Borehole extensometers are highly sensitive instruments which measure aquifer compaction. This infographic explains the various parts of a borehole extensometer, and is modeled after the USGS extensometer at Nansemond, Virginia.
Instrument Table: The instrument table is bolted to the ground, so it rises and falls with land surface.
Steel Pipe: The steel pipe is anchored to the basement rock and doesn't move even when the land surface does.
Fulcrum Arm & Counterweight: The steel pipe may be hundreds to thousands of feet long, and quite heavy! To stop the heavy steel pipe from pulling down on the instruments, or compressing under its own weight, it must be carefully balanced with a counterweight. This makes the steel pipe effectively weightless!
Instruments: As the ground moves, the difference between the ground surface and the unmoving steel pipe is recorded by an instrument called a linear potentiometer.
Borehole: The borehole is drilled from the surface, down through the entire aquifer system and all the way to the basement rock. The steel pipe is anchored at the bottom of the borehole.