Airborne VLF is a passive electromagnetic method that depends on high power marine communication transmitters as the energy source. An anomaly is produced when the large electrical field interacts with local conductors or by a significant change in the resistivity. The VLF field may also concentrate itself along planar features which are parallel to the transmitter, which results in a strong directional bias in the VLF response. To alleviate this effect 2 orthogonal stations are typically monitored.
VLF is generally collected along with magnetic data on a 'best efforts' basis. Frequent maintenance of the transmitters and variable signal strength means that stations may have to be substituted and signal outages may occur. For these reasons interpretation is more qualitative than quantitative, but may still yield useful information at very low cost. The total field map tends to show variations in conductivity, while the quadrature measurement is more responsive to planar features such as faults and contacts.
Goldak employs the reliable, industry standard Herz Totem-2A 2 channel VLF receiver, expanded for a frequency range of 10 to 30 KHz.