April 8, 2013
Surface extensometers measure rock movement over longer distances. The sensor spans tens of metres across numerous fissures.
The sensor is anchored into bedrock on either side of a crack. One end (head assembly) contains a weight that connects to the other end (anchor end) by a steel cable. The whole sensor hangs over a transducer. When the rock moves, the suspended weight shifts. The transducer measures this distance. This information is then read and stored by the datalogger before being telemetered back to our servers. Readings are made hourly. The datalogger/telemetry system is also used by the tiltmeter system.
We selected extensometer locations so the head assemblies (upslope end) and anchors (downslope end) were installed in exposed bedrock, with the extensometer cable roughly parallel to the possible direction of movement.
In October 2004, we had four surface-mounted extensometers installed. We installed a fifth extensometer in 2006. We expect the sensitivity of these extensometers to be about one to two millimetres. Each of the extensometers was custom made and installed by DGSI.
No daily or seasonal changes can be seen in the displacement data. However, we have identified three significant, localized displacements:
A detailed analysis of the data reveals that, in all cases, displacement is strongly related to cold weather. For the June and early September 2005 displacements, we recorded heavy freezing. Furthermore, we measured a dramatic temperature drop during the late October displacement event.