Geophysical Characterisation of Landslide Susceptibility in the Obudu Mountain Area, Cross River State, South South Nigeria

The Obudu Mountain Range in Cross River State is characterised by steep slopes, high rainfall intensities, and deeply weathered basement rocks that create conditions for landslide initiation, with increased frequency of mass movement events having been observed following periods of intense precipitation. This study characterises the subsurface conditions associated with landslide susceptibility using electrical resistivity tomography and refraction seismic across five slope profiles representing different levels of observed instability. ERT profiles of 200 metres were acquired on each slope transect using the Wenner array, with additional 150-metre seismic refraction traverses collocated with selected ERT lines. Inverted resistivity models reveal the presence of a thin, low-resistivity, clay-rich weathered horizon overlying more resistive partially fractured basement at depths of 5 to 18 metres. Zones where the clay-rich layer thickens and the resistivity contrast with underlying material diminishes are identified as high-susceptibility locations consistent with mapped landslide scars. Seismic velocity models indicate that material with compressional velocities below 600 metres per second extends 2 to 4 metres deeper on active landslide slopes than on adjacent stable slopes, attributable to the remoulding and dilation of slope materials during previous failure episodes. The integrated geophysical model provides a basis for landslide hazard zonation supporting spatial planning and road safety management on Obudu mountain roads. Keywords: landslide susceptibility, resistivity, seismic refraction, Obudu Mountain, slope stability