MAPPING OF LINEAR GEOLOGIC STRUCTURES AT MOUNTAIN TOP UNIVERSITY’S PERMANENT SITE, MAKOGI-OBA, OGUN STATE, NIGERIA USING AEROMAGNETIC AND 2D ELECTRICAL RESISTIVITY TOMOGRAPHY DATA

Abstract

Aeromagnetic and 2D Electrical Resistivity Tomography (ERT) data have been processed, integrated and interpreted for the purpose of mapping linear geologic structures at Mountain Top University’s permanent site in Makogi-Oba, Ibafo area of Ogun State which is currently being considered for development. The study was aimed at mapping linear geologic structures whose occurrences are to be given proper consideration in order to optimise land usage in the physical planning of the study area for development. The total magnetic field intensity (TMI) data were reduced to the magnetic equator to remove asymmetries associated with low magnetic latitude anomalies and was subsequently separated into its regional and residual field constituents. The residual field was enhanced using various techniques which included the Total Horizontal Derivative (THD), Standard Euler Deconvoluion and Tilt Derivative (TDR). Four 1110 m long 2D ERT profiles, probing a depth of about 220 m, were also acquired in the study area using the dipole-dipole array. The data were inverted using the “EarthImagerTM” software. The TMI map was observed to be dominated by elongated to linear magnetic anomalies trending mostly in the approximate N – S direction. Though having a low magnetic relief of about 2.6 nT, the study area reflected four magnetic field intensity zones which were characteristically distinct with magnetic intensity decreasing from West to East. The pattern of the decreasing magnetic intensities eastwards seemed to be suggestive of a step-faulting phenomenon with depth to the faulted blocks decreasing eastwards. The residual field map comprised of both elongated and spherical magnetic anomalies, some with relatively high magnetic intensities while others had relative low magnetic intensities. An intersection of magnetic lows in the eastern part of the study area was identified and interpreted to suggest that the study area is a shear zone. The lineaments delineated by the THD comprised of long, intermediate and short lineaments. From the THD map, the western and southern parts consist mostly of long and intermediate lineaments while the northeastern part was found to be dominated by short lineaments. The Euler Deconvolution mostly resolved long lineaments and their appendages but resolved less of isolated short lineaments. The integrated result of the THD and Euler deconvolution derived lineaments appreared to agree in a good part of the study area while in some parts it can be best described as complementary. From the Euler Deconvolution solutions, depth estimates to the linear geologic structures rarely exceeded 200 m except in the central southern and central northern parts. Tilt derivative solutions presented depths to some delineated semi-regional to regional scaled structures as ranging between 102.94 m and 286.76 m. The 2D ERT inverted section imaged three to four geoelectric layers which were interpreted as interclation of sand and clay. The immediate topsoil was found to be clayey and its thickness approached 8 m in some regions. Some structure suspected to be faults, coincident with the delineated magnetic lineaments, were imaged on the 2D ERT sections. The study area was found to be a shear zone having lots of linear geologic structures within it. The northwestern part of the study area appeared to be the least affected by the presence of linear geologic structures and may require less stabilisation for construction. The immediate topsoil being clayey, geotechnical competence tests may be required for the subgrade soil. Groundwater exploration should be concentrated around mapped faults, especially the ones identified on the 2D ERT sections. To optimize the groundwater resources in the area, the linear geologic structures which were all observed to dip towards the north on the 2D ERT profiles should be intersected at their direction of dip.