Geophysical Investigation of Gully Erosion-Prone Zones in Nnewi, Anambra State, South East Nigeria

Gully erosion in Anambra State is a well-documented geomorphological hazard, and Nnewi is among the towns most severely affected, with several residential and commercial areas threatened by advancing gully heads. This study integrates electrical resistivity profiling and seismic refraction to investigate the subsurface conditions predisposing certain zones to preferential gully initiation and expansion. Resistivity measurements along four cross-gully profiles identify subsurface lithological contrasts, particularly sand lenses within clay-rich formations, that facilitate differential infiltration and piping processes. Seismic refraction traverses perpendicular to the main gully axes resolve velocity layering and indicate zones of low compressional wave velocity indicative of loose, poorly consolidated material susceptible to internal erosion. The interface between the overlying red earth and the underlying pale-coloured sand horizon is mapped geophysically and compared with geological cross-sections from exposed gully walls. The combination of a permeable sand layer at 3 to 7 metres depth and a relatively impermeable clay base creates conditions for lateral seepage and progressive internal collapse consistent with observed gully morphology. The geophysical data provide a subsurface framework for understanding gully evolution mechanisms and for targeting surface water diversion and stabilisation measures at the most geotechnically vulnerable locations. Keywords: gully erosion, resistivity, seismic refraction, Nnewi, subsurface lithology