Validating ultra-small low-frequency strain signals recorded by a borehole distributed fibre optic sensing method

In volcanic environment, monitoring nanostrain-level (10^-9) low frequencies (minutes to days) ground deformation is fundamental to detect short-term magma migration preceding and accompanying eruptions and issue alerts for civil protection operations. However, ultra-small slow deformations are also induced by other environmental sources such as Earth tides, rainfall, barometric pressure and air temperature variations which may mask the volcano-related strain signal. The identification and precise estimation of such environmental effects on a strain signal has thus two essential goals: validating/calibrating the signal recorded by the sensor and highlighting the ultra-small volcanic strain after their removal.

We use a full-band distributed strain sensing (FB-DSS) fibre optic method to detect nanostrain-level strain variation of the ground in the minute-to-days timescale. The optical fibre is deployed along a 190 m –deep borehole in the Southern flank of the Etna volcano. The long-term stability and the sensitivity of the FB-DSS method for borehole strain sensing is tested and evaluated by employing well-established techniques used for calibrating and validating strain signals recorded by high-precision borehole sensors. During periods of low volcanic activity, we are able to precisely detect the effects of the Earth tides, the rainfall, the barometric pressure and the air temperature variations. The comparison between the tides recorded by the FB-DSS method and the ones expected from the theory shows that the recorded tides are consistent in terms of both amplitude (10^-9 – 10^-8) and phase. Moreover, variations in the estimated tidal amplitude along depth indicates that the rock stratifies in layers with a different sensitivity to tidal strain. We also detect nanostrain-level variations induced by rainfall events. Such variations are evident especially in the shallow layer (up to 40 m), showing a decreasing admittance with depth from 7 to 1 nanostrain/mm. Barometric pressure variations are clearly detected by the FB-DSS method. The ground response to this source is frequency-dependent showing an admittance ranging from 2 to 11 nanostrain/hPa and consistent with the values estimated from other borehole sensors worldwide. Finally, we also observed surface temperature-induced effects in the very shallow layer (until 10 m) due to diurnal variations of the air temperature.

The observed ultra-small, slow strain variations associated with Earth tides, rainfall, barometric pressure, and air-temperature changes validate the sensitivity and long-term stability of the FB-DSS method implemented for borehole strain sensing. The precise estimation of such effects enabled the identification of ultra-small slow strain changes induced by the Etna eruption on 10^th November 2024, which are consistent with the variations measured by other high-precision borehole sensors already installed at Etna volcano.