CONFIGURATION OF THE BASEMENT TOPOGRAPHY OF ABEOKUTA METROPOLIS USING INTEGRATED GEOPHYSICAL METHODS AND MULTI CRITERIA DECISION ANALYSIS (MCDA) FOR GROUNDWATER EXPLORATION

Abstract

ABSTRACT A widely distributed and large source of fresh water is groundwater, being a renewable natural source with insubstantial evaporation loss, wide distribution and low risk of contamination, playing very vital roles in the environment and human health. In Abeokuta Metropolis where pipe-borne water is out of reach of many people, groundwater becomes the major source of water supply and its exploration in a basement terrain is faced with complex decisions because of its contradictory alternatives. The research aims to evaluate the basement topography of Abeokuta Metropolis to provide a better understanding of the geological features and hydrological indices that support the flow and distribution of groundwater as well as the accumulation of economic mineral deposits. The Multi Criteria Decision Analysis via Analytic Hierarchy Process (MCDA-AHP) is a mathematical based technique employed to correctly predict groundwater by scaling priorities of influencing parameters to make informed economic decisions. Six groundwater influencing factors; coefficient of anisotropy, lineament density, overburden thickness, aquifer thickness, aquifer resistivity and lithology were integrated to predict groundwater potentials of Odeda, Obafemi Owode, Abeokuta North and Abeokuta South Local Government Areas (LGAs). These parameters were derived from the processing, analysis and interpretation of high resolution aeromagnetic and electrical resistivity data over the study area. The aeromagnetic data was sourced from the Nigerian Geological Survey Agency (NGSA) while the electrical resistivity data was acquired in situ, through Vertical Electrical Sounding (VES) technique; at a spacing interval of 1000 ± 100 m across parts of the LGAs, representing the research location. Qualitative interpretation of high resolution aeromagnetic data revealed magnetic responses ranging from high-moderate-low signatures which characterized the geology of the study area into Basement, transition and sedimentary zones. Continuous filtering of aeromagnetic maps through two-dimensional fast Fourier transform further delineated various geologic boundaries of the magnetic field. Quantitative interpretations were applied to map the depth to magnetic sources and evaluate subsurface linear structures referred to as lineaments. Spectral Analysis Technique (SAT), Source Parameters Imaging (SPI) and 3D Euler deconvolution method revealed depth range of 99-340, 133-294 and 150-250 m respectively. These depth results confirmed the presence of near surface intrusive rock minerals like kaolin, feldspar, quartzite, among others. Evaluation of lineaments by CET grid analysis showed the study area to be composed of linear structures predicted to be faults, cracks, fractures, voids and cracks, and their NE-SW orientation is in conformity with the Pan African Orogeny. Interpretation of one hundred and forty four VES data aided computations of hydrogeological parameters and mapping of lithological units. Lateritic sequence, sandstones, clay, continental sands, kaolin deposits and Basement rocks that are granitic or magmatic in nature are the lithological units mapped. The integration of all the groundwater influencing factors using GIS produced a model map that is graded into very low, low, high and very high potentials. Evaluation of the basement topography has demonstrated strong reliability in groundwater potential modelling and it is recommended that the results of this research should be adopted as an information framework in making wise investments in Abeokuta Metropolis.