Assessment of image theory for field evaluation over a multilayer Earth

The complex image theory is used to predict the electromagnetic fields excited by a line source above a horizontally stratified earth. It is shown that the image solution for fields in the air is merely an approximation of one part of the complete rigorous solution. This part is the main branch cut which dominates in the far zone, while other branch cuts and the trapped waveguide modes are completely ignored. It is thus suggested that the image solution be complemented by the trapped waveguide modes which can dominate at high frequencies and in a certain intermediate range of transmitter-receiver spacing.