Abstract
Integrated geophysical techniques involving magnetic and radiometric data were used to investigate the subsurface geometry of Igarra schist belt in Precambrian basement complex of southwestern Nigeria which falls within the latitude range of 7° 1635.6” to 7° 1736.0’ North and longitude range of 6° 0534.1” to 6°05’44”East, with a view to interpret the geology of part of Igarra schist belt and estimate the depth to basement of magnetic source in the area. Six profiles were established with the objective of delineating the metasediments and its structural attributes, and geophysical measurements were made at 5m intervals along these profiles. The results of magnetic datasets were presented as maps and profiles of varying magnetic intensities and the causative rock types. Qualitative interpretation of the magnetic data using Reduction to pole, Analytical signal, first vertical, horizontal and tilt derivatives, upward continuation filters shows what has been interpreted as the signature of a near-vertical fault, trending along a NE-SW and NW-SE direction while radiometric datasets gave geochemical rates of U, Th and K as well as the effect of water contents on the rocks present within the study area. Qualitative and quantitative interpretation of individual magnetic anomalies and geological knowledge of the survey area yielded information on the depth of the geological features (e.g. rock contact, faults or fractures), structure and magnetic properties of rock units. 3D Euler deconvolution and Average radial spectrum were adopted to estimate the depth of the magnetic sources which ranges from – 92.895m to 73.95m; -115.45m to 57.44m and 63m to 44.1m; 88.2m to 81.9 m respectively. The Igarra schist belt has evolved different episodes of orogenies and the metasediments were formed from low grade regional metamorphism of argillaceous sediments such as shales or sediments of varying composition.
Highlights
Based on a study, ground magnetic study is used for detail mapping in order to understand the subsurface geology of an area [1]
The Total Magnetic Intensity (TMI) image show high magnetic susceptible areas in low magnetic values while less magnetic susceptible areas are depicted as high magnetic values except for the analytical signal image and reduction to the pole image where high magnetic susceptible areas are shown as high anomalies
In order to map out the lithology and geological structures of the study area, ground radiometric and magnetic datasets collected over the area were processed and enhanced
Summary
Ground magnetic study is used for detail mapping in order to understand the subsurface geology of an area [1]. The use of integration of geophysical methods such as radiometric, electromagnetic and magnetic techniques as a geological and lithological mapping instruments have been confirmed by numerous authors [3,4].The technique requires measurements of the amplitude of magnetic components at discrete points along traverses distributed regularly throughout the survey area of interest. Three components are measured which are horizontal, vertical and total components. The vertical components and the total components are mostly used in the past studies to delineate faults, fractures, depth to magnetic basement and other geological structures. Total Magnetic Intensity (TMI) or total components were measured at discrete points along traverses in order to investigate on the geologic features like fault, fracture and rock contact distribution within the study area
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