High frequencies travel only on the surface of a metal target, while low frequencies penetrate deeper. The book discusses choosing the optimal operating frequency (typically 3 kHz to 20 kHz for VLF). DIY Coil Construction: The Hardest Part
Overton and Moreland break down the complex mathematics of phase shift, explaining how VLF detectors differentiate between iron (ferrous) and gold or silver (non-ferrous). 2. Pulse Induction (PI) Inside The Metal Detector George Overton Carl Moreland.pdf