Intra-Crater Eruption Dynamics at Nyiragongo (D.R. Congo), 2002–2021

Nyiragongo is one of the rare volcanoes on Earth hosting a lava lake. However, the understanding of its plumbing and lava lake systems remains limited, with, until recently, only sporadic or time-limited historical observations and measurements. Combining dense accurate lava lake and crater floor level measurements based on 1,703 satellite radar images and topographic reconstructions using photogrammetry, we obtain the first reliable picture and time evolution of intra-crater erupted lava volumes between the two last flank eruptions in January 2002 and May 2021. The filling of the crater by lava, initiated in 2002 and continued up to May 2021, is seen as an evidence of a long-term pressure build up of the magmatic system. This filling occurs through irregular pulsatory episodes of rising lava lake level, some of which overflow and solidify on the surrounding crater floor. Pauses of stable molten lava lake level and sudden numerous level drops also marked the summit's eruptive activity. The joint analysis with seismic records available since 2015 revealed that the largest lava lake drops are synchronous with seismic swarms associated with deep magma intrusions, generally preceded by an increase of extrusion rate within the crater. The appearance of a spatter cone in the summit crater in 2016, most likely superficially branched to the lava lake, was a clear marker of the change in eruption dynamics. This first long-term time series of Nyiragongo's crater topography between two hazardous flank eruptions might further help to better decipher Nyiragongo's past and future behavior using multi-parameter observations

Propagation des intrusions basaltiques. Modélisation analogique et suivi temporel par inversion des données de déplacements

We study magma transport in the upper crust by propagation of a buoyancy-driven fluid-filled crack. Two schools of thought formalize the modelling of this phenomenon. They provide a framework to interpret either the geometrical aspects (shape, trajectory) when fluid viscosity is neglected, or the temporal aspect (flow velocity of the fluid) when, resistance to fracturation of the medium is neglected. We use two complementary approaches~: temporal in situ monitoring by inversion of displacement data and analogue modelling, in order to constrain both the geometry and the timing and to discriminate the field of application of each school of thought.We combine InSAR and GNSS data in an original inversion procedure, taking advantage of both the spatial coverage of InSAR and the temporal resolution of GNSS. The method is applied to study the complex propagation (changes of direction and velocity) that led to the eruption of 26 May 2016 at Piton de la Fournaise. This makes it possible to validate the method and provides new constraints on the supply and triggering of this eruption. In the laboratory, we are investigating the influence of fluid viscosity on the velocity and trajectory of a buoyancy-driven fluid-filled crack during ascent in the presence of a heterogeneous stress field. We used gelatine an analogue of elastic host-rock and we show that the addition of salt increases its resistance to fracture. We also show that the trajectory is the result of a competition between the internal pressure, the external stress field and the crack length. Finally, we highlight the influence of the properties of the medium, as well as those of the injected fluid on the propagation velocity and the velocity variations during ascent in the presence of a heterogeneous stress field.Nous étudions le transport du magma dans la croûte superficielle par propagation d'une fracture planaire remplie de fluide sous pression. Deux écoles de pensée formalisent la modélisation de ce phénomène. Elles permettent d'interpréter, soit les aspects géométriques (forme, trajectoire) en négligeant le comportement visqueux du fluide, soit l'aspect temporel (la vitesse d'écoulemenemporalité et de discriminer, in situ et en laboratoire, les champs d'application des écoles de pensée. Nous combinons les données InSAR et GNSS dans une procédure d'inversion originale, tirant avantage, à la fois de la couverture spatiale de l'InSAR et de la résolution temporelle du GNSS. La méthode développée est appliquée à l'étude de la propagation complexe (changements de direction et évolution de la vitesse) ayant conduit à l'éruption du 26 Mai 2016 au Piton de la Fournaise. Ceci permet de valider la méthode et apporte des contraintes nouvelles sur l'alimentation et le déclenchement de cette éruption. En laboratoire, nous cherchons à étudier l'influence de la viscosité du fluide sur la vitesse et la trajectoire d'une fissure sous pression de fluide, isolée, remontant par flottabilité en présence d'un champ de contraintes hétérogène. Nous montrons que l'ajout de sel dans la gélatine utilisée comme analogue de l'encaissant élastique augmente sa résistance à la fracturation. Nous montrons aussi que la trajectoire est le résultat d'une compétition entre la pression interne, le champ de contrainte externe et la longueur de la fissure. Enfin, nous mettons en évidence l'influence des propriétés du milieu et celles du fluide injecté sur la vitesse de propagation et des variations de cette vitesse au cours de la remontée, en présence d'un champ de contrainte hétérogène

Propagation of basaltic intrusions. Analog modeling and time tracking using inversion of ground displacement data

Nous étudions le transport du magma dans la croûte superficielle par propagation d'une fracture planaire remplie de fluide sous pression. Deux écoles de pensée formalisent la modélisation de ce phénomène. Elles permettent d'interpréter, soit les aspects géométriques (forme, trajectoire) en négligeant le comportement visqueux du fluide, soit l'aspect temporel (la vitesse d'écoulemenemporalité et de discriminer, in situ et en laboratoire, les champs d'application des écoles de pensée. Nous combinons les données InSAR et GNSS dans une procédure d'inversion originale, tirant avantage, à la fois de la couverture spatiale de l'InSAR et de la résolution temporelle du GNSS. La méthode développée est appliquée à l'étude de la propagation complexe (changements de direction et évolution de la vitesse) ayant conduit à l'éruption du 26 Mai 2016 au Piton de la Fournaise. Ceci permet de valider la méthode et apporte des contraintes nouvelles sur l'alimentation et le déclenchement de cette éruption. En laboratoire, nous cherchons à étudier l'influence de la viscosité du fluide sur la vitesse et la trajectoire d'une fissure sous pression de fluide, isolée, remontant par flottabilité en présence d'un champ de contraintes hétérogène. Nous montrons que l'ajout de sel dans la gélatine utilisée comme analogue de l'encaissant élastique augmente sa résistance à la fracturation. Nous montrons aussi que la trajectoire est le résultat d'une compétition entre la pression interne, le champ de contrainte externe et la longueur de la fissure. Enfin, nous mettons en évidence l'influence des propriétés du milieu et celles du fluide injecté sur la vitesse de propagation et des variations de cette vitesse au cours de la remontée, en présence d'un champ de contrainte hétérogène.We study magma transport in the upper crust by propagation of a buoyancy-driven fluid-filled crack. Two schools of thought formalize the modelling of this phenomenon. They provide a framework to interpret either the geometrical aspects (shape, trajectory) when fluid viscosity is neglected, or the temporal aspect (flow velocity of the fluid) when, resistance to fracturation of the medium is neglected. We use two complementary approaches~: temporal in situ monitoring by inversion of displacement data and analogue modelling, in order to constrain both the geometry and the timing and to discriminate the field of application of each school of thought.We combine InSAR and GNSS data in an original inversion procedure, taking advantage of both the spatial coverage of InSAR and the temporal resolution of GNSS. The method is applied to study the complex propagation (changes of direction and velocity) that led to the eruption of 26 May 2016 at Piton de la Fournaise. This makes it possible to validate the method and provides new constraints on the supply and triggering of this eruption. In the laboratory, we are investigating the influence of fluid viscosity on the velocity and trajectory of a buoyancy-driven fluid-filled crack during ascent in the presence of a heterogeneous stress field. We used gelatine an analogue of elastic host-rock and we show that the addition of salt increases its resistance to fracture. We also show that the trajectory is the result of a competition between the internal pressure, the external stress field and the crack length. Finally, we highlight the influence of the properties of the medium, as well as those of the injected fluid on the propagation velocity and the velocity variations during ascent in the presence of a heterogeneous stress field

Ascent rates of 3-D fractures driven by a finite batch of buoyant fluid

Propagation of fluid-filled fractures by fluid buoyancy is important in a variety of settings, from magmatic dykes and veins to water-filled crevasses in glaciers. Industrial hydro-fracturing utilises fluid-driven fractures to increase the permeability of rock formations, but few studies have quantified the effect of buoyancy on fracture pathways in this context. Analytical approximations for the buoyant ascent rate facilitate observation-based inference of buoyant effects in natural and engineered systems. Such analysis exists for two-dimensional fractures, but real fractures are three-dimensional (3-D). Here we present novel analysis to predict the buoyant ascent speed of 3-D fractures containing a fixed-volume batch of fluid. We provide two estimates of the ascent rate: an upper limit applicable at early time, and an asymptotic estimate (proportional to t-2/3) describing how the speed decays at late time. We infer and verify these predictions by comparison with numerical experiments across a range of scales and analogue experiments on liquid oil in solid gelatine. We find the ascent speed is a function of the fluid volume, density, viscosity and the elastic parameters of the host medium. Our approximate solutions predict the ascent rate of fluid-driven fractures across a broad parameter space, including cases of water injection in shale and magmatic dykes. Our results demonstrate that in the absence of barriers or fluid loss, both dykes and industrial hydro-fractures can ascend by buoyancy over a kilometre within a day. We infer that barriers and fluid loss must cause the arrest of ascending fractures in industrial settings

Combining InSAR and GNSS to Track Magma Transport at Basaltic Volcanoes

International audienceThe added value of combining InSAR and GNSS data, characterized by good spatialcoverage and high temporal resolution, respectively, is evaluated based on a specific event:the propagation of the magma intrusion leading to the 26 May 2016 eruption at Piton de la Fournaisevolcano (Reunion Island, France). Surface displacement is a non linear function of the geometryand location of the pressurized source of unrest, so inversions use a random search, based on aneighborhood algorithm, combined with a boundary element modeling method. We first invertInSAR and GNSS data spanning the whole event (propagation phase and eruption) to determine thefinal geometry of the intrusion. Random search conducted in the inversion results in two best-fitmodel families with similar data fits. Adding the same time-period GNSS dataset to the inversionsdoes not significantly modify the results. Even when weighting data to provide even contributions,the fit is systematically better for descending than ascending interferograms, which might indicatean eastward flank motion. Then, we invert the GNSS time series in order to derive informationon the propagation dynamics, validating our approach using a SAR image acquired during thepropagation phase. We show that the GNSS time series can only be used to correctly track the magmapropagation when the final intrusion geometry derived from InSAR and GNSS measurements isused as an a priori. A new method to extract part of a mesh, based on the representation of meshesas graphs, better explains the data and better accounts for the opening of the eruptive fissure thana method based on the projection of a circular pressure sources. Finally, we demonstrate that thetemporal inversion of GNSS data strongly favors one family of models over an other for the finalintrusion, removing the ambiguity inherent in the inversion of InSAR data

APPORT DES DONNEES SAR A LA COMPREHENSION ET A LA SURVEILLANCE DES VOLCANS : EXEMPLE DU PITON DE LA FOURNAISE

International audienceLast two decades have proven that remote sensing represents a key tool to improve our knowledge of volcanic systems but also to monitor active volcanoes. Based on the specific case of Piton de la Fournaise, Reunion Island, the most active French volcanoes, we here illustrate how Synthetic Aperture Radar (SAR) data, providing information even in cloudy conditions, make it possible to map eruptive deposits, to quantify their volumes but also to estimate the volcanoes topography (with metric precision) as well as surface deformation fields (with a precision reaching a few millimeters).La télédétection s'est révélée au cours des deux dernières décennies comme un outil essentiel à la fois pour améliorer notre connaissance des systèmes volcaniques mais également pour assurer la surveillance des volcans actifs. En utilisant l'exemple du Piton de la Fournaise, le plus actif des volcans français, nous illustrons ici comment les données radar satellitaires (SAR), dont l'utilisation n'est pas empêchée par la présence de nuages, permettent non seulement de cartographier les dépôts éruptifs et d'estimer leur volume mais aussi de mesurer la topographie de l'édifice volcanique (avec une précision métrique) ainsi que ses déformations de surface (avec une précision atteignant quelques millimètres)

DefVolc: Interface and web service for fast computation of volcano displacement

International audienceDefVolc is a suite of programs and a web service intended to help the rapid interpretation of InSAR data, acquired on volcanoes at an increased frequency thanks to the various dedicated satellites. Our objective is to help to rapidely inverse volcano displacements, whether these displacements result from fractures (sheet intrusions or faults) or massive magma reservoirs. These sources may have complex geometries, and they may deform simultaneously. Moreover, volcanoes are associated with prominent topographies. This makes the analysis of surface displacements complex. To appropriately analyse the InSAR displacements, we conduct inverse modelling, using 3D elastostatic boundary element models and a neighbourhood optimization algorithm. We simultaneously invert non-linear model parameters (source geometry and location) and linear model parameters (source stress changes), and further assess mean model parameters and confidence intervals. In order to speed up the setting up of inversions, we developed a users friendly graphical interface. In order to accelerate the inversions, they run on clusters. A web server is proposed to registered users in order to run the inversions on University Clermont Auvergne clusters. An application is shown for the May 2021 Nyiragongo eruption, for which models were conducted as the intrusive crisis was going on

DefVolc: Interface and web service for fast computation of volcano displacement

International audienceDefVolc is a suite of programs and a web service intended to help the rapid interpretation of InSAR data, acquired on volcanoes at an increased frequency thanks to the various dedicated satellites. Our objective is to help to rapidely inverse volcano displacements, whether these displacements result from fractures (sheet intrusions or faults) or massive magma reservoirs. These sources may have complex geometries, and they may deform simultaneously. Moreover, volcanoes are associated with prominent topographies. This makes the analysis of surface displacements complex. To appropriately analyse the InSAR displacements, we conduct inverse modelling, using 3D elastostatic boundary element models and a neighbourhood optimization algorithm. We simultaneously invert non-linear model parameters (source geometry and location) and linear model parameters (source stress changes), and further assess mean model parameters and confidence intervals. In order to speed up the setting up of inversions, we developed a users friendly graphical interface. In order to accelerate the inversions, they run on clusters. A web server is proposed to registered users in order to run the inversions on University Clermont Auvergne clusters. An application is shown for the May 2021 Nyiragongo eruption, for which models were conducted as the intrusive crisis was going on