In this comment in Earth Science SE I've said:

This is a really cool question! I remember (a long time ago) reading about electromagnetic waves inside conductive soil that travelled near but below the Earth's surface for at least a few miles, and remember seeing a circuit for such a receiver in a book about ham radio circuits.

I'm thinking of a paperback book from the late 1960's or early 1970's white cover with a title perhaps something roughly like 75 circuits for amateur radio operators. Just for example one was A Watt and a Half on 80 Meters. Circuits were mostly if not completely using transistors.

The circuit I'm thinking of was for receiving possible signals that were propagating just under the surface of the Earth. It wasn't saying that this was necessarily "a thing" but did suggest it might be fun to try.

  1. Does this circuit and/or book sound familliar? Any recommendations how to track it down?
  2. Was this actually ever "a thing"? Did people try this? If so, roughly what frequencies might have been looked at?
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    $\begingroup$ Re: 2. IIRC, when operating literally under a rock (i.e. in a cave/mine) that would absorb any reasonable RF emission, existing systems don't actually use radio (as in: radiating waves), but carry a (rather heavy) coil – as one side of a transformer, where someone is somewhere else on that rock with an even bigger coil as secondary side, to pick up. Some signal processing later, and you get a low-info-rate link. This doesn't work with "surface waves" (like the description suggests), because no actual propagating waves are involved, but it would "bundle" all the flux in the high-µ material. $\endgroup$ – Marcus Müller Mar 1 '20 at 9:39
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    $\begingroup$ (just like a transformer core) $\endgroup$ – Marcus Müller Mar 1 '20 at 9:39
  • $\begingroup$ @MarcusMüller Sure a dipole field from a coil will fall off as r⁻³ so a resonant pickup coil and sensitive amplifier will work out to some significant distance. $\endgroup$ – uhoh Mar 1 '20 at 11:12
  • $\begingroup$ Also I realize that I should have said "in poorly conductive soil" but a quote is a quote and I can't change it now. $\endgroup$ – uhoh Mar 1 '20 at 11:27
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    $\begingroup$ You may be thinking of ELF signals in the 3-30 kHz range. A lot of ham stations used those frequencies during WWII when amateur radio was prohibited. They transmitted and received audio by using two ground rods as widely separated as possible. $\endgroup$ – Mike Waters Mar 1 '20 at 19:54

Yes, this most certainly was a thing, and people actually did it.

This type of communication has been done at ELF frequencies in the 3-30 kHz range. Their linear amplifiers were ordinary Hi-Fi audio amps.
I don't know if LF frequencies higher than that are useful for that, but the hams on the 2200m band (137 KHz) use above-ground antennas.

A lot of ham stations used those frequencies during WWII when amateur radio was prohibited. They transmitted and received signals by using two ground rods as widely separated as possible. I used to have an old amateur radio magazine that described this in detail, along with limited distance carrier-current communications through power lines (not recommended!).*

You may very well be able to find radio and electronics magazine articles about this at americanradiohistory.com. For example, I found a complete index to all the articles ever published in W1HR's (SK) Ham Radio magazine, and then downloaded the PDF of the magazine that I wanted from that site.

Some other related Wikipedia articles:

* It might have been an old QST, and ARRL members have free access to all the QSTs ever published. The ARRL's TIS (Technical Information Service) may be able to help you. If you are not an ARRL member, you can buy printed individual articles from them at a reasonable cost.

Older QST magazines can be downloaded from americanradiohistory.com here.

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    $\begingroup$ Wow thank you for taking the time to write this up so thoroughly! I did have things planned for the upcoming week, but now with all of these to read... ;-) $\endgroup$ – uhoh Mar 1 '20 at 23:52
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    $\begingroup$ @uhoh It was fun and interesting for me too! I'm looking forward to you finding a good article and posting it here in an answer. If I had the time and energy to experiment with this --and a nearby neighbor to talk to-- I might have tried this myself. Perhaps a guitar amplifier or a home audio system could serve as both the transmitter and receiver, with the PTT switch operating a transceive relay to swap the microphone input and speaker output. Audio impedance matching transformers would probably be needed, and their value determined by a homebrew audio impedance bridge. $\endgroup$ – Mike Waters Mar 2 '20 at 20:52

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