Active Locating

Tracing a buried line by deliberately applying a known signal to it with a transmitter, then following that signal with a receiver. Active locating is the standard method because the operator controls the frequency and knows which line is being traced.

The transmitter puts a tone on the target line by direct connection, by an inductive clamp around it, or by induction from the surface. The receiver is then tuned to that same frequency and walked over the ground to follow the field radiating from the line. Because the operator chose the frequency, active locating distinguishes the target from everything else nearby in a way passive locating cannot — the trade-off being that it requires an access point or enough coupling to get the signal on in the first place.