Measurement and Modeling of the Indirect Coupling of Lightning Transients into the Sago Mine

This paper describes measurements and analytical modeling of the indirect coupling of electromagnetic fields produced by horizontal and vertical lightning currents into the Sago mine located near Buckhannon, WV. Two coupling mechanisms were measured: direct and indirect drive. Only the results from the indirect drive and the associated analysis shall be covered in this paper. 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. Results from the indirect coupling measurements and our modeling show that vertical lightning currents are not likely to have produced lethal voltages in the Sago mine, but a large, horizontal current above the ground could have created a large voltage on a long, insulated cable. 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.