Ash plume properties retrieved from infrared images: a forward and inverse modeling approach

We present a coupled fluid-dynamic and electromagnetic model for volcanic ash plumes. In a forward approach, the model is able to simulate the plume dynamics from prescribed input flow conditions and generate the corresponding synthetic thermal infrared (TIR) image, allowing a comparison with field-based observations. An inversion procedure is then developed to retrieve ash plume properties from TIR images. The adopted fluid-dynamic model is based on a one-dimensional, stationary description of a self-similar (top-hat) turbulent plume, for which an asymptotic analytical solution is obtained. The electromagnetic emission/absorption model is based on the Schwarzschild's equation and on Mie's theory for disperse particles, assuming that particles are coarser than the radiation wavelength and neglecting scattering. [...] Application of the inversion procedure to an ash plume at Santiaguito volcano (Guatemala) has allowed us to retrieve the main plume input parameters, namely the initial radius $b_0$, velocity $U_0$, temperature $T_0$, gas mass ratio $n_0$, entrainment coefficient $k$ and their related uncertainty. Moreover, coupling with the electromagnetic model, we have been able to obtain a reliable estimate of the equivalent Sauter diameter $d_s$ of the total particle size distribution. The presented method is general and, in principle, can be applied to the spatial distribution of particle concentration and temperature obtained by any fluid-dynamic model, either integral or multidimensional, stationary or time-dependent, single or multiphase. The method discussed here is fast and robust, thus indicating potential for applications to real-time estimation of ash mass flux and particle size distribution, which is crucial for model-based forecasts of the volcanic ash dispersal process.Comment: 41 pages, 13 figures, submitted pape

Volcanic Hot-Spot Detection Using SENTINEL-2: A Comparison with MODIS−MIROVA Thermal Data Series

In the satellite thermal remote sensing, the new generation of sensors with high-spatial resolution SWIR data open the door to an improved constraining of thermal phenomena related to volcanic processes, with strong implications for monitoring applications. In this paper, we describe a new hot-spot detection algorithm developed for SENTINEL-2/MSI data that combines spectral indices on the SWIR bands 8a-11-12 (with a 20-meter resolution) with a spatial and statistical analysis on clusters of alerted pixels. The algorithm is able to detect hot-spot-contaminated pixels (S2Pix) in a wide range of environments and for several types of volcanic activities, showing high accuracy performances of about 1% and 94% in averaged omission and commission rates, respectively, underlining a strong reliability on a global scale. The S2-derived thermal trends, retrieved at eight key-case volcanoes, are then compared with the Volcanic Radiative Power (VRP) derived from MODIS (Moderate Resolution Imaging Spectroradiometer) and processed by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during an almost four-year-long period, January 2016 to October 2019. The presented data indicate an overall excellent correlation between the two thermal signals, enhancing the higher sensitivity of SENTINEL-2 to detect subtle, low-temperature thermal signals. Moreover, for each case we explore the specific relationship between S2Pix and VRP showing how different volcanic processes (i.e., lava flows, domes, lakes and open-vent activity) produce a distinct pattern in terms of size and intensity of the thermal anomaly. These promising results indicate how the algorithm here presented could be applicable for volcanic monitoring purposes and integrated into operational systems. Moreover, the combination of high-resolution (S2/MSI) and moderate-resolution (MODIS) thermal timeseries constitutes a breakthrough for future multi-sensor hot-spot detection systems, with increased monitoring capabilities that are useful for communities which interact with active volcanoes

Dynamics of Mount Nyiragongo lava lake inferred from thermal imaging and infrasound array

Abstract Lava lakes provide a direct observation window into processes which usually remain hidden, such as magma convection and outgassing dynamics. We here report a coupled analysis of thermal infrared footage and infrasound array recordings at Mount Nyiragongo (D. R. Congo), and derive a conceptual model of the lava lake's convective system and outgassing mechanism. We suggest that surface flow results from a horizontal pressure gradient at the surface of the lake, driving the crust from high-pressure regions where hot upwelling magma impinges the surface, to low-pressure regions where cold downwelling magma pulls away from the surface. The ascending current of this convection cell carries gas pockets, which once at the surface, are dragged across the lake into downwelling sinks. Such sinks are characterized by persistent chaotic bubble bursting (spattering), whose intensity and position are tracked from infrasound array analysis. Fluctuations of these are observed, but have not been correlated with oscillations of the lava lake level, nor with the variations of surface velocities, both recorded from infrared footage. We also report the activity of a new eruptive vent, which opened early 2016 near an inner circular fracture of the crater's third terrace. We show that the vent's activity was intermittent, alternating between explosive strombolian activity and effusive activity. The latter produced lava flows which spread on the crater's last terrace before cascading into the active lava lake. Although no significant change in the lake behavior was witnessed while the new eruptive vent was active, increased attention should be addressed as this new activity could reflect over-pressurization of the shallow magmatic system. The variety of phenomena captured by this study complements and expands observations reported at other low-viscosity lava lakes, chiefly Kilauea (Hawai'i) and Erta Ale (Ethiopia). Despite Nyiragongo's more vigorous convective regime (where multiple convective cells can operate simultaneously), we suggest that the mechanisms controlling the surface motion and outgassing are similar at all three systems, pointing to generic processes governing the dynamics of low-viscosity lava lakes

Automatic retrieval of volcanic SO2 emission source from TROPOMI products

Volcanic sulfur dioxide (SO2) satellite observations are key for monitoring volcanic activity, and for mitigation of the associated risks on both human health and aviation safety. Automatic analysis of this data source, including robust source emission retrieval, is in turn essential for near real-time monitoring applications. We have developed fast and accurate SO2 plume classifier and segmentation algorithms using classic clustering, segmentation and image processing techniques. These algorithms, applied to measurements from the TROPOMI instrument onboard the Sentinel-5 Precursor platform, can help in the accurate source estimation of volcanic SO2 plumes originating from various volcanoes. In this paper, we demonstrate the ability of different pixel classification methodologies to retrieve SO2 source emission with a good accuracy. We compare the algorithms, their strengths and shortcomings, and present plume classification results for various active volcanoes throughout the year 2021, including examples from Etna (Italy), Sangay and Reventador (Ecuador), Sabancaya and Ubinas (Peru), Scheveluch and Klyuchevskoy (Russia), as well as Ibu and Dukono (Indonesia). The developed algorithms, shared as open-source code, contribute to improving analysis and monitoring of volcanic emissions from space

Tracking dynamics of magma migration in open-conduit systems

Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long- period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate. During the 2014 flank eruption of Stromboli, this magma input pre- ceded the effusive eruption by several months. When the new lateral effusive vent opened on the Sciara del Fuoco slope, the effusion was accompanied by a large ground deflation, a deepening of the VLP seismic source, and the cessation of summit explosive activity. Such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent. We show how this model successfully reproduces the measured rate of effusion, the observed rate of ground deflation, and the deepening of the VLP seismic source. This study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open-conduit system and to track magma drainage during the effusive crisis, with a great impact on hazard assessment

Shallow magma convection evidenced by excess degassing and thermal radiation during the dome-forming Sabancaya eruption (2012–2020)

We used a large set of satellite- (visible, infrared, and radar images from Planetscope, MODIS, VIIRS, Sentinel2, Landsat 8, and Sentinel 1) and ground-based data (optical images, SO2 flux, shallow seismicity) to describe and characterize the activity of the Sabancaya volcano during the unrest and eruption phases that occurred between 2012 and 2020. The unrest phase (2012–2016) was characterized by increasing gas and thermal flux, sourced by a convective magma column rising along with the remnants of a buried plug still permeable to fluid flow. Conversely, a new conduit, adjacent to the previous one, fed the eruptive phase (2016–2020) which was instead characterized by a discontinuous extrusive activity, with phases of dome growth (at rates from 0.04 to 0.75 m3 s−1) and collapse. The extrusive activity was accompanied by fluctuating thermal anomalies (0.5–25 MW), by irregular SO2 degassing (700–7000 tons day−1), and by variable explosive activity (4–100 events d−1) producing repeated vulcanian ash plumes (500–5000 m above the crater). Magma budget calculation during the eruptive phase indicates a large excess of degassing, with the volume of degassed magma (0.25–1.28 km3) much higher than the volume of erupted magma (< 0.01 km3). Similarly, the thermal energy radiated by the eruption was much higher than that sourced by the dome itself, an unbalance that, by analogy with the degassing, we define as “excess thermal radiation”. Both of these unbalances are consistent with the presence of shallow magma convection that fed the extrusive and explosive activity of the Sabancaya dome

Source mechanisms and dynamics of volcanic pyroclastic emissions : a perspective from Doppler radar (VOLDORAD) and other geophysical data

Cette étude traite de la dynamique des éruptions volcaniques explosives, depuis les mécanismes de sub-surface jusqu’aux processus d’émission et de dispersion des pyroclastes. A cet effet un radar Doppler sol est utilisé (VOLDORAD), lequel renseigne sur la charge / vitesse des ejectas. Les données sont intégrées avec d’autres techniques géophysiques, et des modèles numériques sont développés afin de simuler les émissions pyroclastiques, générer des signaux radar synthétiques, pour finalement améliorer notre compréhension des processus qui leurs sont sous-jacents. L’Arenal (Costa Rica) est utilisé comme volcan cible, où de fréquentes éruptions de faible magnitude émettent des panaches de cendres et des projections balistiques jusqu’à quelques centaines de mètres au-dessus de l’évent. Dans un premier temps, nous combinons des données sismiques et radar afin d’explorer la relation entre les processus de conduit et les émissions pyroclastiques. Leurs interactions complexes sont interprétées via un modèle conceptuel, lequel décrit les fractures parsemant le bouchon de lave comme responsables du dégazage du système, et en retour des signaux sismiques et radar collectés (ces derniers dépendants de la charge en cendres des émissions de gaz). Par ailleurs, nous investiguons la dynamique des émissions pyroclastiques à travers l’étude de radargrammes Doppler. La distribution spatio-temporelle de la vitesse des ejectas indique l’existence de deux phénomènes aux dynamiques distinctes. Des modélisations numériques permettant la reconstruction de signaux synthétiques indiquent qu’il s’agit de l’émission simultanée de blocs balistiques et de panaches de cendres. Une procédure d’inversion de type Monte Carlo couplée d’un algorithme d’optimisation permet de retrouver les radargrammes synthétiques qui reproduisent au mieux ceux observés. Les résultats apportent des contraintes sur divers paramètres éruptifs, tels que les tailles, trajectoires, vitesses des ejectas et des gaz, ainsi que la vitesse / direction de dispersion des panaches de cendres par le vent. Enfin, nous discutons du potentiel des radars Doppler appliqués à la surveillance opérationnelle des émissions volcaniques. En particulier, la possibilité de quantifier les masses éjectées dans l’atmosphère ou retombant sur les flancs du volcan, fournit des paramètres éruptifs à la source pouvant alimenter les modèles de dispersion de panaches de cendres.This study investigates the dynamics of explosive volcanic eruptions, from the sub-surface source mechanisms through to the emission dynamics and downwind dispersal of tephra. To this end, we use a ground-based Doppler radar (VOLDORAD) which informs on the loading / velocimetry of the expelled ejecta. Data are integrated with complementary geophysical techniques, and numerical models are developed to simulate pyroclastic emissions, generate synthetic radar data, and in turn enhance our understanding of the underlying dynamical processes. Arenal (Costa Rica) is used as a case study volcano, where frequent mildly-explosive eruptions commonly expel ash plumes and ballistic projections up to a few hundred meters above the vent. Firstly, we combine seismic and radar data to investigate the link between conduit processes and pyroclastic emissions. A conceptual model is proposed to account for their complex interplay, whereby fractures through a rigid lava cap control the system’s degassing, which in turn governs both the seismic and radar signals (the latter depending on the ash load carried by the gas). Secondly, we investigate the dynamics of pyroclastic emissions from the analysis of Doppler radargrams. Time-velocity distribution of the expelled tephra shows the signature of two distinct phenomena. Numerical modeling and computation of synthetic radargrams show that these are consistent with both ballistic projections and ash plume crossing the beam simultaneously, whose respective mass load can be derived. Inverse modeling using a nearneighborhood Monte Carlo procedure was used to find synthetic Doppler radargrams which best matched the observed ones. The results give constrains on eruptive parameters, such as the size, trajectory, exit velocities and source gas velocities of the ballistics, as well as the speed / direction of the ash cloud drifted by trade winds. Lastly, because Doppler radars are powerful tool for real-time allweather monitoring of volcanic activity, we address issues relative to the operational radar monitoring of ash plumes. In particular, the ability to remotely quantify the mass proportions of ejecta either falling on the slopes of the volcano or prone to be ejected into the atmosphere, gives source eruptive parameters which may feed volcanic ash dispersal models

Dynamique des émissions pyroclastiques et mécanismes à la source : approche couplée par radar Doppler (VOLDORAD) et autres signaux géophysiques

This study investigates the dynamics of explosive volcanic eruptions, from the sub-surface source mechanisms through to the emission dynamics and downwind dispersal of tephra. To this end, we use a ground-based Doppler radar (VOLDORAD) which informs on the loading / velocimetry of the expelled ejecta. Data are integrated with complementary geophysical techniques, and numerical models are developed to simulate pyroclastic emissions, generate synthetic radar data, and in turn enhance our understanding of the underlying dynamical processes. Arenal (Costa Rica) is used as a case study volcano, where frequent mildly-explosive eruptions commonly expel ash plumes and ballistic projections up to a few hundred meters above the vent. Firstly, we combine seismic and radar data to investigate the link between conduit processes and pyroclastic emissions. A conceptual model is proposed to account for their complex interplay, whereby fractures through a rigid lava cap control the system’s degassing, which in turn governs both the seismic and radar signals (the latter depending on the ash load carried by the gas). Secondly, we investigate the dynamics of pyroclastic emissions from the analysis of Doppler radargrams. Time-velocity distribution of the expelled tephra shows the signature of two distinct phenomena. Numerical modeling and computation of synthetic radargrams show that these are consistent with both ballistic projections and ash plume crossing the beam simultaneously, whose respective mass load can be derived. Inverse modeling using a nearneighborhood Monte Carlo procedure was used to find synthetic Doppler radargrams which best matched the observed ones. The results give constrains on eruptive parameters, such as the size, trajectory, exit velocities and source gas velocities of the ballistics, as well as the speed / direction of the ash cloud drifted by trade winds. Lastly, because Doppler radars are powerful tool for real-time allweather monitoring of volcanic activity, we address issues relative to the operational radar monitoring of ash plumes. In particular, the ability to remotely quantify the mass proportions of ejecta either falling on the slopes of the volcano or prone to be ejected into the atmosphere, gives source eruptive parameters which may feed volcanic ash dispersal models.Cette étude traite de la dynamique des éruptions volcaniques explosives, depuis les mécanismes de sub-surface jusqu’aux processus d’émission et de dispersion des pyroclastes. A cet effet un radar Doppler sol est utilisé (VOLDORAD), lequel renseigne sur la charge / vitesse des ejectas. Les données sont intégrées avec d’autres techniques géophysiques, et des modèles numériques sont développés afin de simuler les émissions pyroclastiques, générer des signaux radar synthétiques, pour finalement améliorer notre compréhension des processus qui leurs sont sous-jacents. L’Arenal (Costa Rica) est utilisé comme volcan cible, où de fréquentes éruptions de faible magnitude émettent des panaches de cendres et des projections balistiques jusqu’à quelques centaines de mètres au-dessus de l’évent. Dans un premier temps, nous combinons des données sismiques et radar afin d’explorer la relation entre les processus de conduit et les émissions pyroclastiques. Leurs interactions complexes sont interprétées via un modèle conceptuel, lequel décrit les fractures parsemant le bouchon de lave comme responsables du dégazage du système, et en retour des signaux sismiques et radar collectés (ces derniers dépendants de la charge en cendres des émissions de gaz). Par ailleurs, nous investiguons la dynamique des émissions pyroclastiques à travers l’étude de radargrammes Doppler. La distribution spatio-temporelle de la vitesse des ejectas indique l’existence de deux phénomènes aux dynamiques distinctes. Des modélisations numériques permettant la reconstruction de signaux synthétiques indiquent qu’il s’agit de l’émission simultanée de blocs balistiques et de panaches de cendres. Une procédure d’inversion de type Monte Carlo couplée d’un algorithme d’optimisation permet de retrouver les radargrammes synthétiques qui reproduisent au mieux ceux observés. Les résultats apportent des contraintes sur divers paramètres éruptifs, tels que les tailles, trajectoires, vitesses des ejectas et des gaz, ainsi que la vitesse / direction de dispersion des panaches de cendres par le vent. Enfin, nous discutons du potentiel des radars Doppler appliqués à la surveillance opérationnelle des émissions volcaniques. En particulier, la possibilité de quantifier les masses éjectées dans l’atmosphère ou retombant sur les flancs du volcan, fournit des paramètres éruptifs à la source pouvant alimenter les modèles de dispersion de panaches de cendres

Ballistics and ash plumes discriminated by Doppler radar

International audienc

OSARIS, the "Open Source SAR Investigation System" for Automatized Parallel InSAR Processing of Sentinel-1 Time Series Data With Special Emphasis on Cryosphere Applications

With the advent of the two Sentinel-1 (S1) satellites, Synthetic Aperture Radar (SAR) data with high temporal and spatial resolution are freely available. This provides a promising framework to facilitate detailed investigations of surface instabilities and movements on large scales with high temporal resolution, but also poses substantial processing challenges because of storage and computation requirements. Methods are needed to efficiently detect short term changes in dynamic environments. Approaches considering pair-wise processing of a series of consecutive scenes to retain maximum temporal resolution in conjunction with time series analyses are required. Here we present OSARIS, the “Open Source SAR Investigation System,” as a framework to process large stacks of S1 data on high-performance computing clusters. Based on Generic Mapping Tools SAR, shell scripts, and the workload manager Slurm, OSARIS provides an open and modular framework combining parallelization of high-performance C programs, flexible processing schemes, convenient configuration, and generation of geocoded stacks of analysis-ready base data, including amplitude, phase, coherence, and unwrapped interferograms. Time series analyses can be conducted by applying automated modules to the data stacks. The capabilities of OSARIS are demonstrated in a case study from the northwestern Tien Shan, Central Asia. After merging of slices, a total of 80 scene pairs were processed from 174 total input scenes. The coherence time series exhibits pronounced seasonal variability, with relatively high coherence values prevailing during the summer months in the nival zone. As an example of a time series analysis module, we present OSARIS' “Unstable Coherence Metric” which identifies pixels affected by significant drops from high to low coherence values. Measurements of motion provided by LOSD measurements require careful evaluation because interferometric phase unwrapping is prone to errors. Here, OSARIS provides a series of modules to detect and mask unwrapping errors, correct for atmospheric disturbances, and remove large-scale trends. Wall clock processing time for the case study (area ~9,000 km2) was ~12 h 4 min on a machine with 400 cores and 2 TB RAM. In total, ~12 d 10 h 44 min (~96%) were saved through parallelization. A comparison of selected OSARIS datasets to results from two state-of-the-art SAR processing suites, ISCE and SNAP, shows that OSARIS provides products of competitive quality despite its high level of automatization. OSARIS thus facilitates efficient S1-based region-wide investigations of surface movement events over multiple years.Peer Reviewe