1. USGS FIELD TRIP EXTENSOMETERS

2. FIELD TRIP On Thursday April 17, 2003 the Water Resources in the Tucson Basin class took a trip to see an extensometer. Ryan Scott, from the USGS, came with us.

3. EXTENSOMETERS An extensometer is a tool that is used to measure the level of subsidence, how much the ground has moved to changes in the aquifer. An extensometer has one pole that goes down 1000+ feet, this pole is used to stabilize the two shorter poles that are used to measure the subsidence.

4. EXTENSOMETERS cont. Whenever the two shorter poles move, the center pole becomes more exposed, and this is recorded. The extensometer we saw still used paper and ink to record this movement, even though most extensometers in the United States are now digital. The extensometer we saw showed that there has been very little subsidence in that area over the past twenty years.

5. EXTENSOMETERS cont. There are currently twenty one extensometers in the Tucson area. They measure not only subsidence but also the level of ground water. While the extensometer we saw recorded very little movement, there have been some that have recorded several feet of subsidence.

6. EXTENSOMETERS cont. At the sites where there has been large levels of subsidence, there are often large fissures, or other noticeable change to the land.

7. http://www.hgsubsidence.org/sub_measurement/images/borehole.jpg