Low frequency measurement techniques for coupling into underground cavities

This paper describes measurement techniques of the direct and indirect coupling of electromagnetic fields produced by horizontal and vertical lightning currents into underground mines. Two coupling mechanisms were measured: direct and indirect drive. Indirect coupling results from electromagnetic field propagation through the earth as a result of a cloud-to-ground lightning stroke or a long, low-altitude horizontal current channel from a cloud-to-ground stroke. Unlike direct coupling, indirect coupling does not require metallic conductors in a continuous path from the surface to areas internal to the mine. Indirect transfer function measurements compare extremely well with analytical and numerical models developed for the Sago site which take into account measured soil and rock overburden properties.