Hydrogeophysical Assessment of Subsurface Controls on Gully Erosion Using Electrical Resistivity Methods in Orlu, Southeastern Nigeria

Abstract

Gully erosion constitutes a critical environmental threat in Orlu, southeastern Nigeria, yet the subsurface controls governing this process remain poorly understood. This study employs a quantitative hydrogeophysical approach to elucidate the mechanisms underlying the region's vulnerability to gully erosion. Twelve Vertical Electrical Sounding (VES) stations were deployed using the Schlumberger array to map subsurface resistivity distribution and lithological characteristics. The acquired data revealed significant geological heterogeneity, with resistivity values ranging from 65 to 44,800 Ωm. With increasing depth, resistivity exhibited a consistent upward trend, commencing at 450 Ωm in the moist topsoil and rising sharply to 18,500 Ωm beyond 50 metres. This vertical transition demarcates a shift from stable surface soils to thick, dry, and unconsolidated sandy formations. The results indicate that approximately 68% of the study area falls within a high-risk zone (resistivity exceeding 5,000 Ωm), which correlates precisely with actively forming gully sites. These high-resistivity sandy layers are highly permeable yet lack cohesive strength, functioning as preferential conduits for subsurface flow and internal piping that progressively destabilises the ground from within. By statistically linking resistivity data to erosion behaviour, this research establishes a predictive framework for identifying erosion-prone sites. The findings demonstrate that effective land management and erosion risk assessment in sedimentary regions cannot be accomplished without integrating hydrogeophysical subsurface data.