SPY PIG TRACKING SYSTEMS Operating Instructions Manual page 5

Acoustic systems generate an audible signal, which can be picked up
by external sensors, but only when there is sufficient acoustic coupling
between the transmitter and the wall of the pipe. This usually limits their
use to lines filled with liquid, and as you might expect lines which do not
operate in a high noise environment such as sub-sea lines.
Magnet equipped pigs operate well for tracking. They are not of any use
in locating a stuck pig as it is practically impossible to locate them when
they are not in motion. This is simply due to the physics of the type of
signal generated.
SPY portable systems utilize an electromagnetic transmitter, which is
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attached to, embedded in, or travels with the pipeline pig, and a receiver/
wand that will pick up the signal from those transmitters at a distance
allowing for significant ground cover. This signal is a low frequency signal
that operates in the magnetic range of the electro-magnetic spectrum,
which allows the signal to penetrate the steel pipe wall to transmit a
given distance through the ground.
SPY Inspection Equipment offers two "On-Pipe" systems. One is
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designed to work with our electromagnetic transmitters. The other
system is designed to work with magnet equipped pigs with magnets
of sufficient strength to create a field strength of at least one (1) Gauss
outside of the pipe.
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