Very low frequency (long wave) radio energy from distant transmitters is present in the atmosphere world-wide. The ground provides return paths for this radiation, and buried metallic lines form preferred paths. They then act as aerials re-radiating these signals. The signal strength will vary with coupling to ground, size of line and soil conductivity, the strongest signals emanating from lines with good grounding at each end, or of substantial length so that capacitance coupling is maximised.
These radio frequency signals enable the presence of the conductor to be detected by Radiodetection locators in the Radio mode, which can then find dead power cables or well-balanced high voltage cables which could well be missed by power-frequency-only detectors.
Telecommunication cables and metal pipes also carry these VLF radio signals, so again passive location is effective, but, as with all passive signals, there is no way of identifying the line.
Here is the result of spectrum analysis of the range of frequencies detectable on a typical buried telephone cable, the vertical scale showing the relative signal strength at the frequency shown horizontally. While there is a general broad scatter of frequencies, the two peaks at 16 and 19.6 kHz provide clear tracers for detection by appropriately tuned receivers