Analogue modeling of instabilities in crater lake hydrothermal systems.

International audienceWe carried out analogue experiments on two-phase boiling systems, using a porous vertical cylinder, saturated with water. The base of the cylinder was heated, and the top was cooled, as in a natural hydrothermal system. Previous work had shown that once the two-phase zone reached a certain level, thermal instabilities would develop. We made measurements of the acoustic energy related to boiling, and we found that high levels of acoustic noise were associated with the part of the cycle in which there was upward water movement. We repeated our experiments with a cooling water tank at the top of the system, representing a crater lake. This showed that periodic thermal instabilities still developed in this situation. We then compared our analogue measurements to two natural systems known to exhibit periodic behavior. There is good agreement between the thermal and acoustic cycling seen in our model and the observations made at Inferno Crater Lake in the Waimangu Geothermal area, New Zealand, whose level cycles by nearly 10 m, with a typical period of 38 days. Particularly notable is how in both systems high levels of acoustic noise are associated with rising water level. The much larger Ruapehu Crater Lake, also in New Zealand, cycled with a period of several months to a year for over a decade prior to the 1995 eruption. Strong acoustic and seismic energy usually occurred just before the lake temperature started to rise. This suggests a slightly different model, in which the increasing two-phase flow zone triggers more general convection once it reaches the base of the lake

Is abdominal compression useful in lung stereotactic body radiation therapy? A 4DCT and dosimetric lobe-dependent study

International audiencePurpose : To determine the usefulness of abdominal compression in lung stereotactic body radiation therapy (SBRT) depending on lobe tumor location.Materials and methods : Twenty-seven non-small cell lung cancer patients were immobilized in the Stereotactic Body Frame™ (Elekta). Eighteen tumors were located in an upper lobe, one in the middle lobe and nine in a lower lobe (one patient had two lesions). All patients underwent two four-dimensional computed tomography (4DCT) scans, with and without abdominal compression. Three-dimensional tumor motion amplitude was determined using manual landmark annotation. We also determined the internal target volume (ITV) and the influence of abdominal compression on lung dose-volume histograms. Results : The mean reduction of tumor motion amplitude was 3.5 mm (p = 0.009) for lower lobe tumors and 0.8 mm (p = 0.026) for upper/middle lobe locations. Compression increased tumor motion in 5 cases. Mean ITV reduction was 3.6 cm3 (p = 0.039) for lower lobe and 0.2 cm3 (p = 0.048) for upper/middle lobe lesions. Dosimetric gain of the compression for lung sparing was not clinically relevant. Conclusions : The most significant impact of abdominal compression was obtained in patients with lower lobe tumors. However, minor or negative effects of compression were reported for other patients and lung sparing was not substantially improved. At our institute, patients with upper or middle lobe lesions are now systematically treated without compression and the usefulness of compression for lower lobe tumors is evaluated on an individual basis

Eruptions at Lone Star Geyser, Yellowstone National Park, USA: 1. Energetics and eruption dynamics

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 118 (2013): 4048–4062, doi:10.1002/jgrb.50251.Geysers provide a natural laboratory to study multiphase eruptive processes. We present results from a 4 day experiment at Lone Star Geyser in Yellowstone National Park, USA. We simultaneously measured water discharge, acoustic emissions, infrared intensity, and visible and infrared video to quantify the energetics and dynamics of eruptions, occurring approximately every 3 h. We define four phases in the eruption cycle (1) a 28±3 min phase with liquid and steam fountaining, with maximum jet velocities of 16–28 m s−1, steam mass fraction of less than ∼0.01. Intermittently choked flow and flow oscillations with periods increasing from 20 to 40 s are coincident with a decrease in jet velocity and an increase of steam fraction; (2) a 26±8 min posteruption relaxation phase with no discharge from the vent, infrared (IR), and acoustic power oscillations gliding between 30 and 40 s; (3) a 59±13 min recharge period during which the geyser is quiescent and progressively refills, and (4) a 69±14 min preplay period characterized by a series of 5–10 min long pulses of steam, small volumes of liquid water discharge, and 50–70 s flow oscillations. The erupted waters ascend from a 160–170°C reservoir, and the volume discharged during the entire eruptive cycle is 20.8±4.1 m3. Assuming isentropic expansion, we calculate a heat output from the geyser of 1.4–1.5 MW, which is <0.1% of the total heat output from Yellowstone Caldera.Support comes from NSF (L. Karlstrom, M. Manga), the USGS Volcano Hazards program (S. Hurwitz, F. Murphy, M.J.S. Johnston, and R.B. McCleskey), and WHOI (R. Sohn).2014-02-1

Eruptions at Lone Star geyser, Yellowstone National Park, USA: 2. Constraints on subsurface dynamics

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 8688–8707, doi:10.1002/2014JB011526.We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geyser's eruption cycle. Previously, we described measurements and analyses associated with the geyser's erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5–40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover, ~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of ~10 during eruptions.Funding for USGS team members was provided by the USGS Volcano Hazards Program. R. Sohn's participation was supported by the WHOI Green Technology Program. M. Manga, L. Karlstrom and M. Rudolph did receive salary from the National Science Foundation to spend time on this project.2015-06-0

Challenges in Mars climate modelling with the LMD Mars Global Climate Model, now called the Mars “Planetary Climate Model” (PCM)

The Mars atmosphere Global Climate Model (GCM) developed at the Laboratoire de Météorologie Dynamique [1] in collaboration with several teams around the world (LATMOS, the Instituto de Astrofisica de Andalucia, UAE University, University of Oxford, The Open University), and with the support of ESA and CNES is currently used for many kinds of applications. It simulates Mars from the subsurface to the top of the thermosphere and includes the cycles of dust, water and CO2 that control the current Martian climate as well as a photo-chemical/ionospheric module. The aim of this modeling is high: ultimately to build a numerical simulator based only on universal equations, yet able to consistently reproduce available observations. The goal is to create a realistic virtual planet on which all observed phenomena and climate-induced geological landforms arise naturally. Like for the other similar models in the community, this specific goal is a scientific endeavour by itself. Such a GCM can also provide useful environmental predictions that can be used to process observations or prepare space missions. For this purpose our teams have produced the Mars Climate Database (See Millour et al., this issue) which provides climatologies derived from GCM simulations completed by dedicated tools. The GCM is also used to perform meteorological data assimilation to create an optimal description of the Martian environment obtained by combining observation and model simulations (See e.g. Young et al., Read et al., Holmes et al., this issue)

Volcano-glacier interactions : field survey, remote sensing and modelling. Case study : Nevado del Ruiz, Colombia

Interactions of hot eruptive products with snow and ice have been inferred from geomorphological and glaciological perturbations on ice-clad active volcanoes. These perturbations record a variety of processes, including rapid melting, snow and ice avalanching, surficial abrasion, and mechanical scouring or gullying. The loss of large volumes of snow and ice during eruptions results mainly from : (i) the passage of pyroclastic flows and surges or hot blast on the glacier that caps the mountain, (ii) the contact of subaerial lava flows or tephra with ice or snow, and (iii) the eruptive or geothermal activity which melts the bases of ice caps. Preliminary melting scenarios based on vigorous deposition of hot debris on snow point to melting rate as high as 10-² m/min. Mechanical entrainment and comminution of snow and ice are important processes in triggering lahars. / Les interactions de produits éruptifs chauds avec la neige et la glace ont été déterminées à partir des perturbations géomorphologiques et glaciologiques sur des volcans actifs recouverts de glace. Ces perturbations gardent la trace de différents processus tels que la fonte rapide, les avalanches de neige et de glace, l'abrasion de surface, le creusement par érosion de gorges. La perte d'importants volumes de neige et de glace au cours des éruptions s'explique principalement par (i) le passage d'écoulements pyroclastiques ou de bouffées chaudes sur les glaciers qui recouvrent le sommet de la montagne, (ii) le contact entre les écoulements de lave ou d'autres produits volcaniques et la glace ou la neige, (iii) l'activité éruptive ou géothermique qui fait fondre la base des couvertures de glace. Des scénarios de fonte basés sur un dépôt important de produits chauds sur la neige conduisent à un taux de fonte de 10 mm/mn. L'entraînement mécanique et la comminution de la neige et de la glace sont des processus importants dans le déclenchement des lahars

The effects of hydrothermal eruptions and a tectonic earthquake on a cycling crater lake (Inferno Crater Lake, Waimangu, New Zealand)

International audienceThe lake level and temperature of Inferno Crater Lake, a hot lake in the Waimangu Geothermal Field, vary in quasi-regular cycles with a period of about 30 to 50 days. Between 1970 and 1991, several hydrothermal eruptions at other sites within the Waimangu system affected this cycling, especially the cycle period. This indicated that although the cycling was most evident in one small lake, it was sensitive to any changes in the surrounding geothermal system. After the Raupo Pond eruption, which produced the largest effects, the recovery of the temperature and cycle period was consistent with extra energy diffusing out of the previously proposed heat pipe system. The 1987 Edgecumbe earthquake, whose hypocenter was 50 km from Waimangu, also seems to have significantly affected the hydrothermal system, reducing the cycle period for at least a year afterwards

Interactions volcans glaciers, mesures de terrain, télédétection et modélisation : étude du cas du Nevado del Ruiz

[Departement_IRSTEA]EEE [TR1_IRSTEA]E31-Ingénierie et prévention des risques naturels en montagneExtrait de documentInteractions of hot eruptive products with snow and ice have been inferred from geomorphological and glaciological perturbations on ice-clad active volcanoes. These perturbations record a variety of processes, including rapid melting, snow and ice avalanching, surficial abrasion, and mechanical scouring or gullying. The loss of large volumes of snow and ice during eruptions results mainly from : 1) the passage of pyroclastic flows and surges or hot blasts on the glacier that caps the mountain ; 2) the contact of subaerial lava flows or tephra with ice or snow ; 3) the eruptive or geothermal activity which melts the bases of ice caps. Pyroclastic and lahar deposits, vegetation, and ice and snow were separated on a 1986 SPOT satellite image using remote sensing techniques and extensive field survey at Nevado del Ruiz, Colombia. Geomorphological and hydraulic parameters of transient, mixed avalanches (tephra, snow, and ice) which transformed to lahars suggest non-channelled, unstratified, high-density, gravity driven flows.Les interactions de produits éruptifs de température élevée avec la neige et la glace ont été déduits des perturbations géomorphologiques et glaciologiques observées sur les volcans actifs ayant une couverture glaciaire. L'étude de ces perturbations présente une grande variété de processus dont la fonte rapide, les avalanches de glace ou de neige, l'abrasion superficielle, le fort ravinement mécanique. La disparition d'énormes volumes de glace et de neige au cours des éruptions résulte principalement de ; 1) le passage d'écoulements pyroplastiques et de vagues de nuées de haute température sur les glaciers couronnant le sommet ; 2) le contact de coulées de lave ou de tephra avec glace et la neige ; 3) l'activité éruptive et géothermale qui font la base de la glace ou du manteau neigeux. L'image satellite SPOT 1986 a permis de différencier les dépôts de matériaux pyroplastiques, les lahars, la végétation, la glace et la neige, en utilisant les techniques de télédétection couplées a une minutieuse campagne de terrain. On a noté des traces d'éléments géomorphologiques et hydrauliques ayant tendance a disparaître ainsi que des avalanches mixtes (tephra, neige et glace) qui, transformées en lahars, évoquent des écoulements de haute densité, non canalisés et non stratifiés. En utilisant des ortho-images trois dimensions et les résultats des campagnes de terrain, il a été possible de calculer les surfaces de neige et de glace qui ont été décapées ainsi que les volumes perdus pendant les 20 à 90 minutes de l'éruption du 13 novembre 1985. De tels volumes impliquent des taux de fonte très élevés et des transferts de chaleur extrêmement vigoureux des produits éruptifs à la neige et la glace. Les effets mécaniques d'entraînement et de transmission de chaleur à la neige et la glace sont des processus essentiels dans le déclenchement des lahars