Real-Time Geophysical Monitoring of Particle Size Distribution During Volcanic Explosions at Stromboli Volcano (Italy)

Co-auteur étrangerInternational audienceOf all the key parameters needed to inform forecast models for volcanic plumes,real-time tracking particle size distribution (PSD) of pyroclasts leaving the vent coupledwith plume modeling has probably the highest potential for effective managementof volcanic hazard associated with plume dispersal and sedimentation. This paperpresents a novel algorithm capable of providing syn-emission horizontal size and velocityof particles in real time, converted in mass discharge rates, and its evolution during anexplosion, using thermal infrared videos. We present data on explosions that occurredat the SW crater of Stromboli volcano (Italy) in 2012. PSDs and mass eruption rate(MER) data, collected at frequencies of 40 Hz, are then coupled with particle and gasspeed data collected with traditional image analysis techniques. The dataset is usedto quantify for the first time the dynamics of the explosions and the regime of magmafragmentation. We find that explosive evacuation of magma from a Strombolian conduitduring a single explosion proceeds at a constant rate while the explosive dynamicsare marked by a pattern that includes an initial transient and short phase until thesystem stabilizes at equilibrium. These stationary conditions dominate the emission. Allexplosions begin with a gas jet (onset phase), with maximum recorded vertical velocitiesabove 150 m/s. These high velocities are for small particles carried by the faster movinggas or pressure wave, and larger particles typically have slower velocities. The gas jetsare followed by a particle-loaded plume. The particles increase in number until theexplosion dynamics become almost constant (in the stationary phase). MER is eitherstable or increases during the onset to become stable in the stationary phase. Theshearing at the interface between the magma and the gas jets controls fragmentationdynamics and particles sizes. Quantification of the Reynolds and Weber numberssuggests that the fragmentation regime changes during an explosion to affect particleshape. The algorithm proposed requires low-cost thermal monitoring systems, and lowprocessing capability, but is robust, powerful, and accurate and is able to provide datawith unprecedented accuracy. In general terms, its applicability is limited by the size ofindividual pixels recorded by the camera, which depends on the detector, the recordingdistance, and the optical system, particle temperature, which has to be significantlyhigher than the background