On the comparison of strain measurements from fibre optics with a dense seismometer array at Etna volcano (Italy)

Gilda Currenti,Michael Weber,Philippe Jousset,Rosalba Napoli,Charlotte Krawczyk

doi:10.5194/se-12-993-2021

Abstract

Abstract. We demonstrate the capability of distributed acoustic sensing (DAS) to record volcano-related dynamic strain at Etna (Italy). In summer 2019, we gathered DAS measurements from a 1.5 km long fibre in a shallow trench and seismic records from a conventional dense array comprised of 26 broadband sensors that was deployed in Piano delle Concazze close to the summit area. Etna activity during the acquisition period gives the extraordinary opportunity to record dynamic strain changes (∼ 10−8 strain) in correspondence with volcanic events. To validate the DAS strain measurements, we explore array-derived methods to estimate strain changes from the seismic signals and to compare with strain DAS signals. A general good agreement is found between array-derived strain and DAS measurements along the fibre optic cable. Short wavelength discrepancies correspond with fault zones, showing the potential of DAS for mapping local perturbations of the strain field and thus site effect due to small-scale heterogeneities in volcanic settings.

Highlights

In the recent past, direct measurements of the strain field have been hampered by the complex installation and high maintenance cost of conventional strainmeters
In order to validate the distributed acoustic sensing (DAS) records with the approaches described above, here we focus our analysis on classes of events with a frequency content less than 6 Hz
We selected two types of events (Fig. 2): (i) a volcanic explosion (VE) accompanying the strombolian activity at New South East Crater (NSEC) on 6 July 2019 and (ii) a long-period event (LP) on 27 August 2019 preceding the intensification of the eruptive activity at the summit craters in early September 2019

Summary

Introduction

Direct measurements of the strain field have been hampered by the complex installation and high maintenance cost of conventional strainmeters. Numerical investigations have clearly shown that seismic velocities, gradient displacements and strain dramatically changes at sharp boundaries and in the presence of steep topography (Kumagai et al, 2011; Jousset et al, 2004; Cao and Mavroeidis, 2019). This poses challenges in accurately interpreting strain observations when relying on only a few measurement points. A few field experiments in various environments have been designed to compare DAS strain measurements and indirect strain estimates from co-located or nearby traditional sensors, such as geophones and broadband seismometers (Jousset et al, 2018; Wang et al, 2018; Yu et al, 2019; Lindsey et al, 2020)

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Discussion

Conclusion