Sonde following robot ? electromagnetic field detection

a sewer line camera can have a Sonde on it. the Sonde sends out some sort of signal that can be detected 1-2 meters away, on the surface.

as I understand they can be in clay, plastic or cast iron pipes and work acceptably.

my idea is to have a robot that can follow the Sonde as it travels and the robot could release a drop of dye every now and again.

There is another technology that you can attach to a metal pipe and make a radio antenna from it. I believe 512 hz signal. you would attach to to your water pipe in the street and be able to find the path from the street, under the driveway, up to your house.

the key for this is metal pipe. .or a metal rod/wire to be in the pipe and AC to be applied to create an electromagnetic field . from what I have seen, one end to the transmitter, the other end of the pipe/wire is to a well placed ground.

what I am hoping is to figure out is if it is possible to have a surface robot stay centered over one of these and mark the path