Through-the-Earth (TTE) radio has been proposed for emergency communications in locations inaccessible by conventional means, such as underground mines. While the technology is viable, it is unclear how the signal propagates in inhomogeneous media; neither modelling or obtaining a conductivity distribution in the context of TTE radio has been previously attempted. With a robust model, many practical questions can be answered, such as what is the maximum range or the optimal frequency to use, or where the transmitter and receiver should be ideally placed. To this end, a finite-difference time-domain (FDTD) code was developed and optimized for the forward modelling of TTE radio transmissions. This method is computationally intensive, and to improve performance, it was run on a graphics processing unit (GPU). The code was validated against analytical solutions for simple geometries.
The use of a TTE radio signal to estimate the conductivity structure of an operating environment was demonstrated at an experimental coal mine. A transmitter was set up on the surface, and the signal strength in the underground mine was measured at several locations and frequencies. A four-layer conductivity model, with topography, was fitted to this data, using the FDTD code.
TTE radio signal range has been experimentally observed to increase in the presence of elongated conductors. A thin-wire approximation was added to the FDTD code, and conductivity models were fitted to match observed data for propagation along elevator shafts in multistory buildings and along railway tracks. Multistory buildings present a separate challenge due to their conductive reinforced concrete structure. By representing the floors and walls as thin sheets of a given conductance, observed and modelled data were reconciled for a variety of buildings, including above-ground reinforced concrete constructions, a building with steel floors, and a building partially below ground, and for placements of the transmitter and receiver inside or outside the building.
Further work remains in modelling a wider variety of settings where TTE radio could be used, such as buildings with different materials. Another avenue of research is to improve the performance of the modelling code, by refining the numerical method used.