Electromagnetic Induction (EMI)
Electromagnetic Induction (EMI) is the most widely used technology for locating buried utility services, and is very effective in most soil types. With EMI, you can locate and trace a facility, as well as estimate its depth. Electromagnetic Induction consists of two steps. First, a transmitter is used to transfer an alternating electrical current to the pipe or wire to be located. Next, a receiver is used to analyze the transmitted signal, and localize the position and depth of the facility. The transmitter can transfer the signal to the facility either by a direct connection, or by inducing a signal.
The direct connect method introduces a signal into pipes or cables (or the fluids within pipes) that is radiated from the facility to aid its detection and location. For example, a metal pipe may be used to complete the AC circuit and the resulting electromagnetic field generated is used to locate the pipe. This method requires the utility to be known in advance, and the utility to be accessible at various locations so that the signal can be introduced to the line. The surface-induced method generates a signal at the ground surface that will induce a response in the cable, pipe, or tracer wire underground. For example, the creation of a fluctuating electromagnetic field into the ground will induce a current in a metal pipe. The field due to the induced current can be used to localize the pipe. Unknown pipes can be located using this technique. Also, no direct connection to the pipe is required. Electromagnetic Induction is very complementary to Ground Penetrating Radar.
Limitations of EMI Technology There are certain limitations involved with EMI technology. First, EMI cannot induce a signal in a non-metallic pipe. Second, EMI signals generally cannot travel down a broken tracer wire or if the metal pipe does not have good metal-to-metal contact. Inducing a higher frequency has the potential to jump over a broken tracer wire, but higher frequencies also bring the risk that the signal may jump (or bleed) over to another nearby underground facility.