Geoscience Reference
In-Depth Information
A mean spectrum can be finally calculated from the averaging in the frequency domain of
several consecutive spectra (incoherent integrations) to reduce the noise fluctuations in the
resulting Doppler spectra and improve the detection of the spectral line(s) corresponding to
the volcanic echoes.
5. Insight into the dynamics of Strombolian activity
In addition to measuring eruptive parameters near the emission source, the main
advantage of portable Doppler radars with respect to weather radars resides in the fact
that they can be set up close to an eruptive vent and target just one direction at the base of
the volcanic flow to retrieve near-source parameters. They are thus able to monitor
phenomena of limited spatial dimensions with better spatial resolution and higher
acquisition rate. In this respect, they are particularly useful for studying Strombolian
activity and small scale ash plumes, generally invisible to remote weather radars and
satellites. Although VOLDORAD can also be used to monitor strong ash-laden plumes,
given its wavelength, examples of records on such types of activity are provided
respectively in this section and the next.
5.1 Information retrieved from the shape of Doppler spectra
5.1.1 Ejecta dispersion
Given the relatively low amount of ash produced during typical Strombolian activity, the
echo power is mostly controlled by large blocks that mostly follow ballistic trajectories. An
illustration of the strong control of the ejecta's spatial distribution on the shape of Doppler
spectra is the discrimination between lava bubble outbursts and lava jets (Fig. 7). When the
magma column is high in the conduit, large gas bubbles may deform the lava surface to
form lava bubbles several tens of meters in diameter, whose outburst disrupts the
surrounding lava film and produces the hemispherical ejection of big lava lumps
(decimetric to metric) seen above the crater rim (Fig. 7a). Contrastingly, in the vast majority
of events, gas slug explosions occur within the conduit and produce oriented jets of gas and
more fragmented lava, generally vertical, that can be captured by the radar beam when (if)
pyroclasts go beyond the crater rim (Fig. 7c). Velocity and mass load angular distributions of
pyroclasts are, as expected, more uniform for hemispherical lava bubbles that can burst at
the free surface and more Gaussian-shaped for lava jets that are subjected to conduit wall
friction (Fig. 8). In the case of hemispheric lava bubbles, the uniform mass load and velocity
distribution of pyroclasts over the range of ejection angles produce an equal echo power
over the range of radial velocities, leading to top-hat shaped spectra (Fig. 7b). In the case of
lava jets, the power spectral distribution results from the competing effects of particle
velocities and the distribution of ejection angles. As shown in figure 8b, measured
maximum radial velocities do not result from particles with the highest velocities in the jet
axis, nor from those having the most radial trajectories, but from particles ejected at about
20° to the jet axis (when vertical). Therefore Doppler spectra commonly appear rather
triangular when shown in dB (Fig. 7d). Gouhier & Donnadieu (2010) found that, for most
Strombolians explosions at Etna, 80% of the tephra mass is ejected within a dispersion cone
of about 40°.
