Electromagnetic Location System for Trapped Miners

Electromagnetic signals in very low frequency range (VLF, below 10kHz) can penetrate rock sufficiently in order to locate a miner with nearly no disturbance of the field structure, if the near field condition is fulfilled. The relevant solutions of Maxwell's equations for a magnetic dipole (transmitting antenna to be carried by the miner) in a dissipative medium then are independent of the medium. From these equations the magnetic field strength at any point of reception can be calculated. The key for the location method is the inverse distance function (locus curve), describing the possible location of the dipole from the measured field strength and direction at a point of reception (performed by the rescue team). At least two of these measurements give locus curves intersecting at two points, the correct solution can be found by means of an additional direction information calculated at the points of interest. A system has been built and successfully tested with miner's transmitters which can be activated by a coded signal from the rescue team, a receiver device measuring the field amplitude and field direction and a software package to calculate the miners's location on a notebook directly in the mine.