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.
- Does this circuit and/or book sound familliar? Any recommendations how to track it down?
- Was this actually ever "a thing"? Did people try this? If so, roughly what frequencies might have been looked at?