Extensometer - Static
A borehole extensometer is an instrument that measures vertical land motion caused by changes in aquifer thickness. A borehole is drilled through the entirety of an aquifer system and the steel pipe is anchored at the base to the basement rock. Aquifer compaction is measured by the change in the length of the extensometer pipe which protrudes above the ground surface.
Extensometers measure only changes in aquifer thickness, not other deep-earth causes of subsidence such as glacial isostatic adjustment.
Based on the USGS extensometer at Nansemond, Virginia (USGS 365337076251606 59D 39).