Numerical modeling of 2-D and 3-D geoelectrical resistivity data for engineering site investigation and groundwater flow direction study in a sedimentary terrain

Abstract

In the design of engineering structures, collaborative efforts must be ensured to determine in-situ subsurface soil conditions and depth to competent layers across any proposed site so as to ensure the stability of civil engineering structures, to avoid the devastating effects of structural failure and collapse rampant of late. Geophysical investigations play an important role in the geotechnical detection of subsoil properties. The study presents one of the geophysical methods, electrical resistivity tomography (ERT), which allows for an accurate and spatially instant recognition of the subsurface geology and the phenomena occurring in it. Numerical modeling of an orthogonal set of 2-D apparent resistivity data acquired in an engineering site in a 100 × 70-m² rectangular grid using the Wenner electrode configuration was used to create models of subsurface resistivity which converges with the field data. Six (6) Schlumberger soundings were also performed radially in the site with half-current electrode separation (AB/2) of 120 m. Anisotropy polygon was constructed based on the radial electrical sounding to infer on groundwater flow direction. 2-D apparent resistivity data was processed and inverted using Dipprowin program. The entire set of 2-D apparent resistivity lines were collated into a 3-D data set, and inverted using Earth Imager 3-D software and RES3DINV program to produce a 3-D resistivity model of the subsurface that matches the measured field data. The geological model observed from the 2-D and 3-D resistivity models showed very low resistivity values in the order ρ $\le$ 140 Ωm which indicates a zone of saturation (pore water). Moderately low resistivity values (341–826 Ωm) indicative of porous sand saturated with water (wet sand) and high resistivity values in the order ρ $>$ 1000 Ωm indicative of dry sand were observed in the 2-D and 3-D resistivity models. This interpretation was based on prior geologic information and lithologic logs of the subsurface formation obtained from borehole drilled in the site. 3-D depth slices showed high resistivity values (ρ > 1000 Ωm) from the third to fifth layer at a depth range of 5.38–16.9 m. This implies that construction of civil engineering structures in the site would be favored preferably at the fifth layer at a depth range of 12.5–16.9 m. 2-D images extracted from the 3-D inversion model showed better image clarity and subsurface changes in materials attributed to changes in apparent resistivity values within the subsurface in the vertical plane (x–z and y–z) than in the 2-D inversion sections. Anisotropy polygon of radial sounding showed a dominant resistivity trend in the northeast to the southwest direction along 60°, and indicates that groundwater flow is along 60° (NE–SW) trend. This trend was also observed in iso-resistivity contour map of second and third layers for VES 1–6, which showed low-resistivity trend towards the northeastern part of the site, and implies that rocks in the northeastern part have low resistivity values which implies high porosity and validates that groundwater flow is in this direction, in line with the direction of high porosity.