Infrasound Signal Detection and Back Azimuth Estimation Using Ground‐Coupled Airwaves on a Seismo‐Acoustic Sensor Pair

Abstract

We present a new infrasonic signal detection and back azimuth determination technique that requires just one microphone and one three‐component seismometer. Ground‐coupled airwaves (GCAs) occur when an incident atmospheric acoustic wave impinges on the ground surface and is partially transmitted as a seismic wave. GCAs are commonly detected hundreds of kilometers away on seismic networks and are observed to have retrograde particle motion. Horizontally propagating acoustic waves and GCAs have previously been observed on collocated infrasound and seismic sensor pairs as coherent with a 90° phase difference. If the sensors are spatially separated, an additional propagation‐induced phase shift is present. The additional phase shift depends on the direction from which the acoustic wave arrives, as each back azimuth has a different apparent distance between the sensors. We use the additional phase shift, the coherence, and the characteristic particle motion on the three‐component seismometer to determine GCA arrivals and their unique back azimuth. We test this technique with synthetic seismo‐acoustic data generated by a coupled Earth‐atmosphere 3‐D finite difference code, as well as three seismo‐acoustic data sets from Mount St. Helens, Mount Cleveland, and Mount Pagan volcanoes. Results from our technique compare favorably with traditional infrasound array processing and provide robust GCA detection and back azimuth determination. Assuming adequate station spacing and sampling, our technique provides a new and robust method to detect infrasonic signals and determine their back azimuth, and may be of practical benefit where resources are limited and large sensor networks or arrays are not feasible.

Co-eruptive tremor from Bogoslof volcano: seismic wavefield composition at regional distances

“…1). We note that seismo-acoustic coupled waves (Matoza and Fee 2014;McKee et al 2018) from the eruption did not generally register on these stations. Two short-period seismic stations (OKRE and OKER) located on the north and east side of Okmok volcano were however prone to significant seismo-acoustic coupling over the course of the eruption.…”

“…If the particle motion were elliptical, and the sense of motion (retrograde or prograde) were known, then the 180°ambiguity could be resolved. McKee et al (2018) recently utilized this for the analysis of ground-coupled airwaves on nearly co-located seismic and acoustic sensors. Particle motion is not always elliptical and thus the 180°ambiguity exists in general.…”

Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at Ioto Island, Japan

“…To detect infrasound signals, Ichihara et al (2012) proposed a cross-correlation analysis between the pressure oscillation and ground motion signals, which has been applied successfully with some improvements (e.g., Cannata et al 2013;Matoza and Fee 2014;Nishida and Ichihara 2015;Ichihara 2016;Yukutake et al 2018). McKee et al (2018) extended the method by incorporating the phase shift and the seismic particle motion to estimate the infrasound back azimuth. However, the method is not applicable when the volcano is seismically very active, and the seismic signal dominates infrasound signal in the seismometer record.…”

Seismo-Acoustic Characterization of Mount Cleveland Volcano Explosions

“…While some volcanoes are heavily instrumented and monitored, many eruptions occur in remote regions where minimal local monitoring equipment exists. Therefore, the ability to characterize a volcanic system with limited equipment (e.g., a single seismo-acoustic pair of sensors, McKee et al, 2018) is valuable to the volcano monitoring community.…”