Electromagnetic Location (EML)
Typical equipment sets would involve a multi-frequency locator and a signal generator/transmitter together with sondes, tracer cable and induction clamps.
This technique works well with metallic pipes and conductors. Also drains and ducts open for access. The equipment can be deployed in two modes – active and passive.
There are several ways of using the equipment in active mode.
Direct connection: A signal from the signal generator is applied onto metallic pipe or conductor. This normally involves connecting directly onto the metalwork of an associated fitting such as a valve stem or by using a clamp around the pipe. The signal ‘illuminates’ the pipe so that the locator can follow it route on the ground surface.
Sonding/Tracer Cable: When working with gravity drainage systems or ducts where access is available via manholes a tracer cable can be inserted so the route is traceable. The tracer cable is connected to the signal generator allowing its position to be trackable. A sonde has its own internal transmitter so it is more suited for deeper utilities.
Induction: If direct connection nor access for a tracer cable/sonde is not possible then a signal can be induced from the surface onto a metallic pipe or cable. This assumes that the utility position is visible in at least one point so that the transmitter can be placed over it. This process can be repeated to let long lengths of pipe or conductor be traceable.
Passive mode: Two types of ‘naturally occurring’ signals are found on buried utilities. These signals can be a final check following completion of the active search.
Power: Conductors carrying an electrical load generate electro-magnetic fields around them, which can be detected at ground level.
Radio: Metal pipes and conductors at least 10 metre in length re-radiate low frequency radio waves from submarine communications. These signals can also be picked up with the appropriate detector.