Jean Vandemeulebrouck

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

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 geysers eruption cycle. Previously, we described measurements and analyses associated with the geysers 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.

Periodic behavior of soil CO2 emissions in diffuse degassing areas of the Azores archipelago: Application to seismovolcanic monitoring

Time series of soil CO2 efflux recorded in the Azores archipelago volcanic-hydrothermal areas feature daily and seasonal variations. The recorded CO2 efflux values were lower during summer than in the winter season. The diurnal CO2 efflux values were higher at dawn and lower in the early afternoon, contrary to that observed in biogenic environments. CO2 efflux cycles correlated well with the environmental variables, such as air temperature, wind speed, and barometric pressure, which also showed low- and high-frequency periodicities. Several simulations were performed here using the Transport of Unsaturated Groundwater and Heat 2 (TOUGH2) geothermal simulator to complement the study of Rinaldi et al. (2012). The effects of the water table depth, air temperature perturbation amplitude, and soil thermal gradient contributed to an explanation of the contrasts observed in the diurnal (S1) and semidiurnal (S2) soil CO2 efflux peaks for the different monitoring sites and seasons. Filtering techniques (multivariate regression analysis and fast Fourier transform filters) were also applied to the recorded time series to remove effects of external variables on the soil CO2 efflux. The resulting time series (the residuals) correspond to the best approach to the deep-seated (volcanic/hydrothermal) CO2 emissions and thus should be used in seismovolcanic monitoring programs. Even if no evident correlation can be established yet between the soil CO2 residuals and seismicity over the monitored time, a seismic swarm that occurred around the end of 2008 might have triggered some deviations from the observed daily cycles.

Implications for the thermal regime of acoustic noise measurements in Crater Lake, Mount Ruapehu, New Zealand

Hydrophone measurements of acoustic noise levels in the Crater Lake of Mount Ruapehu, New Zealand were made on 18 January 1991 from an inflatable rubber boat on the lake. The greatest sound pressures were recorded in the 1–10 Hz band, with sound levels generally decreasing about 20 dB per decade from 10 Hz to 80 kHz. The low frequency noise did not have an obvious relationship to the tremor observed at a seismic station within 1 km of the lake. The comparatively low levels of middle and high frequency sound meant that at the time of measurement, direct steam input did not make a significant contribution to the heating of Crater Lake. This is consistent with the earlier conclusion that during the last decade a major part of the heat input of Crater Lake has come from lake water that was heated below the lake and recycled back into the lake.

3D ultra-high resolution seismic imaging of shallow Solfatara crater in Campi Flegrei (Italy): New insights on deep hydrothermal fluid circulation processes

Seismic tomography can be used to image the spatial variation of rock properties within complex geological media such as volcanoes. Solfatara is a volcano located within the Campi Flegrei, a still active caldera, so it is of major importance to characterize its level of activity and potential danger. In this light, a 3D tomographic high-resolution P-wave velocity image of the shallow central part of Solfatara crater is obtained using first arrival times and a multiscale approach. The retrieved images, integrated with the resistivity section and temperature and the CO2 flux measurements, define the following characteristics: 1. A depth-dependent P-wave velocity layer down to 14 m, with Vp < 700 m/s typical of poorly-consolidated tephra and affected by CO2 degassing; 2. An intermediate layer, deepening towards the mineralized liquid-saturated area (Fangaia), interpreted as permeable deposits saturated with condensed water; 3. A deep, confined high velocity anomaly associated with a CO2 reservoir. These features are expression of an area located between the Fangaia, water saturated and replenished from deep aquifers, and the main fumaroles, superficial relief of the deep rising CO2 flux. Therefore, the changes in the outgassing rate greatly affect the shallow hydrothermal system, which can be used as a “mirror” of fluid migration processes occurring at depth.

Satellite monitoring of the vertical temperature profile of Lake Nyos, Cameroon

Abstract The present contribution to Lake Nyos studies, which have been carried out since 1987 by a team of French geophysicists working in close cooperation with the “Institut de Recherches Geologiques et Minieres” (IRGM) of Cameroon, involves a continuous long-duration remote monitoring of the temperature distribution in the depth of the lake. The system which was developed for that purpose, essentially includes: (1) a vertical chain of temperature probes, lifted by an anchored buoy (called Nyossonde); and (2) an associated telemetry, relayed by the Argos transponder on board the NOAA meteorological satellites. The telemetered data are collected, processed and archived, both in France (Lannion, Toulouse, Garchy) and Cameroun (Yaounde). Even though the upper part of the chain was damaged, the lower part is still in operation. Analysis of the telemetered data and comparison with other measurements confirm that the presently described monitoring system is well suited for installation in crater lakes.