Passive Locating

Detecting a buried line from a signal already present on it rather than one the operator applied — typically 50/60 Hz power frequency, or radio energy that has coupled onto long conductors.

Passive locating is fast and needs no transmitter, which makes it a good first sweep of an unknown area: it will show that something conductive is present. What it cannot do is tell you what that something is. A power-mode response could be a service cable, a bonded water main, or a metal fence line, and there is no way to select between them. It is used to find candidates, and then active locating identifies them.