Fitting Jet Noise Similarity Spectra to Volcano Infrasound Data

Abstract

The eruption of gas and other material by volcanoes perturbs the surrounding air and generates pressure waves, including acoustic waves. Due to the large dimensions of volcanoes and volcanic events, the associated sounds are usually in a low-frequency band below human audibility, called infrasound. The physical source process generating the infrasonic waves can vary for different eruptions or even within an eruption. An often used analogy was introduced by Woulff and McGetchin (1976) and describes the volcano acoustic source mechanism as a combination of the monopole, dipole, and quadrupole components. However, as pointed out by Matoza et al. (2013), these analogies may be oversimplified for volcanic sources, especially for eruptions with sustained gas and ash jets and limited sampling of the acoustic wavefield.The term "jet" has been commonly used to describe rapid and sustained gas release from a volcano (Kieffer

Acoustic observations of lava fountain activity during the 2021 Fagradalsfjall eruption, Iceland

“…Relative to other types of eruptive activity such as Strombolian and Vulcanian, acoustic records of basaltic fissure eruptions with Hawaiian-style lava fountains are rare with previous studies limited to events at Etna (Cannata et al, 2009(Cannata et al, , 2011 and Kīlauea volcanoes (Fee et al, 2011;Lyons et al, 2021;Gestrich et al, 2021). Acoustic activity during these events are usually manifested as semi-continuous tremor with broadband frequency spectra characteristics.…”

“…Jet noise frequency spectra exhibit self-similarity, with spectral shape remaining relatively constant and scaling with frequency, diameter, and velocity (Tam, 1998). Studies have shown that this self-similarity can extend to volcanic length scales (metres to hundreds of metres), therefore, similar relationships may exist between volcanic jet noise spectra and vent diameter, jet velocity, and temperature (Matoza et al, 2009(Matoza et al, , 2013McKee et al, 2017;Gestrich et al, 2021). While the spectral shapes are similar, the peak frequencies at volcanoes are generally lower and can be explained through the Strouhal number, a non-dimensional parameter used to describe oscillating flow (Seiner, 1984).…”

“…Using sensor arrays and networks, it is possible to track the activity progression along fissures, including the opening of new vents (Cannata et al, 2011;Fee et al, 2011), and quantifying the effusion rate of highspeed, highly channelised lava flows (Lyons et al, 2021). Acoustic source processes at the vent(s) during fissure eruptions have been modelled as either Strombolian-style explosions (Cannata et al, 2011) or a sustained jet flow (Gestrich et al, 2021).…”

“…Recent studies have demonstrated how eruptions can generate an infrasonic form of 'jet noise' such as those generated by small-scale anthropogenic jet flows (e.g. Matoza et al, 2009;Fee et al, 2010a;Matoza et al, 2013;McKee et al, 2017;Gestrich et al, 2021). Anthropogenic jet noise refers to sounds generated by turbulent exhaust (i.e.…”

“…While the spectral shapes are similar, the peak frequencies at volcanoes are generally lower and can be explained through the Strouhal number, a non-dimensional parameter used to describe oscillating flow (Seiner, 1984). Quantitative comparisons of similarity spectra were carried out on acoustics recorded during the 2018 Kīlauea fissure eruption in which included 80 m high lava fountains (Gestrich et al, 2021). Changes in misfits between the recorded acoustic spectra versus the similarity spectra coincided with variations in eruption dynamics, supporting possible quantitative estimation of eruption flow features (Matoza et al, 2013;McKee et al, 2017;Gestrich et al, 2021).…”