Installation of a high-sensitivity laser strainmeter in a tunnel in central Italy

A new laser interferometer for geophysical purposes is being operated in an underground laboratory in central Italy. A simple electro-optical technique has been used to limit the costs. The main characteristics of this instrument are: high sensitivity ${\text{(Δl/l≈10}}^{\text{−12}})$ , fast sampling rate (up to 6 kHz), large dynamic range (unbounded in principle), the capability of following strain rates as high as ${\text{7×10}}^{\text{−5}}$ s${}^{\text{−1}}$ , and good reliability (two failures in nearly two years).