Articles

Séminaire GM/LGO de Philippe JOUSSET (GFZ Potsdam)

Fibre optic Distributed Acoustic Sensing for seismology, volcanology and submarine research

 

Volcanic and seismic activities produce a variety of phenomena that put population at risk and disrupt our societies. Distributed Acoustic Sensing (DAS) technology has the ability to revolutionize seismic and volcanic monitoring and our understanding of the underlying processes. After a short review of the principle benefits and issues of the new DAS technology and its applications, I will focus on Mount Etna, Italy, a complex volcanic structure, where volcano-tectonic processes interact.

We applied DAS to monitor and explore faults at several locations around Mount Etna volcano in 2018 and 2019, in collaboration with the Instituto Nazionale di Geofisica e Volcanologia (INGV Catania). Distributed Acoustic Sensing (DAS) technology has been tested for the first time in 2018 (and also in 2019), as a new tool for monitoring the complex tectonic and volcanic interactions of Etna volcano from the summit to the sea floor. We connected up to 3 iDAS interrogators, sometimes simultaneously, to optical cables close to the summit, in urban areas and offshore. Each iDAS measured the dynamic strain rate along the whole length of the optical fibre, from the interferometric analysis of the back-scattered light.

At the summit area, we connect an iDAS interrogator inside the Volcanological Observatory of Pizzi Deneri (2800 m elevation close to Etna summit) to record dynamic strain signals along a 1.5 km-long fibre optic cable that we deployed in the scoria of Piano delle Concazze. We recorded signals associated with various volcanic events, local and distant earthquakes, thunderstorm, as well as many other anthropogenic signals (e.g., tourists). To validate the DAS signal, we collocated along the fibre cable multi-parametric arrays (comprising geophones, broadband seismometers, infrasonic arrays). During the survey periods, Etna activity was mainly characterized by moderate but frequent explosive and/or effusive activity from summit craters, as monitored by INGV. Our observations suggest that DAS technology can record volcano-related signals (in the order of tens nanostrain) with unprecedented spatial and temporal resolutions, opening new opportunities for the understanding of volcano processes. I show several recent results.

In urban environments, we took advantage of the existence of fibre optic telecommunication infrastructures, we connected iDAS interrogator to fibre optic cables, known to cross active faults linked to the volcano eastern flank dynamics. We recorded dynamic strain rate along several telecom lines in villages on the slopes of Etna, where faults cross villages (e.g., 4 km cable for about 20 days in Zafferana village; 12 km-long cable running from Linera to Fleri; 40 km-long fibre optic telecommunication cable on the western side of the volcano, at the border between the sedimentary layer and the volcano edifice).

On the sea floor, we connected an iDAS interrogator to a 30-km long fibre within a cable transmitting data from sub-marine instrumentation to INFN-LNS facility at the Catania harbor in collaboration with the University of Brest, the CNRS and IFREMER (FOCUS project). We record dynamic strain signals from local and regional earthquakes and look for faults offsetting the sea floor below the eastern flank of the volcano.

Our preliminary results demonstrate that DAS technology is able to contribute significantly to the monitoring system of earthquake and volcanic phenomena at Etna volcano from the summit to submarine environment, and thereby improves assessment of volcanic and seismic hazard at volcanoes.

Summit of Etna volcano August 2018.

Submarine DAS strain rate record of an earthquake M5.6 in Albania, 21.09.2019

Les webinaires GM (Ifremer)

Pendant la période de confinement où il n’était pas possible d’organiser des séminaires en présentiel, nos voisins et collègues du département Géosciences Marines (IFREMER) ont organisé des webinaires (ou séminaires en distanciel).

Nous étions conviés à participer à ces rendez-vous hebdomadaires riches d’échanges scientifiques.

28 mai 2020, Stephan JORRY (IFREMER)

Origin of modern atolls: Challenging the deeply engrained Darwin’s Theory

4 juin 2020, Stéphanie DUPRE (IFREMER)

The Aquitaine Shelf edge (Bay of Biscay): a primary outlet for microbial methane release

11 juin 2020, Morelia URLAUB (GEOMAR)

Characterising deformation of submerged volcanoes

18 juin 2020, Christophe BASILE (ISTerre – Université de Grenoble Alpes)

Séminaire GM/LGO d’Océane FOIX (GM, Ifremer, Brest)

The 3 ‐ D Velocity Models and Seismicity Highlight Forearc Deformation

Due to Subjecting Features (Central Vanuatu)

 

The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, signifcant uplif of the overriding plate, and the presence of large forearc islands. Volcanic actvity and intermediate ‐ depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subducton of a large seamount chain and an immersed ridge. We used a catalog of P and S arrivals from a local seismological network to construct the frst 3 ‐ D velocity model of the region and to relocate earthquakes beneath the forearc. The 3 ‐ D model reveals a highly heterogeneous velocity distributon in the frst 40 km beneath the surface. Trench ‐ parallel low P and S velocity zones in the upper tens of kilometers beneath the western edges of the two largest forearc islands correlate to the major features entering into subducton and suggest highly fractured and probably water ‐ infltrated features. Trench ‐ parallel high ‐ velocity zones at 5–15 ‐ km depth, further to the east, may be part of a contnuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of contnental remnants. The earthquake distributon is generally heterogeneous, suggestng a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subductng features, where they constrain the angle of subducton to be 15° on average, down to 10–15 ‐ km depth.

6 mars 2020

Séminaire conjoint GM/LGO d’Eric Deville (IFP Énergies nouvelles, Rueil-Malmaison)

H2-rich natural gas seepages on Earth: where do they come from?

 

We present the results of studies (geological context and geochemical characteristics) concerning H 2 -rich gas seepages in different areas of the Earth, comparing offshore and onshore geological contexts, and focusing notably in exhumed terrestrial mantle rocks (ophiolitic rocks) and in intraplate domains (including at depth old continental crust). These gas seepages are in all cases made up of mixtures of 3 main components which are H 2 , CH 4 and N 2 in various proportions, with locally relatively high contents in helium in onshore context. Furthermore, in hydrothermal volcanic systems, the gas mixtures are associated with the presence of CO 2 . The relative contents of the dominant gases show distinct characteristic types which are interpreted as reflecting different zones of gas generation. In all cases, it appears that H 2 generation is mainly favored by the oxidation of Fe II -rich rocks responsible for water reduction (hydrolysis). In absence of CO 2 , H 2 -bearing gases are either associated to ultra-basic springs (pH 10-12) or they seep directly within the fracture systems of the rocks. In volcanic contexts, pH decreases drastically correlated with the presence of acid gases. CH 4 is produced either directly by reaction of dissolved CO 2 with basic-ultrabasic rocks or, in a second step, by H 2 -CO 2 interaction by abiotic or microbial processes. H 2 is present in the gas when no more carbon is available to generate CH 4 (carbon restriction conditions) or when the H 2 -CO 2 association corresponds to an active and dynamic unstable system (case of volcanic gases along mid-oceanic ridges, hot-spot areas and arc systems). N 2 is issued either from water in equilibrium with atmosphere, or, in case of important enrichment in N 2 and He, it is interpreted as issued mainly from the continental crust, respectively by ammonium destabilization within metamorphic clay and radiogenic decay. Experiments have shown that the presence of ammonium favors H 2 generation. This type of study shows that, despite of a common prejudice, H 2 is not so rare in natural environments and this native H 2 can be seen as a potential resource of clean energy commonly associated with helium for which the demand is constantly increasing.

14 février 2020

Séminaire de Luc Bulot (CEREGE / Aix-Marseille Université)

Retour sur le terrain: Une réponse aux limites d’application des échelles stratigraphiques standards à haute résolution