Abstract
The correction of the diurnal magnetic variation is common knowledge in a magnetic survey for geologic exploration. However, a transmission line is avoided in a magnetic survey since the nature of its magnetic field is not known. This study aimed to investigate the magnetic intensity variation around a 330-kV double circuit transmission line daily (15 m from the transmission line) as compared to the diurnal variation within a magnetically quiet location. The result shows for the first two days that the magnetic intensity was high when the sun rose and dipped when the sunset at the magnetically quiet location. The result of the next two days shows the magnetic intensity around the transmission line is less when the sun is rising and peaks when the sun sets, contrary to the nature of the diurnal variation due to solar activities. However, the magnitude of the magnetic variation around the transmission line (with an average intensity of about 79.244 nT) is greater than that of the magnetic variation at the magnetically quiet location (with an average magnetic intensity of about 77.840 nT) at all times. This result provides an interesting insight into the nature of the magnetic variation around the 330-kV transmission line, believed to be a combination of the effects of the magnetic field created by the power cable and solar activities. The magnetic variation, around the power cable, when the sun rises is attributable to the effect of the superposition of the magnetic field due to solar activities and transmission lines, destructive in nature. The findings provide a platform for the determination and the measurements of geomagnetic-induced currents. Due to the findings, the effect of the magnetic field created by the power cable can be corrected on the magnetic data acquired during geophysical explorations.

Key words: Diurnal variation, 330 kV transmission line, magnetic survey, solar activities.