Question: How does the EMAD find signals?

Adam Wootton answered on 11 Nov 2019:

I have to be careful answering this because some of it is a little bit secret, but the basic idea is something called Magnetic Flux Leakage.
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Basically, if we have a magnet, and we run it along a metal like steel, it can turn that steel into another magnet. If you want to really annoy your parents/guardians, you can do this to your cutlery. The science behind that gets very complicated very quickly, but we can use magnets to make more magnets.
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This is what happens in magnetic flux leakage. Most bridges will be made up of steel reinforcing bars like this:

We turn the reinforcing bars into really big magnets, and then record the magnetic field that comes from them. If the magnetic field suddenly changes, then that means that the reinforcing bars could have corroded away, meaning that the bridge would not be as strong as it should be.