ALBERT

Description: Volcanic explosions produce energy that propagates both in the subsurface as seismic waves and in the atmosphere as acoustic waves. We analyse thousands of explosions which occurred at different craters at Etna volcano (Italy) in 2018 and 2019. We recorded signals from infrasound sensors, geophones (GPH), broadband seismometers (BB) and Distributed Acoustic Sensing (DAS) with fibre optic cable. The instruments were deployed at Piano delle Concazze at about 2 to 2.5 km from the active craters, within (or onto) a ~300,000 m2 scoria layer deposited by recent volcanic eruptions. The DAS interrogator was setup inside the Pizzi Deneri Volcanic Observatory (~2800 m elevation). Infrasonic explosion records span over a large range of pressure amplitudes with the largest one reaching 130 Pa (peak to peak), with an energy of ca. 2.5x1011 J. In the DAS and the BB records, we find a 4-s long seismic “low frequency” signal (1-2 Hz) corresponding to the seismic waves, followed by a 2-s long “high-frequency” signal (16-21 Hz), induced by the infrasound pressure pulse. The infrasound sensors contain a 1-2 Hz infrasound pulse, but surprisingly no high frequency signal. At locations where the scoria layer is very thin or even non-existent, this high frequency signal is absent from both DAS strain-rate records and BB/GPH velocity seismograms. These observations suggest that the scoria layer is excited by the infrasound pressure pulse, leading to the resonance of lose material above more competent substratum. We relate the high frequency resonance to the layer thickness. Multichannel Analysis of Surface Wave from jumps performed along the fibre optic cable provide the structure of the subsurface, and confirm thicknesses derived from the explosion analysis. As not all captured explosions led to the observation of these high frequency resonance, we systematically analyze the amplitudes of the incident pressure wave versus the recorded strain and find a non-linear relationship between the two. This non-linear behaviour is likely to be found at other explosive volcanoes. Furthermore, our observations suggest it might also be triggered by other atmospheric pressure sources, like thunderstorms. This analysis can lead to a better understanding of acoustic-to-seismic ground coupling and near-surface rock response from natural, but also anthropogenic sources, such as fireworks and gas explosions.

Description: Distributed Dynamic Strain Sensing (DDSS), also known as Distributed Acoustic Sensing (DAS), is becoming a popular tool in array seismology. A new generation of engineered fibers is being developed to improve sensitivity and reduce the noise floor in comparison to standard fibers, which are conventionally used in telecommunication networks. Nevertheless, standard fibers already have extensive coverage around the Earth’s surface, so it motivates the use of the existing infrastructure in DDSS surveys to avoid costs and logistics. In this study, we compare DDSS data from stack instances of standard multi-fiber cable with DDSS data from a co-located single-fiber engineered cable. Both cables were buried in an area located 2.5 km NE from the craters of Mt. Etna. We analyze how stacking can improve signal quality. Our findings indicate that the stack of DDSS records from five standard fiber instances, each 1.5 km long, can reduce optical noise of up to 20%. We also present an algorithm to correct artifacts in the time series that stem from dynamic range saturation. Although stacking is able to reduce optical noise, it is not sufficient for restoring the strain-rate amplitude from saturated signals in standard fiber DDSS. Nevertheless, the algorithm can restore the strain-rate amplitude from saturated DDSS signals of the engineered fiber, allowing us to exceed the dynamic range of the record. We present measurement strategies to increase the dynamic range and avoid saturation.

Description: Mount Etna (Sicilia, Italy) is one of the most active volcanoes worldwide, located at the boundary between the African and the Eurasian plates. It is characterized by the occurrence of many phenomena such as lava flows, ash eruptions, earthquakes. Its eastern flank is also characterized by a complex system of active faults, associated with an eastward flank movement, up until the submarine environment. As Etna flanks are densely inhabited areas, we aim at better understanding the link between these phenomena to better assess associated hazards and risks. Since 2018, we have been measuring several locations with Distributed Fiber Optic Sensing yearly, enabling us to observe strain at meter-scale spatial interval and on a broad frequency range. We show records and present results from selected cables. Close to the summit active craters, we interrogate dedicated cables, and could analyze the ground response in association with explosions and volcanic tremor. In urban areas, we interrogate telecommunication cables and record local earthquakes. In the submarine area, we interrogate a cable which crosses the North Alfeo fault with several different optical techniques. In the southern flank of the volcano, we show volcanic signals from a cable deployed in a borehole. We also demonstrate how simultaneous multi-fiber measurements can help constrain earthquake hypocenter location. We discuss opportunities and challenges of using fiber optic cable in various environments such as the Etna volcano and beyond, for an integrated vision from deep processes, their interaction with the sub-surface dynamics and the volcano-tectonic structures.

Description: Distributed Dynamic Strain Sensing (DDSS), a.k.a. Distributed Acoustic Sensing (DAS), is becoming a popular tool for volcano monitoring. The sensing method relies on measuring the phase-shift of Rayleigh back-scattered light throughout the fibre due to strain variations in the fibre glass. This provides distributed strain-rate measurements at fine temporal and spatial sampling intervals. During 3 months in 2019, we recorded signals from thousands of mild volcanic explosions from Mt. Etna using a multi-instrument network deployed in an area at ca. 2.5 km distance from the active craters. Infrasound sensors were laying at the surface with a dense array of broadband seismometers (BB). Two types of fibres were also buried ca. 30 cm depth in the non-consolidated scoria from the area. First fibre was a 1.5 km long standard fibre, interrogated with an iDAS unit. The second fibre was a 0.5 km long engineered fibre, interrogated with a Carina unit. Relation between infrasound and DDSS data suggests a ground response of the loose scoria due to the acoustic pressure waves from explosions. Further analysis suggests a non-linear relationship between acoustic pressure and strain-rate data. However, signal saturation is encounter in some of the strain-rate data, which affects the interpretation of the non-linear relation. Therefore, we present an algorithm to correct the signal artefacts, allowing us to restore the true strain-rate signal and exceed the dynamic range limited by the initial DDSS recording parameters. The outcome includes strategies in the selection of acquisition parameters prior to DDSS campaigns to avoid signal saturation.

Description: The GFZ-Landsvirkjun Theistareykir Fibre array is located in the Theytareykir geothermal area, in North Iceland. It is collocated with arrays of broadband seismometers and gravity meters (see e.g., https://doi.org/10.1186/s40517-021-00208-w). The geometry of the fibre array is following the telecom network in the area, and was chosen to test the seismological capabilities of telecom cables in this geothermal environment. We connected an iDAS V2 interrogator from Silixa. The interrogator location is lat=65.898041, lon=-16.966274. The array starts N-S and after 1.5 km, turns towards the East, up to a local transmission antenna station for mobile phones. The length of the path is ~5 km. The length of the cable is actually more than 15 km, as other fibre instance is connected at the transmission antenna station.. Jumps were performed along the cable to geo-locate the channels. The exact location of the fibre can unfortunately not be disclosed. Original recordings at 1000 Hz were downsampled to 200 Hz using a software from INGV-OE (michele.prestifilippo@ingv.it) and are provided in an h5 format. We provide here the first fibre instance (5 km long). The data contain 1 h long recording intervals framing M>5 teleseismic earthquakes recorded in the frame of the global DAS month, an initiative to collaboratively record and share simultaneously recorded DAS data from all over the world (https://www.norsar.no/in-focus/global-das-monitoring-month-february-2023). DAS is an emerging technology increasingly used by seismologists to convert kilometer long optical fibers into seismic sensors.

Description: During February 2023, a total of 32 individual distributed acoustic sensing (DAS) systems acted jointly as a global seismic monitoring network. The aim of this Global DAS Month campaign was to coordinate a diverse network of organizations, instruments, and file formats to gain knowledge and move toward the next generation of earthquake monitoring networks. During this campaign, 156 earthquakes of magnitude 5 or larger were reported by the U.S. Geological Survey and contributors shared data for 60 min after each event’s origin time. Participating systems represent a variety of manufacturers, a range of recording parameters, and varying cable emplacement settings (e.g., shallow burial, borehole, subaqueous, and dark fiber). Monitored cable lengths vary between 152 and 120,129 m, with channel spacing between 1 and 49 m. The data has a total size of 6.8 TB, and are available for free download. Organizing and executing the Global DAS Month has produced a unique dataset for further exploration and highlighted areas of further development for the seismological community to address.