Well, keep in mind that the Swiss 50Hz do fall into the <100kHz designation. I'm aware of it's usage for pinging currently-diving submarines, and in the slight

Well, keep in mind that the Swiss 50Hz do fall into the <100kHz designation. I'm aware of it's usage for pinging currently-diving submarines, and in the slightly higher region, providing wireless bidirectional low-bandwidth comms into deep mines. The power lines do have quite some magnetic stray fields, though. Not that it's too massive; it's not more than a few (3~4) kA at most.