Extensometers and Compaction

Science Center Objects

Extensometers measure the compaction and expansion of the aquifer system, providing depth-specific data that can help CAWSC scientists better understand the rate, extent, and at what depths in the system subsidence is occurring. 

An extensometer measures the one-dimensional (1D) change in thickness of a specified depth interval. In other words, it measures the compaction and expansion of the aquifer system to a specific depth. More than two dozen extensometers in the Central Valley were constructed in the 1950s, 1960s, and 1970s by the U.S. Geological Survey in cooperation with the California Department of Water Resources (Ireland and others, 1984), the early group of which represent the first extensometers ever built in the United States. Additionally, several extensometers have been constructed in the San Joaquin Valley more recently. All of the extensometers are constructed as cable or pipe borehole extensometers.

A green and yellow USGS extensometer installed in a shed

Extensometers are used in land subsidence studies to measure the compaction and expansion of the aquifer system to some depth. This pipe borehole dual-stage extensometer was built in 2008 in San Lorenzo, CA and measures compaction from 10 to about 300 meters. The illustration highlights the features that can be seen in the photograph. (Public domain.)