Fibre-Optic Sensing for Volcano Monitoring on Grímsvötn, Iceland

We present the results of an experiment with Distributed Acoustic Sensing (DAS) on Grímsvötn in Iceland. DAS is a novel detection method that samples the strain wavefield due to ground motion along a fibre-optic cable with high temporal (kHz) and spatial (m) resolution. Consequently, it has the potential to increase our understanding of physical volcanic processes.

 

We deployed a 12 km long fibre-optic cable for one month (May 2021) on Grímsvötn, Iceland’s most active volcano, which is completely covered by the large Vatnajökull ice sheet. The cable was trenched 50 cm into the ice, following the caldera rim and ending near the central point of the caldera on top of a subglacial lake. A large number of hammer blow experiments allow us to estimate the Rayleigh wave dispersion curves, and thickness of the ice layer on top of the volcanic rock.

 

We have discovered previously undetected levels of seismicity, with up to several hundreds of local events per day, using an automated earthquake detection algorithm that is based on image processing techniques. First arrival picks are identified with an automated cross-correlation based algorithm, developed specifically for complex and local events recorded with DAS. The first arrival times, combined with a probabilistic interpretation and the Hamiltonian Monte Carlo algorithm, allow us to estimate event locations and their respective uncertainties, even in the absence of a detailed velocity model. The detection and localisation of the recorded events paints a differentiated picture of Grímsvötn’s volcano-seismicity.

 

The preliminary results of our experiment highlight the potential of DAS for studies of active volcanoes covered by glaciers, and we hope that this research will contribute to the fields of volcano monitoring and hazard assessment.