ERT surveys were conducted using a Syscal Pro Switch 72 system (Iris Instruments). A total of 36 electrodes spaced 10 m apart were deployed simultaneously along each profile, and automated switching protocols controlled the measurement sequence using the Wenner-Schlumberger array (Figure 5).

Figure 5. Schematic diagram of the measurement protocol for an electrical resistivity panel using the Wenner-Schlumberger array (modified after [7]).

In this quadrupole configuration, two current electrodes (A and B) inject a direct current of intensity I into the ground, while two potential electrodes (M and N) measure the resulting potential difference ΔV. The apparent resistivity ρ a is calculated as:

where ρ a is the apparent resistivity, n is the spacing factor, a is the distance between potential electrodes M and N, ΔV is the measured potential difference, and I is the injected current.

Six profiles oriented N70˚, each 1070 m long and spaced 200 m apart, were surveyed. A typical acquisition panel covered 320 m, requiring four roll-along sequences to extend each profile. This line orientation was chosen to perpendicularly intersect the site’s megafractures, which are oriented N160˚ ([9]). The survey grid design was guided by established geophysical criteria, aiming to optimize both coverage and resolution. A 200 m line spacing allows mapping of large site sections along the main structural axes, while a 25 m electrode spacing ensures sufficient lateral resolution to detect narrow subvertical fractures. This configuration thus effectively captures fracture-related conductive anomalies while maintaining optimal operational efficiency.

During the survey, all measurements were conducted using consistent acquisition parameters. The duration of the current injection pulse was fixed at 0.5 seconds. A standard deviation threshold of Q = 0 was applied. The number of measurement cycles ranged from three to five, with three cycles performed when Q < 0 and five cycles when Q > 0. The injection voltage during the measurements was maintained at 400 V.

The collected raw data were processed through iterative digital inversion to image the subsurface geological structures. This inversion employed the Gauss-Newton regularization algorithm ([5]) and was carried out using the Res2DINV software. This program is widely adopted in electrical resistivity imaging due to its effectiveness in inverting complex datasets over heterogeneous layers. The method is based on Equation (2), which minimizes the differences between the observed data and the modeled resistivity at each iteration.

with:

where: m₀, reference model; m_k, model at iteration k; S, sensitivity matrix; λ, regularization factor; D, data weighting matrix; C, stress matrix; d, vector of apparent resistivity data containing N values; f(m_k), direct modeling response m at iteration k; α_x, α_y, α_z, anisotropic weights applied in each spatial direction (x, y, z).

This inversion approach is particularly suitable for basement aquifers, where subsurface transitions are generally gradual ([14]).

Interpretation of the true resistivity sections allowed the identification of the main geoelectrical units. In each section, points spaced at regular 25-m intervals were selected to define litho-logs, representing the vertical succession of units with their respective top and bottom depths. This approach generated a set of spatially distributed “virtual boreholes”.

It is important to note that no borehole logs, pumping data, or water-level observations were available for this site. Therefore, the fractured horizon and hydraulic activity presented in this study are based solely on geoelectrical data and 3D modeling and should be considered as interpretations rather than confirmed hydrogeological conditions.

These data were imported into Surpac software to construct a continuous three-dimensional model. For each geoelectrical unit, corresponding blocks were correlated across all sections. Spatial interpolation between litho-log points was performed using the inverse distance weighting (IDW) method:

where ρ i is the estimated resistivity of block i, ρ j are neighboring values, d_ij is the distance between points, and p is the weighting exponent. This method enabled reconstruction of the volumetric geometry of each unit and the development of a coherent 3D subsurface model.

Interpretation of the true resistivity sections reveals three main geoelectrical units (Figure 6):

A resistive unit R1 composed of scattered blocks;A conductive horizon C;A resistive unit R2.

The three geoelectric units (resistant surface cover, conductive alterite or horizon, and highly resistant sound bedrock) are clearly defined in each section. The resistivity ranges and lithological correlations of these geoelectrical units are summarized in Table 1.

The resistive unit R1 exhibits resistivity values typically ranging from 850 to 1723 Ω∙m, with locally higher values reaching approximately 3000 Ω·m. Based on field observations and geoelectrical interpretation, R1 is interpreted as a superficial lateritic cover composed mainly of scattered cuirass blocks.

Spatially, R1 is discontinuous and fragmentary. It is clearly identified along profiles L00, L200 N, L400 N, and L600 N, where it occurs as localized lenses or blocks. Conversely, it is nearly absent along profiles L800 N and L1000 N, indicating enhanced erosion processes toward the northern part of the site. This heterogeneous distribution reflects the progressive dismantling of the superficial cover during landscape evolution.

Figure 6. True resistivity sections of profiles L00, L200 N, L400 N, L600 N, L800 N, and L1000 N.

Table 1.Resistivity ranges and lithological correlations of the surveyed formations.

The thickness of R1 is generally very limited, rarely exceeding 5 m. This reduced thickness, combined with its lateral discontinuity, strongly limits its volumetric influence within the hydrogeological system.

From a hydrogeological perspective, R1 primarily serves as a protective cover over the underlying conductive unit C, which corresponds to the weathered aquifer. When present, R1 limits direct infiltration of surface water and reduces the risk of contamination. Conversely, its absence or limited development increases exposure of unit C, thereby increasing the shallow aquifer’s vulnerability to anthropogenic pollution. Therefore, R1 does not constitute an aquifer itself but represents the residual upper part of the weathering profile, whose main function is aquifer protection.

Beneath R1 lies the conductive unit C, which is widely developed and represents the most laterally extensive and hydrogeologically significant unit.

Unit C is characterized by low resistivity values, mainly ranging from 25 to 155 Ω∙m, reflecting the presence of electrically conductive materials. Intermediate resistivity levels (155 - 635 Ω∙m) are frequently observed within this unit, indicating lateral and vertical variations related to the degree of weathering, porosity, and water saturation. These resistivity contrasts reflect marked internal heterogeneity.

Lithologically, unit C corresponds to weathered materials derived from basement alteration, composed of clayey-sandy, lateritic, or sandy-clayey formations. Its thickness varies significantly across the site, ranging from a few meters (~5 m) in eroded zones to 35 - 40 m in areas where weathering and/or saturation is more developed. This substantial thickness provides Unit C with significant groundwater storage capacity.

Spatially, unit C is continuous at the site scale, although locally interrupted by higher-resistivity zones corresponding to slightly weathered basement. These internal discontinuities do not compromise the overall coherence of the conductive unit, which remains dominant across all investigated profiles.

Hydrogeologically, unit C represents the main shallow aquifer of the Langbabokohou site. Its variable porosity and permeability favor infiltration and storage of meteoric water, making it a seasonal capacitive reservoir, particularly active during the rainy season. This aquifer is commonly exploited by traditional wells and also contributes to the recharge of deeper boreholes.

At depth lies the resistive unit R2. The transition between unit C and the resistive basement R2 is not abrupt but occurs through an intermediate resistivity horizon (635 - 2000 Ω∙m), interpreted as a fractured basement horizon. This horizon plays a key role in vertical groundwater drainage, ensuring hydraulic connectivity between the shallow aquifer and the deeper fractured basement aquifer.

Unit R2 is characterized by high resistivity values, generally ranging from 1723 to 4973 Ω·m, typical of slightly weathered to fresh crystalline basement with low primary porosity and low electrical conductivity. Relative resistivity homogeneity observed along some profiles indicates massive, weakly fractured bedrock, whereas local variations reflect structural discontinuities.

The depth to the top of unit R2 varies significantly across the site, reflecting an irregular basement morphology. It may be shallow (~5 m) in some areas (profiles L00, L200 N, and L800 N) and reach 30 - 60 m in the central or western sectors (profiles L400 N and L600 N). This variability reflects differential weathering intensity of the basement.

Along most profiles, unit R2 exhibits an intermediate resistivity zone at its top, interpreted as a fractured horizon. This zone corresponds to increased fracturing and microfracturing resulting from tectonic stresses and mechanical weathering, providing a gradual transition between conductive weathered materials and fresh basement.

Hydrogeologically, unit R2 represents the deep basement aquifer, whose productivity depends primarily on fracture density and connectivity. Where the basement is massive and weakly fractured, its contribution to groundwater storage and flow is limited. Conversely, major faults and densely fractured zones, particularly along profiles L1000 N (faults F1 and F2) and the eastern end of profile L600 N, confer high hydrogeologic potential by enhancing groundwater circulation and storage.

Thus, unit R2 forms the structural and hydrodynamic framework of the Langbabokohou aquifer system. Its interaction with the fractured horizon and the conductive weathered unit C controls overall system functioning by ensuring both vertical drainage of infiltrated water and long-term storage within fractured basement zones.

Based on the apparent resistivity pseudo-sections, iso-resistivity maps for levels 2, 7, and 12 were generated (Figure 7). These levels correspond to pseudo-depths of 10 m, 25 m, and 45 m, respectively, in a Wenner-Schlumberger configuration. Overlaying these maps by depth enables assessment of both vertical and lateral subsurface structure.

Figure 7. Overlaid resistivity maps for levels 2, 7, and 12.

At the 10 m pseudo-depth, the resistivity distribution, ranging from 60 to 550 Ω∙m, exhibits strong heterogeneity, with a predominance of conductive zones interpreted as weathered formations and shallow terrains locally saturated with water.

At the 25 m pseudo-depth, resistivity contrasts become more pronounced (100 - 1200 Ω∙m), reflecting a gradual transition from weathered formations to more resistive layers. Conductive zones persist but tend to narrow and align along preferential NNW-SSE directions, suggesting structural control.

At the 45 m pseudo-depth, resistivities generally increase (200 - 3700 Ω∙m), indicating the presence of basement. Resistive domains dominate, while conductive zones, more restricted and well-defined, correspond to deep fractured zones of the basement. These fractures also follow the NNW-SSE orientation observed at 25 m pseudo-depth.

A correlative (vertical) analysis of levels 2, 7, and 12 highlights the rooting of major conductive anomalies, indicated by dashed lines, and reveals fracture structures extending through all investigated levels. The vertical continuity of these fractures indicates hydraulic activity, linking the weathered layer to the fractured basement and enhancing both groundwater flow and the hydrogeological potential of the basement aquifer.

The three-dimensional conceptual model obtained by integrating resistivity sections highlights a well-organized vertical and lateral structure. The volumetric distribution of resistivity contrasts enables the clear identification of four major geoelectric units, whose geometry and connectivity govern the system’s hydrogeological functioning (Figure 8).

Figure 8. 3D conceptual model of the Langbabokohou aquifers.

The lateritic duricrust, characterized by high resistivities, appears as blocky and locally thick at the top of the model. Its lateral discontinuity reflects differential preservation of the duricrust, strongly controlled by surface morphology and erosion processes. Its indurated, low-porosity character suggests a primarily protective role against underlying weathering while locally acting as a partial barrier to direct infiltration of meteoric water. The spatial distribution of this unit indicates that its hydrogeological influence is secondary and strongly dependent on the degree of superficial fracturing.

Beneath the duricrust, the weathered horizon represents the dominant unit in terms of lateral extent. Its variable thickness reflects differential substrate weathering, controlled by lithology, the density of inherited structural discontinuities, and basement topography. This horizon plays a key role as a temporary storage zone and a vertical conduit for infiltrated water to deeper levels.

The fractured horizon constitutes the hydrogeological core of the system. It develops preferentially at the weathered layer-intact basement interface, with an undulating geometry and lateral continuity over much of the model. The relatively low resistivities of this unit indicate a significant increase in secondary porosity, attributable to an open, water-saturated fracture network. The vertical continuity of this horizon, locally connected to the overlying weathered layers, suggests strong hydraulic connectivity and efficient groundwater transfer. Zones where the fractured horizon thickens coincide with depressions in the basement top, indicating pronounced structural control and localized fracture concentration.

The intact crystalline basement, characterized by high resistivities, underlies the model and corresponds to granitic rock, based on field petrographic observations. It is sparsely fractured and poorly permeable. Its irregular geometry reveals substrate relief controlled by deep structures, likely inherited from the regional tectonic history. Basement uplift zones are associated with thinning of the weathered and fractured horizons, reflecting unfavorable hydrogeological conditions. Conversely, basement subsidence areas favor the development of a thicker weathered cover and a more interconnected fractured horizon, thereby constituting preferential zones for groundwater circulation and accumulation.

Overall, the 3D conceptual model demonstrates spatial coherence between basement structuring and the distribution of conductive units. The lateral and vertical continuity of conductive zones suggests the presence of major fracture corridors that act as preferential hydraulic conduits. This architecture aligns with conceptual models of basement aquifers, in which borehole productivity is primarily controlled by the thickness and connectivity of the fractured horizon and by the presence of a sufficiently developed weathered cover to ensure recharge.

True resistivity sections, level-specific maps, and the 3D conceptual model reveal a characteristic vertical and lateral organization of the subsurface typical of crystalline basement aquifers in humid tropical environments. The succession of resistive-conductive units (R1-C-R2), together with their geometry and connectivity, reflects a polyphase weathering profile controlled by substrate lithology, weathering intensity, and inherited tectonics.

The superficial lateritic duricrust (R1), though locally well-expressed (>850 Ω∙m), is spatially discontinuous and thin, indicating advanced dismantling due to differential erosion and current morphodynamics. This observation aligns with West African studies showing that duricrust preservation strongly depends on topography and surface stability ([1]; [4]). Its near absence in northern profiles exposes the weathered layer directly, reinforcing lateral heterogeneity.

Beneath, the conductive horizon C dominates both laterally and in thickness (locally 35 - 40 m). Measured resistivities (25 - 155 Ω∙m) and higher intermediate values indicate pronounced internal heterogeneity, reflecting variations in weathering degree, material texture, and water content. This structure corresponds to progressive weathering models in crystalline basement, where weathered layers evolve vertically from a conductive clay-sandy horizon to coarser, more resistive levels ([16]; [6]).

Level-specific resistivity maps further highlight lateral structuring. At 10 m, resistivity heterogeneity reflects variable surface formations and locally saturated zones. At 25 m, conductive lineaments aligned NNW-SSE suggest structural control, likely due to inherited fractures or shear zones. At 45 m, these trends persist within the basement, where conductive anomalies mark deep fractures. The directional coherence and vertical continuity of these anomalies indicate deep structures traversing the weathering profile, confirming that basement topography and fracture distribution govern the organization of geoelectric units ([12]; [15]).

The geoelectric architecture at Langbabokohou reflects typical basement-aquifer functioning, controlled by interactions among the weathered cover, the fractured horizon, and the intact basement. The conductive horizon C functions as a capacitive shallow aquifer, temporarily storing meteoric water and gradually transferring it to deeper levels. Its lateral extent and local thickness make it a strategic reservoir, particularly for shallow wells.

Discontinuous lateritic duricrust limits direct recharge locally. Where absent or thinned, the shallow aquifer is exposed, increasing vulnerability to anthropogenic contamination. This partial protection is typical of tropical weathered environments, where duricrust provides localized rather than continuous shielding ([13]; [10]).

The transition from conductive weathered layers to resistive basement R2, represented by an intermediate resistivity horizon (635 - 2000 Ω∙m), is key to hydro-dynamic functioning. This fractured horizon, extensively developed at the weathered layer-basement interface, features enhanced secondary porosity from fracturing and mechanical weathering. Its lateral continuity and vertical connection with overlying layers enable efficient drainage toward the deep aquifer. Thickened zones, often coinciding with basement depressions, are hydrogeologically favorable, whereas basement uplifts, with thinned weathered and fractured layers, are less favorable. These patterns are consistent with recent studies of West African basement aquifers ([4]; [3]).

The intact crystalline basement (R2), although generally resistive and poorly permeable, becomes hydrogeologically significant locally where affected by faults or dense fractures, as observed in profiles L600 N and L1000 N. The vertical continuity of conductive anomalies from weathered layers to fractured basement confirms hydraulic activity and links shallow and deep aquifers. These fracture corridors serve as preferential drains, enhancing both flow and long-term storage.

Overall, the 3D conceptual model highlights a hierarchical aquifer system in which functioning relies on the complementarity of a capacitive shallow aquifer and a fractured deep aquifer. Borehole productivity is primarily controlled by the thickness and connectivity of the fractured horizon and by the presence of a sufficiently developed weathered cover to ensure recharge. This organization aligns with current conceptual models of West African basement aquifers and supports the relevance of integrated geoelectrical approaches for sustainable groundwater management.