Three-axial Fiber Bragg Grating Strain Sensor for Volcano Monitoring

Wien

Fiber optic and FBGs sensors have attained a large diffusion in the last years as cost-effective monitoring and
diagnostic devices in civil engineering. However, in spite of their potential impact, these instruments have found
very limited application in geophysics. In order to study earthquakes and volcanoes, the measurement of crustal
deformation is of crucial importance. Stress and strain behaviour is among the best indicators of changes in the
activity of volcanoes .. Deep bore-hole dilatometers and strainmeters have been employed for volcano monitoring.
These instruments are very sensitive and reliable, but are not cost-effective and their installation requires a large
effort. Fiber optic based devices offer low cost, small size, wide frequency band, easier deployment and even the
possibility of creating a local network with several sensors linked in an array.
We present the realization, installation and first results of a shallow-borehole (8,5 meters depth) three-axial Fiber
Bragg Grating (FBG) strain sensor prototype. This sensor has been developed in the framework of the MED-SUV
project and installed on Etna volcano, in the facilities of the Serra La Nave astrophysical observatory. The
installation siteis about 7 Km South-West of the summit craters, at an elevation of about 1740 m. The main goal
of our work is the realization of a three-axial device having a high resolution and accuracy in static and dynamic
strain measurements, with special attention to the trade-off among resolution, cost and power consumption.
The sensor structure and its read-out system are innovative and offer practical advantages in comparison with
traditional strain meters. Here we present data collected during the first five months of operation. In particular, the
very clear signals recorded in the occurrence of the Central Italy seismic event of October 30th demonstrate the
performances of our device.