Abstract
A detailed ground magnetic survey was carried out in a basement complex underlain Modomo community in southwestern Nigeria with a view to delineating the subsurface structures, estimate the overburden thicknesses and assess the relevance of the geophysical method in groundwater investigation in the locality. The total field component of the earth’s magnetic field was measured at station intervals of 10–100 m along access routes within the study area. The data were corrected for diurnal variation and offset and subsequently reduced to the magnetic equator (RTE). Data enhancement techniques including the second vertical derivative, total horizontal derivative and Euler deconvolution were applied to the RTE data to map edges and estimate depths to the structures. Overburden thicknesses were estimated from 2D magnetic subsurface modeling along eight profiles. The magnetic survey derived lineaments (structures) and overburden thicknesses were validated with resistivity survey derived 2D structures and overburden thicknesses and borehole log data from the study area. Twenty-four lineaments with lengths ranging from 150 to 777 m were identified from the magnetic map. The lineament orientations were E–W, ENE–WSW, WNW–ESE, NNW–SSE, NW–SE and NE–SW directions. Many of the identified lineaments correlated significantly with structures derived from the 2D resistivity images. Comparable thicknesses were observed between correlated magnetic derived overburden thicknesses (4.41–29.4 m) and depths from wells and boreholes (5.48–27.1 m). The study concluded that the magnetic method could be reliably used for overburden thickness estimation and structure mapping required in groundwater potential assessment in a typical basement complex terrain.
Highlights
Hydro-geophysical investigation, most especially in groundwater development-related problems, often involves assessment of aquifer potential, water quality and pollution assessment and aquifer vulnerability/protective capacity study among others (Kinnear et al 2013; Orakwe et al 2018; Olorunfemi and Oni 2019)
The reduced to the magnetic equator (RTE) map (Fig. 5) shows a magnetic field intensity that varies from − 370.77 to 219.43 nT with series of high (A–I) and low (1–9) magnetic anomalies
The electrical resistivity method is often engaged in predrilling groundwater investigation while the magnetic method is rarely used
Summary
Hydro-geophysical investigation, most especially in groundwater development-related problems, often involves assessment of aquifer potential, water quality and pollution assessment and aquifer vulnerability/protective capacity study among others (Kinnear et al 2013; Orakwe et al 2018; Olorunfemi and Oni 2019). The electrical resistivity and electromagnetic methods have proven to be the most useful methods because the hydrogeological parameters such as aquifer porosity and permeability and the concentration of the saturating fluid significantly determine the electrical resistivity/ conductivity that is measured (Zohdy 1969; Ogilvy 1970; Paillet 2013) These methods enable large-scale aquifer characterization that may not be possible through conventional (borehole drilling, pump testing and core sampling) techniques in a minimally/or noninvasive and cost-effective manner (Robinson et al 2013; Ruggeri et al 2013; Deng et al 2016)
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