A Synthetic Study to Determine Adequate Infrasound Network Configurations for Resolving Source Directionality

Abstract:

Acoustic source inversions have the potential to provide estimates of volume and mass flow rates, increasing our ability to monitor volcanic eruptions and characterize emissions. These inversions can also give insight into acoustic source directivity when the monitoring network is sufficient. This directionality may be similar to that of ballistics and emissions of gas ejected during an explosion, which are some of the most hazardous eruption products to estimate for proximal safety. Iezzi et al. (2019) showed that a network comprised of infrasound sensors aboard one tethered aerostat and five sensors on the ground provided enough data to resolve acoustic directivity for explosions at Yasur volcano, Vanuatu that was consistent with ballistic directionality. However, it was found that more airborne sensors are required to uniquely determine the directionality in three dimensions using numerical acoustic source inversion methods.

Here we conduct a synthetic study in which infrasound sensors are placed around the source to determine which configuration of infrasound sensors is required to adequately estimate the infrasonic multipole source mechanism (monopole and dipole) uniquely at a given frequency. We use finite-difference time-domain methods to obtain the numerical Green’s functions for each synthetic station, where an acoustic source is placed at the vent location and propagated over topography. The numerical Green’s functions and synthetic waveforms are used to invert for the acoustic source mechanism to determine if the original source time function can be retrieved. A variety of airborne sensor configurations are tested for our source inversions, including vertical lines of sensors, horizontal rings of sensors around the vent, and horizontal lines of sensors moving away from the vent. The ability to recover a directional infrasound source is explored considering the influences of environmental factors, as well as dipole source strength and orientation. The optimal ground and aerial configurations to best estimate acoustic directionality found in this synthetic study will help guide future deployment configurations around active volcanoes and anthropogenic explosions.

Iezzi, A, Fee, D, Kim, K, Jolly, A, & Matoza, R (2019), 3-D Acoustic Multipole Waveform Inversion at Yasur Volcano, Vanuatu, JGR.