Magnetic and Resistivity Imaging of a probable Fault within the Precambrian Crystalline Rocks in Ogbomoso, Southwestern Nigeria

Abstract

A linear geological structure which is suspected to be a fault within the Precambrian Basement Complex rocks of Ogbomoso, Southwestern Nigeria is found in the geological map of Ogbomoso Southwest Sheet 222. The ground magnetic and electrical resistivity methods were used to investigate the structure and determine its physical characteristics. Five (5) traverses with lengths varying from 700–800 m and trending NNW-SSE (N338oW) were established perpendicularly (ENE-WSW, N68oE) to the suspected fault. Total magnetic field data were acquired at 10 m interval, corrected for diurnal variation and inverted using the Geosoft Oasis Montaj software package. Dipole-Dipole resistivity data were acquired on the traverses using a dipole length of 20 m with expansion factor n varying from 1 to 5 and modelled to generate 2D resistivity structures with the DIPPRO™ software package. Thirty-seven (37) depth sounding data were acquired, quantitatively interpreted and the results used to generate geoelectric sections along the suspected geologic features. Three (3) parallel linear features F1-F1, F2-F2 and F3-F3 characterized by peak negative (-110 to -20 nT) magnetic anomalies typical of thin and thick dykes (in low magnetic latitude) and low resistivity vertical discontinuities (˃ 188 m deep, 30–153 m wide) within 2D resistivity structures, were identified across the five traverses. F1-F1 coincided with the suspected fault, F1-F1. F2-F2 and F3-F3 are parallel and suspected to be syngenetic with F1-F1. The absence of a significant vertical displacement, typical of normal faulting, in the basement bedrock on the 2D geoelectric sections corroborated the inference of strike slip faulting from the geological map.