Subsurface Probing by High-Frequency Measurements of the Wave Tilt of Electromagnetic Surface Waves

An overview of the tilt of an electromagnetic surface wave propagating along the surface of the earth is given. High-frequency wave-tilt measurements can have important applications in low conductivity regions such as desert and permafrost areas. A specific application includes determining the depth of permafrost. Past applications of wave-tilt measurements were primarily with lower frequencies in regions with high ground conductivities. Thus low frequencies (<1 MHz) were needed to probe to significant depths. Wave-tilt calculations show that for low surface conductivities, it may be feasible to probe to significant depths using high frequencies (>1 MHz). This enables resolution of thicknesses of layers, a result that has not been possible at low frequencies in the higher conductivity media. It is noted that differences in wave tilt occur for vertically and horizontally polarized surface waves. These wave-tilt differences are useful because they indicate profile characteristics that are sensitive in different ways to the two polarizations.