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A compass or magnetometer by themselves can only detect/measure magnetic fields, and thus would normally only be able to detect ferrous metals and (possibly) some active electrical circuits in addition to the earth's magnetic field. That in itself can be useful at times. However, if you had a circuit with an induction coil on it (any size or shape), meant only to induce current in nearby metals, could you

a. Get a baseline measurement of the reaction of the compass or magnetometer to the induction circuit.

b. Move an assembly of the magnetometer/compass with the induction circuit around to

c. detect non-ferrous metals by the different reaction the compass/magnetometer has from its baseline when the induction circuit and sensors get near non-ferrous metals?

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Yes, it is quite commonly done, sometimes even with large sources/detectors operating at 100km/h or more ~100m or so above the ground.

The ground is more-or-less conductive so you also see the fields from currents induced in the earth. And any other nearby metal as well.

Secondary and primary fields can be distinguished by phase. ahem the devil is definitely in the details.

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  • \$\begingroup\$ What do you mean by the devil in the details? \$\endgroup\$ – cluemein Apr 12 '18 at 20:04
  • \$\begingroup\$ It means that in practice it is non-trivial and there is a lot of proprietary knowledge and techniques required to get good results. \$\endgroup\$ – Spehro Pefhany Apr 12 '18 at 22:03
  • \$\begingroup\$ How do you distinguish phase? Can a magnetometer or compass detect that by themselves? If not, what is needed to detect and measure it? \$\endgroup\$ – cluemein Jun 12 '18 at 20:43

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