Uncovering soil piping vulnerability using direct current geophysical techniques in Awka, Anambra State, Nigeria

Abstract

In recent times, there have been increasing reports of soil subsidence occurring in various parts of Anambra State, Nigeria. The formation of soil pipes in the subsurface, which has been reported by many researchers globally, is the chief cause of these subsidence incidents. Thus, this research aims to use Direct Current (DC) geophysical techniques to delineate the spatial distribution, pattern, and characteristics of these soil pipes at two sites: Awka Site 1 and Awka Site 2 both in Awka South LGA of Anambra state, Nigeria. Two electrodes array configurations - the dipole-dipole array for analyzing the Electrical Resistivity Tomography (ERT) and the Schlumberger array for analyzing the Vertical Electrical Sounding (VES) - were employed. Nine ERT profiles and eighteen soundings were carried out. The results from the ERT survey divided the subsurface into six distinct structures. The high-resistivity 3000-30000Ωm, the eroded structure with dry pore spaces, which mainly occur at the top of the profile, was interpreted as having the right conditions for the formation of soil pipes. The result from the VES survey revealed two to five different geo-electrical sections and fourteen distinctive sounding curves that are characterized by the vertical changes in the subsurface. The weathered soil with resistivity ranging from 1200-30000Ωm, which aggregates very close to the surface 0-5m of the study areas, was interpreted as having the best soil formations that allow the building of soil pipes. The 2D and 3D iso-resistivity maps obtained from the data of the VES show that soil pipes follow the NW-SE direction. This coincides with the stress direction, fluid migration paths, and sloppy terrains of the study areas.