4.12 Locating Electromagnetically
When locating electromagnetically, active/conductive locating is preferable to passive/inductive locating.
The preferred method of actively applying a signal onto a facility is to use direct connection. Direct connection is the process of connecting a direct lead from the transmitter to the target facility and connecting a ground lead from the transmitter to a ground point to complete a circuit. This process provides the strongest signal on the line and is less likely to “bleed over” to adjacent facilities than other methods of applying a signal. This method allows a greater range of frequency and power output options. It is good practice to use the lowest frequency possible at the lowest power output possible to complete the locate. If direct connection is not possible, use of an induction clamp (coupler) is the most effective method of applying a locate signal onto the target conductor. This method is more limiting for the choices of frequency and power outputs than direct connection. Using an induction clamp is not as effective at transmitting a signal as direct connection, can only be used within certain frequency ranges, and must use a higher power output. The least-preferred method is induction or broadcast mode on a transmitter. This usually results in a weak signal that will “bleed over” to any conductor in the area.
Practice Statement B:
When electromagnetic locating is not possible, radar-based technologies can be used.10
In cases where non-conductive utilities cannot be located using electromagnetic means, radar-based methods such as ground penetrating radar and associated technologies can be used to determine the location of such utilities. It is important to note that these technologies are not applicable in all areas or conditions, because conductive soils and materials obscure radar signals. Users of these technologies should have the degree of knowledge and training required to operate the associated equipment and/or to interpret the results. Applicable radar frequencies range from 200 MHz to 900 MHz, where higher frequencies provide higher resolution but shallower depth of penetration.