Far field radiation involves electromagnetic energy transferred through space. Cave communicators now use a FM carrier between 70kHz and 130kHz. A similar loop is often used on the surface. Electrodes are punched into the surface and connected to a 50 watt transceiver. It is the induction field that is sensed by the underground antenna acting the same as the secondary winding of a transformer.
Voice communication is employed in most cave situations.
This is especially true in rescue operations since CW is not known by most SAR communicators. It would seem that CW would be a better mode adaptation since it could be received at even greater underground distances.