Electromagnetic Direction Finding Experiments for Location of Trapped Miners

Direction finding techniques for determining the position on the surface directly above a small electromagnetic (loop) transmitter were tested at two hardrock mines and two coal mines in the fall of 1971. The actual locations of the underground transmitters in each case were projected to the surface using mine maps and conventional surveying techniques. A crossed loop direction finding receiver was developed to respond to the magnetic field. A null in the horizontal magnetic field was used to identify the surface location directly above the transmitter. Evaluation of this system at five locations in the four mines revealed location errors of 13 meters, 7 meters, 3 meters, 2 meters and 0. 5 meters respectively. Theoretical computations of magnetic field strengths based on transmitter frequency, ground conductivity and transmitter depth are shown along with measured profiles for two of the four mines visited. The effects of uniformly sloping terrain and dipole tilt are also examined theoretically and the results are shown to be consistent with the field measurements made under these conditions. Using known theoretical corrections for hill slope, it is shown that the measured source location can be improved to where it is within three meters of the actual location.