Hi dear J-P.
Appreciable efforts to draw these illustrations. thank you, but in one image you have mentioned this statement:
"Electronic instruments cannot the MFD signal from more than 10 Cm distance from the brass probes in the ground."
I should tell that I seriously object to this part of your statements; it's not right.
I have one of these MFDs made by Notsi(Bulgarian) I don't know you remember or not.
At that time(6-7 years ago) I borrowed one portable frequency meter from my e.e. friend to check signal output and distance of signal traveling. I remember well it could show the exact frequency set on transmitter for 2 meters beyond MFD probes, even when I put frequency-meter probes in a very thick ceramic vase inserted into soil it could easily penetrate to thick ceramic.
My e.e friend said: " this is an ordinary Fr-meter, I'm sure if use a sensitive Fr-meter you can detect sent signal so much more than this".
Best Regards.