Large-Scale DFOS at Cuolm da Vi

Cuolm da Vi is one of the largest active slope instabilities in the Alps (see Figure 1), consisting of ~150 × 10⁶ m³ of unstable rock moving at approximately 10–20 cm per year in the central area. Currently, the yearly displacement values are relatively stable, but the instability poses long-term risks to infrastructure and communities. It is therefore continuously monitored using a variety of remote sensing and radar methods. However, these surface-based systems provide limited information on subsurface dynamics, while understanding the internal driving mechanisms is crucial for improving hazard assessment. 

To address this challenge, we trenched and deployed a 6.5 km fibre-optic cable network in summer 2022 (see Figure 2), suitable for Distributed Acoustic and Strain Sensing (DAS & DSS) measurements, across both the active and more stable parts of the slope instability (see Figure 3). Using this configuration, we collected more than seven months of continuous DAS data, as well as numerous short-term and long-term DSS recordings. 

One of the key advantages of fibre-optic sensing in such a large-scale instability monitoring context is its ability to provide very dense and continuous measurements over kilometre-scale distances. At Cuolm da Vi, this allows us to, for example, monitor a wide variety of seismic signals that would have been difficult to detect with traditional (sparse) seismic networks alone. 

Using several months of continuous DAS recordings, we identified more than 1,200 microseismic events occurring within the slope area. These signals likely reflect internal deformation processes such as fracturing or slip along existing weakness zones. Using a Matched Field Processing approach, adapted to DAS data, we were able to localize these signals and study their spatial and temporal behaviour (see Figure 4). 

Interestingly, the detected seismic activity clusters around certain fracture systems and zones of weakened rock. In addition, periods of increased seismic activity appear to coincide with environmental triggers such as heavy rainfall or snowmelt. These observations provide new insights into how external factors influence the internal stress state and deformation of the Cuolm da Vi slope.

Beyond this specific site, the project highlights the potential of fibre-optic sensing for long-term monitoring of unstable slopes. By turning kilometres of optical fibre into dense sensing arrays, fibre-optic technology opens new possibilities for slope instability research and monitoring across scales ranging from metres to kilometres, and from milliseconds to months.

Author 
Tjeerd Kiers 

Institution 
Institute of Geophysics, ETH Zurich 

Collaborators 
Cédric Schmelzbach, Hansruedi Maurer, Florian Amann, Pascal Edme, Antonio Pio Rinaldi, Yves Bonanomi, Johan Robertsson 

Funding 
European Union Horizon 2020 – Marie Skłodowska-Curie Grant Agreement No. 955515