I don't know
But if the resonance effect is a capacitive effect, the signal will not appear with high frequencies of infrared radiation. It will be lost within the carrier signal,
so it is suitable for measurement only based on the intensity of reflection
. ( Capacitive modulation is used for intermediate frequencies (AM)) . This may be an explanation that they prefer to use 250 kHz receiving resonance circuits in long-range metal detectors. Coincidentally, this frequency is also used to detect electrical signals.
Magnetism cannot be relied upon to detect precious metals because they are non-magnetic metals. Most suitable for ferrous metals.
