Subsurface EM transmission in tunnel waveguides


  • J. R. Wait,

  • D. A. Hill


The concept that electromagnetic waves can be guided in tunnel-like structures in the earth is now being exploited for telecommunication purposes by groups in Belgium, France, England, Canada, and the US. The relevant propagation phenomena are much more sophisticated than first envisaged. Some of the complexities are due to the complicated structure of tunnels and haulageways that were designed for ease of transportation rather than communication. Nevertheless, much progress has been made by utilizing idealized models with varying degrees of complexity. A significant aspect of this study is how an antenna interacts with the tunnel walls. For the communication problem, we wish to minimize this coupling with the environment but, for a remote sensing tool, this interaction can be used to determine the conductivity and permittivity of the external earth media. An important finding is that the input resistance of an electric dipole is approximately a constant for frequencies less than about 25 MHz for a typical tunnel radius of 2 m. In contrast, for the same conditions, the input resistance of the magnetic dipole or small loop varies approximately as the square of the frequency. Such results are shown to be in qualitative agreement with an earlier analysis of dipoles over a dissipative half space and dipoles located in insulated cavities.