Use of Geophysics in Ground Stability Assessment for Mining Operations in Enugu State

This study examines the use of geophysical methods for ground stability assessment in coal mining operations in Enugu State, South East Nigeria. Enugu State hosts Nigeria's oldest coal mining operations, with both active and legacy underground workings that create ground stability and subsidence risks for communities, infrastructure, and active mine operations. Near-surface geophysical methods can identify old mine workings, zones of undermined overburden, fracture networks, and water-bearing zones that represent ground stability hazards. This study applies ERT, seismic refraction, and microgravity surveying across two active and three legacy mine sites in Oji River, Awgu, and Enugu South local government areas. ERT data are acquired using 96-electrode dipole-dipole arrays with 3-metre electrode spacing. Microgravity is measured at 10-metre station intervals on a 50 by 50 metre grid over subsidence-prone areas. Seismic refraction characterises P-wave velocity profiles to identify zones of reduced velocity associated with void spaces. Historical mine plan maps are used for geophysical interpretation guidance. Findings reveal low-resistivity anomalies below 5 ohm-metres and gravity deficits of 0.05 to 0.3 milligal over legacy void zones, with co-located seismic velocity minima confirming weakened rock mass. Three previously unrecorded void clusters are identified within 20 metres of surface beneath active roadways. One site shows active subsidence currently affecting structures. The study recommends mandatory geophysical ground stability surveys before infrastructure construction or rehabilitation over legacy mining areas, and periodic monitoring surveys at active mine sites.

Keywords: ground stability, mining, ERT, microgravity, Enugu State.