Geophysical Investigation of Landslide Susceptibility Zones in the Idemili River Valley, Anambra State

The Idemili River valley in Anambra State experiences recurrent landslides during intense rainfall events, causing human casualties and infrastructure damage, yet the subsurface conditions governing slope instability remain poorly understood. This study investigates landslide susceptibility zones in the Idemili River valley using electrical resistivity tomography and seismic refraction combined with slope stability analysis. Ten ERT profiles and six seismic refraction lines were established across slopes with documented landslide histories and adjacent undisturbed control sites. ERT data were inverted using RES2DINV, and seismic data were processed with SeisImager software. The seismic refraction profiles define three velocity layers corresponding to weathered soil (280 to 450 m/s), partially saturated saprolite (600 to 1,100 m/s), and competent bedrock (2,100 to 3,200 m/s). ERT tomograms reveal low-resistivity zones (5 to 20 ohm-m) within the slope interiors, interpreted as perched water tables and saturated colluvial fills that reduce effective shear strength. Potential slip surfaces are inferred from the contact between low-resistivity saturated material and the underlying higher-resistivity competent rock. Limit equilibrium analysis using geophysical input parameters shows factors of safety dropping below 1.0 along two slopes under simulated 100-year return period rainfall intensity. These slopes are flagged as high-priority instability zones requiring drainage improvement and vegetation management. The study demonstrates the value of combining electrical and seismic geophysics with slope stability analysis for landslide hazard assessment.

Keywords: ERT, seismic refraction, landslide susceptibility, slope stability, Idemili valley