Investigating the Susceptibility of Tunnel Erosion in Southern Nigeria using Integrated Geophysical Methods

Abstract

There has been a growing incidence of soil subsidence in various regions of Anambra State, located in southern Nigeria. These subsidence events primarily stem from the formation of natural tunnels or soil pipes within the subsurface. To delve deeper into this issue, the present study employed two geophysical techniques, namely the Very Low Frequency Electromagnetic (VLF-EM) method and the Electric Resistivity Method utilizing the Dipole-Dipole array. These methods were utilized to investigate the characteristics and spatial distribution of soil pipes at two specific sites: Awka Site 1 and Awka Site 2, both situated within Anambra State, Nigeria. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed. On each profile, the inphase and outphase were collected using the Abem Wadi Meter after a confirmed connection to the external satellite, while nine dipole-dipole profiles were carried on the same location. Four profiles with transverse lengths of 100 m and 5 m spacing were surveyed (Figures 3 and 4). The data analysis from the VLF-EM survey reveals that a significant portion (80%) of the low conducting zones observed in the pseudosection originates from the top of the profile, indicating a downward trend in the formation of soil piping. The VLF-EM result also inferred that the subsurface voids in the study areas extend vertically downward up to 10 meters, with an average horizontal extension exceeding 0.5 meters. This highlights the extensive spatial reach of the subsurface voids and emphasizes their potential impact on the surrounding environment. The dipole-dipole survey conducted in the study area identified six distinct structures, with the eroded formation being particularly noteworthy. This structure, characterized by resistivity ranging from 1200–30000 Ωm, plays a significant role in creating favorable conditions for soil piping. Furthermore, the presence of strong dispersive soils increases the likelihood of soil piping occurrences within this structure.