Channel characterisation and system design for sub surface communications
Sub-surface or through-the-earth communication using electromagnetic fields - and specifically magnetic induction equipment - plays a key role in search and rescue systems used in the mining industry and, increasingly, by cavers and pot-holers. Similar equipment is used for radio-location, sub-surface surveying and geophysical measurements. The use of fast desktop computers allows a mathematical model of the propagation to be investigated in detail, demonstrating the preferred orientation of the antennas and the existence of an optimum frequency that is dependent on depth and other parameters. Computer simulations demonstrate a reduction in accuracy of radiolocation at skin depth distances, and methods of correcting this inaccuracy are introduced. The transmitter and receiver antennas for portable induction loop systems are usually aircored loops or magnetic-cored solenoids, tuned to resonance. However, the preferred antenna often depends on the intended use of the communication system, with untuned antennas having an advantage in some situations. The use of toroid (anapole) structures and rotating magnets as transmitters is discussed briefly. A figure of merit - the specific aperture - is introduced as an aid to antenna design. Internal (amplifier) noise can be reduced by noise-matching, although special account has to be taken of the inductive antenna. Internal noise is often swamped by external (atmospheric) noise, for which several mitigation strategies are discussed. Differing up-link and down-link noise performance may dictate different antennas. External noise is frequently characterised by the atmospheric noise temperature ratio, but data derived using the standard electric field antenna cannot be applied to a study of magnetic noise. The design of a wide-band low-frequency channel sounder is described, with which it is intended to perform a detailed channel evaluation using a binary sounding sequence. A simple method of calculating the inverse of such a sequence is introduced, for which cross-correlation with the inverse sequence at the receiver results in a system identification signal that is used to maintain synchronism with the transmitter. The extreme wideband nature of the system results in a low efficiency, which is countered by using signal-averaging techniques at the receiver. Preliminary results are reported, in which the sounder was used to capture background noise.