Very low frequency - electromagnetic (VLF-EM) and Offset Wenner resistivity survey of spring sources for groundwater development

Abstract

Geophysical mapping involving VLF Electromagnetic profiling along seven profiles, ten Offset Wenner and two azimuthal soundings were employed in the study of perennial spring sites at Iloyin community in Akure metropolis, southwest Nigeria. Geophysical methods are useful in mapping areas of natural water resource for groundwater abstraction or construction of containment such as dam or fishpond. Linear inversion of the Electromagnetic measurements involved application of the Fraser and Karous-Hjelt filter on the measured real components of the field data and generation of pseudo sections. The azimuthal sounding data were presented as azimuthal graphs in both Cartesian and Polar coordinates in conjunction with the polar plots of the Offset Wenner resistances (RD1 and RD2). The topsoil is generally thin with resistivity values ranging between 65 to 612 Ohm-m and thickness from 0.5 to 2.9 m. The underlying layer with resistivity value ranging from 25 to 468 ohm-m is the main aquifer recognized as quartzite bed and quartzite impregnated clayey aquifer. The Aquifer layer thickness varies between 0.9 to 32.6 m. The basement rocks, which graduate from highly weathered/fractured to fresh bedrocks, are characterized by resistivity values ranging from 827 to 11064 Ohm-m. Aquifer units in the area comprise brecciated quartzite, clayey sand and fractured bedrock. The topsoil is highly loose and unprotected which makes the aquifer in the study area susceptible to infiltration and groundwater being prone to pollution. Key words: Spring, electromagnetic, Offset Wenner, Azimuthal resistivity, filtering.