Three-dimensional resistivity tomography of Vulcan's forge, Vulcano Island, southern Italy

International audience9,525 DC resistivity measurements were taken along 9 profiles crossing the volcanic edifice of La Fossa di Vulcano (the forge of God Vulcan in ancient Roman mythology), Vulcano Island (Italy) using a total of 958 electrode locations. This unique data set has been inverted in 3D by minimizing the L2 norm of the data misfit using a Gauss-Newton approach. The true 3D inversion was performed using parallel processing on an unstructured tetrahedral mesh containing 75,549 finite-element nodes and 398,208 elements to accurately model the topography of the volcanic edifice. The 3D tomogram shows a very conductive body (>0.1 S/m) comprised inside the Pietre Cotte crater with conductive volumes that are consistent with the position of temperature and CO2 anomalies at the ground surface. This conductive body is interpreted as the main hydrothermal body. It is overlaid by a resistive and cold cap in the bottom of the crater. The position of the conductive body is consistent with the deformation source responsible for the observed 1990-1996 deflation of the volcano associated with a decrease of hydrothermal activity

Importance of structural history in the summit area of Stromboli during the 2002–2003 eruptive crisis inferred from temperature, soil CO2, self-potential, and electrical resistivity tomography

International audienceThe 2002-2003 eruptive crisis of Stromboli volcano in the Aeolian Islands raised the question of how to assess the stability of the flanks of this volcanic edifice during such a crisis. To provide a response to this question, we analyzed a detailed fluid flow mapping plus the reiteration of a profile located in the vicinity of the active vents using the self-potential method, temperature data, soil-gas (CO2) measurements, and electric resistivity tomography. Coupling the interpretation of these methods that are sensitive to the flow of gas and water in the ground indicates the position of areas of mechanical weakness. In addition, they can be used to monitor the change in the discharge of fluids associated with these features before and during the 2002-2003 eruptive crisis. Our results emphasize the importance of old structural boundaries, such as the Large Fossa crater, in the development of the new set of fractures observed during the 2002-2003 eruptive crisis. Between October 2002 and January 2003, the use of CO2 soil-gas technique evidenced an increase in the discharge of CO2 outside the Large Fossa crater boundaries, along the failure boundary of the southern Sciara del Fuoco area. Self-potential and temperature measurements made before the 2002-2003 eruptive crisis reveal significant changes along the main structural boundaries of the Fossa area. The development of these anomalies is interpreted as an increase of the permeability of the structure from May 2000 to May 2002. Between January 2003 and March 2003 the reiteration of self-potential, temperature, and CO2 measurements shows an increase of fluid discharge along weakness planes located inside the Large Fossa crater boundary. They evidence no change outside this structural boundary. The importance of the Large Fossa crater boundary in controlling the deformation and fluid flow from January to March 2003 has been attested by the development of the fractures inside the Large Fossa crater boundary, and also with a network of electrooptical distance measurement stations located inside and outside this ancient crater. This multidisciplinary approach to fluid flow assessment before and during an eruptive crisis is complementary to geodetic measurements of the deformation of the edifice. It demonstrates for the first time the powerful potential of combining electrical resistivity tomography, self-potential, temperature, and soil CO2 measurements in assessing the position of the planes of mechanical weakness in a volcanic edifice

Heat flux measurement from thermal infrared imagery in low-flux fumarolic zones: Example of the Ty fault (La Soufrière de Guadeloupe)

International audienceMonitoring the geothermal flux of a dormant volcano is necessary both for hazard assessment and for studying hydrothermal systems. Heat from a magma body located at depth is transported by steam to the surface, where it is expelled in fumaroles if the heat flow exceeds 500 W/m2. If the heat flow is lower than 500 W/m2, steam mainly condensates in the soil close to surface and produces a thermal anomaly detectable at the surface. In this study, we propose a method to quantify low heat fluxes from temperature anomalies measured at the surface by a thermal infrared camera. Once corrected from the atmospheric and surface effects, thermal infrared images are used to compute (1) the excess of radiative flux, (2) the excess of sensible flux and (3) the steam flux from the soil to the atmosphere. These calculations require measurements of atmospheric parameters (temperature, wind velocity and humidity) and estimations of surface parameters (roughness and emissivity). This method has been tested on a low-flux fumarolic zone of the Soufrière volcano (Guadeloupe Island -- Lesser Antilles), and compared to a flux estimation realized from the thermal gradient measurements into the soil. The two methods show a good agreement and a similar precision (267 ± 46 W/m2 for the thermal infrared method, and 275 ± 50 W/m2 for the vertical temperature gradient method), if surface roughness is well calibrated

Structure of an active volcano associated with a resurgent block inferred from thermal mapping: The Yasur–Yenkahe volcanic complex (Vanuatu)

International audienceSubsurface thermal measurements provide a valuable tool to map hydrothermal-fluid release zones in active volcanic areas. On the Yasur–Yenkahe volcanic complex (Tanna Island, Vanuatu archipelago), fumaroles and hot springs abound, signs of upraising heat fluxes associated to a well-developed hydrothermal activity. Combination of high resolution mapping of ground thermal anomalies with geomorphological analysis allows the characterization of the structural relationships between the active Yasur volcano and the Yenkahe resurgent block.A complex system of heat release and hydrothermal fluid circulation below the Yasur–Yenkahe complex is evidenced. Circulation, though propagating vertically as a whole, is funneled by stratification. Thus, the main thermal fluid released is almost exclusively concentrated along structural limits that break the seals induced by the stratified nature of the ground. Three types of medium/high temperature anomalies have been evidenced: (1) broad hydrothermalized areas linked with planar stratification that favor lateral spreading, (2) linear segments that represent active faults, and (3) arcuate segments related to paleo-crater rims. The limit between the Yasur volcano and the Yenkahe resurgent block is characterized by an active fault system accommodating both the rapid uplift of the Yenkahe block and the overloading induced by the volcano weight. In such a setting, faults converge below the cone of Yasur, which acts as a focus for the faults. Evidence of such structures, sometimes hidden in the landscape but detected by thermal measurements, is critical for risk assessment of flank landslides

Lead isotopes behavior in the fumarolic environment of the Piton de la Fournaise volcano (Reunion Island)

International audienceThe recent activity of the Piton de la Fournaise volcano offers a rare opportunity to address the issue of Pb isotope behavior in volcanic fumaroles, as the composition of the degassing source is accurately and precisely known. Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100 degrees C include Na-K sulfate (aphthitalite), Ca-Cu sulfate (e.g., gypsum), Na sulfate (thenardite), Ca-Mg-Al-Fe fluoride (e.g., ralstonite) and native sulfur. The high-temperature deposits show trace element patterns typical of volcanic gas (with Pb concentration up to 836 ppm) while the low-temperature deposits are depleted in most volatile elements (Pb <1 ppm) with the exception of Pd and Tl (in fluorides) and Se (in native sulfur). Only for low-temperature fluoride samples do Pb isotope compositions plot significantly outside the field of lavas. The isotopic shift is ascribed to leaching ubiquitous unradiogenic phases (e.g., sulfides) by acidic gas condensates. The similarity in Pb isotope signature between lavas and sublimate samples more representative of the gas phase (sulfates) indicates that the net fractionation of Pb isotopes resulting from volatilization and condensation processes is smaller than the precision of Pb isotope measurements (better than 60 ppm/a.m.u.). The absence of net fractionation could result from negligible isotope fractionation during Pb volatilization followed by extensive condensation of gaseous Pb, with possibly significant isotopic fractionation at this stage. Although this scenario has to be refined by more direct measurement of the gas phase, and its general applicability tested, it suggests that a small fraction (<10\%) of initially volatilized Pb ultimately escapes to the atmosphere, while the remaining dominant fraction is trapped in sublimates. As sublimates are rapidly dissolved and entrained by runoff, the fumarolic environment appears as a factory efficiently transferring isotopically unfractionated Pb from magmas towards the hydrological system and seawater. Resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge. High-precision Pb isotope measurements rest not only on instrumental performance but also on high-yield chemistry, as Pb isotopes drastically fractionate (800 ppm/a.m.u.) upon elution on anionic resin. For 50\% Pb recovery, the estimated isotopic bias is plus or minus 60-80 ppm/a.m.u., depending on which of the early (isotopically light) or late (isotopically heavy) Pb fraction is lost. (c) 2012 Elsevier Ltd. All rights reserved

Geochemical survey of hot springs around Ubinas, the most historical active volcano in southern Peru

Ubinas (5675 m a.s.l.) is known as the most active volcano in southern Peru during historical times, with 23 minor eruptive or intense fumarolic crises since 1550. Due to its intense explosive recurrence and to the high proximity of the homonymous village (4000 inhabitants), only 6 km far away from the summit crater, Ubinas appears as one of the highest priority in volcano monitoring in southern Peru. The purpose of this survey has been (1) in a first step to evidence the geochemical characteristics of the hot springs located around the volcano in order to define (2) in a second step an appropriate fluid geochemical monitoring program. (1) The analysis of the waters collected around the volcano displays evident geochemical trends that can be interpreted as mixing processes between 3 end-members: fresh meteoric waters, magmatic fluids, sometimes interacting with a deep chloride reservoir. (2) Since 1999, we began a geochemical water monitoring program of two hot springs located in the lower flank of the volcano. This hydrothermal water have been characterized by a constancy in sulfate and chlorine concentrations before the big earthquake (Mw 8.4) occurred in Peru on June 23rd, 2001, causing hundreds of fatalities and serious damages along all the southern Peruvian coast. After this event, one new hot spring appears near to our sampling area and sulfate and chlorine display an increase in concentration. This phenomenon could be attributed to a consequence of the earthquake of June 23rd, 2001, increasing the permeability of the fault system crossing the volcano. In this way, an higher permeability in the soil along preferential regional faults could allow a better interaction between the ascending magmatic gases (in particular HCl and SO2/H2S gas species responsible respectively of chlorine and sulfate concentration in hydrothermal waters) and the shallower fresh meteoric waters

Spatio-temporal evolution of temperature and fluid flow through a new “thermo-lithological” boundary; the case of a pit crater of Karthala volcano (Comoros archipelago) refilled on January 13th 2007 by a lava flow

International audienceOn January 13 th 2007 the effusive eruption of Karthala volcano, in the Comoros islands, emitted a lava flow which has been perfectly constrained inside the cylindrical shape of the Choungou Chagnoumeni pit crater; 225m in diameter and with a thickness of about 7m. The good knowledge of the geometry of this lava flow allowed modeling its cooling, with the Rheolef code, with a good accuracy. The results of the thermal modeling were compared with other measurements performed on the field: (1) a high resolution electrical resistivity tomography profile crossing the boundary of the crater and (2) the thermal monitoring of temperature with eleven sensors, spaced 1 meter apart, at 30 cm depth, perpendicularly to the boundary of the crater. Thanks to this multidisciplinary approach it has been possible to display that, in the first order, the resistivity distribution in the soil 2 years and 5 months after the eruption is directly influenced by the residual temperature and not by the structural boundary. During the first three years of cooling, an inversion in the shallow lateral temperature gradient was observed in temperature data set collected at 30 cm depth

Estudio del sistema hidrotermal y estructural del volcán Ticsani mediante el método de potencial espontáneo

Este trabajo presenta una investigación llevada a cabo en el volcán Ticsani ubicado en el segmento norte de la Zona Volcánica de los Andes Centrales (70 ° 36'O, 16 ° 44'S, 5408 m). El objetivo de este trabajo es estudiar la estructura del volcán Ticsani, por el intermedio del método de Potencial Espontaneo (SP). Se realizaron 5850 medidas en todo el complejo volcánico del Ticsani. En base a estos datos, se ha identificado 2 zonas de mayor interés: (1) una transición entre las anomalías de la zona Sur y Norte, relacionada con una estructural importante de colapso lateral del complejo Norte del volcán hacia el oeste y (2) una zona de subida preferencial de fluidos hidrotermales a lo largo de una estructural de forma cuasi elíptica, relacionable a una posible caldera en el área de la cumbre. La originalidad de este estudio ha sido de evidenciar que no existe una anomalía positiva en la cumbre del volcán Ticsani (como identificado de forma “clásica” en otros volcanes del Perú; como El Misti o el Ubinas), pero tener una anomalía positiva que está bordeando la parte de la cumbre. Este resultado ha permitido mostrando que el complejo de domos que constituye el volcán Ticsani, tiene una estructura interna que drena los fluidos hidrotermales de forma más compleja que otros estratovolcanes como El Misti o el Ubinas. La localización de estas anomalías positivas de SP podrán ser utilizadas en el futuro para colocar instrumentación de monitoreo del sistema hidrotermal del volcán Ticsani

Fluid circulation pattern inside La Soufrière volcano (Guadeloupe) inferred from combined electrical resistivity tomography, self-potential, soil temperature and diffuse degassing measurements

International audienceAfter a drastic decline in 1983, hydrothermal activity at La Soufrière lava dome (Guadeloupe, Lesser Antilles) has been progressively increasing in the summit area since 1992, raising the threat of a renewed eruptive activity. To better constrain the geometry of the hydrothermal system, an extensive high-resolution self-potential survey was performed on the dome and three multi-method profiles combining electrical resistivity tomography, self-potential, ground thermometry and soil CO 2 diffuse degassing measurements were carried out to cover its southern periphery in January 2011. Results indicate that hydrothermal ascending flows are currently restricted to a proximal area including the dome and its very vicinity. The extension of hydrothermal alteration inferred from electrical resistivity tomography reflects the presence of a heat source just below the dome. A first-order correction of topography-related self-potential variations allows the identification of major hydrothermal fluid circulations pathways, as well as significant meteoric infiltration zones. Local shifting of hydrothermal fluids towards the dome periphery is favored by the presence of major axes. The regional La Ty fault appears as the major axis draining large volumes of hydrothermal and magmatic fluids. However hydrothermal activity remains confined inside a collapse structure surrounding the dome, that formed in the last 9000 years as a result of recurrent edifice collapses, the latest occurring at the onset of the 1530 AD eruption. The combination of these qualitative results with structural analysis leads to a synthetic model of magmatic and hydrothermal fluids circulation inside the dome, which may be useful for the assessment of potential hazards associated with a renewal of fluid pres-surization, and a possibly associated partial flank-failure

Estudio estructural y del sistema hidrotermal de los volcanes Sabancaya y Hualca-Hualca mediante el método de Potencial Espontáneo

El volcán Sabancaya, considerado el segundo volcán más activo del Perú forma parte del complejo Volcánico Ampato-Sabancaya (CVAS), está ubicado a 80 Km en dirección NNO de la ciudad de Arequipa (15°47’ S; 71°72’W; 5976 msnm) en el sur del Perú. El presente estudio tiene como finalidad determinar estructuras importantes que se encuentran ocultas por material volcánico y el efecto que generan estas estructuras sobre la señal del Potencial Espontaneo (PE); además, estudiar el sistema hidrotermal del volcán Sabancaya, aplicando uno de los métodos geofísicos más antiguos y conocidos, pero poco usado en la vulcanología, como es el PE. La aplicación de este método nos ha permitido conocer la estructura interna del área del CVAS y volcán Hualca-Hualca, así como determinar las dimensiones del sistema hidrotermal