Abstract
Acoustic pressure is largely used to monitor explosive activity at volcanoes and has become one of the most promising technique to monitor volcanoes also at large scale. However, no clear relation between the fluid dynamics of explosive eruptions and the associated acoustic signals has yet been defined. Linear acoustic has been applied to derive source parameters in the case of strong explosive eruptions which are well-known to be driven by large overpressure of the magmatic fluids. Asymmetric acoustic waveforms are generally considered as the evidence for supersonic explosive dynamics also for small explosive regimes. We have used Lattice-Boltzmann modeling of the eruptive fluid dynamics to analyse the acoustic wavefield produced by different flow regimes. We demonstrate that acoustic waveform well reproduces the flow dynamics of a subsonic fluid injection related to discrete explosive events. Different volumetric flow rate, at low-Mach regimes, can explain both the observed symmetric and asymmetric waveform. Hence, asymmetric waveforms are not necessarily related to the shock/supersonic fluid dynamics of the source. As a result, we highlight an ambiguity in the general interpretation of volcano acoustic signals for the retrieval of key eruption source parameters, necessary for a reliable volcanic hazard assessment.
Highlights
Acoustic pressure is largely used to monitor explosive activity at volcanoes and has become one of the most promising technique to monitor volcanoes at large scale
The motion of the fluid produced by a volcanic explosion may be strongly directed upward and is in general affected by non-linear phenomena related for instance to compressibility and development of flow instability
The wavefield computed with Lattice Boltzmann method (LBM) is always consistent with the one predicted by the linear theory, when asymmetric source time function (SRT) derived by the Friedlander equation is used
Summary
Acoustic pressure is largely used to monitor explosive activity at volcanoes and has become one of the most promising technique to monitor volcanoes at large scale. The local air velocity induced by the volumetric flux should be much smaller than the sound speed (cs), equivalent to low acoustic Mach number (Ma = u/cs 1) reducing the application of this theory to the sub-sonic explosive dynamics These conditions are typically violated in the presence of strong pressure transients, like shock waves that propagates at supersonic speeds. Infrasonic signals characterized by an asymmetric waveform have been explained as produced by the diffraction of crater rim[20], whereas in some cases the asymmetric waveform is shown to be remarkably similar to the Friedlander waveform[15] In this latter case, the striking similarity between waveforms may represent the evidence of a blast wave generated by the supersonic dynamics of small but still violent Strombolian explosions[15] and it is confirmed by the slightly (345–437 ms−1) supersonic propagation speed of the measured acoustic front. We show that the blast/shock wave dynamics or the diffraction around the source is not a necessary requirement for explaining the asymmetric waveform of acoustic waves sometimes observed at explosive volcanoes during moderate Strombolian explosions
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