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).

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