Detection of Underground Tunnel Networks Using Integrated Geophysical Techniques in Nsukka Campus, UNN

Reports of subsurface tunneling and related underground structural features within portions of the University of Nigeria, Nsukka campus have raised concerns about the stability of certain buildings and infrastructure elements. This study applies an integrated suite of geophysical methods, including ground penetrating radar, electrical resistivity tomography, and microgravity, to detect and map subsurface tunnel networks and voids within the UNN campus. GPR data were acquired along a grid of 48 profiles using a 100 MHz antenna to image features in the 0 to 10 m depth range. ERT measurements were conducted at fourteen selected locations using the dipole-dipole array. Microgravity measurements were performed at 5 m station spacing across a 2.5 hectare investigation area using a CG-6 Autograv gravimeter. GPR radargrams reveal linear discontinuities and diffractions at depths between 1.5 m and 6 m in a consistent orientation, suggesting elongated subsurface features. ERT tomograms identify low-resistivity corridors that are interpreted as air or rubble-filled cavities within the natural geological matrix. Microgravity Bouguer anomaly residuals show elongated negative anomalies of amplitude negative 0.15 to negative 0.45 mGal co-located with GPR and ERT anomalies, confirming mass deficits consistent with tunnel or cavity structures. Three tunnel segments are delineated with widths of 0.8 to 1.5 m at depths of 2.5 to 5.5 m. These results inform campus infrastructure management decisions and provide a methodology for similar investigations at other institutions.

Keywords: ground penetrating radar, microgravity, electrical resistivity, tunnel detection, UNN campus