Geoelectric investigation of a site for a proposed airport in Ebonyi State, southeast Nigeria

Abstract

A geoelectric investigation, involving 1-D Vertical Electrical Sounding (VES) with the Schlumberger array and 2- D Electrical Imaging with the dipole-dipole array, was carried out at a site proposed for an airport in Ebonyi State. This was with a view to generating near and subsurface images required for the  assessment of the suitability of the site for civil engineering construction works. Fifty six (56) VES  stations were occupied along eight (8) traverses while the dipole-dipole profiling was carried out along  segments of three of the traverses. The VES curves were quantitatively interpreted and the results were used for the generation of geoelectric sections and isopach map of the lateritic layer. Four subsurface layers were delineated. These include the topsoil (lateritic in most places), lateritic shale, shale/clay and siltstone. The topsoil resistivity values varied from 24-5898 Ωm while the thicknesses ranged from 0.7 - 4.6 m. The lateritic shale second layer resistivities and thicknesses were 101-473 Ωm and 1.6 - 8.3 m respectively. The clay/shale third layer showed thicknesses and resistivity values of 3.6 – 108.3 m and 1 – 98 Ωm respectively. The fourth layer was siltstone with resistivity values ranging from 53 -3170 Ωm. The 2-D resistivity structures delineated two subsurface layers: a merged topsoil/lateritic shale top layer and the underlying clay/shale layer. No subsurface geologic structure was delineated. The high resistivity lateritic topsoil/lateritic shale and the basal siltstone constituted the two competent layers while the low resistivity shale/clay, the incompetent horizon. The upper competent lateritic layer could host light-medium weight civil engineering structures while heavy structures would need to be anchored on friction piles.

Keywords: Geoelectric Investigation, Subsurface Sequence, Subsoil Competence, Structure Foundation.