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| Titel |
The potential of continuous near-field recording of infrasound produced by volcanoes in Vanuatu for probing the the state of the atmosphere. |
| VerfasserIn |
Christelle Zielinski, Sylvie Vergniolle, Philipson Bani, Michel Lardy, Alexis Le Pichon, Damien Ponceau, Francis Gallois, Pascal Herry, Sylvain Todman, Esline Garaebiti |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040594
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| Zusammenfassung |
| Active volcanoes in the Vanuatu archipelago are a natural source of infrasound, which can be
used to monitor the propagation of the waves in the atmosphere, recorded at several hundred
of kilometers away. While Lopevi is only producing significant infrasonic waves when an
eruptive column of several kilometers high is emplaced in the atmosphere, Marum and
Benbow both on Ambrym island and Yasur on Tanna island are permanently generating
infrasonic waves.
We initially installed, in 2008, an acoustic triangular network on Ambrym volcano to
detect strong volcanic explosions, relatively close to the vents, at 2 km. The lack of strong
explosions during our 6 months of recordings, with recorded acoustic pressure
< 10 Pa, prevented us to use these explosions as natural sources for tracing the
propagation path in the atmosphere towards the station IS22 installed at Noumea
(New Caledonia), at several hundred of kilometers away. However, the comparison
between measurements performed at 2 km with those performed for 3 days at a
few hundred of meters from Benbow quantifies the potential of using a triangular
acoustic network at a safe distance for monitoring volcanic activity even when very
small.
Yasur volcano is an outstanding source of infrasonic waves, as its explosions are always
sufficiently strong to be recorded at Noumea (New Caledonia). One microbarometer, installed
at 300 m from its crater since october 2003, has now recorded several years of
activity. The volcanic sound reaches the station IS22 at Noumea during the austral
summer, allowing us to compare near-field and far-field signals for a very long
period. Our observations in the far-field shows that its volcanic activity is relatively
stable, as confirmed by our near-field measurements. We have also performed for a
week infrasonic measurements almost directly above the crater to further explore
the quality of our continuous measurements in a safe location at a distance of 300
m.
In the absence of appropriate modelling of the sound wave, we have used a dimensionless
analysis, which relates acoustic power and the velocity of gas-ejecta mixture. Our multi-years
recordings show several sudden increases in gas flux over one week as well as a more
progressive evolution, over several months. The gas flux varies between 280 m3/s and 1100
m3/s, in agreement with visual observations.
The acoustic time series is analysed by detecting explosions with a method based on
wavelet decomposition. The frequency is remarkably constant, showing that the gas volume
at the vent does not change significantly over the years. However huge variations exist in the
number of explosions (up to a factor 5) and in the mean acoustic pressure (up to a
factor 4). Furthermore three different regimes can be distinguished and a sharp
transition exists between them (less than a day). In the first one, the activity is quiet
with both a low number of explosions (20 per hour) and a low acoustic pressure
(50 Pa). The second regime is characterised by a high acoustic pressure and a low
number of explosions, while the third one is exactly the opposite. Regimes 2 and 3
are found in alternation, showing that the transition is reversible. We suggest that
the different regimes reflect sudden variations of the gas volume fraction in the
conduit.
Our near-field and far-field measurements, when combined, show the potential of
infrasound from volcanoes as a natural and continuous source of sound to estimate more
precisely how large the errors in the upper wind models are, in a region where there is a lack
of routine measurements. |
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