The research, published in the journal Discover Soil, was carried out by Donald Okechukwu Nnebedum of the Department of Geology at the University of Nigeria, Nsukka. The target was an edifice approaching completion in the Nsukka urban area, where fractured foundation soils had been suggested as a primary cause of structural distress. The site sits within the Campano-Maastrichtian succession of the Anambra Basin and is underlain by the Maastrichtian Ajali Formation, a unit composed of medium to coarse-grained sandstones and conglomerates, including angular to subangular quartz-arenites. Fresh samples of this formation are typically whitish with iron staining, but weather to a brownish colour at the surface, a sign of its high iron content. Outcrop evidence suggests the formation is between 25 and 50 metres thick, with fluvio-deltaic and fluvial channel depositional features such as trough and planar crossbedding, mud-draped foresets, and Ophiomorpha trace fossils.

Crucially, the region has a documented history of tectonic deformation. Post-Santonian events have left two fracture trends, trending northwest-southeast and northeast-southwest, in the Campano-Maastrichtian formations around nearby Enugu. These pre-existing weaknesses matter because even though cohesionless coarse-grained sandstones have lower bearing pressure limits of about 440 kilopascals, and medium-grained sandstone soils about 240 kilopascals, fractures can slash that strength dramatically, posing an extreme threat to any structure founded directly above them. Classical bearing capacity analyses, such as Prandtl’s and Terzaghi’s methods, assume a homogeneous soil mass and may simply miss fracture-induced heterogeneity altogether, which is precisely where geophysics earns its keep.

The study deployed two techniques side by side on identical survey plans. Electrical resistivity imaging, or ERI, is the established workhorse for mapping near-surface inconsistencies, and its effectiveness in fracture detection is well documented. Its companion, induced polarization imaging, or IPI, measures a subtler phenomenon. When current is injected into the ground and then abruptly switched off, the measured voltage does not vanish instantly. A residual polarization voltage, known as overvoltage, decays over time as electrical charges stored in the subsurface gradually dissipate. The magnitude of this effect, expressed as chargeability in milliseconds, reflects how well the ground acts like a capacitor. In non-mineralized soils, this response arises from electrolytic or membrane polarization, in which positively charged ions concentrate at electrolyte boundary layers and extend along the pore walls of clastic rocks.

That physics is exactly what makes the method sensitive to fractures. Open fractures enhance porosity and create conduits for fluid movement, and the groundwater filling them behaves as a weak electrolyte rich in dissolved minerals. Fracture zones therefore act like capacitors, accumulating charge that decays slowly once the current is cut off, producing anomalously high chargeability. Fractures that juxtapose two separate blocks or bring chemically dissimilar fluids together can amplify the effect further, and even clay-filled fractures generate a polarization response when current is interrupted. According to Archie’s law, which governs electrolytic conduction in clastic rocks, increasing porosity also drives down bulk resistivity, so fractured zones should appear as abrupt low-resistivity features in resistivity images. The two techniques thus provide independent, complementary fingerprints of the same geological defect.

Four ERI surveys and four IPI surveys were executed along parallel lines crossing the foundation zone, using the same Wenner alpha electrode configuration for both. A base electrode spacing of 10 metres was expanded through separation factors of one, two, and three, reaching a maximum spacing of 30 metres and a median depth of investigation of about 15.57 metres, adequate for non-high-rise foundation studies. Data were acquired with an ABEM SAS 1000 Terrameter configured for simultaneous resistivity and time-domain induced polarization measurement, offering resistivity accuracy of about one percent and chargeability precision of 0.1 milliseconds. Stainless steel electrodes kept contact resistance below 5 kilo-ohms, current injection ranged from 50 to 150 milliamperes, and four to eight stacking cycles per reading boosted the signal-to-noise ratio, with reciprocal measurements on ten percent of data points confirming repeatability errors below three percent. Current and potential cables were routed separately to suppress electromagnetic coupling, a refinement previously shown to improve time-domain IP data quality in multi-electrode systems.

The raw apparent resistivity and apparent chargeability data were organized in block model format and inverted with a least-squares approach using RES2DINV software, a format noted to deliver good solutions for fault modelling. Reliability was assessed carefully. A depth of investigation index, computed by comparing inversions initialized with reference half-spaces of 100 and 1000 ohm-metres, indicated values below 0.2 and therefore well-resolved model parameters to depths of 12 to 14 metres, comfortably enclosing the interpreted fracture zones at 5 to 12 metres depth. Interpreted fracture positions carry lateral uncertainty of roughly 1.5 metres and depth uncertainty of about 1 metre, while absolute resistivity and chargeability values vary by roughly 12 to 18 percent and 9 to 15 percent respectively. Importantly, the spatial coincidence of low-resistivity and high-chargeability anomalies remained stable across perturbation tests.

The results were emphatic. On survey line one, the inverted resistivity section revealed an inclined zone of low resistivity, below 600 ohm-metres, at offset distances of roughly 70 to 90 metres, and the corresponding chargeability image showed an anomalously high zone of 7.77 to 8.98 milliseconds against a background of just 0.5 to 3.0 milliseconds at the same position. Survey line three, on the opposite side of the site, mirrored the pattern, with a low-resistivity anomaly near 203 ohm-metres or less between 75 and 105 metres and high chargeability of 11.6 to 24.8 milliseconds between 80 and 105 metres. Together, the paired lines outlined a fracture trending north-northwest to south-southeast, dubbed the Alpha fracture. Survey lines two and four mapped a second defect, the Beta fracture, trending northeast-southwest, with line two showing resistivity anomalies from 107 to 695 ohm-metres and chargeability from 200 to 395 milliseconds, and line four showing resistivity of 169 to 388 ohm-metres and chargeability between 195 and 296 milliseconds, plus a smaller zone exceeding 603 milliseconds. Signal-to-background ratios for all IPI anomalies exceeded the accepted threshold of 2.0, confirming that the signatures were genuine subsurface features rather than noise artifacts.

The decisive test came from the ground itself. A post-survey site visit revealed pronounced fissures on the foundation floor at the same spatial locations the imaging had flagged, with one set trending north-south, consistent with the Alpha fracture, and another set trending east-west, matching the Beta fracture. These surface expressions tally with the two post-Santonian fault trends previously identified in the Campano-Maastrichtian formations of the Anambra Basin. The study’s multi-evidence framework, combining co-located anomalies, stable inversion convergence, cross-line replication, and surface correspondence, demonstrates that conventional two-dimensional time-domain induced polarization imaging, a cost-effective and easily deployed technique, can stand shoulder to shoulder with electrical resistivity imaging in mapping fracture-induced heterogeneity beneath foundations. The author notes that future work should incorporate targeted borehole data to link geophysical responses directly to specific fracture properties, but for engineers worried about what lurks beneath their next project, the message is clear: the ground has an electrical memory, and it will tell you where it is broken.

Subject of Research: Use of 2D induced polarization imaging to detect fractures in foundation soils at a failing building site in Nsukka, Nigeria

Article Title: Efficacy of 2D induced polarization in detecting fractures in foundation soils

Article References: Nnebedum, D. O. (2026). Efficacy of 2D induced polarization in detecting fractures in foundation soils. Discover Soil, 3(1), Article 108. https://doi.org/10.1007/s44378-026-00268-z

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00268-z

Keywords: induced polarization, electrical resistivity imaging, foundation soils, fractures, geophysics, Nsukka, Anambra Basin, Ajali Formation, chargeability, Wenner array, site investigation, building failure