Components of a rod extensometer include anchors, rods with protective tubing, and a reference head. The anchors, with rods attached, are installed downhole. The rods span the distance between the anchors and the reference head, which is installed at the borehole collar. Measurements are obtained at the reference head with a sensor or a depth micrometer, either of which measures the distance between the top (near) end of the rod and a reference surface.
A change in this distance, found by comparing the current measurement to the initial measurement, indicates that movement has occurred. Movement may be referenced to a downhole anchor that is installed in stable ground or to the reference head, which can be surveyed. The resulting displacement data can be used to determine the zone, rate, and acceleration of movements, or to calculate strain.
Source:http://www.slopeindicator.com/instruments/ext-rod.php
Link to report of this instrument: ETR