Low Frequency Magnetic Induction (LFMI) transponders have been used for more than half a century to precisely radiolocate “ground zero” points on the earth’s surface lying directly above beacons placed at key spots in sub-surface passages. The main use of LFMI has been for the purpose of precisely aligning sub-surface maps with a surface topographic map.

Specific ground zero points have been used to create new cave and mine entrances and by farmers to drill wells to tap the water in underground pools. An LFMI transponder is a two-part system consisting of a sub-surface 3.5 kHz AC magnetic field generator and a surface component consisting of a phase-locked loop directional antenna. The magnetic field created by the transmitter looks like the field produced by a classic “bar” magnet standing on one end. The field directly above the beacon is exactly vertical. In use, the receive loop is held vertically and slowly rotated to find the direction of weakest signal (the null). Ground Zero lies along the Line Of Position (LOP) passing thru the plane of the loop. Ground Zero lies at the point where the Beacon signal nulls in all directions with the Receive loop held precisely vertical with a bubble level. The same technology can be used, at slightly higher frequency, to transmit voice. Through-rock range can be as great as 1200 meters, depending on rock conditions and regional weather. The signal is not attenuated significantly by water or ice. Stone Aerospace will build custom through-rock locators and tranceivers based on customer specifications.

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