Arcs and sparks
As usual your analysis is quite good JP.
The big problem w/ arcs and sparks is they don't necessarily have to go to metal - anything conductive would work like damp soil that has enough ionic activity to do some conduction.
One thing that may be fruitful is to have a high voltage probe like regular metal detectors use a coil. The voltage would not have to be high enough to do arcing. Then monitor the HV probe field for anomalies. Conductive metal in the ground would tend to distort the probe field. Might even be special distortions relative to different types of metal.
Goldfinder
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