrops (Figure 5(a) & Figure 5(b)). Non-productive wells are located in areas where resistivity ranges from 400 to 2500 Ω·m, but high resistivity alone does not explain the absence of water. The most promising location would be the station 100 meters away, where the resistivity is less than 150 Ω·m; given the presence of water in the production well, a resistivity value of less than 150 Ω·m is indicated at our study site. The Loumbila site is shown in the imagery (Figure 6(a) et Figure 6(b)) a zone of weathered granite with resistivities ranging from 150 to 400 Ω∙m and a thickness ranging from 1 to 10 m, and a fractured zone with resistivities ranging from 400 to 2000 Ω∙m and a thickness ranging from 9 to 35 m.

The tomography shows that the majority of non-productive wells intersect zones with low conductivity or no conductive zones, as well as dry or unfed fractures. A comparison of data from productive and non-productive wells makes it possible to define a range of resistivity values conducive to obtaining a productive well at the study site. The resistivity must be less than 150 Ω∙m, which confirms that a favorable zone is linked to its conductivity; however, low resistivity is sometimes due to clay or other materials. To ensure good performance, all implementation steps must be carried out meticulously, from site selection using satellite imagery to analysis of the site’s environment and geophysical surveys. The study identified four zones: the first exhibits resistivity values below 150 Ω∙m, characteristic of a favorable zone containing water; followed by a second layer with values ranging from 150 to 400 Ω∙m, representing altered zones; a third layer with values ranging from 400 to 2000 Ω∙m, representing zones of fissured and fractured bedrock, and a fourth layer with resistivities greater than 2000 Ω∙m, characterizing the sound bedrock. The results obtained are consistent with those of [13], stipulating that the resistivity values of the weathered zone and the fractured zone traversed by the boreholes are, for all sites, between 150 and 400 Ω∙m and between 800 and 2000 Ω∙m, respectively which are the target ranges for acceptable hydrodynamic properties.

ERT is capable of accurately characterizing the various structures, accurately determining the depth of the sound bedrock, and thus the thickness of the aquifers [7][11][21] and the identification of potential infiltration areas [22]. Across all ERT images, it is evident that resistivity increases with depth, and the bottom layer appears to have higher resistivity, as indicated by other geophysical methods [23].

To achieve saturation, water must be stored; to ensure adequate storage capacity, the storage zone must be extensive. Thus, resistivity alone does not guarantee a sustainable, productive water drilling a fracture is needed to feed the low-resistivity zone.

In fact, the presence of a significantly thick weathered layer is essential for achieving and maintaining good flow rates during operation [22]. Crystalline rocks are generally highly fractured because they are prone to breaking, but the rock’s texture and structure play a key role in their productivity [24]. For bedrock aquifers in general, the thickness of the weathered zone is a factor in productivity. The weathered zone should not only be as thick as possible, but also as sandy as possible, rather than clayey. [13].

The fluid-filled sac in the imaging (Figure 4(a)) behaves like a confined aquifer, whose hydrogeological conditions are shaped by geological structures, aquifer characteristics, and land-use practices. These factors influence permeability and recharge rates [25]. However, it is essential to consider not only the amount of precipitation but also its timing and distribution, as these factors significantly influence groundwater potential [26][27].

The objective of this study was to demonstrate the value of tomography in characterizing the rock aquifers of Ziniaré and Loumbila in order to improve the selection of drilling sites. The imaging profiles made it possible to identify indicators by grouping them into four classes. The first class has resistivity values below 150 Ω∙m, characterizing a favorable zone containing water. The second class, with values between 150 and 400 Ω∙m, represents weathered zones. The third class, with values ranging from 400 to 2000 Ω∙m, represents areas of fissured and fractured bedrock. The fourth class, with resistivities greater than 2000 Ω∙m, characterizes sound bedrock.

The results show that combining tomography data with drilling data improves the identification of areas suitable for new drilling placements.

For future drilling placements in the basement zone, several factors such as geology, structure, lithology, and existing drilling data must be examined. In order to select a drilling site not solely based on resistivity, thickness, and weathering, but by combining all of this information while taking into account the type of faults or fractures to predict the drilling performance.