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eruption near-source dynamics through their Doppler capability: commercial micro rain
radars, that are continuous-wave frequency-modulated and working at 24 GHz (Seyfried &
Hort, 1999; Hort et al., 2003, 2006) and the VOLDORAD system, an L-band pulsed volcano
Doppler radar (e.g., Dubosclard et al., 1999, 2004; Donnadieu et al., 2005). Being set up at a
chosen location and aiming directly at the emission source (instead of rotation scanning),
these compact radar systems can advantageously sound the gas thrust region and provide
source eruptive parameters like eruption velocities, but also capture short-lived weak
explosive activity, not visible to satellites or weather radars. They have higher temporal (<1 s)
and spatial resolutions (tens to hundreds of meters) and higher sensitivity. A comparison of
some characteristics of weather radars and transportable volcano Doppler radars is
presented in Table 1.
Acquisition
rate
Volume
scanned
Power
consumption
Frequency
bands
Location
Max. range
Min. range
Weather
radars
Fixed
100-300 km
Few km
Few min
km 3
100s of kW
S, C, X, Ka
Portable
radars
Few mW to
few 10s of W
Chosen
10-15 km
10s-100s m
 1 s
10 4 -10 8 m 3
L, X, Ka
Table 1. Characteristics of weather and transportable radars for the monitoring of volcanic
eruptions. Note in particular the difference in temporal and spatial resolution.
Hort and Seyfried (1998) and Seyfried and Hort (1999) measured mean vertical velocities of
about 10 m/s for 12 lava jets during very low activity at Stromboli volcano with a
commercial portable FM-CW radar Doppler anemometer 200-300 m away from the eruptive
vent. Using the same instrument, Hort et al. (2003) found an increase in eruption duration,
much higher velocities and indirect evidence of mean particle size decrease after a rain
storm. Gerst et al. (2008) reconstructed the 4D velocity (directivity) of Strombolian eruptions
at Erebus and Stromboli from 3 FM-CW radars. FM-CW radars have a narrower field of
view (around 1° or so at 3 dB) and can thus target a precise sector of the volcanic emission
but, on the other hand, lack the integrated information of longer wavelength pulse radars
with a wider beam aperture and deeper range gates. L-band frequency signals are very little
attenuated by hydrometeors or volcanic particles and can sound the interior of very dense
particle-laden plumes. VOLDORAD also has a higher temporal resolution (<0.1 s).
Donnadieu et al. (2005) showed very detailed time series of power and maximum radial
velocities of a Strombolian explosion at Etna and an ash plume at Arenal, acquired at high
rate (<0.1 s) with VOLDORAD. Donnadieu et al. (2003, 2005) and Dubosclard et al. (2004)
further showed evidence of strong correlation between volcanic tremor and maximum radar
velocities for several Strombolian episodes, suggesting the influence of gas bubble dynamics
in the conduit on tremor generation at Etna. Using VOLDORAD, Gouhier & Donnadieu
(2008) first quantified the mass of tephra of Strombolian explosions at Etna (50-200 tons)
from a new power inversion method. From the analysis of the shape of Doppler spectra of
200 Strombolian explosions, Gouhier & Donnadieu (2010) found that 80% of the load is
ejected within a 40° dispersion cone and that, for 2/3 of the explosions, ejecta are distributed
uniformly within this cone. Using measured maximum radial velocities, at-vent particle and
gas velocities can be retrieved, and source gas fluxes estimated when the vent diameter is
known (Gouhier & Donnadieu, 2011). Comparing thermal data with records from a FM-CW
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