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This was mentioned by geof_junk in another thread and had a little Google.  
 

Found this  https://www.phys.k-state.edu/reu2011/nnorvell/Metal_Detector_Research.html

I don’t really understand the technical side of metal detectors.  Does this have any application to current day detectors?  Will it help cancel out ground noise more?  Will the current crossing/not crossing the ‘bridge’ tell you something about what is under one of the receive coils.  
 

Although I don’t understand it, I am amazed and a little in awe of those that do  👍

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