When locating plastic or concrete pipes, ducts and drains there is no way of detecting and tracing them electromagnetically by current flow, unless a tracer wire is inserted or laid near to the line. This can then have a signal imposed upon it by active location methods.

The alternative is to insert a signal transmitter known as a Sonde into the line, and use a Radiodetection receiver to detect and pinpoint its position from above. The sonde element comprises nothing more than ferromagnetic rod surrounded by a coil, which is energised with a closely controlled signal frequency by a battery-powered oscillator. Rod, oscillator and battery are usually built as a self-contained watertight unit, designed to be attached to a drain rod, floats, or other appropriate means of traversing along a length of pipe. The frequency used can be chosen to match that of a signal generator, so that the same receiver can be used to locate the position of the sonde, and trace its progress from above ground.

There are certain important differences in the characteristics of the magnetic field produced by a sonde in comparison with that produced by current in a length of line.

The significance of them is dealt with in Sonde Signal Characteristics article. The distance over which a sonde can be detected is a function of its transmitting power, which is necessarily related to its size. A tiny sonde for small diameter lines inevitably has a limited transmission range by comparison with a large sonde for use in deep sewers.

As a sonde is approached with a horizontal aerial, a rising signal will be detected as it comes within range. This will then fall off again to null, so giving the impression that it has been passed. But if the traverse is continued, the signal will rise again to a much higher level, the peak of which is above the sonde. It will then fall to null again, followed by a small rise, and then fade away. The small peaks either side of the nulls may therefore mislead the unwary; it is essential to find the higher peak between the two nulls to pinpoint the sonde position.