Hi FrancoItaly
seems you have the correct attitude concerning real research and free available "helping together technical development methods" unlike most others here who play the big keepers of the holy grail or better: of the "holy but just grass finding cow"
Of course as long as you don't have tested this device through and through it could be a pure waste of time, work, money, motivation and other ressources for those who wanna have it or built it.
So please find some clever "excluding" test-ideas so you can go shure it really detects metal only.
There are simply too much magnetical, electrostatic, radio-frequency and even "cosmic" radiation and fields out there and its highly doubtful that metal objects which are in tight contact with wet soil will attract or reflect EM-energy very good.
If this stuff is not tested on a sophisticated basis it can detect virtually "anything" - and we really dont wanna find "anything"!
The whole stuff reminds me on my own electronic experiments as I was a child where the sound or noise starts when i moved my hand near some special wires (without touching). And wireless energy transfer from one coil to a nearby ferrite-coil-circuit containing a little lamp by radio-waves looked almost like magic to me.
The higher the amplification circuit, the more signal stuff it can detect. Especially if the device is grounded through the body and some metallic contacts at the handle.
However its good
if the forum now is coming back to its technical basis after all this time containing just worthless opinion discussions without any hard provable facts.