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International audience; Muography uses muons contained in the natural cosmic rays to determine the density of rock volumes. The measurements consist of counting the muons emerging from the target to determine the screening effect produced by the rock. Since the larger the rock thickness, the smaller the number of muons able to cross, the time resolution that can be achieved by muography to monitor density changes is on the order of one or two weeks for kilometer-sized volcanoes. This limitation of the method can be reduced by joining muography with high time-resolution measurements like passive seismic monitoring. In the case of structural imaging, muography benefits from the fact that muon trajectories are linear, making the tomography problem simpler than for other geophysical techniques like electrical resistivity tomography. Experiments performed on La Soufrière of Guadeloupe volcano are described to show how muography can be used to contribute to structural imaging of a highly heterogeneous lava dome and to detect abrupt transient hydrothermal phenomena likely to produce dangerous explosive events.

En el presente artículo se expondrán los resultados de las investigaciones sobre las causas y consecuencias de la casi extinción del árbol de la quina por la actividad agrícola, con base en la relevancia de este árbol que quedó registrada en la tradición «Los polvos de la condesa», de Ricardo Palma. También se analizarán las propiedades de la quina en el ámbito de la medicina desde la época del Perú virreinal, en la que eran utilizadas para las diferentes patologías propias de la época, hasta la actualidad. Nos interesa resaltar la importancia de la difusión de la información sobre el árbol de la quina para preservarlo de las actividades agrícolas que son llevadas de manera indiscriminada e inconsciente, y que provocan que la quina esté en riesgo de la casi extinción.

The rocks forming a volcano are typically saturated or partially-saturated with liquid. However, most experiments aimed at better understanding the mechanical behaviour of volcanic rocks have been performed on dry samples, and therefore most large-scale models designed to explore volcanic stability have used parameters representative for dry rock. We present a combined laboratory and modelling study in which we (1) quantify the influence of water on the mechanical behaviour of variably altered dome rocks from La Soufrière de Guadeloupe (Eastern Caribbean) and (2) use these new laboratory data to investigate the influence of water on dome stability. Our laboratory data show that the ratio of wet to dry uniaxial compressive strength (UCS) and Young's modulus are ~0.95–0.30 and ~1.00–0.10, respectively. In other words, the rocks were all weaker when saturated with water. We also find that the ratio of wet to dry UCS decreases with increasing alteration (the wt% of secondary minerals). Micromechanical modelling suggests that the observed water-weakening is the result of a decrease in fracture toughness (KIC) in the presence of water. We also find that the ratio of wet to dry KIC decreases with increasing alteration, explaining why water-weakening increases with alteration. To explore the influence of water saturation on dome stability, we numerically generated lava domes using the experimental data corresponding to dry unaltered and altered rock, in Particle Flow Code. The strength of the dome-forming rocks was then reduced to values corresponding to wet conditions. Our modelling showed that, although the stability of the unaltered dome was not influenced by water saturation, large displacements were observed for the altered dome. Additional modelling in which we modelled a buried alteration zone within an unaltered dome showed that higher displacements were observed when the dome was water saturated. We conclude that (1) the presence of water reduces the UCS and Young's modulus of volcanic rock, (2) larger decreases in UCS in the presence of water are observed for altered rocks, and (3) large-scale dome stability modelling suggests that the stability of a dome can be compromised by the presence of water if the dome is altered or contains an altered zone. These conclusions highlight that the degree of alteration and water saturation should be monitored at active volcanoes worldwide, and that large-scale models should use values for water-saturated rocks when appropriate.

SUMMARY The controlled source electro-magnetic (CSEM) method is increasingly used for in-land and off-shore subsurface characterization. Given its complex dependence between data and the parameters of interest, there is a crucial need for performant numerical algorithms that can simulate the CSEM response of 3-D geological structures. Here, we present two finite element (FE) algorithms for simulating the CSEM response in 3-D media with isotropic conductivity. A primary/secondary field approach is used to avoid the singularity introduced by the source. The primary field is computed semi-analytically for a horizontally layered model and different sources. The secondary field is obtained by discretizing the diffusive frequency-domain Maxwell’s equations with non-conforming FE. The two numerical algorithms are specifically designed to work on distributed-memory computers: (1) an iterative procedure with domain decomposition and (2) a direct and global algorithm. We evaluate their performance by computing their speed up on parallel processors, and solving problems with realistic conductivity structures. We also compare the accuracy of the solutions with published results on canonical models. The results shown here demonstrate the functionality of the two methodologies presented for specific cases when computing 3-D CSEM solutions.

Volcano slope stability analysis is a critical component of volcanic hazard assessments and monitoring. However, traditional methods for assessing rock strength require physical samples of rock which may be difficult to obtain or characterize in bulk. Here, visible to shortwave infrared (350-2500 nm; VNIR-SWIR) reflected light spectroscopy on laboratory-tested rock samples from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria) and La Soufriere de Guadeloupe (Eastern Caribbean) volcanoes was used to design a novel rapid chemometric-based method to estimate uniaxial compressive strength (UCS) and porosity. Our Partial Least Squares Regression models return moderate accuracies for both UCS and porosity, with R2 of 0.43-0.49 and Mean Absolute Percentage Error (MAPE) of 0.2-0.4. When laboratory-measured porosity is included with spectral data, UCS prediction reaches an R2 of 0.82 and MAPE of 0.11. Our models highlight that the observed changes in the UCS are coupled with subtle mineralogical changes due to hydrothermal alteration at wavelengths of 360-438, 532-597, 1405-1455, 2179-2272, 2332-2386, and 2460-2490 nm. These mineralogical changes include mineral replacement, precipitation hydrothermal alteration processes which impact the strength of volcanic rocks, such as mineral replacement, precipitation, and/or silicification. Our approach highlights that spectroscopy can provide a first order assessment of rock strength and/or porosity or be used to complement laboratory porosity-based predictive models. VNIR-SWIR spectroscopy therefore provides an accurate non-destructive way of assessing rock strength and alteration mineralogy, even from remote sensing platforms. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

International audience; The tensile strength of volcanic rocks is an important parameter for understanding and modelling a wide range of volcanic processes, and in the development of strategies designed to optimise energy production in volcanic geothermal reservoirs. However, despite the near-ubiquity of hydrothermal alteration at volcanic and geothermal systems, values of tensile strength for hydrothermally altered volcanic rocks are sparse. Here, we present an experimental study in which we measured the tensile strength of variably altered volcanic rocks. The alteration of these rocks, quantified as the weight percentage of secondary (alteration) minerals, varied from 6 to 62.8 wt%. Our data show that tensile strength decreases as a function of porosity, in agreement with previous studies, and as a function of alteration. We fit existing theoretical constitutive models to our data so that tensile strength can be estimated for a given porosity, and we provide a transformation of these models such that they are a function of alteration. However, because porosity and alteration influence each other, it is challenging to untangle their individual contributions to the measured reduction in tensile strength. Our new data and previously published data suggest that porosity exerts a first-order role on the tensile strength of volcanic rocks. Based on our data and observations, we also suggest that (1) alteration likely decreases tensile strength if associated with mineral dissolution, weak secondary minerals (such as clays), and an increase in microstructural heterogeneity and (2) alteration likely increases tensile strength if associated with pore- and crack-filling mineral precipitation. Therefore, we conclude that both alteration intensity and alteration type likely influence tensile strength. To highlight the implications of our findings, we provide discrete element method modelling which shows that, following the pressurisation of a dyke, the damage within weak hydrothermally altered host-rock is greater and more widespread than for strong hydrothermally altered host-rock. Because the rocks in volcanic and geothermal settings are likely to be altered, our results suggest that future modelling should consider the tensile strength of hydrothermally altered volcanic rocks.

The collapse of a volcanic flank can be destructive and deadly. Hydrothermal alteration is common to volcanoes worldwide and is thought to promote volcano instability by decreasing rock strength. However, some laboratory studies have shown that not all alteration reduces rock strength. Our new laboratory data for altered rhyodacites from Chaos Crags (Lassen volcanic center, California, USA) show that pore- and crack-filling mineral precipitation can reduce porosity and permeability and increase strength, Young's modulus, and cohesion. A significant reduction in permeability, by as much as four orders of magnitude, will inhibit fluid circulation and create zones of high pore fluid pressure. We explored the consequences of pore fluid pressurization on volcano stability using large-scale numerical modeling. Upscaled physical and mechanical properties for hydrothermally altered rocks were used as input parameters in our modeling. Results show that a high-pore-pressure zone within a volcano increases volcano deformation and that increasing the size of this zone increases the observed deformation. Hydrothermal alteration associated with mineral precipitation, and increases to rock strength, can therefore promote pore pressurization and volcano deformation, increasing the likelihood of volcano spreading, flank collapses, and phreatic/phreatomagmatic explosions. We conclude that porosity-decreasing alteration, explored here, and porosity-increasing alteration can both promote volcano instability and collapse, but by different mechanisms. Hydrothermal alteration should therefore be monitored at volcanoes worldwide and incorporated into hazard assessments.

International audience; The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.

El presente artículo realizará un análisis sobre la presencia de plantas medicinales y la labor hospitalaria de los bethlemitas, ambos temas encontrados en los relatos «El abad de Lunahuaná» y «Los Barbones» respectivamente. Estos textos forman parte de Tradiciones peruanas, obra célebre de Ricardo Palma que fue escrita producto de su investigación histórica, basada en documentos y también en tradiciones orales a las cuales introducía algunos elementos de ficción. A través de este texto nos interesa propiciar las investigaciones universitarias especialmente respecto al uso de plantas medicinales, resaltando sus bondades e importancia en el campo de la medicina hasta la actualidad. De igual manera, se busca destacar la participación activa de la orden religiosa de los bethlemitas en el ejercicio de la medicina, tanto en su labor hospitalaria, dedicada al cuidado del paciente, como en sus saberes en el área de la farmacología, generando una relación entre el pasado y el presente por medio de estos relatos.

Fil: Macchioli Grande, Franco. CONICET, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata; Argentina. Fil: Zyserman, Fabio. CONICET, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata; Argentina. Fil: Monachesi, Leonardo. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina. Fil: Jouniaux, Laurence. Institut de Physique du Globe de Strasbourg (UMR 7516), Université de Strasbourg et CNRS, Strasbourg; Francia. Fil: Rosas Carbajal, Marina. Institut de Physique du Globe de Paris; Francia. In glacial studies, properties such as glacier thickness and the basement permeability and porosity are key to understand the hydrological and mechanical behaviour of the system. The seismoelectric method could potentially be used to determine key properties of glacial environments. Here we analytically model the generation of seismic and seismoelectric signals by means of a shear horizontal seismic wave source on top of a glacier overlying a porous basement. Considering a one-dimensional setting, we compute the seismic waves and the electrokinetically induced electric field. We then analyse the sensitivity of the seismic and electromagnetic data to relevant model parameters, namely depth of the glacier bottom, porosity, permeability, shear modulus and saturating water salinity of the glacier basement. Moreover, we study the possibility of inferring these key parameters from a set of very low noise synthetic data, adopting a Bayesian framework to pay particular attention to the uncertainty of the model parameters mentioned above. We tackle the resolution of the probabilistic inverse problem with two strategies: (1) we compute the marginal posterior distributions of each model parameter solving multidimensional integrals numerically and (2) we use a Markov chain Monte Carlo algorithm to retrieve a collection of model parameters that follows the posterior probability density function of the model parameters, given the synthetic data set. Both methodologies are able to obtain the marginal distributions of the parameters and estimate their mean and standard deviation. The Markov chain Monte Carlo algorithm performs better in terms of numerical stability and number of iterations needed to characterize the distributions. The inversion of seismic data alone is not able to constrain the values of porosity and permeability further than the prior distribution. In turn, the inversion of the electric data alone, and the joint inversion of seismic and electric data are useful to constrain these parameters as well as other glacial system properties. Furthermore, the joint inversion reduces the uncertainty of the model parameters estimates and provides more accurate results.

The intense hydrothermal activity occurring at La Soufrière de Guadeloupe, French Lesser Antilles, is a major concern for volcanologists at the Volcanological and Seismological Observatory of Guadeloupe and for the local population living nearby. Partly hosted within the andesitic lava dome, this hydrothermal system, continuously fueled with meteoric waters, is indeed responsible for both violent phreatic eruptions (the last major event occurred in 1976-1977), and fast rock alteration due to hot and acid fluid circulation that worsens the risk of a partial volcano flank collapse. Since 2015, the deployment of particle trackers built in IP2I, Lyon, to perform dynamic muon imaging of the lava dome structure has allowed to increase the knowledge of the hydrothermal system dynamics. Recently a new generation of hodoscopes equipped with 4 plastic scintillator matrices have been installed around the volcano to scan different areas of the lava dome. Here we will present a new versatile method to process the data acquired by these detectors, and estimate the corresponding density radiographies. Particle trajectories are reconstructed by performing a fit of the recorded hits in the impacted scintillator bars using a Random Sampling Consensus procedure (RANSAC). This algorithm is specifically built for discriminating outlier points ("noisy" hits) in the data. Thus, it allows to significantly improve the muon signal/noise separation, and obtain higher quality estimates of the particle trajectories. To measure the performances of this RANSAC-based processing algorithm, and its ability to reconstruct muon tracks, we have developed a dedicated GEANT4 simulation of our 4-panel detector, interfaced with the MC generator CRY to simulate the incident flux of each component of the atmospheric showers (muonic, hadronic and electromagnetic).In addition to the simulation outputs, we will notably show preliminary results obtained from six months of real data (∼107 reconstructed events), recorded in a telescope, equipped with four 16x16 scintillator bars matrices, located at the volcano south-west. This configuration allows to increase the precision on the 2D density radiography of the central part of the scanned region, and offers two additional 3-panel sub-configurations, which leads to three different radiographies.The 2D density radiographies will then be combined to serve as input for numerical modelling, conjointly with other geophysics data sets (e.g from gravimetry surveys), in order to obtain a 3D bulk density distribution model of the lava dome. An unprecedented resolution is expected thanks to the uniqueness of the different data sets available and the quality of the event reconstruction. This will lead to a better characterization of the hydrothermal fluid circulation impact on rock alteration and the resulting partial flank collapse hazard.

The hydrothermal system of the andesitic volcano La Soufrière in the Basse-Terre island of Guadeloupe, is an ever evolving and highly dynamic system that is characterized by a variety of surface manifestations such as thermal springs, fumaroles and the Tarissan acid boiling pond (TAS) (Villemant et al.,2014). These produce halogen-rich surface emissions that makes it difficult to interpret subtle perturbations in the magmatic reservoirs using traditional geochemical and geophysical monitoring techniques (Moretti et al., 2020). A challenging situation for monitoring a volcano that has in recent history experienced six phreatic eruptions, the latest being the 1976-1977 eruptive crisis followed by a renewal of unrest in 1992 with the latest accelerated unrest episode occurring in 2018 (Komorowski et al., 2005, Moretti et al.,2020). Concurrently, TAS exhibited reduced Cl/Br ratios from ~1000 to ~300 from 18 January 2018 to 23 November 2020, suggesting marine water as a possible salinity source into the hydrothermal system. Hence, there is a critical need to fully conceptualize and appreciate the sources, full evolution and dynamic response of the hydrothermal system at La Soufrière. The research begins with investigating the potential input of ocean water into the hydrothermal system to aid in developing an exploitable geochemical database for prospective geochemical modelling analysis, leading to possible inferences on the influence of salinity on; 1) the gas-water-rock interactions in the shallow hydrothermal reservoir 2) scrubbing effects and 3) forcing conditions responsible for the measured surface gas emissions. We investigate this by numerically modelling flow transport of NaCl brines (wt.% 5, 25 and 35) using TOUGH2 software. Brines were modelled to represent volcanic- or marine- sourced brines at high temperature and pressure conditions (300°C - 350°C and 195Pa respectively). Steady state solutions of varying mass injections of orders of 1.0E-4 kg/s/m2 and 1.0E-5 kg/s/m2 resulted with the brines concentrating at heights of ~1100m a.s.l. at the summit and exiting the system, depending on the adopted permeability values. Thus implying that strong permeability contrasts due to sealing effects promoted by argillic alterations influences the trapping of brines in the upper edifice.

Volcanoes are subject to intense fluid circulation, altering primary rock properties. The rock alteration can be due to “cold” water-driven weathering or “hot” hydrothermal fluids. Such alteration is facilitated by the efficient circulation of fluids through fractures and the connected pore-network. Fluid-driven alteration can manifest as mineral dissolution and precipitation, ultimately changing the properties of the host rock, such as strength and elasticity. Alteration-induced geomechanical changes are complex due to the large range of protolith porosity (e.g. 0.01-0.8) and permeability (e.g. 10-10 to ≤10-18 m2). For example, fresh pyroclastic rocks can initially have low strength, which can be ‘reinforced’ by mineral precipitation. In contrast, dense lava rocks can decrease their strength due to pore space enlargement and development of secondary clays, oxides, sulfides and sulfates.This study analysed lab-tested samples from Ruapehu, Merapi, Whakaari, Chaos Crags, Ohakuri, Styrian Basin and La Soufrière de Guadeloupe volcanoes to provide new insights into the controls on geomechanical properties due to weathering and hydrothermal alteration. The volcaniclastic and lava rocks range from basaltic to rhyolitic in composition and encompass surface weathering and intermediate and advanced argillic alteration styles. The physical, geomechanical, Visible-Near Infrared (VNIR; 350-1000 nm), and Shortwave Infrared (SWIR; 1000-2500 nm) properties of the rocks were measured on core samples. Porosity, P-wave velocity, and uniaxial compressive strength ranged from 0.02-0.67, 88-5800 m/s, and 0.1-312 MPa, respectively. Partial Least Squares Regression (PLSR) was employed to successfully predict physical and mechanical properties using VNIR-SWIR spectroscopy data. The PLSR-based prediction models highlighted a handful of spectral bands around 400-600 nm, 1400 nm and 2200-2300 nm, indicating that hydrated secondary minerals were responsible for the observed geomechanical changes. The proposed method using VNIR-SWIR spectroscopy can lead to a new way of mapping physical and geomechanical properties at outcrop-scale using field spectrometers, and at volcano-scale using airborne and satellite remote sensing.

The tensile strength of volcanic rock exerts control over several key volcanic processes, including fragmentation, magma chamber rupture, and dyke propagation. However, and despite this importance, values of tensile strength for volcanic rocks are relatively rare. It is also unclear how their tensile strength is modified by rock physical properties such as porosity, pore size, and pore shape and ongoing processes such as hydrothermal alteration. We present here the results of systematic laboratory and numerical experiments designed to better understand the influence of porosity, microstructural parameters (pore size, shape, and orientation), and hydrothermal alteration on the tensile strength of volcanic rocks. Our data show that tensile strength is reduced by up to an order of magnitude as porosity is increased from 0.01 to above 0.3, highlighting that porosity exerts a first-order control on the tensile strength of volcanic rocks. Our data also show that pore diameter, pore aspect ratio, and pore orientation can also influence tensile strength. Finally, our data show that hydrothermal alteration can decrease tensile strength if associated with mineral dissolution and weak secondary minerals, or increase tensile strength if associated with pore- and crack-filling mineral precipitation. We present a series of theoretical and semi-empirical constitutive models that can be used to estimate the tensile strength of volcanic rocks as a function of porosity or alteration intensity. To outline the implications of our data, we show how tensile strength estimations can influence predictions of magma overpressures and assessments of the volume and radius of a magma chamber, and we explore the influence of alteration using discrete element method modelling in which we model the amount and distribution of damage within variably-altered host-rock surrounding a pressurised dyke. It is our hope that the experiments, models, and understanding provided by our study prove useful for modellers that require the tensile strength of volcanic rocks for their models.

The characterization of volcanic hydrothermal systems (VHS) is fundamental for the early detection of precursors to phreatic or magmatic eruptions and for understanding hazards related to slope instabilities. Since these phenomena can be related to pore-fluid dynamics within the volcanic edifice, monitoring the spatial distribution of the different fluid phases (water, air, vapor) is of great importance. Ambient noise seismic interferometry has been employed for this task, correlating temporal changes in seismic velocities with variations in the water table depth in volcanic areas. However, this technique usually considers that above this depth, the pore space within the rock is totally occupied by a gaseous phase. This, in turn, implies that the body wave velocities and the density of the unsaturated zone are constant values, which is expected to impact on the determination of the water table depth. In this work, we assess the influence of partial saturation in the unsaturated zone of a VHS on ambient seismic noise, by employing a comprehensive rock physics model based on a saturation profile given by the Van Genuchten model. We focus on the sensitivity of Rayleigh waves, which are usually considered to be the most important contribution to the ambient seismic noise. We consider an altered andesite layer overlying a half-space consisting of a relatively unaltered andesite representing the volcanic basement, which is representative of the VHS of La Soufrière de Guadeloupe volcano (Eastern Caribbean, France). We base our rock physics model on Gassmann’s equations to compute body wave velocities as a function of fluid saturation. We compute Rayleigh wave phase velocities for different positions of the water table and analyze their relative difference with respect to a reference scenario that corresponds to the mean value of the water table depth in this region. Our results suggest that the existence of a partial water saturation distribution could affect the Rayleigh wave velocities, and that this effect depends on the range of frequencies considered and the degree of hydrothermal alteration of the medium. For highly altered andesite, characterized by a higher porosity and a lower rock-mass stiffness, the relative variation in velocity obtained with a partial saturation distribution can be up to twice or down to half of the variation for the scenario corresponding to a constant saturation, depending on the frequency range considered. The depth sensitivity kernels of the Rayleigh wave phase velocities exhibit significant variations within regions with variable water content for frequencies between 4 and 6 Hz, which indicates that these seismic waves are able to distinguish the presence of a partial saturation distribution. These results indicate that relative velocity differences derived from ambient noise interferometry provide the possibility of inferring spatial distributions of water content and, therefore, density variations within VHS. This technique therefore emerges as a useful tool to inform on volcanic hazards related to hydrothermal activity, such as erratic explosions and partial flank collapses.

International audience; In this chapter, we describe basic features and give some current applications of the most popular detection technology used in muography: the scintillator-based muon detectors, widely used not only in volcanology, where their properties find natural applications, but also in geosciences, archeology, non-invasive industrial control, civil engineering, homeland security, nuclear non-proliferation and more. As we will emphasize in the following sections, there are many advantages in the use of scintillators, which are known to be robust – and therefore usable in harsh environmental conditions – and offer real-time analogic measurement capabilities with a good space and time resolution. The design of such detectors is flexible and may be used in many different ways depending on the target under study, the field conditions, the modularity of the detectors etc. Throughout this chapter, we will focus on one particular muon detector (also referred to as “muon telescope”) originally designed to study the active volcanic dome of the Soufrière of Guadeloupe to show the generic features of this detection technique.

International audience; Particularly in the presence of a hydrothermal system, many volcanoes output large quantities of heat through the transport of water from deep within the edifice to the surface. Thus, heat flux is a prime tool for evaluating volcanic activity and unrest. We review the volcanic unrest at La Soufrière de Guadeloupe (French West Indies) using an airborne thermal camera survey and in situ measurements of temperature and flow rate through temperature probes, Pitot-tube and MultiGAS measurements. We deduce mass and heat fluxes for the fumarolic, ground and thermal spring outputs and follow these over a period spanning 2000–2020. Our results are compared with published data and we performed a retrospective analysis of the temporal variations in heat flux over this period using the literature data. We find that the heat emitted by the volcano is 36.5 ± 7.9MW, of which the fumarolic heat flux is dominant at 28.3 ± 6.8 MW. Given a total heated area of 26 270 m2, this equates to a total heat flux density of 1366 ± 82 W/m2, which is amongst the highest established for worldwide volcanoes with hydrothermal systems, particularly for dome volcanoes. A major change at La Soufrière de Guadeloupe, however, is the development of a widespread region of ground heating at the summit where heat output has increased from 0.2 ± 1 MW in 2010 to 5.7 ± 0.9 MW in 2020. This change is concurrent with accelerating unrest at the volcano and the emergence of two new high-flux fumaroles in recent years. Our findings highlight the importance of continued and enhanced surveillance and research strategies at La Soufrière de Guadeloupe, the results of which can be used to better understand hydrothermal volcanic systems the world over.

International audience; Volcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to volcano instability. We present a study that combines laboratory experiments, geophysical data, and large-scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufrière de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro-thermo-mechanical modeling code, show (a) the importance of using upscaled values in large-scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low-strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.

After 25 years of gradual increase, volcanic unrest at La Soufriere of Guadeloupe reached its highest seismic energy level on 27 April 2018, with the largest felt volcano-tectonic (VT) earthquake (M-L 4.1 or M-W 3.7) recorded since the 1976-1977 phreatic eruptive crisis. This event marked the onset of a seismic swarm (180 events, 2 felt) occurring after three previous swarms on 3-6 January (70 events), 1 st February (30 events, 1 felt) and 16-17 April (140 events, 1 felt). Many events were hybrid VI's with long-period codas, located 2-4 km below the volcano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La Soufriere. Elastic energy release increased with each swarm whereas inter-event time shortened. At the same time, summit fractures continued to open and thermal anomalies to extend. Summit fumarolic activity increased significantly until 20 April, with a maximum temperature of 111.4 degrees C and gas exit velocity of 80 m/s, before declining to similar to 95 degrees C and similar to 33 m/s on 25 April. Gas compositions revealed increasing C/S and CO2/CH4 ratios and indicate hydrothermal P-T conditions that reached the critical point of pure water. Repeated MultiGAS analysis of fumarolic plumes showed increased CO2/H2S ratios and SO2 contents associated with the reactivation of degassing fractures (T = 93 degrees C, H2S/SO2 approximate to 1). While no direct evidence of upward magma migration was detected, we attribute the above phenomena to an increased supply of deep magmatic fluids that heated and pressurized the La Soufriere hydrothermal system, triggering seismogenic hydro-fracturing, and probable changes in deep hydraulic properties (permeability) and drainage pathways, which ultimately allowed the fumarolic fluxes to lower. Although this magmatic fluid injection was modulated by the hydrothermal system, the unprecedented seismic energy release and the critical point conditions of hydrothermal fluids suggest that the 2018 sequence of events can be regarded as a failed phreatic eruption. Should a similar sequence repeat, we warn that phreatic explosive activity could result from disruption of the shallow hydrothermal system that is currently responsible for 3-9 mm/y of nearly radial horizontal displacements within 1 km from the dome. Another potential hazard is partial collapse of the dome's SW flank, already affected by basal spreading above a detachment surface inherited from past collapses. Finally, the increased magmatic fluid supply evidenced by geochemical indicators in 2018 is compatible with magma replenishment of the 6-7 km deep crustal reservoir feeding La Soufriere and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.