Principles of cave radiolocation

The basic principles of radiolocation are simple. Rock absorbs radio waves at commonly-used frequencies (which is why your GPS and cell phone don’t work underground). What has been proven to be useful is a magnetic signal produced by a circular coil underground. With the plane of the coil horizontal*, the imaginary lines of the magnetic field curve from one side to the other like iron filings between the poles of a bar magnet:

At the point on the surface directly vertical to the transmitter the lines of magnetic force will be vertical. This point is conventionally called “ground zero”. Seen from above, these lines will appear to radiate outwards from ground zero:

The receiver loop is held vertically, and the signal heard in the earphones will vary depending on how the loop is orientated in relation to the lines of the magnetic field. The more lines that go through the loop, the stronger the signal. When the loop is exactly aligned with the magnetic field lines, the signal strength will be at a minimum, and this is called the “null”:

When a null is found with the receiver, the plane of the loop antenna is pointing directly towards ground zero. When the loop is exactly at ground zero, the magnetic field lines are exactly vertical and therefore the signal will be nulled whatever direction the receiver loop is pointing in, as long as the loop is exactly vertical

*If the loop is placed at an angle from true horizontal, ground zero will not be directly above it. This error will be about a third of the angle of inclination subtended at the surface. For more information, see Advanced Radiolocation Topics.