Induction

Applying a locating signal to a buried line without touching it, by setting the transmitter on the ground above the line and letting the field couple onto it. Used when no access point exists.

Induction is the fallback when there is nothing to clip to and nothing to clamp around. It works, but it is the least selective way to energize a line: the transmitter couples onto everything conductive within range, not only the intended target, so bleedover is common and the operator has to work harder to confirm which trace is real. Standing the transmitter well back from the receiver and sweeping the area to identify every response before committing to a mark are the usual disciplines.