Infrasound Studies Research Articles

Probing a planet from the subsurface to the atmosphere with infrasound data

The analysis of infrasound waves has a significant potential for retrieving a range of geophysical parameters across various scales, such as atmospheric structures, characteristics of surface and buried sources, and seismic velocity structures. This potential was recently showcased in infrasound studies that illustrated the ability to retrieve Earth's shallow velocities and Mars' near-surface atmospheric winds from remote acoustic observations. Consequently, infrasound is becoming an essential component of planetary missions, providing insights into the interiors and atmospheres of other terrestrial planets in the solar system. However, utilizing infrasound data requires efficient forward wave simulation techniques and an accurate description of waveform sensitivity to source and medium parameters. Even under ideal conditions, many inverse problems associated with infrasound-based probing are inherently ill-posed, necessitating regularization or other approaches to ensure reliable results. In this contribution, we highlight recent seismo-acoustic research findings, addressing atmospheric probing at both smaller and global scales, as well as innovative methods to accelerate infrasonic wave propagation modeling.

Examining Infrasound Propagation at High Spatial Resolution Using a Nodal Seismic Array

AbstractInfrasound—acoustic waves in the atmosphere below 20 Hz—is a useful monitoring tool. Topography and atmospheric structure strongly control infrasound propagation, and at common source–receiver distances neither of these effects can be ignored when quantitative source constraints are sought. Detailed spatial measurements of the infrasound wavefield would inform propagation models and improve source estimates. However, the “large‐N” deployment strategy now well‐known in seismology has not yet been realized for infrasound studies. Here, we use the 900‐node seismic array from the 2014 Imaging Magma Under St. Helens (iMUSH) experiment as a proxy for a large‐N infrasound network, by leveraging acoustic–seismic coupled arrivals. The active‐source component of iMUSH consisted of 23 shallowly buried explosions around Mount Saint Helens volcano; these explosions produced epicentral infrasound recorded on the nodes. We find that the bulk presence of ground‐coupled infrasound on the nodes is controlled by wind noise and source–receiver distance, with observed arrivals for eight explosions. Explosions with the most extensive coupling produce complex spatial waveform patterns across the array. These patterns are related to both topographic and atmospheric propagation effects, as well as spatially variable site (coupling) effects. We compare our observations to simple topographic diffraction and high‐resolution wind advection models, and full‐wave numerical simulations. We find strong spatial correlations between (a) coupled arrival strength and modeled topographic obstruction and (b) coupled arrival time and along‐path winds. Our seismoacoustic analyses and results are applicable to other existing and future nodal seismic data sets and can expand the utility of such deployments.

Stratospheric Balloon Observations of Infrasound Waves From the 15 January 2022 Hunga Eruption, Tonga

AbstractThe 15 January 2022 eruption of the Hunga volcano (Tonga) generated a rich spectrum of waves, some of which achieved global propagation. Among numerous platforms monitoring the event, two stratospheric balloons flying over the tropical Pacific provided unique observations of infrasonic wave arrivals, detecting five complete revolutions. Combined with ground measurements from the infrasound network of the International Monitoring System, balloon‐borne observations may provide additional constraint on the scenario of the eruption, as suggested by the correlation between bursts of acoustic wave emission and peaks of maximum volcanic plume top height. Balloon records also highlight previously unobserved long‐range propagation of infrasound modes and their dispersion patterns. A comparison between ground‐ and balloon‐based measurements emphasizes superior signal‐to‐noise ratios onboard the balloons and further demonstrates their potential for infrasound studies.

Infrasound and Gravity Waves Over the Andes Observed by a Pressure Sensor on Board a Stratospheric Balloon

AbstractThe study of infrasound (acoustic) and gravity waves sources and propagation in the atmosphere of a planet gives us precious insight on atmosphere dynamics, climate, and even internal structure. The implementation of modern pressure sensors with high rate sampling on stratospheric balloons is improving their study. We analyzed the data from the National Aeronautics and Space Administration Ultra Long Duration Balloon mission (16 May to 30 June 2016). Here, we focus on the balloon's transit of the Andes Mountains. We detected gravity waves that are associated to troposphere convective activity and mountain waves. An increase of the horizontal wavelengths from 50 to 70 km with increasing distance to the mountains is favoring the presence of mountain waves. We also report on the detection of infrasounds generated by the mountains in the 0.01–0.1 Hz range with a pressure amplitude increase by a factor 2 relative background signal. Besides, we characterized the decrease of microbaroms power when the balloon was flying away from the ocean coast. These observations suggest, in a way similar to microseisms for seismometers, that microbaroms are the main background noise sources recorded in the stratosphere even far from the ocean sources. Finally, we observed a broadband signal above the Andes, between 0.45 and 2 Hz, probably associated with a thunderstorm. The diversity of geophysical phenomena captured in less than a day of observation stresses the interest of high rate pressure sensors on board long‐duration balloon missions.

Recent Developments and Applications of Acoustic Infrasound to Monitor Volcanic Emissions

Volcanic ash is a well-known hazard to population, infrastructure, and commercial and civil aviation. Early assessment of the parameters that control the development and evolution of volcanic plumes is crucial to effective risk mitigation. Acoustic infrasound is a ground-based remote sensing technique—increasingly popular in the past two decades—that allows rapid estimates of eruption source parameters, including fluid flow velocities and volume flow rates of erupted material. The rate at which material is ejected from volcanic vents during eruptions, is one of the main inputs into models of atmospheric ash transport used to dispatch aviation warnings during eruptive crises. During explosive activity at volcanoes, the injection of hot gas-laden pyroclasts into the atmosphere generates acoustic waves that are recorded at local, regional and global scale. Within the framework of linear acoustic theory, infrasound sources can be modelled as multipole series, and acoustic pressure waveforms can be inverted to obtain the time history of volume flow at the vent. Here, we review near-field (<10 km from the vent) linear acoustic wave theory and its applications to the assessment of eruption source parameters. We evaluate recent advances in volcano infrasound modelling and inversion, and comment on the advantages and current limitations of these methods. We review published case studies from different volcanoes and show applications to new data that provide a benchmark for future acoustic infrasound studies.

Acoustic characterization of a portable infrasound source

A trailer-able, pneumatic infrasound source is described that can produce tones with frequencies as low as 0.25 Hz. The device is based on compressed gas air flow which is released into the atmosphere from a pair of 500 gallon reservoirs, pressurized to 200 psi (1377 kPa) and modulated by pair of rotating ball valves, with 2 in. (5 cm) diameter ports. Positive air flow is released twice per revolution, so the device is a siren. In addition to the fundamental frequency, the siren also produces a series of harmonics of the fundamental tone, enabling simultaneous measurements to be made at multiple frequencies, up to 20 Hz. This instrument was developed to support in-situ calibrations of infrasound sensors and arrays as well as to provide a controllable source for other infrasound studies, such as signal insertion loss measurements of wind noise suppression structures and characterization of infrasound array azimuthal directivity. The physical attributes of the source are described and results of acoustic measurements of sound pressure levels and azimuthal directivity are presented. The results are also compared to estimates made using improved versions of a previously presented aero-acoustic model (Pneumatic Infrasound Source: Theory and Experiment, POMA 19, 045030 [2013] and papers 4pPA3 and 4pPA4. J. Acoust. Soc. Am. 134 [2013]). [Work supported by the U.S. Army Space and Missile Defense Command/Army Forces Strategic Command (USASMDC/ARSTRAT).]

Introduction to an Open Community Infrasound Dataset from the Actively Erupting Sakurajima Volcano, Japan

Online Material: Movie of time‐synced visual and infrasound data of two explosions at Sakurajima Volcano, Japan. Active volcanoes are significant sources of seismic and acoustic radiation. Extensive work has shown that infrasound is an effective tool to study and monitor active volcanoes. Infrasound is now a regular tool utilized by volcano observatories to aid in volcano monitoring and hazard mitigation. Infrasound‐based studies are able to provide important information on eruption dynamics and to develop quantitative models of volcanic eruptions. In addition, volcanoes provide excellent acoustic sources for atmospheric propagation studies and can be used to infer atmospheric dynamics and structure (see Johnson and Ripepe, 2011; Fee and Matoza, 2013 for recent reviews on volcano infrasound). The number of infrasound sensors and studies is also increasing rapidly, including those deployed as part of the USArray Transportable (seismic) Array. Lastly, seismic and acoustic records provide complementary information on both volcanic and nonvolcanic sources and elastic wave propagation in the Earth and atmosphere (Arrowsmith et al. , 2010). Although the number of volcano infrasound stations and research studies has increased substantially in recent years, many questions remain unresolved. For example, current work is focused on the spatial and temporal variability of the atmosphere and how it affects infrasound propagation at distances of a few to thousands of kilometers (Johnson et al. , 2012; Fee and Matoza, 2013; Lacanna et al. , 2014). Other studies are beginning to assess the effects of complex topography and volcanic crater morphology on infrasound signals recorded at both local and regional distances (e.g., Matoza, Garces, et al. , 2009; Kim and Lees, 2011; Lacanna and Ripepe, 2012). To date, most volcano infrasound studies assume linear sound propagation from the source, yet this assumption may not be valid for all volcanic explosions and has not been tested …

Application of the Monopole Source to Quantify Explosive Flux during Vulcanian Explosions at Sakurajima Volcano (Japan)

A primary goal in volcano seismology is to characterize source motions internal to a volcano in terms of their representative forces. In a similar manner, much volcano infrasound research strives to recover eruptive force time histories corresponding to material accelerations occurring at Earth’s free surface. These motions may correspond to explosive emission of gas and pyroclasts (e.g., Banister, 1984), rapid ground distensions of a volcanic dome (e.g., Johnson and Lees, 2010), and/or gravity driven rock fall or pyroclastic flows (e.g., Yamasato, 1997). When free surface motion is unsteady it imposes stresses upon the surrounding atmosphere, which are propagated as acoustic airwaves. Typically, the sounds produced by volcanic phenomena are recorded with low‐frequency infrasound sensitive microphones. The intervening atmosphere is relatively homogeneous and nonattenuating for infrasound propagation of a few kilometers, therefore the recorded excess pressures closely represent volcano source time functions. Such force time histories are equivalent to volumetric accelerations of the atmosphere at or near to the source. Many volcano explosion signals begin with intense, impulsive bipolar pulses (compression followed by rarefaction) with durations of a few seconds or less (e.g., Morrissey and Chouet, 1997). These volcano explosion waveforms are often similar to chemical explosion signals recorded in the far field and are suggestive of a rapid release and expansion of gases (Kitov et al. , 1997). Several volcano infrasound studies have made use of the Lighthill (1978) simple acoustic source to characterize ejection of fluid from the volcanic vent (e.g., Johnson, 2003). Volcanic explosions generate bipolar infrasound pulses with durations of a few seconds (Firstov and Kravchenko, 1996; Johnson, 2003) that have been attributed to simple acoustic sources. A simple acoustic source, or a monopole, corresponds to a point approximation of fluid injection or extraction. Although most volcanic vents possess length scales of tens of meters to …

Three-dimensional volcano-acoustic source localization at Karymsky Volcano, Kamchatka, Russia

We test two methods of 3-D acoustic source localization on volcanic explosions and small-scale jetting events at Karymsky Volcano, Kamchatka, Russia. Recent infrasound studies have provided evidence that volcanic jets produce low-frequency aerodynamic sound (jet noise) similar to that from man-made jet engines. For man-made jet noise, noise sources localize along the turbulent jet flow downstream of the nozzle. Discrimination of jet noise sources along the axis of a volcanic jet requires high resolution in the vertical dimension, which is very difficult to achieve with typical volcano-acoustic network geometries. At Karymsky Volcano, an eroded edifice (Dvor Caldera) adjacent to the active cone provided a platform for the deployment of five infrasound sensors in July 2012 with intra-network relief of ~600m. The network was designed to target large-scale jetting, but unfortunately only small-scale jetting and explosions were recorded during the 12-day experiment. A novel 3-D inverse localization method, srcLoc, is tested and compared against a more common grid-search semblance technique. Simulations using synthetic signals show that srcLoc is capable of determining vertical solutions to within ±150m or better (for signal-to-noise ratios ≥1) for this network configuration. However, srcLoc locations for explosions and small-scale jetting at Karymsky Volcano show a persistent overestimation of source elevation and underestimation of sound speed. The semblance method provides more realistic source locations, likely because it uses a fixed, realistic sound speed of ~340m/s. Explosion waveforms exhibit amplitude relationships and waveform distortion strikingly similar to those theorized by modeling studies of wave diffraction around the crater rim. We suggest that the delay of acoustic signals and apparent elevated source locations are due to raypaths altered by topography and/or crater diffraction effects, implying that topography in the vent region must be accounted for when attempting 3-D volcano acoustic source localization. Though the data presented here are insufficient to resolve small-scale jet noise sources, similar techniques may be successfully applied to large volcanic jets in the future.

Frequency response and design parameters for differential microbarometers

The study of infrasound is experiencing a renaissance since it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty. Source identification is one of the main topics of research which involves detailed knowledge on the source time function, the atmosphere as medium of propagation, and the measurement system. Applications are also foreseen in using infrasound as passive probe for the upper atmosphere, taking the field beyond its monitoring application. Infrasound can be conveniently measured with differential microbarometers. An accurate description of the instrument response is an essential need to be able to attribute the recorded infrasound to a certain source or atmospheric properties. In this article, a detailed treatment is given of the response of a differential microbarometer to acoustic signals. After an historical introduction, a basic model for the frequency response is derived with its corresponding poles and zeros. The results are explained using electric analogs. In addition, thermal conduction is added to the model in order to capture the transition between adiabatic and isothermal behavior. Also discussed are high-frequency effects and the effect of external temperature variations. Eventually, the design parameters for differential microbarometers are derived.

A climatology of infrasound detections in northern Norway at the experimental ARCI array

The study of infrasound is experiencing a renaissance in recent years since it was chosen as a verification technique for the Comprehensive Nuclear-Test-Ban Treaty. Currently, 60 infrasound arrays are being installed to monitor the atmosphere for nuclear tests as part of the International Monitoring System (IMS). The number of non-IMS arrays also increases worldwide. The experimental ARCES infrasound array (ARCI) is an example of such an initiative. The detectability of infrasound differs for each array and is a function of the array location and configuration, the state of the atmosphere, and the presence of natural and anthropogenic sources. In this study, a year of infrasound data is analyzed as recorded by ARCI. Contributions of the atmosphere and the sources are evaluated in both a low- (0.1–1.0 Hz) and high-frequency (1.0–7.0 Hz) pass-band. The enormous number of detections in the low-frequency band is explained in terms of the stratospheric wind and ocean wave activity and compared with the detection of microseism. Understanding the detectability in the low-frequency band is of utmost importance for successfully applying infrasound as a verification technique since small-sized nuclear test will show up in this frequency range.

Infrasonic signature of the 2009 major sudden stratospheric warming

The study of infrasound is experiencing a renaissance since it was chosen as a verification technique for the Comprehensive Nuclear‐Test‐Ban Treaty. The success of the verification technique strongly depends on knowledge of upper atmospheric processes. The ability of infrasound to probe the upper atmosphere starts to be exploited, taking the field beyond its monitoring application. Processes in the stratosphere couple to the troposphere and influence our daily weather and climate. Infrasound delivers actual observations on the state of the stratosphere with a high spatial and temporal resolution. Here we show the infrasonic signature, passively obtained, of a drastic change in the stratosphere due to the major sudden stratospheric warming (SSW) of January 2009. With this study, we infer the enormous capacity of infrasound in acoustic remote sensing of stratospheric processes on a global scale with surface based instruments.

Infrasound studies of hurricanes.

Hurricanes, whether in the Gulf of Mexico or the open ocean, are known to produce infrasound in the microbarom band (around 0.2 Hz). Infrasound technology is an excellent choice for monitoring hurricanes because portable, sensitive, and relatively low‐cost infrasound arrays can be deployed quickly in areas out of danger from storm winds and waves while still monitoring storm‐related signals continuously. The National Center for Physical Acoustics is developing a Gulf Coast hurricane monitoring system involving permanent and portable infrasound arrays. This discussion will focus the results of infrasound studies of the 2008 hurricane season, along with background information on the history and theory of hurricane microbaroms. The latest results of theoretical calculations about the microbarom source and possible effects of storm winds on bearings will also be presented.

Listening to the secret sounds of Earth's atmosphere

A new global network is breathing life into a dormant branch of geophysics. The study of infrasound, or long‐period acoustic signals in the atmosphere was bustling in the 1950s and 1960s. Prior to 1963, almost all nuclear tests occurred in the atmosphere. After 1963, the USSR and U.S. signed the Limited Test Ban Treaty (LTBT), which eliminated all atmospheric nuclear tests. During the era of atmospheric nuclear testing, infrasound research was in demand, since the massive explosions produced strong, long‐period acoustic waves that were globally observed and could be used to locate and describe the nuclear tests. Interest in this branch of geophysics waned with the end of atmospheric testing.

An infrasound experiment system for industrial hygiene.

An infrasound experiment system has been constructed in order to investigate the effects of infrasound on human health from the point of view of industrial hygiene and to establish a method to assess it in working environments. Some measurements were carried out to evaluate the acoustic performance of the system in the low frequency sound range, including infrasound, and to clarify the points to be improved. As a result, it was found that the system had some weak points in the audible frequency range. They were considered to be caused by the large capacity of the test chamber, and some countermeasures were required. On the other hand, in the infrasound range, it appeared that the higher harmonics of infrasound possibly affected the experiments and some limitations might be found necessary. But the frequency response of the chamber and the spatial uniformity of the reproduced infrasound proved to be satisfactory for the experiments in this range. Consequently it was concluded that the system could contribute to the study of infrasound.

Analytical Review of the Electromechanical Piston and the Modified Moore Variometer Used in Infrasound Studies

The electromechanical system, which is an integral part of the infrasonic calibration and testing facility at the Institute of Geophysics and Planetary Physics at the University of California, has been studied analytically. An equation taking into account the electrical and mechanical parts as a closed system has been formulated, and the resulting frequency response and sensitivity curves compared with those obtained experimentally. A similar study was conducted on the fluid dynamics involved with the infrasonic sensor (the modified Moore Variometer) used in our infrasound studies. The results yielded a better understanding of capabilities of this device and verified its evaluation. The calibration facility itself has the capability of testing and evaluating the behavior of any infrasonic sensor in the frequency range of 1/1000–1000 cps, and pressure variations of 10−2–103 μbar, in the ambient atmosphere. The facility has also the capability of testing and calibrating the infrasonic sensors under high-altitude conditions, enabling the simulation of atmospheric density up to 90 km and of varied temperature conditions.