The Physics of Metal Detector Depth Limits
Metal detectors operate on the principle of electromagnetic induction. The search coil transmits a low-frequency electromagnetic field into the ground. When this field encounters a metal target, it induces small circular electric currents (eddy currents) in the metal. These eddy currents generate their own secondary electromagnetic field, which is received by the search coil and processed by the detector control box.
The maximum depth at which a target can be detected depends on several variables: search coil diameter, operating frequency, soil mineralization, target size/conductivity, and detector sensitivity.
The primary rule of search coils is: detection depth is proportional to coil size. A larger coil transmits a deeper magnetic field, but is less sensitive to very small items. A rough physical benchmark is that a search coil can detect a coin-sized target at a depth equal to its diameter (e.g., a 10-inch coil can detect a coin at roughly 9 to 10 inches under clean soil conditions).