Measuring the full strain tensor using a low-cost optical borehole strainmeter

Monitoring volcanic deformation is crucial for understanding eruptive activities and associated hazards. While various surface deformation measurement tools like GPS, INSAR and tiltmeters are widely used on volcanoes, strainmeters offer unique means to detect volcanic transient phenomena, deformation source location and dynamics (Bagagli et al., 2017; Bonaccorso et al., 2023), including the categorization of phenomena (Carleo et al., 2023). However, their widespread use has been hindered by cost considerations and the challenges associated with on-field calibration. Additionally, strainmeters are recording deformation from shallow water levels, body tides, pressure changes and snowfall, that makes difficult the separation among the involved processes.

To address these challenges, we propose a novel, moderate-cost, high-resolution borehole strainmeter based on optical measurement. Strain is quantified by measuring the diameter change of a sphere using optical interferometry (Seat et al. 2012), providing a 10^-10 strain resolution across a broadband [0; 2.5] kHz range. This innovative strainmeter (Chery 2021, patent FR2106959) features six uniaxial strain gauges anchored inside a concrete spherical shell, enabling the reconstruction of the six components of the 3D strain tensor. The full measurement of the strain tensor should enhance our understanding of deformation dynamics both at borehole vicinity and in the far-field. Indeed, the knowledge of the full straintensor should allow the differentiation between near-field and far-field perturbations due to the distinct nature of their strain perturbation in time and space.

A seventh measurement gauge, free of shell deformation, facilitates the filtering of physical variations associated to laser wavelength, temperature and air pressure. To ensure accuracy and reliability of the strainmeter, an in-situ calibration system involving sphere pressurization is integrated. This innovative feature should help to determine borehole heterogeneity and to minimize long term drift.

In November 2023, a first strainmeter prototype has been successfully installed on the Larzac Plateau (France) at the multi-instrument observatory (OSU OREME and H+/OZCAR network). We will present first results of the instrument and will discuss the future perspectives.