Electromagnetic Induction (EMI)
Electromagnetic Induction (EMI) is the most widely used technology for locating buried services and is very effective in most soil types and conditions. With EMI technology, you can locate and trace utilities, as well as estimate their depths. 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 that is radiated from the facility to help 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, by creating 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.
The EM scanner is a good first tool to use on large sites. The scanners are simple to use and easy to carry. Data can be collected much faster and the device does not have to be in contact with the ground as does GPR. Rather than pushed or dragged like a GPR device, users can walk faster and maneuver around obstacles more quickly. It’s far easier to collect data with the carried EM on sites where there may be overgrown grass, tall weeds, or rocks.
Once uncovered, each detected utility that lies below the surface is then identified on top by spray paint or flags of bright colors, and each color tells the next person at the site what is there. Red indicates that electric lines are below. Orange denotes communication such as telephone, cable TV, fiber optic lines. Yellow is gas and oil; blue shows us where the water lines are, and green is usually sanitary sewers, storm sewers and drain lines while White is used to mark proposed excavation.
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. Another limitation is that it cannot be used in proximity (5 to 20 ft) to aboveground obstructions like buildings, cars, and fences. This makes it less useful for smaller urban sites like gas stations, where there tend to be numerous aboveground obstructions. It also cannot provide exact information on the target’s depth, shape, and orientation; however, the data is easy to view, process, and overlay on maps such as Google Earth.