| All sorts of things happen at lower frequencies, and some of them are not easily explained. A few years ago, one of the caving journals reported that a group of cavers, who were several hundred metres below the surface somewhere in southern Europe, had received BBC R4 on 198 kHz although the signal was not audible on the surface.
On a somewhat higher frequency (31 m broadcast) I once received Radio Nederland at good strength when I was between 30 and 100 m below the surface in the Yorkshire Dales. No other stations were received at any strength in that band. One of my colleagues, professionally engaged in radio communications, suggested there might be some sort of fault in the rock. This was probably true, but we were unable to determine whether that caused the unusual propagation.
John F5VLF
On 23 Jun 2011, at 10:43CEST, Roger Lapthorn wrote: It is some while since I last did any earth mode (conduction through the ground) tests although I intend to resume these in a few weeks' time after my holiday to Iceland. One of the tests will be earth mode at much higher frequencies such as around 30 and 70kHz as well as at even lower frequencies below 100Hz. I will try not to trigger WW3 by TXing around 82Hz though.
Today I noticed some workmen in the road outside my house with measuring kit and listening gear. They were checking for leaks in water pipes which occur usually where sections of pipe are linked. As I'd always assumed my earth mode was utilities assisted via the metal water pipes from my house and in the road I asked the question what sort of pipes were in the road. I was surprised to hear these are not metal at all but are asbestos cement pipes. The did not believe any tracer wires were present either.
So, I am a bit puzzled now about what exactly my earth mode signals are coupling into. All the indications were that I was using the water pipes but now I have some doubts. The other utilities in the road are electricity, gas and CATV which is fibre optic. Phones come in via overhead wires. I think the answer is that the coupling to buried utilities must be more complex than I had assumed and that the earth mode signal is coupled into a variety of pipes and cables.
73s
Roger G3XBM
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