I was reading in older article about cave radios (see: http://caves.org.uk/radio/comms_in_caves.html) and the author (David Gibson) made a statement that left me with a question.
"Low frequencies are attenuated less than high frequencies - long wave broadcasts such as BBC Radio 4 (198kHz) can be detected at the bottom of deep caves. Unfortunately, detecting a radio signal is easier than transmitting it - a 200kHz signal would require an antenna some 750m long, which is normally only feasible for mining installations."
My question is: at what depth have such broadcasts been detected?
Have recent experiments been done with ground stations to attempt regionalized underground triangulation using ultra low frequency radio? I know that this is NOT useful for communication. What I'm interested in is direction, distance, and location. With the current trend of local grottos buying caves and establishing preserves there are possible locations for maintaining a 750m antenna and a radio for cave triangulation. This of course would only be one-way communication much the same as GPS.