Aeromagnetic, ground magnetic and vertical electrical sounding (VES) methods were used to delineate the basement structures around Iwaraja area, southwestern Nigeria. In doing this, an aeromagnetic sheet 264 of the Geological Survey of Nigeria (GSN) and digitized aeromagnetic map from Nigeria Geological Survey Agency (NGSA, 2008) were interpreted. Also sixteen ground magnetic profiling were carried out, mostly in the E-W direction. A total of sixty seven vertical electrical soundings using Schlumberger electrode configuration was occupied. Qualitative interpretation of the aeromagnetic and ground magnetic profiles suggest varying magnetic intensities from different sources producing the anomaly. Quantitative interpretation aided by the manual half slope and automated Euler deconvolution techniques yielded information on depth to fracture, ranging between 6 and 38 m. Vertical electrical sounding results helped in the delineation of four subsurface geologic layers, also basement depressions that coincides with fracture zones were mapped in the area. The top soil layer resistivity ranges from 50 to 2359 Ωm. The weathered layer (clay, clayey sand, sand and sandy clay) resistivity ranges from 35 to 4935 Ωm while the fractured basement ranges from 152 to 981 Ωm and the fresh basement resistivity ranges from 1132 to 22821 Ωm. A major basement ridge (Rs1) trending approximately in the NE-SW direction was also delineated.
Key words: Basement structures, anomaly signature, Euler deconvolution, aeromagnetic, Iwaraja, depth to fracture zone.
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