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FBG strain sensors for volcano monitoring

Stress changes at active volcanoes are among the best indicators of changes in the activity of the system, and its possible evolution towards critical stages. In volcano geodesy, encouraging results have been obtained though borehole strain-meters. However, they are not easy to install, especially in the harsh conditions usually encountered in the summit zone of an active volcano.

Advancements in opto-electronics have allowed the development of low-cost fiber optic sensors, reliable, rugged and compact, which are particularly suitable for field applications. In particular, strain sensors based on optical Fiber Bragg Grating (FBG) are the most promising to monitor active volcanoes. FBGs are constructed by introducing a periodic modulation of the refractive index in the core of an optical fiber. When illuminated by broadband (white) light, the grating produces a narrow-band of reflected light, centered on the so-called Bragg wavelength (Fig. 1a). If the fiber experiences axial strain, the Bragg wavelength will shift and there is a linear relationship between strain and reflected wavelength shift (Fig. 1b).

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Figure 1 – a) Working principle of FBG. b) FBG response as a function of strain.

In the framework of Sub-Task 2.2.2 of MED-SUV, it has been developed a prototype FBG strain sensor for preliminary measurements in the field. The prototype was installed by INGV-OE and MATEC on the south-eastern slope of Etna, at an elevation of about 1800 m a.s.l. (Fig. 2). The instrument was installed across the surface trace of the fracture system that opened during the 1989 eruption. The reading unit and the acquisition system were hosted in a semi-underground metal box and powered through solar panels connected to trickle-charged batteries. This test installation was mainly aimed at (i) checking the performances of the prototype strain sensor in out-of-the-lab conditions and in the hostile environment in the summit zone of Etna ; (ii) informing the design specifications of the final, multi-axial device for volcano monitoring.

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Figure 2 - Google Earth map showing the position of the installation site on the southernmost edge of the 1989 fracture system (Etna).

Preliminary analysis of the data acquired in the framework of this test installation (Fig. 3) have confirmed the good performances of the FBG strain sensor.

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Figure 3 – Data sequence acquired through the FBG strain sensor on Etna between 28 May and 17 July 2014.

Authors : Nicolò Beverini (1), Massimo Calamai (1), Daniele Carbone (2), Francesco Francesconi (3), Salvo Gambino (2), Renzo Grassi (3), Alfio Alex Messina (4), Enrico Maccioni (1), Mauro Morganti (5), and Fiodor Sorrentino (3)

(1) Dipartimento di Fisica, Università di Pisa, Pisa, Italy ; (2) INGV - Osservatorio Etneo - Sezione di Catania, Catania, Italy ; (3) Marwan Technology Srl, Pisa, Italy ; (4) INGV - Sezione Roma2, Roma, Italy, (5) Istituto Nazionale di Fisica Nucleare, Pisa, Italy and Accademia Navale di Livorno, Livorno, Italy

Contacts : Daniele Carbone – INGV – Sezione di Catania, Osservatorio Etneo (daniele.carbone[at]ingv.it) ; Fiodor Sorrentino - Marwan Technology Srl, Pisa, Italy

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Last update : 11-07-2016