Technical term – passive detectors
Passive detectors measure anomalies in the earth magnetic field, caused by “ferro metal” objects in the soil. Passive detectors can register and measure these differences, and have a standard measuring range of 2 to 3 m (or more, for large objects like aerial bombs).
“Ferro metal” objects in soils alter the trajectory and density of the surrounding earth magnetic field lines. A passive detector can register and measure these alterations. Passive detectors cannot detect "non-ferro metals" (as opposed to active detectors).