Borehole extensometers are a more site specific method of measuring land subsidence. These instruments consist of a pipe or cable anchored at the bottom of a well casing. Pipe or cable extend from the bottom of the well, through geologic layers susceptible to compaction, to the ground surface. The pipe or cable is connected to a recorder that measures the relative distance between the bottom of the bore hole to the ground surface. These instruments are capable of detecting changes in land surface elevation to 1/100th of a foot. When land subsidence and water depth monitoring activities are paired together, hydraulic and mechanical properties of the aquifer system can be determined. DWR monitors 11 extensometers in the Sacramento Valley.
For more information regarding land subsidence monitoring vist: http://www.water.ca.gov/groundwater/landsubsidence/LSmonitoring.cfm