MAGNETIC MAPPING OF IJEBU-IJESA AREA, SOUTHWESTERN NIGERIA

Abstract

Magnetic mapping was conducted over the Precambrian Basement complex of Ijebu-Ijesa area and its environs employing aeromagnetic and ground magnetic methods. The quality checked aeromagnetic total magnetic intensity data using butterwort filter was digitized along fourteen (14) west - east traverses while the ground magnetic data was collected along three (3) west – east traverses. Depth estimation techniques involving half slope, maximum slope and the freeware Euler deconvolution techniques were used to estimate geomagnetic depths which were later used to generate geomagnetic sections. From these geomagnetic sections probable fracture/fault zones and contact zones where delineated. Furthermore, the aeromagnetic datasets over the study area were processed and enhanced using Oasis MontajTM software. The enhancement of the dataset included reduction to the equator (RTE), first order vertical and horizontal derivatives, upward and downward continuation and pseudo-gravity transformation. From the result of the study, estimated depth to the magnetic sources from aeromagnetic data using maximum slope, half slope and freeware Euler deconvolution techniques are in the range of 81 – 1092, 126 – 693, and 60 – 820 meters respectively while for the ground magnetic data they are 36 – 200, 13 – 69, and 5 – 27 meters respectively. Power spectrum analysis classified the depth measured as low (50 to 180 m), intermediate (280 to 300 m) and high (180 to 300 m). The probable lateral extent estimated from the geomagnetic sections over the rock types range from100 m to 750 m while range of depth is from 126 to 693 m. The deepest portion coincides with depressions and is mostly structurally controlled. Development of structures (Fractures / Faults) is more pronounced on the quartz Schist than on the migmatite gneiss and granite gneiss. 3D Euler deconvolution was carried out with different structural indices (0.0, 0.5, 1.0, 1.1, 2.0 and 3.0) all of which represent different structural models. These structural indices coincide with different geometry such as pipes, cylinders, dykes and spheres. However, the most suitable that coincide with the geology of the study area are 0, 1, 2 and 3 all of which are relevant to mineral investigation and resource development. Pseudo gravity transformation delineated more dense materials that are relevant for solid mineral investigation. Finally, a structural map and a modified geologic map of the study area were then produced using the data generated. It was found that the Iwaraja fault was mapped as a boundary between Quartz schist and Migmatite gneiss and it stretches across the entire study area (approximately 2.5 km) and trends in the NE-SW direction.