Very Low Frequency (VLF)
Many coastal nations operate long wavelength, or very low frequency (VLF), radio stations, principally for communication with and navigation by submerged submarines. These stations operate at high power, up to 1,000 kW, at assigned frequencies in the range 15 to 29 kHz. Antennas are up to 1,000 feet in height. Because of high power and relatively low attenuation of long wavelengths, these signals are, generally, detectable around the world. Stations within 3,000 or so miles normally come in at high signal strength. The electromagnetic emissions from the antennas have an electric field component oriented parallel to the line between antenna and measurement point, and the magnetic field component is perpendicular to this line.
The galvanic currents that flow in the ground by reason of this broadcast field are refracted downward at the earth's surface. They are as indicated above, regional in nature. This "sheet" of current tends to flow toward, conductive bodies and flow away from, or diverge around, resistive bodies. These disturbances in current flow are expressed as anomalies in the associated magnetic and electric fields. In addition to galvanic currents, secondary, eddy or vortex currents, are induced to flow in any conductor that is present in the ground or near the surface. Vortex currents flow in closed loops on the surface of the conductors, and these alternating currents, in turn, create secondary magnetic fields. Because vortex currents flow in closed loops, and galvanic currents converge and diverge, it is intuitively obvious that VLF readings at the earth's surface can be either positive or negative. If the earth had no relative conductive and resistive zones, there would, naturally, be no secondary fields, and the primary fields over the homogeneous earth would be uniform, that is, no local anomalies. Over a non-uniform earth - the real world - primary fields converge and diverge, and secondary fields develop as a function of conductivity contrast, nature, orientation and depth of the conductor. VLF anomalies are qualitatively interpreted.
In the qualitative approach, if there are no anomalous conditions in the earth, then there are no VLF anomalies. And where anomalous conditions exist, the resultant field is warped and has a secondary contribution superposed. Thus, by measuring strength (amplitude) and various components, the magnetic field, at least, is fully described, and the information on the underground conductors is expressed. VLF is very useful in delineating the presence of groundwater in fractured crystalline rock
Traverses with the VLF were conducted across lineation features thought to be the effect of faulting.
On multiple traverses, the VLF data sets indicate an anomaly which suggest a possible fault.