Absorption rates of u.l.f. modes guided in the Earth´s crust

The Earth-crust waveguide is a propagation medium of potential usefulness in long-range, low-data-rate u.l.f. communication. Absorption rates for u.l.f. proper modes guided by this stratification are quite low (as opposed to what happens at higher frequencies such as v.l.f. and e.l.f.), even with a conductivity of 10¿5 S/m in the guiding duct. Improper modes, if a way could be found to excite them with a finite-aperture source, would be characterised by comparable absorption rates with a conductivity in the duct as high as 10¿4 S/m.