Application of geoelectrical resistivity imaging and VLF-EM for subsurface characterization in a sedimentary terrain, Southwestern Nigeria

Geoelectrical resistivity imaging and very-low frequency electromagnetic (VLF-EM) geophysical techniques were integrated to characterize the subsurface as part of preliminary investigations for groundwater resource assessment, development, and management in a sedimentary terrain, southwestern Nigeria. Six parallel 2D geoelectrical resistivity field data were collected using Wenner array; the VLF-EM data were equally collected along the same traverses. In addition, four vertical electrical soundings (VES) were conducted on the site using Schlumberger array to provide layering information. The plots of filtered in-phase and quadrature components of the VLF-EM data as well as their corresponding Fraser and Karous-Hjelt pseudo-sections are presented. The observed apparent resistivity data for the 2D traverses were inverted to produce 2D inverse model resistivity and then collated to 3D data set, which was inverted to obtain 3D inverse model resistivity of the subsurface. Iso-resistivity surfaces for 750, 1,000, and 1,500 Omega m extracted from the 3D inverse model show the 3D distribution of these resistivities. High-resistivity layer at depth range of 10.2-16.4 m and thickness ranging from 11.0 to 21.0, which overly the aquifer unit, delineated in the VES and 2D/3D resistivity models could not be distinctly discriminated in the Fraser and Karous-Hjelt pseudo-sections of the VLF-EM data. However, some conductive linear anomalies thought to be fissures or joints, which could serve as the main conduit path for groundwater recharge, were delineated in the Fraser and Karous-Hjelt pseudo sections. Thus, the use of geoelectrical resistivity or VLF-EM technique alone is inadequate to characterize the subsurface features in the study site; consequently, the integration of 2D and 3D resistivity imaging with VLF-EM technique enhanced the degree of reliability of the subsurface characterization in the study site.