International Telemetering Conference Proceedings / October 22-25, 2001 / Riviera Hotel and Convention Center, Las Vegas, Nevada / This paper presents the results of a feasibility study undertaken by the University of Salzburg (Austria), investigating the autonomous acquisition of environmental data in a global network. A suggested application which is used as the basis of this paper is a volcano monitoring system which would be able to track the activity of a volcano and act as a disaster warning system. The background Volcano observation data required for such a system is covered, before discussing the concepts for sensor data acquisition, storage and processing. A final analysis is then presented of the opportunities for the transmission by packet radio (both terrestrial and satellite).

Telica Volcano, Nicaragua, is a persistently restless volcano with high rates of seismicity that can vary from less than ten events to over a thousand events per day. Low-frequency (LF) events dominate the seismic catalogue and seismicity rates at Telica show little clear correlation with periods of eruption. As such, traditional methods of forecasting of volcanic activity based on increases in seismicity and recognition of LF activity are not applicable. A single seismic station has been operating at Telica since 1993, and in 2010 we installed a broadband seismic and continuous GPS network (TESAND network) at Telica. In this study we investigate the seismic characteristics surrounding a nine-month period of phreatic to phreatomagmatic explosions in 1999, and also from the initial three-and-a-half year deployment of the TESAND network, including a three-month phreatic vulcanian eruptive period in 2011. We demonstrate that pertinent information can be obtained from analysis of single-station data, and while large seismic networks are preferable when possible, we note that for many volcanoes this is not possible. We find unusual patterns of seismicity before both eruptive periods; rather than a precursory increase in seismicity as is observed prior to many volcanic eruptions, we observe a decrease in seismicity many months prior to eruption. We developed a new program for cross-correlation of large seismic data catalogues and analysed multiplet activity surrounding both eruptive periods. We observed that the formation of new multiplets corresponds to periods of high event rates (during inter-eruptive periods) and high percentages of daily events that belong to a multiplet. We propose a model for the seismicity patterns observed at Telica, where changes in seismicity are related to a cyclic transition between open-system degassing and closed-system degassing. Periods of open-system degassing occur during non-eruptive episodes and are characterised by high event rates, a broad range of frequency content of events and high degrees of waveform correlation. A transition to closed-system degassing could be due to sealing of fluid pathways in the magmatic and/or hydrothermal system, or due to magma withdrawal. Periods of closed-system degassing are characterised by low event rates, higher frequency contents and low degrees of waveform correlation. Eruptive periods may then represent a transition from closed-system degassing to open-system degassing, however the system must also be capable of transitioning to open-system degassing without eruption. These observations have important implications for volcano monitoring and eruption forecasting at persistently restless volcanoes. Rather than a precursory increase in seismicity as is often observed prior to eruption at other volcanoes, our observations indicate that phreatic eruptions at Telica occur after a decrease in seismicity, a corresponding change in the frequency content of events, and a decrease in waveform correlation. These changes may represent a period of closed-system degassing that could culminate in phreatic eruptions. The inclusion of real-time analysis of variations in frequency content and multiplet activity provides critical information for volcano monitoring institutions.

The presence of a hydrothermal system at Taal volcano is evident from the presence of a craterlake (Main Crater Lake, MCL), a caldera lake (Lake Taal) and several hot springs on the flanksof Taal volcano island and in the crater. Taal MCL, covering an area of 1.2 km², is acidic (pH= 3), warm (T = 30-33 °C) and its composition is dominated by Cl, Na and SO4. This thesisaims at understanding the geochemistry of Taal volcano’s hydrothermal system and the wayCO2 is transported through the hydrothermal system and MCL towards the atmosphere.The long-term geochemical evolution of MCL indicates that the hydrothermal system is madeof two reservoirs, one being volcanic and one geothermal in origin. The geothermal componentin Taal MCL has stayed rather constant since 1991, while the volcanic component hasdecreased.The low pH makes Taal volcano the perfect natural laboratory to study the behaviour of CO2,because there is no dissociation of CO2. A combined approach of total CO2 flux measurementsvia accumulation chamber and gaseous CO2 flux measurements via echo sounder shows thatmore than 90% of the total CO2 output of Taal volcano is due to the influx of dissolved CO2,migrating from the hydrothermal system to MCL via thermal springs under the lake surface.After verification of both horizontal and vertical homogeneity of dissolved CO2 concentrations,a continuous monitoring station was installed in 2013, measuring dissolved CO2 using aninfrared gas analyser protected by an ePTFE membrane, as well as several meteorological andenvironmental parameters. Several environmental and lacustrine processes influence CO2transport in MCL, including stratification, solar heating and rainfall.Taal volcano regularly goes through periods of unrest, characterised by seismic swarms,ground deformation and increased carbon dioxide flux. In 1991-1994, this was accompaniedby geochemical changes in MCL, including pH decrease and F, Si and Fe concentrationincrease. These changes can be attributed to an intrusion of magma to shallow levels less thanone kilometre deep. More recent unrests do not show these geochemical changes and are likelycaused by pressure changes in the hydrothermal system. The permanent monitoring stationrecorded hourly data on the 2015 unrest and showed that abnormally high CO2 concentrationswere recorded before the start of seismic or deformation activity, which makes continuous CO2monitoring a very valuable addition to current monitoring activities at Taal volcano. / La présence d’un système hydrothermal au volcan Taal se manifeste par la présence d’un lac de cratère (Main Crater Lake, MLC) ainsi qu’un lac de caldera (Lake Taal) et de multiples sources d’eau chaudes sur les flancs et dans le cratère. Le MCL, avec une surface de 1.2 km², est acide (pH = 3), chaud (T = 30-33 °C) et composé principalement de Cl, Na et SO4. Le but de cette thèse est de comprendre la géochimie du système hydrothermal du Taal et la manière dont le CO2 est transporté à travers de celui-ci ainsi qu’à travers le MCL vers l’atmosphère. L’évolution géochimique à long terme indique que le système hydrothermal est composé de deux réservoirs, un d’origine volcanique et un autre d’origine géothermale. Le composant géothermal est resté plutôt constant depuis 1991, tandis que le composant volcanique a diminué. Le pH plutôt bas fait que le volcan Taal est le laboratoire naturel parfait pour étudier le comportement du CO2, parce qu’il n’y a pas de dissociation de CO2. Une approche combinée du flux de CO2 total via chambre d’accumulation, et flux de CO2 gazeux via echo sondeur montre que plus que 90% du flux de CO2 total est dû au CO2 dissout, qui migre depuis le système hydrothermal au MCL via des sources thermales sous la surface du lac. Après vérification de l’homogénéité horizontale et verticale du CO2 dissout, une station de monitoring en continu a été installée en 2013. Cette station mesure le CO2 dissout à l’aide d’un analyseur de gaz infrarouge protégé par une membrane en ePTFE, ainsi que de multiples paramètres météorologiques et environnementaux. Le transport de CO2 dans le MCL est influencé par plusieurs processus environnementaux et lacustre, comprenant la stratification, l’échauffement solaire et la pluie. Le volcan Taal connait régulièrement des périodes de crises caractérisées par une activité sismique, par une déformation du sol et par un flux élevé du CO2. En 1991-1994, ceux-ci ont été accompagnés par des changements géochimiques du MCL, comprenant une diminution du pH et une augmentation de la concentration de F, Si et Fe. Ces changements peuvent être attribués à une intrusion superficielle de magma à moins d’un kilomètre de profondeur. Les crises plus récentes ne montrent pas ces changements en géochimie et sont probablement causés par des changements de pression dans le système hydrothermal. La station de monitoring en continu a enregistré des données toutes les heures pendant la crise en 2015 et a montré que des concentrations particulièrement élevées en CO2 dissout ont été enregistrées avant le début de l’activité sismique et de déformation. Ceci a montré que le monitoring en continu du CO2 est une addition très précieuse aux activités de monitoring du volcan Taal. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Dans cette thèse, nous étudions le problème de la localisation de sources sismo-volcanique, à partir des données enregistrées par des réseaux de capteurs composés de nouveaux sismomètres à trois composantes (3C). Nous nous concentrerons sur le volcan Ubinas, l'un des plus actifs au Pérou. Nous développons une nouvelle approche (MUSIC-3C) basée sur la méthode MUSIC permetant de retourner les 3 paramètres utiles (lenteur, azimut et incidence). Pour valider notre méthodologie, nous analysons des sources synthétiques propagées en tenant compte de la topographie du volcan Ubinas. Dans cette expérience, les données synthétiques ont été générées pour plusieurs sources situées à différentes profondeurs sous le cratère Ubinas. Nous utilisons l'algorithme MUSIC-3C pour les relocaliser. Nous traitons également des données réelles provenant d'une expérience de terrain menée sur le volcan Ubinas (Pérou) en 2009 par les équipes de recherche de l'IRD-France (Institut de Recherche pour le Déveleppment), UCD l'Irlande (projet VOLUME) et l'Institut de Géophysique du Pérou (IGP). Nous utilisons l'algorithme MUSIC-3C pour localiser les événements explosifs (type vulcanien), ce qui nous permet d'identifier et d'analyser les processus physiques de ces événements, à la suite de cette analyse, nous avons trouvé deux sources pour chaque explosion situées à 300 m et 1100 m en dessous du fond du cratère actif. Basé sur les mécanismes éruptifs proposés pour d'autres volcans du même type, nous interprétons la position de ces sources ainsi que les limites du conduit éruptif impliqué dans le processus de fragmentation. / In this thesis, we study the seismo-volcanic source localization using data recorded by new sensor arrays composed of three-component (3C) seismometers deployed on Ubinas stratovolcano (Peru). We develop a new framework (MUSIC-3C) of source localization method based on the well-known MUSIC algorithm. To investigate the performance of the MUSIC-3C method, we use synthetic datasets designed from eight broadband isotropic seismic sources located beneath the crater floor at different depths. The fundamental scheme of the MUSIC-3C method exploits the fact of the cross-spectral matrix of 3C array data, corresponding to the first seismic signal arrivals, provides of useful vector components (slowness, back-azimuth and incidence angle) from the seismic source. Application of the MUSIC-3C method on synthetic datasets shows the recovery of source positions. Real data used in this study was collected during seismic measurements with two seismic antennas deployed at Ubinas volcano in 2009, whose experiment conduced by volcanic teams of IRD-France (l'Institute de Recherche pour le Déveleppment), Geophysics group University College Dublin Ireland and Geophysical Institute of Peru (IGP). We apply the MUSIC-3C algorithm to investigate wave fields associated with the magmatic activity of Ubinas volcano. These analysis evidence a complex mechanism of vulcanian eruptions in which their seismic sources are found at two separated sources located at depths of 300 m and 1100 m beneath the crater floor. This implies the reproduction of similar mechanisms into the conduit. Based on the eruptive mechanisms proposed for other volcanoes of the same type, we interpret the position of this sources as the limits of the conduit portion that was involved in the fragmentation process.