Integrated Geochemical and Geophysical Approach to Mineral Prospecting – A Case Study on the Basement Complex of Ilesa Area, Nigeria - Advances in Data, Methods, Models and Their Applications in Geoscience - page 80

Fig. 5. Map showing the association ratio of Cd - Zn in the study area.
5.3 VLF- EM data, analysis and results
The Very Low Frequency (VLF) electromagnetic method uses powerful remote radio
transmitters set up in different parts of the world for military communications (Klein and
Lajoie, 1980). In radio communications terminology, VLF means very low frequency, of
about 15 to 25 kHz. Relative to frequencies generally used in geophysical exploration, these
are actually very high frequencies. The radiated field from a remote VLF transmitter,
propagating over a uniform or horizontally layered earth and measured on the earth's
surface, consists of a vertical electric field component and a horizontal magnetic field
component each perpendicular to the direction of propagation.
These radio transmitters are very powerful and induce electric currents in conductive bodies
thousands of kilometers away. Under normal conditions, the fields produced are relatively
uniform in the far field at a large distance (hundreds of kilometers) from the transmitters.
The induced currents produce secondary magnetic fields that can be detected at the surface
through deviation of the normal radiated field. The VLF method uses relatively simple
instruments and can be a useful reconnaissance tool. Potential targets include tabular
conductors in a resistive host rock such as faults in limestone or igneous terrain. The depth
of exploration is limited to about 60% to 70% of the skin depth of the surrounding rock or
soil. Therefore, the high frequency of the VLF transmitters means that in more conductive
environments, the exploration depth is quite shallow; for example, the depth of exploration
might be 10 to 12 m in 25-Ωm material (Milsom, 1989). Additionally, the presence of