In a simplified sense, borehole extensometers are used to measure vertical displacements in the ground. More specifically, they are deeply anchored wells that are used to measure compaction of the interval between the bottom of the extensometer and land-surface. An extensometer is the only mechanical method available to measure the compaction of the aquifer material.
Within the Houston-Galveston region, 14 borehole extensometers are operated and maintained by the USGS. These extensometers were constructed as wells drilled to various depths (650 to 3,300 feet below ground surface) and anchored in stable strata with a cement plug in order to measure compaction within different aquifers (Kasmarek, et al., 2015). The outer casing is equipped with slip-joints to maintain well integrity by preventing damage from subsidence and the inner pipe is attached to a cement plug at the bottom of the borehole. An interval approximately 10- to 20-ft above the cement plug is screened to allow water to flow into the cased well. The extensometers use analog (chart) recorders, which are connected to the inner pipe, to continuously measure the change between a reference point on the inner pipe and the land-surface elevation. This measurement is the compaction of specific aquifers at depth.
GPS stations have been installed at extensometers, including placement on the inner pipe to monitor movement of the cement plug and also on the surface to monitor land-surface movement around the extensometer.
It has been documented since the 1980s that aquifer compaction results in subsidence and therefore the compaction data provided by the extensometers is critical to understand groundwater withdrawal, depressurization of the aquifer, and associated subsidence. Although the accuracy and importance of this measurement method is impressive, the installation of an extensometer is rather expensive compared to a GPS PAM or CORS site. These data sets have been invaluable in understanding the causal relations between aquifer depressurization and aquifer compaction.
Over time, as technologies have evolved, the HGSD has moved toward more cost-efficient and equally accurate forms of measurement such as GPS technologies and remote sensing to measure land subsidence. Although the HGSD has determined that the cost effectiveness of GPS stations is better than a broad network of extensometers, the HGSD has recently worked with local and county agencies to install extensometers to learn more about the compaction of frontier and recently developed aquifer units such as the Jasper aquifer.
Cross-sectional view of a borehole extensometer. Source: Kasmarek, M.C., Ramage, J.K., and Johnson, M.R., 2016, Water-level altitudes 2016 and water-level changes in the Chicot, Evangeline, and Jasper aquifers and compaction 1973–2015 in the Chicot and Evangeline aquifers, Houston-Galveston region, Texas: U.S. Geological Survey Scientific Investigations Map 3365, pamphlet, 16 sheets, scale 1:100,000, http://dx.doi.org/10.3133/sim3365