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
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
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
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.