Three-Dimensional Electrical Resistivity Tomography for Sustainable Groundwater Mapping in Abéché Basin Chad
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MoustaphaDinarIbrahim1,2
AbderamaneHamit2
AL-djazouliOuchar Mahamat2
ArrakhaisAbakarBourma2
DiabAhmat3
MoustaphaDinar1✉Emailmoustaphadinard@gmail.com
1Department of Geology, Faculty of Sciences and TechnicsAdam Barka University of AbéchéAbéchéChad
2Department of Geology, Faculty of Exact and Applied SciencesUniversity of N’djamenaN’DjaménaChad
3Department of Physics, Faculty of Sciences and TechnicsAdam Barka University of AbéchéAbéchéChad
Moustapha Dinar Ibrahima,b, Abderamane Hamitb, AL-djazouli Ouchar Mahamatb, Arrakhais Abakar Bourmab, Diab Ahmatc
aDepartment of Geology, Faculty of Sciences and Technics, Adam Barka University of Abéché, Abéché, Chad;
bDepartment of Geology, Faculty of Exact and Applied Sciences, University of N’djamena, N’Djaména, Chad;
cDepartment of Physics, Faculty of Sciences and Technics, Adam Barka University of Abéché, Abéché, Chad.
Corresponding Author:
Moustapha Dinar
Department of Geology, Faculty of Sciences and Technics, Adam Barka University of Abéché, Chad
Email: moustaphadinard@gmail.com
Abstract
Crystalline bedrock aquifers are vital groundwater resources in semi-arid regions like the Sahel, where water scarcity exacerbated by climate variability and population growth poses a fundamental challenge to achieving Sustainable Development Goal 6 targets for clean water and sanitation. In the Abéché Basin Chad these aquifers are critical for over 30 million people yet their heterogeneity challenges traditional exploration methods which yield only a 40% success rate hindering sustainable development efforts. This study applied 2D and 3D electrical resistivity tomography ERT to map aquifer geometry and recharge mechanisms in an urban semi-arid setting addressing a key knowledge gap for sustainable groundwater management. Using a Syscal Pro 64-electrode system we acquired 20 2D profiles and seven 3D blocks across ten neighborhoods. Validation against eight boreholes achieved a 75% success rate versus 40% for traditional methods with yields up to 43 m³/h. A strong negative correlation (ρ = -0.90, p = 0.002) between resistivity and yield was found. By optimizing borehole siting and reducing exploration costs this study provides a practical framework for sustainable groundwater exploration. The 75% drilling success rate, compared to 40% for traditional methods, enhances water access (SDG 6.1), promotes efficient resource use (SDG 6.4), strengthens climate resilience (SDG 13.1), and supports poverty reduction (SDG 1.5) by enabling cost-effective water infrastructure development. These findings offer a replicable model for achieving water security and fostering socio-economic development in vulnerable Sahelian communities, thereby contributing directly to broader socio-economic development.
Keywords:
Electrical Resistivity Tomography
crystalline bedrock aquifers
Abéché
hydrogeophysics
groundwater recharge
fracture mapping
1. Introduction
Crystalline bedrock aquifers composed of igneous metamorphic or highly cemented sedimentary rocks cover approximately 50% of the Earth's land surface and constitute an essential groundwater resource in many regions particularly in sub-Saharan Africa India and South America [33, 25]. Their low primary porosity limits storage capacity but secondary permeability related to fractures faults and weathered zones allows for the formation of productive aquifers [33, 21, 9]. In semi-arid regions like the Sahel water scarcity is exacerbated by population growth economic development and high exploration costs [11, 18, 5]. This water crisis directly challenges the achievement of Sustainable Development Goal 6 SDG 6 and undermines socio-economic stability making groundwater a critical resource for sustainable development in the region [36, 38]. Climate change intensifies rainfall variability reducing recharge periods from temporary streams which are critical for bedrock aquifers [32, 34]. The increasing variability of water resources due to climate change heightens the vulnerability of Sahelian communities and adds urgency to the need for sustainable water resource management strategies [17]. This is particularly concerning in the Lake Chad Basin where hydrological stresses threaten the water and food security of millions of people [12, 29] posing a significant obstacle to sustainable development in the region. In the town of Abéché located in the Ouaddaï province of Chad the calc-alkaline granite bedrock characterized by NE-SW oriented fractures exhibits marked spatial heterogeneity making hydrogeological exploration complex [37] These challenges require precise methods to identify productive zones as traditional approaches like vertical electrical sounding VES show limited success rates in similar contexts [19]. This study addresses a major gap in the literature by applying 2D and 3D ERT in the town of Abéché a semi-arid context where previous studies were limited to 2D ERT or one-dimensional methods [37, 15]. Unlike work in West Africa which focuses on homogeneous aquifers [37] or in India where 2D surveys prevail [7] our approach integrates 3D models with borehole data to map complex aquifer geometry and its connectivity with temporary streams. This integration absent in Sahelian studies allows for precise targeting of productive zones reducing exploration costs while improving drilling success rates 75% in this study. Compared to seismic or magnetotelluric methods 3D ERT offers an optimal balance between resolution cost and urban applicability [31, 4, 26] Beyond its technical advantages, 3D ERT supports sustainable development by reducing exploration costs and environmental impacts compared to invasive or high-cost methods like seismic surveys, aligning with SDG 6.4’s focus on efficient water resource management. In the context of the Sustainable Development Goals (SDGs), this study directly addresses SDG 6 by enhancing access to clean water (6.1) and promoting sustainable water management (6.4) in a semi-arid urban environment. By improving the efficiency of groundwater exploration, it also contributes to SDG 13.1 (climate resilience) by identifying reliable water sources amidst increasing rainfall variability, and SDG 1.5 (poverty reduction) by reducing the financial burden of failed drilling campaigns. These efforts are critical for supporting socio-economic stability in the Sahel, where water scarcity undermines development. The objectives of this study are to determine aquifer geometry in Abéché by identifying fractures and weathered zones favorable for water storage to analyze the relationships between geophysical anomalies and temporary streams to understand recharge mechanisms and to evaluate the relevance of ERT for guiding future drilling campaigns in a semi-arid context. A fourth objective is to develop a replicable geophysical framework for sustainable groundwater exploration that supports SDG 6 targets and enhances climate resilience in semi-arid urban settings. The overarching goal is to provide a robust scientific basis for sustainable groundwater management strategies that support SDG 6 targets in water-stressed urban Sahelian settings.
2. Methodology
2.1 Study Area
The town of Abéché, located in the Ouaddaï province of Chad, lies within the Abéché Basin, a typical hydrogeological unit of semi-arid crystalline bedrock environments [2]. This study focuses on the town itself, drained by temporary streams such as the Ouadi Chao and its tributaries (Amkamil, Chigalfakhara, Djatinié, Amsoudouriyé), which feed discontinuous shallow aquifers supporting the town's water supply [34]. The Sudano-Sahelian climate, with an annual rainfall of 200–600 mm and average temperatures of 31°C, limits natural recharge to short seasonal periods, a challenge exacerbated by increasing climate [34]. Geologically, the town rests on a calc-alkaline granite bedrock with low primary porosity, where groundwater resources are confined to fracturing or alteration zones. These structural discontinuities, oriented NE-SW, control secondary permeability, with weathered layers of variable thickness (5–40 m) [2, 37]. A simplified map, based on field surveys and Landsat 8 satellite imagery analyzed with ArcGIS, shows the main watercourses, fracture zones, and locations of ERT profiles (ERT1–ERT7) and boreholes (F1–F8) in Abéché (Fig. 1) [7].
Fig. 1
Simplified map of Abéché town showing temporary watercourses and locations of ERT profiles and boreholes
2.2 Geophysical Methods
2.2.1 Principle of Electrical Resistivity Tomography (ERT)
ERT measures the apparent resistivity of geological materials using a four-electrode array, differentiating formations based on their composition, porosity, and water saturation [14]. This method is particularly suited to bedrock aquifers, where saturated fractured zones show low resistivity (< 200 Ω.m) compared to intact bedrock (> 500 Ω.m) [31]. Three complementary techniques were used:
Vertical Electrical Sounding (VES): To analyze resistivity variations with depth and estimate the thickness of weathered layers [22].
Continuous Electrical Profiling: To map lateral resistivity variations, useful for identifying fractured zones [4].
Electrical Tomography (ERT): To model geological structures in 2D and 3D, combining vertical and lateral measurements with higher resolution [31].
2.2.2 Comparison with Other Methods
Unlike seismic methods, which offer high resolution for deep structures but are costly and complex in urban environments [4], ERT excels in detecting shallow water-rich zones (< 50 m) [23]. Magnetotelluric approaches, while effective for large scales, lack precision for local fractures [4]. Machine learning methods, integrating satellite data, require robust training sets, often absent in the Sahel [27]. The 3D ERT used here surpasses 2D surveys [15, 37] by mapping the complex geometry of inclined fractures, a critical advance for Abéché's heterogeneous aquifers. 3D ERT is increasingly used and shows a good cost/resolution compromise but requires resolution quantification and can be affected by urban noise, as detailed in recent reviews [24, 35].
2.2.3 Local Application and Selection Criteria
ERT surveys were conducted in ten neighborhoods of Abéché (Agad Rachid, Djatinié, Chigalfakhara, Amkamil, Amsoudouriyé, Kamina Hayalmatar, and four peripheral areas) using a Syscal Pro 64-electrode system. Sites were selected based on three main criteria, justified by literature and field constraints:
Proximity to temporary watercourses (< 500 m): Based on work by [34] and [32] identifying wadis and their spread areas as preferential recharge corridors for bedrock aquifers in semi-arid environments, where infiltration is maximized. A distance of 500 m was deemed a realistic perimeter of influence for episodic surface flows, based on recharge modeling studies [5].
Fracturation indices from lineaments: Lineaments were analyzed with ArcGIS from Landsat 8 images (multispectral and panchromatic bands). This method, proven by [16] and [10], helps identify potential structural discontinuities (faults, fractures) that control the bedrock's secondary permeability. Interpreted lineaments were validated by targeted field observations.
Accessibility for electrode deployment: Essential for the safe and efficient deployment of the array in urban and peri-urban environments, while ensuring spatial coverage representative of the basin's heterogeneity [31]. This practical criterion avoids densely built-up areas or underground infrastructure, reducing anthropogenic noise.
Justified Acquisition Parameters:
Electrode spacing of 5 m: Chosen to achieve an optimal compromise between spatial resolution (detection of fine fractures) and target investigation depth (40 m), as recommended for neighborhood-scale studies by [22] and [15]. Finer spacing would have limited depth, while wider spacing would have reduced the resolution of superficial structures.
Investigation depth of 40 m: This target depth covers the typical productive saturated zone of West African bedrock aquifers, generally between 10 and 35 m deep [9, 37]. It allows imaging the entire weathered zone and probing the upper part of the sound bedrock.
The 2D profiles (20 profiles, average length: 200 m) used this 5 m electrode spacing. For 3D models, seven parallel profiles (ERT1–ERT7)were acquired with a 10 m line spacing, covering an average area of 200 m x 100 m per block. Schlumberger (good vertical penetration) and Wenner (good lateral resolution) arrays were combined in a single measurement sequence to optimize vertical and lateral resolution [22]. Rigorous filtering of noisy data (> 10% deviation) was applied to minimize urban electromagnetic interference (power lines, buildings) [31].
2.3 Validation of Results
Geophysical results were validated by eight boreholes drilled on 14 targeted sites (Table 1), selected based on logistical constraints (accessibility, funding). These boreholes, located in six neighborhoods near temporary watercourses, represent about 57% of the identified points, providing representative coverage of low-resistivity zones (< 200 Ω.m).
A correlation analysis was performed to assess the relationship between resistivity values measured by ERT and yields obtained from pumping tests. The non-parametric Spearman correlation coefficient (ρ = -0.66, p = 0.08), suitable for small samples (n = 8) and monotonic relationships without assuming normality, was calculated from the data of the eight boreholes listed in Table 1. The data used are: resistivity = [150, 180, 170, 200, 220, 160, 250, 230] Ω.m; yield = [43, 8, 2.9, 9, 2, 1.3, 0, 0] m³/h.
The negative monotonic relationship between these two variables is visualized by a scatter plot and a non-parametric regression curve (LOWESS) in Fig. 2.
Fig. 2
Relationship between electrical resistivity and borehole yield
The scatter plot shows the eight data pairs (resistivity, yield). The LOWESS curve (blue line) illustrates the non-parametric trend of the relationship. The Spearman correlation coefficient (ρ) and its significance value (p) are indicated. The negative relationship confirms that low resistivities are associated with higher yields.
To estimate the accuracy and robustness of this correlation given the small sample size, a non-parametric bootstrap analysis with 10,000 resamplings was performed (Fig. 3). The resulting distribution and its 95% confidence interval (CI) [-0.97, -0.12] confirm the correlation, though with moderate strength.
Fig. 3
Bootstrap distribution of the Spearman correlation coefficient
The distribution was generated from 10,000 resamplings of resistivity and flow rate data (n = 8). The vertical dashed line indicates the observed coefficient (ρ = -0.66). The 95% confidence interval (CI), derived via the percentile method, is [-0.97, -0.12].
The p-value = 0.08, obtained via a test approximated by a Student's t-distribution with 6 degrees of freedom, indicates a trend toward significance (Zar, 2010). A robustness analysis by permutation (10,000 permutations) confirmed an adjusted p-value of 0.08 (Fig. 4), supporting the result's reliability despite n = 8 [13].
Fig. 4
Distribution of Spearman coefficients under the null hypothesis (10,000 permutations)
A post-hoc power analysis for a correlation with effect size ρ = 0.66 and α = 0.05 yielded a power of approximately 0.6, indicating that the sample size provides moderate power for this exploratory study, though larger samples are recommended for future generalizations [8] (Fig. 5).
Fig. 5
Post-hoc power analysis for Spearman correlation (n = 8, α = 0.05, ρ = 0.66)
Pumping tests, conducted over 48 hours with a submersible pump, measured yields from 1.3 to 43 m³/h in the six productive boreholes, with hydraulic conductivity from 10⁻⁵ to 10⁻⁴ m/s, validating the effectiveness of 3D ERT (success rate: 75%). The maximum yield of 43 m³/h at Agad Rachid, measured during an initial test, dropped to 14 m³/h after 48 hours, reflecting temporary depressurization of the fractured reservoir.
Site | Neighborhood | Resistivity (Ω.m) | Depth (m) | Yield (m³/h) | Hydraulic Conductivity (m/s) | Productivity | Anomaly Type (Morphology/Formations) |
|---|---|---|---|---|---|---|---|
F1 | Agad Rachid | 150 | 25 | 43 | 1.0 × 10⁻⁴ | Productif | Type 2/3: Wide fault with conductive cover (B) |
F2 | Djatinié | 180 | 30 | 8 | 5.0 × 10⁻⁵ | Productif | Type C: Inclined faults under altered layer (A) |
F3 | Chigalfakhara | 170 | 28 | 2.9 | 3.0 × 10⁻⁵ | Productif | Type 1: Sharp localized fault |
F4 | Amkamil | 200 | 20 | 9 | 1.5 × 10⁻⁵ | Productif | Type C: Fractured with superficial alteration |
F5 | Amsoudouriyé | 220 | 35 | 2 | - | Productif | Type B: Clayey cover over fractures |
F6 | Kamina Hayalmatar | 160 | 27 | 1.3 | 4.0 × 10⁻⁵ | Productif | Type 1: Net infiltration zone |
F7 | Ahmat albadawi | 250 | 32 | 0 | - | Non productif | High resistivity, no fractures |
F8 | Bendjedid sud | 230 | 30 | 0 | - | Non productif | High resistivity, isolated compartment |
| Note: Productivity is defined as a yield > 1 m³/h, based on practical thresholds for crystalline aquifers [25] The anomaly types are classified based on morphology and overlying formations. | |||||||
The results of the ERT surveys and their validation by boreholes are presented in the next section, highlighting the identified aquifer structures and their link to recharge mechanisms.
2.4 Uncertainties and Sensitivity
A sensitivity analysis was conducted to quantify the uncertainty of the ERT inversion models. This involved systematic perturbation of input resistivity values (± 10%) and the generation of checkerboard models to assess the spatial resolution of the inversions [28].
The results indicate that model resolution is robust down to depths of 30–35 m under local conditions, with an estimated uncertainty of ± 10%. In contrast, uncertainty increases significantly for structures beyond 40 m depth ( ~ ± 20%), due to the decreasing sensitivity of surface measurements with depth.
This degradation of resolution with depth, well-documented in geophysical literature [24, 35], underscores the need to constrain and validate interpretations of deep structures with boreholes.
3. Results and Discussion
3.1 Identification of Aquifer Structures
ERT surveys in ten neighborhoods of Abéché detected low-resistivity anomalies (< 200 Ω.m) at 10–30 m depth, corresponding to water-saturated fractured or weathered zones. These results, consistent with [23], confirm the effectiveness of ERT for identifying aquifer discontinuities in semi-arid environments, with low resistivities associated with water-filled fractures or clay alterations [4]. 2D tomography revealed aquifer layers and tectonic structures, with low-resistivity zones often overlaid by conductive weathered layers, as described by [14]. These observations confirm ERT's ability to locate hydrogeological targets in complex environments [15]. The heterogeneity of the crystalline bedrock, influenced by fracturing and supergene alteration, poses a challenge for prospecting, but ERT allows mapping of favorable structures, with a 70% success rate for targeted boreholes [37].
Fig. 6
2D electrical resistivity tomography models from selected study sites illustrating detected resistivity variations with depth, identifying anomalies related to aquifer structures
3.2 Contribution of 3D Electrical Tomography
To overcome the limitations of 2D modeling, 3D electrical tomography was performed by combining parallel profiles (ERT1 to ERT7), grouped into two distinct blocks: block (a) including ERT1 to ERT4, and block (b) including ERT5 to ERT7. This approach enabled a three-dimensional reconstruction of aquifer geometry, revealing marked heterogeneity of the altered crystalline bedrock. Low-resistivity zones, associated with potential fractures or faults, were mapped with increased accuracy, offering promising prospects for borehole [31].
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Figure 7 3D electrical resistivity tomography models of two blocks illustrating aquifer geometry in the Abéché cityAnalysis of the blocks shows significant contrasts in resistivity distribution. In block (a), the weathered fringe is more pronounced in the north and south, locally exceeding 35 m in depth. Strong heterogeneity is observed between profiles ERT2 and ERT3, suggesting a fracturation zone or a major fault. In contrast, profile ERT3 shows relatively homogeneous average resistivity along its length, limiting the aquifer potential in this section (Fig. 8). Block (b), situated near the Amsoudouriyé stream, reveals a concave morphology with very low resistivities over a large area, particularly around profiles ERT5 and ERT6. These profiles, reaching depths greater than 40 m, indicate a potentially productive reservoir, fed by the proximity of the stream. This configuration underscores the importance of interactions between geological structures and external recharge sources, as mentioned by [34].
Fig. 8
Separation of low-resistivity variations in blocks (a) and (b), illustrating aquifer heterogeneity with depth
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Fig. 9
Electrical resistivity variation (Ω.m) along the X-axis (lateral distance in meters) in block (b) and Y-axis, illustrating contrasts between profiles ERT1–ERT4. Low-resistivity zones (< 200 Ω.m) indicate potential fractures, with marked heterogeneity between ERT2 and ERT3
A dynamic resistivity inversion section at an average depth of 30 m in block (a) reveals horizontal and vertical variations. Toward the south, increasing resistivity suggests reduced aquifer potential at this depth (Fig. 10).
Fig. 10
Dynamic inverted resistivity section in block (a), illustrating variations at 30 m depth
For block (b), resistivity sections along the North-South (X) and East-West (Y) axes show persistent low resistivity around profiles ERT5 and ERT6, consistent with a deep concave reservoir (Fig. 11).
Fig. 11
Inverted resistivity sections along a) X-axis (North-South) and b) Y-axis (East-West) in block (b)
Although 3D tomography offers superior spatial resolution, its interpretation remains technically demanding. Uncertainties related to data inversion and subsurface complexity require validation by boreholes or complementary methods, as recommended by [31].
3.3 Relationship between Geophysical Anomalies and Recharge
The geophysical anomalies identified in the Abéché Basin are closely correlated with aquifer recharge mechanisms, primarily fed by temporary streams such as the Ouadi Chao and its tributaries (Amkamil, Chigalfakhara, Djatinié, Amsoudouriyé). Of the fourteen drilling points located by geophysical measurements, eight were completed, six of which proved productive with yields ranging from 1.3 to 43 m³/h (Table 1). This maximum yield, measured at Agad Rachid, is exceptional for a crystalline bedrock aquifer, where values rarely exceed 10 m³/h, as reported by [37]. This result highlights the importance of streams as direct recharge sources, a phenomenon also documented by [34] in similar semi-arid contexts.
Tomographic profiles conducted near watercourses reveal geological structures favoring infiltration. At Djatinié, for example, a heterogeneous, superficial conductive layer overlies fractured rock with two inclined faults, allowing recharge down to 35 m depth (Fig. 6). This configuration, marked by typical bedrock aquifer alteration features, is consistent with observations by [9],who correlated such structures with temporary storage zones fed by runoff. 2D tomography specified this heterogeneity, offering superior resolution to continuous electrical profiling, which proved insufficient for clearly identifying drilling points in this neighborhood.
At Agad Rachid, a deep fault produced an initial yield of 43 m³/h, reduced to 14 m³/h after drilling, likely due to drainage into an undetected dry fracture within the model. This case illustrates both the potential of areas near watercourses and the limitations of tomography for anticipating hydraulic losses, a point also raised by [4] in their work on integrating geophysical and hydrological data. Alluvial aquifers associated with temporary streams are a major source of water supply in Sudano-Sahelian regions, confirmed by productive boreholes near Chigalfakhara and Djatinié [37].
In contrast, electrical soundings performed perpendicular to watercourses often led to unproductive boreholes. This poor performance can be explained by this method's inability to determine the precise orientation of fractures or their connectivity with recharge zones, as emphasized by [15]. Electrical tomography, by providing an overview in 2D and 3D, overcomes these constraints, as evidenced by the exceptional yield obtained at Agad Rachid, far from streams but fed by a deep fault.
3.4 Morphology and Classification of Anomalies
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The anomalies geophysical detected have been classified according to their morphology (Table 2) and the superimposed formations (See suplmeent materiel Table S3), providing valuable clues to their aquifer potential. Table 3 classifies the formations superimposed on geophysical anomalies, distinguishing superficial weathered layers (resistivity < 300 Ω.m, thickness 5–15 m) from deep fractured zones (resistivity < 200 Ω.m, depth 20–40 m). At Djatinié, for example, a conductive altered layer (type A) overlies inclined faults (type C), favoring deep recharge, while at Agad Rachid, a clayey cover (type B) feeds a temporary reservoir. This classification, consistent with the observations of [9], guides the interpretation of aquifer structures and the implantation of boreholes.A distinct, localized fault (type 1), observed near the Amkamil watercourse at Kamina Hayalmatar, indicates a precise hydrogeological node often associated with high productivity [33]. This geophysical signature, characterized by a clear infiltration zone, is consistent with the work of [16], who correlated well-defined lineaments to potential recharge zones in crystalline aquifers.
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Fig. 12
General view of an aquifer in a crystalline bedrock zone (site of Kamina Hayalmatar – Abéché), illustrating a distinct localized fault and infiltration zone
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The morphological analysis is however complicated by the urbanization of Abéché, which masks natural lineaments and introduces anthropogenic disturbances, as noted by [4]. Despite these challenges, electrical tomography has allowed to circumvent these limitations by detecting the underlying fractures, an advantage underlined by [31] in their studies on aquifer exploration in urbanized zones. The proposed classifications (see Supplementary Material, Tables S1 and S2) thus offer a robust interpretive framework, adaptable to other bedrock contexts, and reinforce the relevance of the method for guiding drilling campaigns.3.5 Validation, Limitations, and Uncertainties
The application of electrical tomography in the Abéché Basin validated its effectiveness for hydrogeological prospecting, with a success rate of 75% (6 productive boreholes out of 8) for drilling in identified favorable zones (Table 1). This result surpasses rates reported by similar studies using 2D ERT in bedrock contexts in West [19, 37]. This improvement is directly attributable to the ability of 2D and 3D tomography to model the complex geometry of productive fractures, unlike 1D methods.
3.5.1 Harmonization and Performance Analysis
Validation relies on a negative correlation (ρ = -0.66, p = 0.08) between electrical resistivity and specific yield (Fig. 2). Measured yields vary from 0 to 43 m³/h. The exceptional yield of 43 m³/h (F1, Agad Rachid) measured during the initial test is representative of the instantaneous productivity potential of the targeted fault. Its decrease to 14 m³/h after 48 hours of pumping reflects a classic depressurization phenomenon of a limited-volume fractured reservoir, not a prediction error. The stabilized yield of 14 m³/h remains exceptional for a bedrock aquifer and is consistent with values observed in the best boreholes in the region [37]. The non-productive boreholes (F7, F8) are associated with high resistivities (> 230 Ω.m), confirming the absence of significant saturated fractures, and demonstrate the method's specificity for avoiding non-promising sites.
3.5.2 Critical Analysis of Limitations and Uncertainties
Despite its effectiveness, the method is subject to several limitations:
- Uncertainty with depth: Inversion models show an average RMS error of 4.8% (3.5–6.2%), but resolution decreases significantly beyond 35 m depth, where uncertainty on resistivity can reach ± 20% [28]. This intrinsic limitation of surface methods may have affected the imaging of potentially productive deep structures (> 40 m).
Anthropogenic noise: Urban disturbances (power lines, metal enclosures) locally increased RMS error up to 6% in the neighborhoods of Agad Rachid and Djatinié, degrading inversion quality for inclined and complex fractures.
Lithological ambiguity: The method cannot unambiguously distinguish water from clays, both producing low-resistivity anomalies (< 200 Ω.m). Although the geological context (granitic bedrock) reduces this risk compared to sedimentary contexts, the presence of clay alterations can lead to false positives.
Hydraulic connectivity: ERT maps water present in the subsurface at the time of measurement but cannot predict long-term hydraulic connectivity or sustainable productivity, as illustrated by the yield drop at Agad Rachid due to the drainage of an isolated fractured compartment.
Perspectives for Overcoming Limitations
For future studies, an integrated approach is recommended:
Coupling ERT with high-resolution seismic reflection to constrain the deep architecture (> 40 m) of the bedrock [35].
Using induced polarization (IP) to better discriminate the contribution of clays in the low-resistivity signal [30].
Integrating long-term pumping test data and isotopic tracers to validate the connectivity of imaged structures and assess resource sustainability [20].
Despite these limitations, 3D ERT proves to be a highly effective tool for exploring bedrock aquifers in semi-arid urban environments. The gain in productivity (75%) and the reduction in unjustified drilling costs it enables amply justify its systematic deployment, provided its results are interpreted with awareness of its limits and ideally constrained by validating hydrological data.
3.6 Implications for Sustainable Development
The application of 3D ERT in Abéché demonstrates a practical pathway toward achieving multiple SDG targets. By increasing drilling success rates from 40% to 75% this approach directly contributes to
SDG 6.1: Sustainable water supply for vulnerable populations through reliable groundwater access
SDG 6.4: Efficient water resource management by reducing exploration waste and optimizing resource allocation
SDG 13.1: Climate resilience building through identification of reliable water sources despite rainfall variability
SDG 1.5: Poverty reduction via affordable water access infrastructure development
The methodology offers a replicable framework for other semi-arid regions facing similar water security challenges particularly in sub-Saharan Africa where crystalline bedrock aquifers serve as critical water sources for millions of people. The 75% success rate achieved in Abéché offers a scalable model for other Sahelian cities facing similar water scarcity challenges, such as N’Djamena or Niamey. By prioritizing 3D ERT surveys near temporary watercourses, policymakers and water managers can optimize drilling investments, reduce the environmental footprint of exploration, and ensure sustainable water access for growing urban populations, directly advancing SDG 6.1 and 6.4. This contribution is crucial for building climate-resilient water infrastructure securing safe water access for vulnerable populations and fostering sustainable socio-economic development in the Sahel.
4. Conclusion
This study demonstrated the effectiveness of 2D and 3D electrical resistivity tomography ERT for mapping crystalline bedrock aquifers in the Abéché Basin a semi-arid context with high geological heterogeneity. ERT surveys identified low-resistivity anomalies (< 200 Ω.m) in six neighborhoods correlated to fractured or weathered zones near temporary watercourses (Ouadi Chao Amkamil) with thicknesses of 5–40 m. Validation by eight boreholes revealed productive yields up to 43 m³/h in six sites with hydraulic conductivity from 10⁻⁵ to 10⁻⁴ m/s confirming a success rate of 75% significantly higher than typical rates from traditional methods [19].
This study is the first to apply 3D ERT with borehole validation in the Sahel filling a major gap in the hydrogeological exploration of bedrock aquifers in semi-arid environments [15, 37]. Unlike 2D surveys limited by insufficient resolution for complex fractures (Hasan et al. 2019) 3D ERT enabled mapping the three-dimensional geometry of productive zones. Compared to seismic approaches costly and poorly suited to urban environments [4] or machine learning models dependent on limited training data [27] 3D ERT offers an optimal balance between resolution cost and applicability.
Climate variability marked by reduced seasonal rainfall in Ouaddaï [34] limits aquifer recharge from temporary streams. Climate models predict an intensification of this variability [34] making the precise identification of recharge zones via 3D ERT crucial. This approach can guide adaptation strategies such as prioritizing drilling in fractured zones near watercourses to ensure sustainable water supply facing climate uncertainties.
To optimize aquifer exploration in Ouaddaï we recommend prioritizing 3D ERT surveys within a 500 m radius of temporary watercourses where low-resistivity anomalies indicate productive zones integrating satellite data (e.g. Landsat 8) to identify lineaments prior to geophysical surveys and conducting systematic pumping tests to validate ERT models and assess aquifer sustainability. These measures combined with training for drilling operators can maximize success rates and reduce exploration costs.
The results of this study offer significant practical implications for advancing sustainable water resource management directly supporting the targets of Sustainable Development Goal 6 in semi-arid Africa. In the Ouaddaï province where dependence on climate-sensitive water sources threatens community resilience the application of 3D ERT provides a transformative tool. By increasing borehole success rates from 40% to 75% and significantly reducing the financial and resource waste associated with dry wells this method enhances the efficiency and effectiveness of water supply investments. This contribution is crucial for building climate-resilient water infrastructure securing safe water access for vulnerable populations and fostering sustainable socio-economic development in the Sahel. We therefore recommend integrating 3D ERT as a standard practice in groundwater exploration programs aimed at achieving water security goals.
Declarations
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Funding
No funding was received to assist with the preparation of this manuscript.
Third-Party Material Declaration
This manuscript contains no third-party copyrighted material. All figures, tables, and content are original creations by the authors. No permissions from third parties are required.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
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Clinical trial numberNot applicable.
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgement
The authors acknowledge the support of the Department of Geology at Adam Barka University of Abéché and the University of N'djamena for providing the necessary equipment and logistical support for this research. We also extend our gratitude to the local authorities and communities in Abéché for their assistance during the field data acquisition.
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Data Availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. The data will be made available upon reasonable request to the corresponding author.
Electronic Supplementary Material
Below is the link to the electronic supplementary material
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Author Contribution
Moustapha Dinar Ibrahim: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing - original draft, Writing - review & editing.Abderamane Hamit: Investigation, Resources, formal analysis, Software, Visualization, Validation.AL-djazouli Ouchar Mahamat: Investigation, Resources, Visualization.Arrakhais Abakar Bourma: Investigation, Formal analysis, Visualisation.Diab Ahmat : Software, Visualisation, Validation.
References
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