Three‐Dimensional Acoustic Multipole Waveform Inversion at Yasur Volcano, Vanuatu

Abstract

Acoustic waveform inversions can provide estimates of volume flow rate and erupted mass, enhancing the ability to estimate volcanic emissions. Previous studies have generally assumed a simple acoustic source (monopole); however, more complex and accurate source reconstructions are possible with a combination of equivalent sources (multipole). We deployed a high‐density acoustic network around Yasur volcano, Vanuatu, including acoustic sensors on a tethered aerostat that was moved every ∼15–60 min. Using this unique data set we invert for the acoustic multipole source mechanism using a grid search approach for 80 events to examine volume flow rates and dipole strengths. Our method utilizes finite‐difference time‐domain modeling to obtain the full 3‐D Green's functions that account for topography. Inversion results are compared using a monopole‐only, multipole (monopole and dipole), simulations that do not include topography, and those that use a subset of sensors. We find that the monopole source is a good approximation when topography is considered. However, initial compression amplitude is not fully captured by a monopole source so source directionality cannot be ruled out. The monopole solution is stable regardless of whether a monopole‐only or multipole inversion is performed. Inversions for the dipole components produce estimates consistent with observed source directionality, though these inversions are somewhat unstable given station configurations of typical deployments. Our results suggest that infrasound waveform inversion shows promise for realistic quantitative source estimates, but additional work is necessary to fully explore inversion stability, uncertainty, and robustness.

Volume Flow Rate Estimation for Small Explosions at Mt. Etna, Italy, From Acoustic Waveform Inversion

“…These observations suggest that, for the data considered here, a monopole source provides a realistic and sufficiently accurate estimate of VFRs (supporting information Figure S5). This is consistent with previous results from Iezzi et al () at Yasur and Fee et al () at Sakurajima. One important implication of this observation, if confirmed to have more general validity, is that if waveform inversion was implemented for use in real‐time volcano monitoring, a monopole‐only source would provide realistic estimates of atmospheric VFR.…”

“…(c) Detail of the time history of volume flow rate (top panel) and horizontal dipole (middle and bottom panels) for EV1 in (a). Also shown (top panel) a comparison between the volume flow time series with no detrending applied (black dashed line), with simple linear detrending following the methods of Iezzi et al (, yellow dashed line), and after application of FWZPZF detrending following the methods of Johnson and Miller (); (d) Azimuth of the horizontal dipole for EV1 in (a).…”

“…Infrasound waveform inversion using numerical GFs, accounting for topographic effects, was performed in the past at Sakurajima Volcano (Japan), assuming a monopole source (Kim et al, ; Fee et al, ) and recently at Yasur Volcano, for a multipole source (Iezzi et al, ). Vulcanian activity at Sakurajima exhibits peak pressures of 10–400 Pa at a distance of 2.3 km from the vent and produce VFR of 6 × 10 5 –1.5 × 10 6 m 3 /s, 2 orders of magnitude larger than the small degassing explosions we observe at Etna.…”

Changes in Crater Geometry Revealed by Inversion of Harmonic Infrasound Observations: 24 December 2018 Eruption of Mount Etna, Italy

“…We model the acoustic radiation as a baffled piston in a half space. In recent years, numerous studies have shown that topographic scattering is important, even for near‐field observations (Fee et al., 2017; Iezzi et al., 2019; Kim & Lees, 2011, 2014). We contend that we can neglect topographic scattering here due to the low frequencies that we consider and because the spectral content is similar across all stations in the network (Figure S1; Cannavò et al., 2019).…”

Volcanic ballistic projectile deposition from a continuously erupting volcano: Yasur Volcano, Vanuatu

“…As part of concurrent studies [Iezzi et al 2019;Jolly et al 2017;Matoza et al 2017], a temporary infrasound and seismic network was set up around the cone. Two stations from this network (YIB22 and YS01), lo-cated~700 m from the vent area, are used in this pa-Volcanica 3(2): 183 -204. doi: 1 .3 9 9/vol.…”