Two High-Sensitivity Laser Strainmeters Installed in the Canfranc Underground Laboratory (Spain): Instrument Features from 100 to 0.001 mHz

Abstract

Two laser strainmeters are being operated in the Canfranc Underground Laboratory (LSC, Central Pyrenees, Spain) at about 350 m depth. One of the two laser strainmeters (GAL16, striking 76°) is located about 670 m from the Spanish entrance of a decommissioned train tunnel, along the side wall of one of the bypasses connecting a recent highway tunnel to the train tunnel. The other strainmeter (LAB780, striking -32°) is located about 780 m from the Spanish entrance of the train tunnel, inside two narrow side halls parallel to and built at the same time as the train tunnel. Their mechanical and optical setups derive from a previous installation at Gran Sasso, Italy, with some changes and improvements. Here we show the main instrument features in the frequency range of 100–0.001 mHz. At frequencies lower than 4 mHz, strain noise compares well with the best laser strainmeters made till now, while at higher frequencies strain noise is higher than at Kamioka, Japan, probably because of frequency instabilities of the laser source. Environmental (air temperature and pressure) effects on measured strain are quite small; thus, signal-to-noise ratio in the tidal bands is unusually high. In particular, diurnal Ψ₁ and Φ₁ tides clearly emerge from noise even using a 2-year-long strain record, giving the opportunity to improve previous determinations of the Free Core Nutation parameters from strain data as soon as more data are acquired. The features of the LSC strainmeters allow investigating the Earth in a very broad frequency range, with a signal-to-noise ratio as good as or better than that of the best laser strainmeters in the world.