On modal propagation of high frequency electromagnetic waves in straight and curved tunnels

Abstract

Modal propagation in tunnels within the earth environment is considered for the case of rectangular and semicircular shaped tunnels excited by high frequency electromagnetic fields. It is shown that when the applied frequency is sufficiently high, simple forms of the modal fields exist for tunnels with rectangular cross section. Simple expressions for the modal attenuation rates derived previously (Emslie et al. (1975)) are assessed against numerical solutions to the modal equations. The case of a tunnel with a semicircular cross section is treated such that the flat surface is considered as a slightly imperfect electric conductor. The case of a curved tunnel is studied using the Airy function representation of the modal fields. Modal propagation constants in curved tunnels for different polarizations are obtained.