IOSR Journal of Applied Physics (IOSR-JAP) e-ISSN: 2278-4861.Volume 8, Issue 3 Ver. II (May. - Jun. 2016), PP 90-98 www.iosrjournals.org DOI: 10.9790/4861-0803029098 www.iosrjournals.org 90 | Page Ground Magnetic Survey of the Charnokitic Dykes in the Areas Around Omu-Ijelu, Southwestern Nigeria D.O. Ogagarue 1 , J.E. Emudianughe 2 1,2 (Department of Earth Sciences, Federal University of Petroleum Resources, P.M.B. 1221, Effurun, Nigeria). Abstract: A ground magnetic survey was carried out in the areas around Omu-Ijelu in the basement complex of southwestern Nigeria, using the Proton precession magnetometer. The aim was to map the magnetic mineralization of the charnokitic dykes in the area and determine their approximate burial depths and lateral extents. Total magnetic field intensity data was acquired along four irregularly spaced E-W traverses at stations spaced 10 m along each traverse. Diurnal correction was applied to the data after which they were smoothed using a running mean technique. Polynomial fitting was used to estimate the regional magnetic field in the area, and this was removed from the smoothed data to obtain the residual magnetic field. Two major dykes with very high positive magnetic intensity were delineated in the eastern and western parts of the area at subsurface depths of 25 m and 18.75 m, respectively. The dykes may have been caused by multiphase intrusions emplaced over time. The results also show two distinct zones of weakness, fracturing and/or shearing associated with large negative magnetic anomalies. One of these is massive, and straddles NE-SW across the entire breath of the survey area. These zones may be the result of residual forces arising from deformation, and tend to infer that intense tectonic activities occurred in the area in the past. They are likely to be the main acquiferous zones and therefore prospective zones for hydrogeologic activities in the area. Keywords: Basement complex, Charnokitic dykes, Magnetometer, Diurnal correction, Magnetic anomalies. I. Introduction The earth has a magnetic field which, at any point on the surface of the earth, is a vector defined by magnitude and direction. About 99% of the geomagnetic field is thought to originate within the earth, from convection processes in the fluid, outer core of the earth. The remaining 1% contribution arises from sources external to the earth, and results mainly from the interaction of ionospheric currents with the solar wind. Certain rocks and minerals found within the earth are magnetized by induction by the geomagnetic field, and they retain a permanent magnetization resulting from the induced field at temperatures below the Curie point. Within the area occupied by these rocks and minerals, there is an additional magnetic field such that the total magnetic field is the vector sum of the retained localized magnetic field, called remanent magnetization, and the inducing geomagnetic field. The magnitude, otherwise known as intensity of the total magnetic field at any point on the earth’s surface therefore constitutes the sum of the localized magnetic field and geomagnetic field at that point, and the direction of the field, being a vector, is described by the magnetic inclination and declination at that point. The magnitude or intensity of the acquired magnetization, I , is proportional to the magnetic field causing the magnetization, and is given by: H k I  (1) where, k  magnetic susceptibility  H strength of the magnetic field causing the magnetization The ratio of the scalar magnitudes of the component of the remanent magnetization to the component of the inducing geomagnetic field is called the Koenigsberger ratio, Q, for the rock [1] and is given by: in I rm I Q  (2) where, rm I = scalar magnitude of magnetism retained in I = scalar magnitude of inducing geomagnetic field The localized magnetic field, otherwise known as the residual magnetic anomaly can be determined by removing the effect of the geomagnetic field from the total magnetic field intensity recorded at a station. Magnetic survey is a natural source geophysical exploration method employed to map rocks and ore- bearing minerals, including other subsurface geologic features such as faults, contact zones and intrusions on the