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I've heard the term "lift-off effect" mentioned a few times on the forum (in relation to coil design). Can someone explain what this is in layman's terms?
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It is a term borrowed from non-destructive testing with eddy sensors, which are basically the same as IB coils in metal detectors. Lift-off describes what happens as the coil is lowered to the ground. Consider a classic concentric coil with TX, RX and TX-bucking windings. As the coil gets close to the ground, the big TX field from the TX coil becomes distorted from mineralization but the smaller TX field from the bucking coil is not initially distorted. This causes an upset in the induction balance and an RX signal is seen. As the coil gets even closer to the ground, the TX field from the bucking coil then becomes distorted, and this can actually bring the IB back to a better null.
This is one example of lift-off. DD coils can also experience lift-off but with a different mechanism. The DOD coil used on the GPZ is fairly immune to lift-off because the RX is a butterfly coil, which is why bobbing the coil is ineffective in establishing a proper GB. Same with a Bigfoot coil.
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Makes sense. Thanks for the explanation.Comment
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