Mapping of Subsidence Zones and Soil Settlement Risk Using Ground-Based LiDAR and Resistivity in Enugu-Ezike, Igbo-Eze North

Land subsidence driven by underground coal and limestone dissolution has been documented in parts of Enugu-Ezike, posing significant risk to building foundations and road infrastructure in the area. This study integrates ground-based LiDAR scanning and electrical resistivity tomography to map active and incipient subsidence zones in Enugu-Ezike, Igbo-Eze North. LiDAR data were acquired along ten scan stations using a Leica ScanStation C10 terrestrial laser scanner, providing millimetre-precision surface topography and deformation mapping over approximately 3 square kilometres of affected terrain. Electrical resistivity tomography was conducted at twenty sites identified from the LiDAR survey as exhibiting anomalous surface depression or cracking. Two-dimensional inversion of ERT data was performed using RES2DINV software. LiDAR-derived digital elevation models reveal surface depressions of 0.15 to 1.2 m amplitude distributed in clusters consistent with pillar collapse in shallow coal workings. ERT profiles beneath the most severe subsidence areas reveal cavities as low resistivity voids (2 to 8 ohm-m) at depths of 5 to 20 m, interpreted as collapsed mine galleries filled with water and debris. The spatial distribution of subsidence features correlates strongly with historical mining records from the defunct Okpara Coal Mine. A risk zonation map categorizes the study area into high, moderate, and low subsidence risk classes based on cavity size, depth, and proximity to surface infrastructure. These results are critical for guiding land use restrictions and stabilisation measures in Enugu-Ezike.

Keywords: LiDAR, electrical resistivity tomography, land subsidence, coal mine, risk zonation