Philosophiae Doctor - PhD / The El Tatio geyser field (ETGF) is the largest known geothermal field in Chile, forming part of a wide spectrum of extremophilic habitats in the country. The ETGF is NaCl rich, with high concentrations of toxic elements such as Li, As and Cs, which are contributed mainly by volcanic activities in the region. Most previous studies in the area have focused on the geology and geochemistry for mining purposes, as well as on the search for geothermal resources for power generation. Very little is currently known about the composition of the microbial communities of the ETGF, which makes the study reported here of particular novelty.A metagenomic approach, involving the amplification of 16S rRNA gene phylogenetic markers from metagenomic DNA was used to investigate seven different sites within the geyser field. The sample sites were characterized by high temperatures (80-85 °C) and a range of pH values (6.3-8). Various molecular methods, including clone library construction and PCR-DGGE analyses were used to target a wide range of microbial populations within the ETGF sites. Multivariate analysis was also applied to assess differences in the microbial diversity from different sites and to correlate microbial diversity with environmental conditions. Culture-dependent screening of novel nanoarchaeal species was also undertaken.These were coupled with PCR and other detection methods such as fluorescent in situ hybridization (FISH) to trace the presence of nanoarchaeal signals from enriched cultures.The results have shown that the ETGF encompasses a limited microbial diversity represented by only 30 dominant phylotypes, and most likely due to the toxic chemical content of the geyser field. The microbial representatives identified were assigned to OTUs from archaeal,nanoarchaeal and bacterial taxonomic groups. The dominant microbial taxa included members of the Proteobacteria, Firmicutes, Aquificae, Actinobacteria, Euryarchaeota(Halobacteriales, Archaeoglobales), Crenarchaeota (Thermoproteales, Desulfurococcales),together with uncultured representatives of the bacteria, archaea and nanoarchaeota. Notably,representatives of mesophilic, thermophilic and hyperthermophilic taxonomic groups were all detected in ETGF samples. This is attributed to various factors such as temperature gradients and dispersal mechanisms (e.g. natural forces such as rain and volcanic activities). Principal component analysis (PCA) showed significant differences (P < 0.05) in the microbial diversity of the ETGF samples, with principal components (based on the sequenced species from both 16S rRNA clone libraries and PCR-DGGE profiles) explaining up to 62.7% of variance. Furthermore, CCA showed that the differences in phylogenetic diversity were most influenced by temperature and salinity. This was also confirmed by the sequencing results,which showed that hyperthermophilic and haloarchaeal taxa were dominant in the ETGF sites. However, conductivity and pH were also found to contribute to variations in the microbial diversity of the experimental samples, with TDS (total dissolved solids) being a less influential factor. Attempts to generate nanoarchaeal-host co-cultures, and to recover sufficient nanoarchaeal genomic DNA for fosmid and/or large insert cloning for comparative genome analysis, were unsuccessful.This study is the first to employ metagenomic approaches to analyse the microbial diversity of sites in the ETGF, and has expanded our knowledge of microbiota present in this geyser field.

Methanogenesis, a metabolic pathway unique to Archaea, is severely inhibited by the reduced form of arsenic (As). Despite this inhibition, methanogenic Archaea are present in some hydrothermal features at the El Tatio Geyser Field (ETGF), a high-arsenic site with 100+ hydrothermal features, including boiling pools, geyers, fumaroles, and springs. The ability of methanogenic Archaea and other microorganisms to withstand elevated arsenic concentrations, and a variety of other extreme environmental conditions at ETGF, may be due to unique adaptations or syntrophic relationships with other microorganisms. ETGF is situated in the Andes Mountains at an altitude of ~4300 meters. UV radiation is elevated in this region and air temperatures fluctuate widely. Most hydrothermal waters discharge at ~85˚C, the local boiling point, and rapidly evaporate due to the arid climate. This concentrates hydrothermal salts and metals, including arsenic (As) and antimony (Sb). Additionally, dissolved inorganic carbon (DIC) concentrations are extremely low in most features and may limit life. Water chemistry analyses done for this study show variability in dissolved constituents between features that are consistent over time. Variations may be due to the source or residence time of waters, and differences in chemistry could be responsible for the presence or absence of methanogenic Archaea at hydrothermal sites. The overlying control on microbial diversity and community composition may be water geochemistry, and potentially specific constituents. The goals of this study were to detect novel microbial taxa at ETGF, including novel methanogens, as well as to document microbial community composition at select hydrothermal features. The distribution and diversity of microorganisms at each feature was analyzed phylogenetically and within an ecological context in order to determine physicochemical and biological controls on community composition. Additionally, a model methanogen was used in laboratory analyses to determine how concentrations and oxidation states affected growth and methane production. This methanogen, Methanothermobacter thermautotrophicus, is found at ETGF, Yellowstone, and other hydrothermal fields, and thrives in high-temperature environments. MPN (most probable number) analyses show that culturable biomass from multiple sites contain metabolically active methanogens. These results support the biogenicity of dissolved methane detected in the field. 16S rRNA surveys of Archaea at four sites show that Archaea are diverse, and archaeal community composition varies across features. Phylogenetic tree construction indicates that Archaea from ETGF group together, suggesting that the isolation and broad environmental constrains on ETGF have some control on phylogenetic diversity. Laboratory analyses of As and Sb concentrations on M. thermautotrophicus suggest that Sb may decrease the inhibition of methanogenesis by As by preventing the formation of As(III) from As(V). Statistical analyses correlating microbial community composition and structure to physicochemical parameters show that archaeal and bacterial communities relate to different variables; with Bacteria correlating to water temperature, and Archaea correlating to dissolved constituents such as hydrogen gas and sulfate. / text

Carbon Dioxide (CO₂) sequestration requires that the injected CO₂ be permanently trapped in the subsurface and not leak from the target location. To accomplish this, it is important to understand the main mechanisms associated with CO₂ flow and transport in the subsurface once CO₂ is injected. In this work CO₂ seeps at the Crystal Geyser site were studied using modeling and simulation to determine how CO₂ geochemically reacts with formation brines and how these interactions impact the migration of CO₂. Furthermore different scenarios for CO₂ migration and seepage along the Grand Wash fault are studied and the possible outcomes for these different scenarios are documented. The GEM (Generalized Equation-of-State Model) from CMG Ltd. was used to perform the simulation studies. A 2-D model was built without geochemical reactions to mainly study the mechanism associated with dissolution of CO₂ gas. The process of CO₂ release from the brine as the fluid mixture flows up along the fault was modeled. Then, 3-D models with geochemical reactions were built for CO₂ migration corresponding to two different sources of CO₂ - deep crustal ₂ and CO₂-dissolved in groundwater. In both these cases, CO₂ reacted with the aqueous components and minerals of the formation and caused carbonate mineralization. In the case of deep crustal CO₂ source, there were vertical patterns of calcite mineralization simulated along the fault that indicated that calcite mineralization might be localized to isolated vertical flow paths due to vertical channeling of CO₂ from the crust. In the case of CO₂-dissolved groundwater flowing along the sandstone layers, calcite mineralization is spread over the entire fault surface. In this case, the groundwater flow is interrupted by the fault and there is vertical flow along the fault until a permeable sandstone layer is encountered on the other side of the fault. This vertical migration of CO₂-saturated brine causes a release in pressure and subsequent ex-solution of CO₂. As a result, modeling allowed us to establish difference in surface expression of CO₂ leakage due to two different CO₂ migrations scenarios along the fault and helped develop a scheme for selecting appropriate model for CO₂ leakage based on surface observation of travertine mounds. A key observation at the Crystal Geyser site is the lateral migration of CO₂ seep sites over time. These migrations have been confirmed by isotope studies. In this modeling study, the mechanism for migration of seep sites was studied. A model for permeability reduction due to precipitation of calcite was developed. It is shown using percolation calculations that flow re-routing due to permeability alterations can result in lateral migration of CO₂ seeps at rates comparable to those established by isotope dating. / text

Le geyser d'Old Faithful dans le Parc National de Yellowstone, aux États-Unis, est l'undes geysers les plus connus au monde. La cyclicité de ses éruptions est étudiée depuis lesannées 60 a_n de comprendre sa dynamique. En e_et, le caractère bimodal de la fréquencede ses éruptions intriguent les scienti_ques qui cherchent à en connaître les causes.Les enregistrements sismiques réalisés à la surface du geyser démontrent des signauximpulsionnels dont l'origine fut identi_ée par Sharon Kedar. Ainsi, en 1992, S. Kedar etses collègues ont déployé plusieurs capteurs sismiques dans le but d'étudier la source dessignaux sismiques de type tremor enregistrés à la surface du dôme. Ils ont ainsi identi_éla source du signal sismique enregistré à la surface du geyser comme étant des signauxde cavitation de bulles. La cavitation se produisant à la surface du niveau de l'eau dansle conduit, les localisations des sources sismiques réalisées à partir des enregistrements desurface peuvent être reliées au niveau de l'eau dans le conduit.Dans un premier temps nous avons proposé de localiser les sources sismiques desenregistrements à partir de la méthode du Matched Field Processing (MFP) provenantde l'acoustique sous-marine. Plusieurs algorithmes du MFP ont été testés pour pouvoirlocaliser au mieux les sources sismiques. La bonne concordance des résultats obtenus avecchacun des algorithmes a permis d'obtenir un grand nombre de localisations des sourcesau cours du cycle. Les positions déterminées avec les di_érents algorithmes du MFP ontpermis de mettre en évidence deux zones d'activité hydrothermale du geyser associéesà di_érentes périodes du cycle éruptif, telles que le remplissage du conduit avant leséruptions et l'alimentation du geyser en eau une fois la vidange du conduit e_ectuée.Dans un second temps, l'analyse des variations de vitesse des signaux sismiques estproposée pour suivre des changements des propriétés du dôme du geyser, comme des variationsde pression avant l'éruption. Pour cela, une nouvelle méthode basée sur les mesuresde phases instantanées est suggérée. Les résultats obtenus montrent des faibles changementsde vitesse, pouvant être associés à la mise en pression du dôme ou à l'augmentationde la température du milieu avant l'éruption en surface. / The geyser of Old Faithful in the National Park of Yellowstone, in USA, is one of themost famous geysers in the world. The cyclic behavior of the geyser is studied since the60's with the aim to understand its dynamics. In fact, the bimodal nature of the frequencyof the eruptions raises questions and scientists want to know the causes of this behavior.The seismic signals recorded at the surface of the geyser present pulses whose origin wasidenti_ed by Sharon Kedar. Thus, in 1992, S. Kedar and his colleagues deployed severalseismic sensors in order to study the source of the seismic signals, which are tremor-like,recorded at the surface of the edi_ce. They identi_ed the source of the seismic signalrecorded at the surface of the geyser that they related to bubbles collapse. The bubblescollapse takes place at the surface of the water level in the conduit, thus the localizationsof the seismic sources determined with the records made at the surface would be relatedto the water level in the conduit.In a _rst time we proposed to locate the seismic sources of the records using theMatched Field Processing (MFP), a method used in ocean acoustics. Several algorithmsof the MFP were tested to better localize the seismic sources. The good agreement ofthe di_erent results obtained with each technique allowed to obtain a big number oflocalizations of the sources through the cycle. The locations determined with di_erentalgorithms of MFP allowed to highlight two areas of hydrothermal activities of the geyserlinked to di_erent periods of the eruption's cycle, as the _lling-up of the conduit beforeeruptions and the feeding of the geyser with water once the discharge of the conduitaccomplished.In a second time, the analysis of velocity's changes of the seismic records is proposedto follow changes in the properties of the edi_ce of the geyser, and pressure changes beforean eruption for example. To do that, a new technique based on the measurement of theinstantaneous phases is suggested. The results obtained show weak changes of velocity,that can be related to the pressure buildup of the edi_ce or to the increase of temperaturein the medium before an eruption.

Background: Electricity tariffs charged by Eskom have sharply increased over the past three years, with a 25% annual increase approved by Nersa until April 2012. There is no indication on what to expect in the future with regard to electricity tariffs. Many South Africans are searching for ways to save on their monthly electricity bills by seeking out alternative water heating systems. Solar geysers became a popular investment option, but this might not be the best options available on the market. Purpose: The purpose of this study is to determine the most financially viable investment option in order to reduce electricity cost when it comes to water heating systems for use in households. This is done by comparing the capital expenditure and operational cost needed with the financial benefits generated by the investment, taking into consideration the size of the household. Design and method: A literature study was done on the different alternative water heating systems in order to obtain a better understanding of how these systems operate and what savings they can generate. Different investment appraisals were identified and a literature review was performed in order to identify the most appropriate investment appraisals for the purpose of this study. It was found that the net present value, equivalent annual annuity, internal rate of return, modified internal rate of return, accounting rate of return, discounted payback period and the economic value added were the best investment appraisal methods to use for the purpose of this study. Findings and conclusion: It was found that the five investment options identified in the literature review would all, to some extent, be financially viable to implement within households with high as well as low volume hot water consumption. All the investment appraisals gave positive outcomes. The conclusion was made that a saving will be generated on the monthly electricity bill no matter what alternative water heating system were to be installed in the place of a conventional geyser. Recommendations: It is recommended that a household with low volume hot water consumption should install a time switch as this investment option renders the highest IRR, MIRR, ARR and discounted payback period. The second best investment option for a household with low volume hot water consumption is a heat pump and the third best option is a gas geyser. For a household with high volume hot water consumption, the best investment options is again a time switch, as this renders the best IRR, MIRR, ARR and discounted payback period. The second best investment option is a heat pump, with a gas geyser as the third best investment option. Value of the research: This study focuses on five alternative water heating systems for a household within South Africa in times where electricity charges sharply increase. The financial viability of each of the alternatives is determined through various investment appraisals and the best option can be identified by comparing the outcomes of the alternatives. Furthermore, each individual is able to determine the viability of the alternatives by using the Excel model attached to this study and by inputting his/her own variables, where applicable. Research limitation: Limited literature was available on the different alternative water heating systems. No indication could be found of the maintenance cost of the different water heating systems. Assumptions had to be made with regard to households, although no two households are the same. Areas for further research: The same study could be performed, but with the focus on small businesses and large organisations. Furthermore, a study could be performed to determine the appropriate discount rate for individuals as well as the maintenance cost for water heating systems. / Thesis (MCom (Management Accountancy))--North-West University, Potchefstroom Campus, 2012

The Little Grand Wash normal fault near Green River, eastern Utah, hosts a series of naturally occurring CO₂ seeps in the form of active and extinct CO₂-charged springs distributed along the fault zone. I have studied the association of fault structure with CO₂-related alteration as an analogue for the long-term (1,000- to 10,000-year) effects of leakage through faults in CO₂ sequestration reservoirs. Structure and alteration in a portion of the Little Grand Wash fault zone were mapped at a 1:700 scale in order to determine the association of faulting with CO₂-related diagenesis. I combined structural and diagenetic mapping were combined with laboratory analyses of mineralogical, isotopic and textural changes in order to assess controls on the migration of CO₂ traveling up the fault and its effects on the fault itself. The fault zone is 200 m wide at its widest and contains 4-5 major subparallel fault segments that form multiple soft- and hard-linked relay ramps. The area includes a travertine deposit and related sandstone alteration: outcrop-visible coloration, porosity-occluding calcite cement and veins occasionally so abundant that they obliterate the rock fabric. Structural mapping shows that the travertine is located at an intersection of major fault segments constituting the hard link of a 450-meter-long relay ramp. Sandstone alteration is confirmed to be related to the CO₂ seep by mapping its distribution, which shows a decrease in concentration away from the travertine, and by the unique isotopic signature of calcite cement near the travertine. At distances greater than 25 m from the travertine intense alteration disappears, though scattered fault-subparallel veins and patchy, burial-related calcite cement remain. Intense alteration is limited to major fault overlaps and does not permeate the fault zone along its entire length, nor does it extend outside the zone. This indicates that rising CO₂-laden fluids do not flow uniformly through the entire fault zone, but that vertical flow is channeled at fault intersections. In thin section, porosity near the travertine has been extensively or completely occluded by calcite cement. Permeability in some conduit samples is less than 1 mD, three or four orders of magnitude lower than sandstone away from the travertine. In active CO₂ conduits, such reduction in porosity and permeability would occlude the preferred flow conduit and ultimately restrict upward flow of CO₂-charged water. X-ray diffraction detects small amounts of goethite and hematite and a decrease in chlorite-smectite in altered conduit sandstones. Calcite is abundant, but many authigenic minerals predicted by geochemical models of CO₂ influx into sandstone reservoirs are not observed, including kaolinite, aragonite, dolomite, siderite, ankerite or dawsonite. This difference between observed and predicted mineral occurrence likely results from differences in mineral kinetics between natural and laboratory systems. Prediction of leakage risk based on fault geometry improves the ability to assess the suitability of potential carbon sequestration reservoirs, many of which will be faulted. The point seep nature of leakage through a fault zone limits the amount of CO₂ that can escape over time and also enables targeted surface monitoring for CO₂ escape into the atmosphere--both critical for ensuring the effectiveness of injection projects and earning the trust necessary for carbon sequestration to gain public acceptance. The point seep nature of leakage also accelerates the rate at which conduits may seal through mineralization, since precipitation from a large volume of fluid is focused in a narrow conduit. The presence of multiple fossil and active seep locations along the Little Grand Wash fault, active at different times in the geologic past, indicates that cementation may be effective in sealing single conduits but that fault systems with complex geometry such as Little Grand Wash may continue to leak for a long period of time. / text

Background: Electricity tariffs charged by Eskom have sharply increased over the past three years, with a 25% annual increase approved by Nersa until April 2012. There is no indication on what to expect in the future with regard to electricity tariffs. Many South Africans are searching for ways to save on their monthly electricity bills by seeking out alternative water heating systems. Solar geysers became a popular investment option, but this might not be the best options available on the market. Purpose: The purpose of this study is to determine the most financially viable investment option in order to reduce electricity cost when it comes to water heating systems for use in households. This is done by comparing the capital expenditure and operational cost needed with the financial benefits generated by the investment, taking into consideration the size of the household. Design and method: A literature study was done on the different alternative water heating systems in order to obtain a better understanding of how these systems operate and what savings they can generate. Different investment appraisals were identified and a literature review was performed in order to identify the most appropriate investment appraisals for the purpose of this study. It was found that the net present value, equivalent annual annuity, internal rate of return, modified internal rate of return, accounting rate of return, discounted payback period and the economic value added were the best investment appraisal methods to use for the purpose of this study. Findings and conclusion: It was found that the five investment options identified in the literature review would all, to some extent, be financially viable to implement within households with high as well as low volume hot water consumption. All the investment appraisals gave positive outcomes. The conclusion was made that a saving will be generated on the monthly electricity bill no matter what alternative water heating system were to be installed in the place of a conventional geyser. Recommendations: It is recommended that a household with low volume hot water consumption should install a time switch as this investment option renders the highest IRR, MIRR, ARR and discounted payback period. The second best investment option for a household with low volume hot water consumption is a heat pump and the third best option is a gas geyser. For a household with high volume hot water consumption, the best investment options is again a time switch, as this renders the best IRR, MIRR, ARR and discounted payback period. The second best investment option is a heat pump, with a gas geyser as the third best investment option. Value of the research: This study focuses on five alternative water heating systems for a household within South Africa in times where electricity charges sharply increase. The financial viability of each of the alternatives is determined through various investment appraisals and the best option can be identified by comparing the outcomes of the alternatives. Furthermore, each individual is able to determine the viability of the alternatives by using the Excel model attached to this study and by inputting his/her own variables, where applicable. Research limitation: Limited literature was available on the different alternative water heating systems. No indication could be found of the maintenance cost of the different water heating systems. Assumptions had to be made with regard to households, although no two households are the same. Areas for further research: The same study could be performed, but with the focus on small businesses and large organisations. Furthermore, a study could be performed to determine the appropriate discount rate for individuals as well as the maintenance cost for water heating systems. / Thesis (MCom (Management Accountancy))--North-West University, Potchefstroom Campus, 2012

Thesis (MTech (Faculty of Engineering))--Cape Peninsula University of Technology, 2009.Included bibliographical references (p. 98-99).

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data.

Le geyser d'Old Faithful dans le Parc National de Yellowstone, aux États-Unis, est l'undes geysers les plus connus au monde. La cyclicité de ses éruptions est étudiée depuis lesannées 60 a_n de comprendre sa dynamique. En e_et, le caractère bimodal de la fréquencede ses éruptions intriguent les scienti_ques qui cherchent à en connaître les causes.Les enregistrements sismiques réalisés à la surface du geyser démontrent des signauximpulsionnels dont l'origine fut identi_ée par Sharon Kedar. Ainsi, en 1992, S. Kedar etses collègues ont déployé plusieurs capteurs sismiques dans le but d'étudier la source dessignaux sismiques de type tremor enregistrés à la surface du dôme. Ils ont ainsi identi_éla source du signal sismique enregistré à la surface du geyser comme étant des signauxde cavitation de bulles. La cavitation se produisant à la surface du niveau de l'eau dansle conduit, les localisations des sources sismiques réalisées à partir des enregistrements desurface peuvent être reliées au niveau de l'eau dans le conduit.Dans un premier temps nous avons proposé de localiser les sources sismiques desenregistrements à partir de la méthode du Matched Field Processing (MFP) provenantde l'acoustique sous-marine. Plusieurs algorithmes du MFP ont été testés pour pouvoirlocaliser au mieux les sources sismiques. La bonne concordance des résultats obtenus avecchacun des algorithmes a permis d'obtenir un grand nombre de localisations des sourcesau cours du cycle. Les positions déterminées avec les di_érents algorithmes du MFP ontpermis de mettre en évidence deux zones d'activité hydrothermale du geyser associéesà di_érentes périodes du cycle éruptif, telles que le remplissage du conduit avant leséruptions et l'alimentation du geyser en eau une fois la vidange du conduit e_ectuée.Dans un second temps, l'analyse des variations de vitesse des signaux sismiques estproposée pour suivre des changements des propriétés du dôme du geyser, comme des variationsde pression avant l'éruption. Pour cela, une nouvelle méthode basée sur les mesuresde phases instantanées est suggérée. Les résultats obtenus montrent des faibles changementsde vitesse, pouvant être associés à la mise en pression du dôme ou à l'augmentationde la température du milieu avant l'éruption en surface.