CHAPTER 7: GEOLOGICAL MAPPING USING VLF RADIO FIELDS
The fact that electrical properties of the ground affect the behavior of radio waves has been known for years. Indeed measurements of the conductivity and dielectric constant of the earth using “wave-tilt” techniques were first performed in the 1930’s [Feldman (1933), Smith-Rose (1933), Barfield (1934)]. These early measurements were, however, made at relatively high frequencies which resulted in a shallow depth of penetration. It was not until 1963 that Paal (1965) observed that radio waves at VLF frequencies (technically the 3–30 kHz band, but in fact limited to 15–25 kHz by the available high powered transmitters) could be used to prospect for electrically conductive orebodies. By surveying over known shallow orebodies in Sweden with a calibrated field-intensity meter tuned to VLF stations, Paal showed that the horizontal VLF magnetic field was greatly enhanced over subsurface conductors at exactly the same location where the modulus of the vertical magnetic field component became a minimum. A large rotation in the compass bearing of the horizontal magnetic field coincided with the location of the maximum in this component. Such behavior was recognized by Paal as being consistent with the response from a current, induced by the radio field, running along the top edge of the target. Furthermore, additional measurements of the field strength made in mines, although disturbed by machinery, cables, ore, etc., suggested that the horizontal field strength at a depth of 275 m was still about 25 percent of the value at the surface, from which he concluded that orebodies should be detectable under Swedish conditions to depths of about 100 m, and that prospecting could be carried out from the surface or the air.
In 1964 Ronka (Paterson and Ronka, 1971) introduced the first commercially available ground VLF instrument, and within a few years similar instruments were available from other manufacturers. By 1969 several airborne VLF systems were also being flown commercially. All of these instruments, ground or air, basically measured either the magnetic field tilt-angle or the vertical or horizontal magnetic field strengths so as to detect the presence of localized electrically conductive targets. A different approach was taken by Collett and Becker (1967). Their Radiohm is basically a magnetotelluric type of instrument which uses VLF transmitters (rather than atmospheric noise fields) as a source of signal. This instrument provides the advantage of a coherent source and allows the phase angle between the horizontal electric and magnetic fields to be accurately measured and used for interpretation. Unlike the earlier instruments which worked solely with the magnetic field components, the Radiohm directly measures the wave impedance, from which the terrain resistivity can be derived, for use in general geologic mapping even in highly resistive terrains. At about the same time Barringer (1973) commenced work on the airborne Radiophase and E-Phase VLF systems. These systems used the vertical electrical field as a phase reference and E-Phase was also unique in that it measured the quadrature component of the horizontal electric field, from which once again the terrain resistivity can be derived. Finally in 1974, Tilsley (1973) suggested the use of a portable VLF transmitter as a supplement to the regular VLF transmitters, which could shut down without notice. A portable transmitter is also useful when the electromagnetic field components from the available VLF transmitters are in poor coupling with the target. A summary of the various VLF instruments is given in Table 1.