Interfels magnetic probe extensometer is used to monitor soil or rock settlement and heave in excavations, foundations, dams, and embankments, tunnels, shafts and sheet piles. Data from the extensometer indicate the depths at which settlement has occurred as well as the total amount of settlement.

The complete system incorporates a probe which traverses along an access tubing using a graduated tape wound in a tape reel fitted with built in light and beeper. A number of magnetic targets are fitted to the outside of the access tube which are in contact with the surrounding soil and move up or down as dictated by ground movement. When the probe enters the target's magnetic field, a reed switch closes and the beeper and light indicators are activated. The magnet levels where light and beeper are activated are read from the graduated tape.

The depth of each magnetic target is referenced to a fixed datum magnet when the bottom of access tubing is anchored in stable ground (e.g. bedrock). Otherwise topographic measurement is taken from the measuring point at the top of the access tube.