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the infrasonic signature. In one example, a ~200-m deep gas-filled cavity above the Halema’uma’u lava lake at Kilauea, Hawai’i, was successfully modeled as a large Helmholtz reso- nator producing a sustained spectral peak at ~0.5 Hz (Fee et al., 2010). Flow-induced oscillations through shallow volcanic cavity structures have also been hypothesized as a source of harmonic infrasound (Matoza et al., 2010). Although of inter- est for improving understanding of how Hawaiian volcanoes work and contributing to local volcano monitoring systems, these types of signals are generally weaker and do not tend to propagate long distances in the atmosphere.
At the other end of the spectrum is explosive volcanism. However, even within the general category of “explosive vol- canism,” there is a great variety of processes and infrasound signals. These processes range from discrete blasts lasting only a few seconds to sustained jetting activity that can last for tens of minutes to hours in duration (Figure 6a).
Perhaps the most well-known explosive eruption style is termed plinian, which produces the tall eruptive columns of pyroclasts reaching high in the atmosphere, first described by Pliny the Younger for the AD 79 eruption of Vesuvius. Volca- nologists schematically divide plinian eruption columns into three altitude regions. The lowermost section of a plinian vol- canic eruption column represents a momentum-driven, tur- bulent, free-shear jet flow. Above the jet-flow region (termed the gas-thrust region), the flow transitions with altitude into a thermal buoyancy-driven volcanic plume (the convective region), which rises higher in the atmosphere by entraining surrounding air (Figure 5). Eventually, neutral buoyancy is achieved and the flow transitions into a laterally spreading umbrella region dominated by advection and diffusion.
Recent work has suggested that the gas-thrust region of plin- ian eruptions, a large natural jet flow, generates a low-frequen- cy (infrasonic) form of the aeroacoustic jet noise produced by smaller scale anthropogenic jets, such as the exhausts of jet engines and rockets (Matoza et al., 2009, 2013; Fee et al., 2013; Taddeucci et al., 2014). Jet noise is the noise generated by a turbulent jet flow itself. Jet noise has been characterized in laboratory and field aeroacoustics studies by considering how acoustic signal properties vary as a function of angle to the jet axis and jet operating parameters such as the jet veloc- ity, diameter, temperature, density, and nozzle geometry. The spectral shapes of observed volcano infrasound signals are in approximate agreement with the spectra of laboratory jets but shifted to lower frequencies (Matoza et al., 2009; Figure 6b). However, the observed volcanic signals have additional com- plexities not present in the pure-air laboratory data. These
features may result from multiphase flow containing solid particles and liquid droplets, very high temperatures, and complex volcanic vent morphology (Matoza et al., 2013). In addition, similarity between waveforms from volcanic erup- tion infrasound and jet flow from supersonic jet engines and solid rocket motors has been observed (Fee et al., 2013). Erup- tions from Nabro volcano, Eritrea; Stromboli volcano, Italy; and Calbuco volcano, Chile, produced infrasound waveforms consisting of asymmetric, shock-like pressure pulses with positive waveform skewness, similar to those associated with the “crackle” phenomenon in audible noise from superson- ic, heated jet and rocket engines (Gee et al., 2007; Fee et al., 2013; Goto et al., 2014; Figure 6c). High-speed visual (Yokoo and Ishihara, 2007; Genco et al., 2014) and infrared (Delle Donne and Ripepe, 2012) video has been employed to im- age supersonic shock waves emanating from explosions and reconstruct the resultant acoustic waves. Differencing of im- ages from sustained volcanic jets has revealed repeated shock waves emanating from the edges of the jet (Genco et al., 2014; Taddeucci et al., 2014).
The infrasound produced by volcanic jet flows is in some ways similar, but is not perfectly analogous, to the jet noise produced by laboratory jets and flight-vehicle exhaust. To further develop understanding of volcanic jet aeroacous- tics, laboratory (e.g., Médici and Waite, 2016) and numeri- cal (e.g., Cerminara et al., 2016) studies are required. Labo- ratory and numerical studies will also need to be coupled with novel field deployments that improve observational constraints on volcanic jet noise. For instance, volcanic jet flows are vertically oriented, such that sampling directivity in the acoustic wavefield of a volcanic jet is challenging. In- frasound sensors are usually located on the ground surface and thus sample the acoustic wavefield in a limited angular range in the upstream jet direction.
Toward this goal, a recent field experiment employed infra- sound sensors aboard a tethered aerostat (a helium balloon- kite hybrid) at the active Yasur volcano, Vanuatu (Jolly et al., 2017). The aerostats carried 2-3 infrasound sensors sus- pended on a string with 10-20 m vertical spacing, with each aerostat deployment lasting ~20 min to an hour. The sensors were lofted to a position ~200 to 300 m above the active vent and <100 m above the crater rim at 38 tethered positions, sampling angular ranges of ~200° in azimuth and ~50° in takeoff angle. Comparison with synchronous recordings of ground-based infrasound sensors enabled determination of an anisotropic radiation pattern that may be related to path effects from the crater walls and/or source directional-
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