Abstract-Recent research into very large, regularly shaped, geological structures has shown that in the 100 kHz to 10 MHz frequency range electromagnetic waveguide behaviour is observed when the material forming the structure is not too lossy (conductivity σ < 0.0001). While mode formation and modal behaviour in electromagnetic waveguides is very well understood, much of the literature describes high frequency structures for which it can generally be assumed that the loss tangent of the wave guiding medium (tan δ) is very much less than unity. In this case, wave attenuation is small and can generally be considered to be insignificant. This is not true for large low frequency waveguides, such as those formed by geological strata, and little seems to have been reported in the literature on the nature of modes in waveguides of this description. The paper takes the form of a parametric study aimed at ascertaining the limitations to modal formation in waveguides, for which tan δ is greater than unity, by revisiting the basic equations describing electromagnetic wave propagation in lossy media. The theoretical predictions are supported by modelling studies on large waveguide strata formed from material layers with dimensions typical of a geological structure such as a coal seam or oil-wet, strata-bound, petroleum reservoir.

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