Magnetic surveying as an aid to geological mapping: a case study from Obafemi Awolowo University campus in ILE-IFE, southwest Nigeria

Abstract

A magnetic map of the Obafemi Awolowo University (OAU) Campus, in Ile-Ife, was produced from ground magnetic measurements on all the roads and footpaths. The study was aimed at improving our knowledge of the structural disposition of the lithologies within the area. The magnetic data obtained were subjected to diurnal corrections, removal of the International Geomagnetic Reference Field, reduction to the magnetic equator, pseudogravity transformation and residualization to enhance the anomalies. The residual magnetic and pseudogravity anomaly maps were qualitatively and quantitatively interpreted using the total horizontal derivative, second vertical derivative, pseudogravity transformation, Euler deconvolution, and 2-D forward modeling techniques. The derivative maps showed that the area is clearly divided into three main magnetic (and pseudogravity) anomaly zones, demarcated by five contact locations (C1 to C5), which had close associations with the main lithologies of grey (or banded) gneiss, granite-gneiss and mica schist. The contact C1 indicated that the granite-gneiss might not be as extensive in the northwest as suggested by the geological map, while C4 and C5 indicated that the mica schist might in fact be less extensive, and the grey gneiss more extensive in the south. Two faults, F1 and F2, mapped by earlier workers in the north and south of the area respectively were centered along two broad quasi-linear NE-SW trending zones with relatively higher magnetic susceptibility and density than the host rocks. The faults were strike-slip faults, steeply dipping and could be associated with a zone of prominent magnetic low and pseudogravity high. The depth to the fault zones derived from Euler deconvolution solutions and forward modeling ranged from 18 to 40 m. The study concluded that the magnetic character of the two fault zones suggested zones of weakness in the upper crust through which more mafic rocks might have infiltrated.

Keywords: Ground magnetic, Pseudo-gravity, Subsurface Geology, Strike-slip Fault