Summary
Seismic signals from volcanoes are useful for understanding the current status of volcanoes. They can provide valuable information to assess the current hazard situation from possible eruptions. In particular, the mechanism of low-frequency signal components is still discussed controversially. Conventional fibre-optic telecommunications cables laid on land or under the sea can now be used to detect these signals. This was shown by researchers from the National Institute of Geophysics and Volcanology (INGV) in Italy, the German Research Centre for Geosciences (GFZ) and the University of Catania (UniCT) (Italy) in a study recently published in the journal Scientific Reports. The investigations took place on the Italian island of Vulcano off Sicily. The researchers used the so-called DAS (Distributed Acoustic Sensing) technology to obtain distributed strain data along 16 km optical fibre, every 4 meters. To analyse the large amounts of data, they developed new algorithms and used artificial intelligence methods.
Background: Volcanic risk assessment
Volcanic risk assessment requires geophysical, geochemical and geological information obtained by scientific instruments on the flanks and summits of volcanoes. In particular, the low frequency, very long period (VLP) and long period (LP) seismic signals in volcanoes have been the subject of scientific debate for several decades. Understanding their generation mechanisms is a key element for assessing the activity state of volcanoes and thus for developing warning systems.
Previous technology
Until now, broadband seismometers have been the main instruments used to study seismic signals and the processes of their generation in volcanoes. They are usually installed on the volcano building to record events and estimate their source. However, on small volcanic islands, the underwater environment requires the installation of instruments (OBS, Ocean Bottom Seismometers) that are particularly expensive and difficult to operate and maintain.
New approach also for low-frequency signals
As an alternative, it is now possible to use already installed fibre optic telecommunication cables in combination with the so-called DAS technology (Distributed Acoustic Sensing). This technology is also suitable for detecting the very long-period and long-period seismic signals, which has now been demonstrated by a research team led by Gilda Currenti from the National Institute of Geophysics and Volcanology (INGV) in Italy and Philippe Jousset from the GFZ German Research Centre for Geosciences on the Italian island of Vulcano, which belongs to the Lipari Islands off Sicily. They published their results in the scientific journal Scientific Reports. Last year, they had already reported that the method is basically suitable for volcano monitoring.
In their studies, they used the submarine fibre optic connection between the TIM central telecommunication station at Vulcano island and Milazzo in the north-east of Sicily, which stretches for about 50 kilometres along the seabed. Optical cables comprise several fibres in parallel for broad-band telecommunications. DAS requires one free fibre, not used for telecommunication.
DAS technology and its advantages
These optical fibres are used in combination with DAS (Distributed Acoustic Sensing) technology as seismic sensors. In this case, a DAS device (“iDAS” interrogator from Silixa, UK) was set up in the central telecommunication station and connected to one free fibre. The device comprises a laser which sends light pulses into the fibre and records the backscattered signal. Vibrations and movements in the subsurface deform the fibres. These strain fluctuations distort the properties of the backscattered light, creating a strain signal. From its analysis, information about the Earth's movement can be derived remotely. This method makes it possible to record signals with high spatial resolution of about four metres and high temporal resolution of about one kilohertz.
“With the ability to interrogate submarine cables even over long distances on the order of tens of kilometres, DAS devices transform fibre optics into a dense array of distributed sensors that are easier to manage than conventional sensors. This capability enables easy and quick intervention to capture signals useful for rapid response to volcanic crises,” says Gilda Currenti, INGV.
The current measurement campaign
The measurement campaign of the study now presented extended from 13 January to 14 February 2022. Signs of unrest had already been observed at the volcano since September 2021, with CO2 degassing and the occurrence of long-period and very long-period events.
During the month-long survey, the researchers recorded 1488 events with a wide variety of waveforms composed of two main frequency bands (from 0.1 to 0.2 Hz and from 3 to 5 Hz) with different relative amplitudes.
Challenge: Large amounts of data
Because the acquisition of DAS signals generates a huge amount of data, the method poses a challenge for storage, access and processing from a computer science perspective. During the volcano experiment, data had been continuously collected, all in all about 20 terabytes.
Therefore, the team developed new computer solutions to collect, manage and analyse the extremely large amounts of data. To do this, it used the latest technological advances in high-performance computing (HPC) and artificial intelligence.
The team compared its results with those of conventional measurement and evaluation methods.
Results and assessment of the new method
Based on the spectral signature and classification, the researchers propose a model for volcanic activity to interpret the data, in which gas accumulates in the hydrothermal system, a hot gas-water mixture that exists inside certain types of volcanoes, and is released to the surface through a series of fractures.
“The study has shown that interrogation of the underwater fibre optic cable connecting the island of Vulcano to Milazzo, combined with specific processing algorithms, can effectively contribute to seismic monitoring and understanding the origin of low-frequency seismic signals generated by hydrothermal activity at Vulcano. This example opens the door to monitoring unknown submarine volcanoes”, resumes Philippe Jousset, GFZ.
Original study:
Currenti, G., Allegra, M., Cannavò, F., Jousset, P. et al. Distributed dynamic strain sensing of very long period and long period events on telecom fiber-optic cables at Vulcano, Italy. Sci Rep 13, 4641 (2023). DOI: 10.1038/s41598-023-31779-2
Related publications:
https://www.gfz-potsdam.de/en/press/news/details/vulkanueberwachung-mit-glasfaserkabeln
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