Geoscience Reference
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5.2.2 Eruptive velocities
Initial velocities are of great interest because they control the height reached by the
pyroclasts and are related to the gas overpressure. In measuring particle velocities
continuously and at high rate, Doppler radars potentially hold information on these
parameters and on the detailed kinetics of the jets. Although the variations of measured
radial velocities closely reflect the kinetics of the volcanic jet, retrieving absolute initial
particle velocities from measured maximum radial velocities is not straightforward, in
general, because (i) the latter are associated with oblique trajectories, as illustrated in figure
8b-c, (ii) the distance between the range gate's lower boundary and the emission source
must be taken into account; it also controls which particle size induces the measured
maximum velocities (Gouhier & Donnadieu, 2011). In order to confidently retrieve initial
(at-vent) velocities, eruption models with suitable laws are needed, along with accurate
knowledge of the sounding geometry and crater configuration. Velocity calibration can be
further supported by an analysis of video-derived velocities. By sounding the volcanic
emission near their source, it turns out often that maximum radial velocities, commonly in
the range of a few tens to 160 m/s are not very different from eruptive velocities during
Strombolian activity at Etna for example. Gouhier & Donnadieu (2011) analyzed 247
Strombolian explosions during the paroxysmal phase of the July 4 2001 eruptive episode of
Etna's SE crater and found time-averaged values of 95 ±24 m/s for initial particle velocities,
37.6 ±1.9 m/s for the bulk jet velocity, and 118 ±36 m/s for the initial gas velocity. Note that
the initial gas velocity is highly dependent upon the chosen model law, as the gas velocity
decrease with height is exponential and more quantitative observations are needed, using
complementary techniques like high-rate thermal infrared imagery.
5.2.3 Detailed dynamics at short time scales
Much can be learned from the analysis of time series of echo power and velocities on the
eruption dynamics at various time scales, ranging from a single explosion through to an
entire eruptive episode, to a series of eruptions. Figure 10 shows such time series for a
Strombolian explosion at Yasur volcano, Vanuatu, similar to that shown in figure 11.
The maximum along-beam velocities associated with rising ejecta show a very sharp
increase up to a peak near 120 m/s within 0.21 s, attesting to initial accelerations of over 560
m/s 2 (i.e. 57 g ). Measured velocities then regularly decrease for 6-7 s, then reach a plateau at
20-30 m/s with sparse, short fluctuations up to 50 m/s. Interestingly, during the strong
velocity phase, ample fluctuations associated with short power increases can be seen
pulsating every 1.5 s, suggesting rapid variations in the discharge rate. This could be
caused either by the successive explosions of trains of closely packed gas bubbles rather
than a single large slug, or by flow oscillations related to the high speed motion of a
compressible fluid and conduit irregularities. The sharp onset in echo power in the 344 m
range gate for both P- and P+ shows that rising particles with motion components both
toward and away from the antenna (pointing downward) contribute to the signal. The
former are dominant as the bulk flow is vertically directed. The first break in slope is likely
associated with ejecta leaving the gate through its upper boundary. P- then gently increases
up to its peak amplitude nearly 3 s later, as more pyroclasts fill up the probed volume.
About 6 s after the onset, P+ becomes dominant whereas P- starts to decrease, indicating the
contribution from block fallout. Comparing with figure 11, it can be inferred that most of the
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