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Lineament-Density-Guided Vertical Electrical Sounding for Geoelectric Characterization and Groundwater Potential Mapping of a Basement Complex Terrain, Southwestern Nigeria

This study demonstrates that integrating lineament-density mapping from remote sensing and aeromagnetic data with Vertical Electrical Sounding (VES) effectively identifies fracture-enhanced groundwater potential and validates structural favorability models in the Precambrian basement complex of southwestern Nigeria, offering a reliable strategy for siting boreholes in areas previously plagued by low yields.

Original authors: OLUBUNMI PHILIP OLUWAJANA, MURAINA ZAID MOHAMMED, cyril chibueze okpoli, oladotun afolabi oluwajana, temidayo olamide ALE

Published 2026-08-12

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: OLUBUNMI PHILIP OLUWAJANA, MURAINA ZAID MOHAMMED, cyril chibueze okpoli, oladotun afolabi oluwajana, temidayo olamide ALE

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Hidden Map Beneath Our Feet

Imagine the Earth's crust in some places is like a giant, solid block of concrete. It's hard, unbroken, and holds almost no water inside the rock itself. But in other places, like the basement complex terrains of southwestern Nigeria, this "concrete" has been battered by millions of years of wind, rain, and heat. This weathering process cracks the rock, creating a hidden, sponge-like layer of broken stones and soil right beneath the surface. This is where the water hides. Finding it is like trying to locate a specific, wet sponge inside a massive, dry pile of rocks without being able to dig a hole first.

To solve this puzzle, scientists use a method called "Vertical Electrical Sounding" (VES). Think of it as sending a giant, invisible electrical pulse into the ground. Just as a flashlight beam changes when it hits a mirror versus a piece of cloth, electricity travels differently through wet clay, dry sand, or solid rock. By measuring how the electricity bounces back, scientists can build a 3D picture of the layers underground. But there's a catch: the ground is huge, and you can't measure every single spot. So, researchers need a clue to tell them where to look. That clue is "lineaments." If you look at a map from space, you might see long, straight lines where the land has cracked or shifted. These are the "fault lines" or "cracks" in the Earth's skin. The big question for anyone needing water is: Do these surface cracks point to the hidden, water-filled sponges deep down?

The Crack Hunt at Adekunle Ajasin University

This paper tells the story of a team of scientists who went to the Adekunle Ajasin University campus in southwestern Nigeria to answer that exact question. The campus had a problem: they had drilled many water wells, but a lot of them were dry or gave very little water. It was like trying to find a treasure chest by digging holes at random, only to keep hitting empty spots. The researchers decided to try a smarter approach. They first used satellite images and magnetic maps to draw a "crack map" of the area, identifying zones where the lineaments (the surface cracks) were very dense and zones where they were sparse. They then set up 20 specific spots to test the ground, deliberately choosing locations that ranged from "very few cracks" to "tons of cracks."

Using their electrical sounding equipment, they sent pulses into the ground at these 20 spots and listened to the echoes. They found that the ground wasn't just a flat layer of dirt; it was a sandwich of different materials. Usually, there was a thin top layer of soil, a middle layer of weathered rock (the "sponge"), and a bottom layer of hard, fresh rock. The thickness of that middle "sponge" layer was the key.

The results were like a treasure map that actually worked. In the areas where the satellite map showed a high density of cracks (the "very high" lineament zones), the electrical tests confirmed that the weathered, water-holding layer was much thicker. These spots, which the team called "moderate-to-good" prospects, were often right next to the few wells on campus that actually worked. Conversely, in the areas where the surface looked smooth and had very few cracks, the electrical tests showed a very thin "sponge" layer sitting directly on top of hard, dry rock. These were the "poor" spots, matching the locations of the failed, dry wells.

However, the story has a twist that keeps things from being too simple. While the "crack map" was great at finding where the water might be, it didn't tell the whole story about how safe that water would be. The researchers calculated a "protective capacity" score for each spot, which measures how well the ground above the water filters out pollution. They found that almost everywhere, this score was "weak" or "poor." It's like finding a great swimming hole, but realizing the fence around it is full of holes, so anyone could jump in and throw trash. Even in the best spots, the ground was too sandy and thin to stop surface dirt from seeping into the water.

There was also one very strange spot (Station 14) where the ground was incredibly resistant to electricity, suggesting a giant vein of quartzite rock. Even though this spot was in a "crack-heavy" zone, it wasn't a water source at all; it was just a hard, dry rock formation. This proved that just because a map shows a lot of cracks doesn't guarantee water; sometimes it just points to a different kind of rock.

In the end, the paper suggests that using satellite crack maps is a brilliant first step to narrow down where to drill, saving time and money. It successfully identified the "good" spots and the "bad" spots based on the thickness of the weathered layer. But the authors warn that this map isn't a magic wand. You still need to do the electrical tests to confirm the water is there, and you absolutely need to check if the ground above it is thick enough to protect the water from pollution. The "crack map" gets you to the right neighborhood, but you still need to knock on the right door and check the locks before you start digging.

Technical Summary: Lineament-Density-Guided Vertical Electrical Sounding for Geoelectric Characterization and Groundwater Potential Mapping

Problem Statement
Groundwater development in Precambrian basement complex terrains, such as the study site at Adekunle Ajasin University in Akungba-Akoko, southwestern Nigeria, faces significant challenges. The parent igneous and metamorphic rocks possess negligible intrinsic porosity and permeability; thus, economic aquifers exist only in zones of secondary porosity created by weathering, fracturing, and faulting. Locating these zones is notoriously difficult using surface observation alone due to regolith and vegetation cover. This difficulty has resulted in a documented history of low-yield and failed boreholes on the university campus. While Vertical Electrical Sounding (VES) is a standard geophysical method for resolving near-surface layering, its effectiveness is often limited by the one-dimensional, point-specific nature of the data when station locations are chosen arbitrarily or logistically rather than based on structural models.

Methodology
This study employed a targeted survey design to validate a structural favorability model derived independently from Landsat-8 remote sensing and aeromagnetic lineament mapping (reported in a companion paper).

Site Selection: Twenty Schlumberger-array VES stations were sited specifically at lineament intersections. These locations were deliberately chosen to span the full spectrum of composite lineament-density classes, from very-low to very-high, and included sites proximal to both productive and failed existing boreholes.
Data Acquisition: Apparent resistivity data were collected using an Ohm-mega resistivity meter with a maximum current-electrode half-spacing (AB/2) of 100 m.
Data Processing: Data were interpreted using a two-step process: initial partial curve matching against standard master curves, followed by iterative computer-assisted inversion using WinResist software.
Analysis: The study classified geoelectric curves into standard families (H, K, A, HA, KH, etc.) and computed Dar-Zarrouk parameters—Longitudinal Conductance ( $S$ ) and Transverse Resistance ( $T$ )—to assess aquifer protective capacity and transmissivity potential. Aquifer protective capacity was qualitatively classified using the threshold scheme of Oladapo et al. (2004).

Key Results

Geoelectric Stratigraphy: The survey delineated three-to-four-layer sequences consisting of topsoil, a weathered layer, a fractured/weathered basement transition zone, and fresh basement. Basement depths ranged from approximately 6.4 m to 73.8 m. Resistivities varied from ~47 $\Omega\cdot$ m in clay-rich weathered zones to over 40,000 $\Omega\cdot$ m in unweathered basement and a locally intercalated quartzite body.
Curve Type Distribution: The dominant curve types were H/HA (45% of stations) and K/A (40%).
- High Lineament Density: Stations in high-density corridors were dominated by H/HA-type curves, indicating thicker, more conductive weathered/fractured horizons. These were classified as moderate-to-good groundwater prospects.
- Low Lineament Density: Stations in low-density zones were dominated by K/A-type curves, characterized by thin overburden directly overlying resistive, unfractured basement, classified as poor prospects.
Dar-Zarrouk Parameters:
- Protective Capacity ( $S$ ): The majority of stations (17 of 20) fell into the "weak-to-poor" protective capacity class ( $S < 0.2$ mho). Only three stations reached the "moderate" class. None reached "good," "very good," or "excellent" classes.
- Transverse Resistance ( $T$ ): High $T$ values were observed at stations with thick, highly resistive basement or quartzite units (e.g., VES 14), rather than exclusively at stations with good groundwater prospects.
Anomalies: Station 14, located in a high-density lineament zone, revealed an anomalously resistive layer (interpreted as quartzite) with extremely high transverse resistance but low storage potential, demonstrating that high lineament density does not always equate to a productive aquifer.

Key Contributions

Validation of Structural Siting: The study provides empirical evidence that composite lineament density, derived from remote sensing and aeromagnetic data, serves as a practical first-pass criterion for siting VES surveys and boreholes in basement complex terrains.
Integration of Structural and Geoelectric Data: It demonstrates a spatial correlation where high lineament density corresponds to thicker, more conductive weathered zones (H/HA curves) and productive borehole locations, while low density corresponds to thin overburden and failed boreholes.
Critical Assessment of Protective Capacity: The research highlights a critical limitation in relying solely on structural indicators or weathered-zone thickness. Despite identifying "good" groundwater prospects based on thickness, the Dar-Zarrouk analysis revealed that the aquifer protective capacity across the study area is predominantly weak-to-poor, indicating a high vulnerability to surface contamination.
Lithological Caveats: The study identifies that high lineament density can sometimes indicate resistive, non-aquiferous lithologies (such as quartzite veins) rather than productive fracture zones, cautioning against the exclusive use of lineament density or $T$ values without lithological context.

Significance and Claims
The authors conclude that composite lineament density is a useful, non-invasive tool for prioritizing sites for borehole investigation in structurally analogous basement terrains. However, the paper modestly asserts that this structural data is insufficient on its own. The findings underscore the continued necessity of direct geoelectric verification (VES), explicit curve-type classification, and Dar-Zarrouk protective-capacity assessment to resolve site-specific exceptions and contamination vulnerabilities that structural data alone cannot reveal. The study advocates for a multi-criteria approach combining lineament density, VES curve character, protective capacity, and wellhead protection assessments for final site selection, rather than relying on any single indicator.

The Hidden Map Beneath Our Feet

The Crack Hunt at Adekunle Ajasin University

Technical Summary: Lineament-Density-Guided Vertical Electrical Sounding for Geoelectric Characterization and Groundwater Potential Mapping

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