Detecting volcano unrest at Etna using borehole distributed fibre optic sensing

In 2024 an innovative Distributed Fiber Optic Sensing prototype has been set up to interrogate a fiber optic cable installed in a 190-m deep borehole on the southern Etna flank about 5 km away from the summit crater. We use a full-band distributed strain sensing (FB-DSS) fibre optic method, implemented using a reference-based Rayleigh backscatter correlation approach, in which each acquisition is compared with previously recorded reference data to retrieve distributed strain changes with long-term stability at nanostrain-level.

The local strain response is assessed by comparing the distributed signals against natural and controlled deformation sources. Thanks to the nanostrain level sensitivity, variations induced by Earth tide and environmental parameters, including temperature, precipitation and atmospheric pressure, are clearly visible and in agreement with theoretical expectations.

The strain residuals, achieved after the removal of the Earth tide components, show up deformation related to Etna volcano activity. On the morning of 10^th November 2024 Etna experienced a weak lava fountain preceded by a short and small seismic swarm. Despite the tiny deformation induced by the volcano unrest, the FB-DSS prototype was able to discern strain variations on the order of 125 nanostrain over 2 h (0.02 nanostrain/s). The strain variations are in agreement with dilatometer and tilt signals recorded by the permanent high-precision deformation network of Etna. No displacements above the background noise level are observed in the GPS data. The joint analysis and modeling of the deformation dataset from the FB-DSS and permanent network allows to track the eruptive activity, constraints the magmatic processes and estimate source parameters. Our findings demonstrate that the FB-DSS approach concurs in bridging the seismo-geodetic bandgap, while offering important advantages over conventional borehole point sensors.