Adaptive Digital Communications for Sub-surface Radio Paths

Low frequency sub-surface radio is used for communications with miners and cavers. The signal attenuation is dependent on frequency and on the electrical conductivity of the ground. In addition, the optimum frequency for communications is dependent on depth below the surface; and on the degree of noise and cochannel interference. An adaptive communications system makes use of a channel-sounding technique by transmitting a wideband signal and deriving the signal/noise ratio (SNR) as a function of frequency. The communications system can thereby alter its transmission frequency, and other parameters, to optimise SNR under different conditions. The same principles are relevant to applications in sub-surface radiolocation, archaeology and geophysics.