Theory relating to remote electromagnetic probing of a nonuniform‐thickness coal seam

Abstract

The effect of lateral inhomogeneities on electromagnetic remote probing of layered structures is considered. The excitation is taken to be an incident transverse electric (TE) plane wave. The fields, the surface admittance, and the wave tilt depend upon the angle of incidence, the distance from the air‐coal interface, the electrical contrast, and the layer profile (e.g., sinusoidal, step, and slant profiles). Both low‐ and high‐frequency cases are illustrated and compared with corresponding results having no lateral inhomogeneities. Results indicate that the surface admittance and the fields tangential to the surface are “good” indicators of the local structure, whereas the field normal to the surface and the wave tilt are not. However, the wave tilt and the field normal to the surface give good indication of subsurface anomalies. As the observation point is moved away from the air‐coal interface, the “information content” regarding the lateral inhomogeneities rapidly decreases. Thus, measurements performed on the surface, or measurements dependent upon the fields at the surface, e.g., reflection coefficients, are more sensitive to lateral inhomogeneities than measurements performed away from the surface.