Monitoring underground movements
On 16 September 2015 at 22:54:33 (UTC), an 8.3-magnitude earthquake struck off the coast of Chile. 11,650 km away, at CERN, a new-generation instrument – the Precision Laser Inclinometer (PLI) – recorded the extreme event. The PLI is being tested by a JINR/CERN/ATLAS team to measure the movements of underground structures and detectors.
The Precision Laser Inclinometer during assembly. The instrument has proven very accurate when taking measurements of the movements of underground structures at CERN.
The Precision Laser Inclinometer is an extremely sensitive device capable of monitoring ground angular oscillations in a frequency range of 0.001-1 Hz with a precision of 10-10 rad/Hz1/2. The instrument is currently installed in one of the old ISR transfer tunnels (TT1) built in 1970. However, its final destination could be the ATLAS cavern, where it would measure and monitor the fine movements of the underground structures, which can affect the precise positioning of the giant detector.
The device, initially proposed by Julian Budagov and Mikhail Lyablin with colleagues from the Joint Institute for Nuclear Research (JINR), is currently being developed and tested at CERN by a joint JINR-CERN team in collaboration with Jean-Christophe Gayde’s team in the Large Scale Metrology section (EN-MEF-SU) and Beniamino Di Girolamo, the former ATLAS Technical Coordinator. The instrument developed by the JINR team is a new kind of ground oscillation detector, which is able to record any angular seismic activity in the Earth’s surface accurately. The Chilean earthquake’s angular signal in µrad registered by the PLI is shown in the image below on a Coordinated Universal Time abscissa. The maximum amplitude of the signal corresponds to an angular variation of 40 µrad, compared with the background of micro-seismic ground motions at a level of ~0.1 µrad and corresponding to the superposition of seismic waves travelling both through the Earth and on its surface.
The earthquake that struck Chile on 16 September at 22:54 UTC was recorded 15 minutes later by the PLI, a new precision instrument designed to follow any ground movement.
For confirmation of the PLI’s measurements, teams at ATLAS compared the PLI data with the seismogram of the earthquake recorded by a seismometer located in Chile and obtained from the Incorporated Research Institutions for Seismology. By comparing the two graphs, the experts were also able to evaluate the expected delay of the signal caused by different speeds of propagation of surface- and body-seismic waves. For the body waves the arrival time at CERN was about 15 minutes later and for the surface waves it was approximately 60 minutes, showing an agreement with the wave speed ranges expected according to existing literature.
| Also the LHC tunnel moves under the seismic wave The circumference of the LHC tunnel oscillated by 0.2 mm when the waves from the Chile earthquake arrived. The signals were recorded on the beams for around one hour (figure on the left). Later that day the operators also saw a smaller earthquake, again from Chile (figure on the right).
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