Sonde

A small battery-powered transmitter on a push rod or camera head, fed through a non-metallic line and tracked from the surface. It is how a plastic water service, a sewer lateral, or an empty conduit gets located when there is nothing conductive to put a signal on.

A sonde transmits from a single point rather than along a line, so the operator locates it as a discrete position, records depth, then advances the rod and repeats — building the route point by point. It is the standard answer for non-metallic pipe and empty duct, and its practical limits are access and reach: the line has to have an open end the rod can enter, it has to be reasonably clear, and the sonde has to stay within receiver range of the surface.