Quote:
Originally Posted by ArmySGT.
Ultra Low Frequency traveling through bedrock.
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Wikipedia has this to say about ULF range Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced electrode pairs hammered into the soil. At the receiving end, the signal is detected as a weak electric current between a further pair of electrodes. Using weak signal reception methods with PC-based DSP filtering with extremely narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10-100 W and electrode spacing of around 10–50 m.
You are acommo guy so probably have more information to share