Infrasound Propagation Research Articles

Infrasound transmission in the human ear: Implications for acoustic and vestibular responses of the normal and dehiscent inner ear.

The transmission of infrasound within the human ear is not well understood. To investigate infrasound propagation through the middle and inner ear, velocities of the stapes and round window membrane were measured to very low frequencies (down to 0.9 Hz from 2000 Hz) in fresh cadaveric human specimens. Results from ear-canal sound stimulation responses show that below 200 Hz, the middle ear impedance is dominated by its stiffness term, limiting sound transmission to the inner ear. During air-conduction, normal ears have approximately equal volume velocities at the oval (stapes) and round windows, known as a two-window system. However, perturbing the impedance of the inner ear with a superior canal dehiscence (SCD), a pathological opening of the bone surrounding the semicircular canal, breaks down this simple two-window system. SCD changes the volume velocity flow in the inner ear, particularly at low frequencies. The experimental findings and model predictions in this study demonstrate that low-frequency auditory and vestibular sound transmission can be affected by a change in the inner-ear impedance due to a SCD.

Generation of Infrasound Waves at Sites of Underground Mine Collapses

AbstractIn 2015 and 2016, collapse‐induced earthquakes, accompanied by surface subsidence, occurred at two underground mines in South Korea and were followed by impulsive infrasound signals that were detectable at distances greater than 200 km. To explain the infrasound generation caused by the collapses, we hypothesize that excess pressure was released by the rapid outflow of an underground air mass into the atmosphere caused by the sudden implosion of earth into the underground space. We tested this idea with a physical model that assumes a displaced air volume as a monopole source with reduced pressure. The estimated volumes of the two collapses were ~1.7 × 105 and ~7.3 × 105 m3, respectively, which agrees with the collapse volumes inferred from surface subsidence data. The infrasound generation model we used is applicable to the monitoring of other catastrophic failures within manmade or natural subsurface cavities.

Simulating the Propagation of Infrasonic Waves and Estimating the Energy of the Chelyabinsk Meteoroid Explosion Observed on February 15, 2013

Results obtained from simulating the propagation of infrasonic waves from the Chelyabinsk meteoroid explosion observed on February 15, 2013, are given. The pseudodifferential parabolic equation (PDPE) method has been used for calculations. Data on infrasonic waves recorded at the IS31 station (Aktyubinsk, Kazakhstan), located 542.7 km from the likely location of the explosion, have been analyzed. Six infrasonic arrivals (isolated clearly defined pulse signals) were recorded. It is shown that the first “fast” arrival (F) corresponds to the propagation of infrasound in a surface acoustic waveguide. The rest of the arrivals (T1–T5) are thermospheric. The agreement between the results of calculations based on the PDPE method and experimental data is satisfactory. The energy E of the explosion has been estimated using two methods. One of these methods is based on the law of conservation of the acoustic pulse I, which is a product of the wave profile area S/2 of the signal under analysis and the distance to its source E I [kt] = 1.38 × 10–10 (I [kg/s])1.482. The other method is based on the relation between the energy of explosion and the dominant period T of recorded signal E T [kt] = 1.02 × (T [s]2/σ)3/2, where σ is the dimensionless distance determining the degree of nonlinear effects during the propagation of sound along ray trajectories. According to the data, the explosion energy E I,T ranges from 1.87 to 32 kt TNT.

Local Infrasound Variability Related to In Situ Atmospheric Observation

AbstractLocal infrasound is widely used to constrain source parameters of near‐surface events (e.g., chemical explosions and volcanic eruptions). While atmospheric conditions are critical to infrasound propagation and source parameter inversion, local atmospheric variability is often ignored by assuming homogeneous atmospheres, and their impact on the source inversion uncertainty has never been accounted for due to the lack of quantitative understanding of infrasound variability. We investigate atmospheric impacts on local infrasound propagation by repeated explosion experiments with a dense acoustic network and in situ atmospheric measurement. We perform full 3‐D waveform simulations with local atmospheric data and numerical weather forecast model to quantify atmosphere‐dependent infrasound variability and address the advantage and restriction of local weather data/numerical weather model for sound propagation simulation. Numerical simulations with stochastic atmosphere models also showed nonnegligible influence of atmospheric heterogeneity on infrasound amplitude, suggesting an important role of local turbulence.

Application of an updated atmospheric model to explore volcano infrasound propagation and detection in Alaska

Winds and temperature gradients greatly affect the long-range propagation of infrasound. The spatio-temporal variability of these parameters must therefore be accurately characterized to correctly interpret recorded infrasound at long distances, specifically to differentiate between source and propagation effects. Here we present the first results of an open source reanalysis model, termed Alaska Volcano Observatory Ground-to-Space (AVO-G2S), constructed to accurately characterize the atmosphere and model long-range infrasound propagation from volcanic eruptions in Alaska. We select a number of case studies to examine recent eruptions of Alaskan volcanoes whose ash emissions posed a threat to air traffic, including the two most recent eruptions of Pavlof Volcano and two typical explosions from Cleveland Volcano. Strong tropospheric ducting and low noise at the station during the 21 July 2015 explosion of Cleveland Volcano led to an automated detection of the explosion at an infrasound array 992 km away, whereas low signal-to-noise ratio for the 6 November 2014 Cleveland Volcano explosion helps explain the non-detection in real-time of a predicted strong stratospheric arrival. For the November 2014 Pavlof eruption, discrepancies between local seismic data and a distal infrasound array 460 km away cannot be solely explained by changes in atmospheric conditions, though some features of the complex propagation predictions follow the trends in long-range infrasound signals. The most recent eruption of Pavlof Volcano in March 2016 shows minimal changes in propagation conditions throughout the eruption and therefore indicates that the signals detected at long-range primarily reflect source processes. These results show how detailed examination of the acoustic propagation conditions provides insight into detection capability and eruption dynamics. Future work will implement AVO-G2S and high-resolution long-range infrasound propagation modeling in real-time for Alaskan volcanoes of interest.

Determining essential scales and associated uncertainties for regional atmospheric infrasound propagation by incorporating three-dimensional weather model forecasts

Understanding infrasound propagation is important for geophysical and military applications. Infrasound signatures can be detected from larger sources such as nuclear detonations and from smaller sources such as bridges, dams, and buildings. Infrastructure sources produce signals of lower amplitude, leading to more regional (up to 150 km) propagation. However, current methods for calculating regional infrasound propagation involve assumptions about the atmosphere, such as horizontal homogeneity, that deviate from more realistic environmental conditions and decrease the accuracy of the infrasound predictions. To remedy this issue, we have interfaced three-dimensional forecasts from the Weather Research and Forecasting (WRF) meteorological model with range-dependent parabolic equation propagation models. To test the improvement of infrasound propagation predictions with more realistic weather data, we conducted sensitivity studies with different propagation ranges and horizontal resolutions and compared the...

Realistic terrain boundary conditions for the three-dimensional finite-difference parabolic equation method applied to infrasonic propagation

Infrasound propagation in the atmosphere can be significantly influenced by environmental parameters such as meteorology and topography. An accurate modelling of these parameters and their effects can improve signal processing of infrasound arrivals and development of more accurate prediction tools. Over the last few decades, much attention has been given to the integration of complex atmospheric features, namely, absorption, turbulence, density, and refraction, into the numerical methods, while topographic effects have usually been neglected. In this respect, the Parabolic Equation (PE) method has been continuously improved to deal with complex configurations encountered in various applications, such as atmospheric waveguides, ocean acoustics and electromagnetics. We have further extended the Beilis-Tappert (BTPE) generalized coordinate transform to three-dimensional propagation in order to account for out-of-plane scattering by irregular three-dimensional topography, in the presence of air density varia...

A Green’s function parabolic equation description of infrasound propagation in an inhomogeneous and moving atmosphere

Accurate atmospheric infrasound propagation models must account for variations in local sound speed and ambient flow. This paper describes and demonstrates a method to extend the Green’s function parabolic equation (GFPE) to account for 3D high Mach number ambient flow in addition to an inhomogeneous atmosphere. Predictions of infrasonic propagation using the resulting GFPE model are then compared with predictions using other models.

Reflections on Los Alamos infrasound research

Researchers at Los Alamos National Laboratory began work in infrasound research in 1982 under Department of Energy funding. This beginning was to investigate the generation and propagation of surface ground motion generated atmospheric infrasound signals. Infrasound arrays were operated to measure the infrasound signals at near regional distances. The program was successful and continued until the end of US underground nuclear testing in 1992. During this time, data were collected on earthquakes, bolides and on various man-made high explosive tests. As work on the Comprehensive Nuclear Test Ban Treaty was continuing, Los Alamos, working with Sandia National Laboratory, fielded a prototype infrasound array following Comprehensive Nuclear Test Ban Treaty guidelines, which operated from mid 1997 to 2007. More recent work emphasizes seismo-acoustic research in explosion monitoring, improving infrasound propagation models for better source location and analysis and the development of Python based tools. In this talk, I will highlight some aspects of the earlier and current Los Alamos activity.Researchers at Los Alamos National Laboratory began work in infrasound research in 1982 under Department of Energy funding. This beginning was to investigate the generation and propagation of surface ground motion generated atmospheric infrasound signals. Infrasound arrays were operated to measure the infrasound signals at near regional distances. The program was successful and continued until the end of US underground nuclear testing in 1992. During this time, data were collected on earthquakes, bolides and on various man-made high explosive tests. As work on the Comprehensive Nuclear Test Ban Treaty was continuing, Los Alamos, working with Sandia National Laboratory, fielded a prototype infrasound array following Comprehensive Nuclear Test Ban Treaty guidelines, which operated from mid 1997 to 2007. More recent work emphasizes seismo-acoustic research in explosion monitoring, improving infrasound propagation models for better source location and analysis and the development of Python based tools. In thi...

An investigation of infrasound propagation over mountain ranges.

Linear theory is used to analyze trapping of infrasound within the lower tropospheric waveguide during propagation above a mountain range. Atmospheric flow produced by the mountains is predicted by a nonlinear mountain gravity wave model. For the infrasound component, this paper solves the wave equation under the effective sound speed approximation using both a finite difference method and a Wentzel-Kramers-Brillouin approach. It is shown that in realistic configurations, the mountain waves can deeply perturb the low-level waveguide, which leads to significant acoustic dispersion. To interpret these results, each acoustic mode is tracked separately as the horizontal distance increases. It is shown that during statically stable situations, situations that are common during night over land in winter, the mountain waves induce a strong Foehn effect downstream, which shrinks the waveguide significantly. This yields a new form of infrasound absorption that can largely outweigh the direct effect the mountain induces on the low-level waveguide. For the opposite case, when the low-level flow is less statically stable (situations that are more common during day in summer), mountain wave dynamics do not produce dramatic responses downstream. It may even favor the passage of infrasound and mitigate the direct effect of the obstacle.

Visualized localization and tracking of debris flow movement based on infrasound monitoring

During the initiation and movement of debris flow, low frequency infrasound is generated and emitted with characteristics such as strong penetration, low attenuation, and a high propagation speed that is faster than the debris flow. Monitoring infrasound has been used in debris flow detection and early warning systems; however, although the current infrasound-based warning system of debris flow can provide an alarm when debris flow occurs, it cannot locate and track the movement of debris flow in real time, and cannot determine the arrival time of debris flow to its potential victims. Thus, the potential applications of low frequency infrasound in debris flow disaster prevention and reduction have not yet been fully explored. In this study, we constructed an optimal triangular monitoring array based on factors such as the terrain of the monitoring area and characteristics of infrasound propagation. Combined with the time-delay estimation method, we determined and established an acoustic source localization model based on the time difference of arrival. Furthermore, with the help of a GIS platform, a visualized localization and tracking system for debris flow movement was developed to achieve real-time monitoring of debris flow. The performance of the system on acoustic source localization was validated using a simulation test and long-term field monitoring of debris flow at Jiangjia gully in Dongchuan, Yunnan province, China. Both the simulation and field test results showed that the system has high localization accuracy and strong real-time response. The results of the monitoring system could provide more accurate warning information of debris flow to local government and residents, allowing them to take appropriate mitigation measures well in advance to reduce the loss of life and property caused by debris flows.

Infrasound propagation in tropospheric ducts and acoustic shadow zones.

Numerical computations of the Navier-Stokes equations governing acoustic propagation are performed to investigate infrasound propagation in the troposphere and into acoustic shadow zones. An existing nonlinear finite-difference, time-domain (FDTD) solver that constrains input sound speed models to be axisymmetric is expanded to allow for advection and rigid, stair-step topography. The FDTD solver permits realistic computations along a given azimuth. It is applied to several environmental models to examine the effects of nonlinearity, topography, advection, and two-dimensional (2D) variations in wind and sound speeds on the penetration of infrasound into shadow zones. Synthesized waveforms are compared to a recording of a rocket motor fuel elimination event at the Utah Test and Training Range. Results show good agreement in the amplitude, duration, and spectra of synthesized and recorded waveforms for propagation through 2D atmospheric models whether or not topography, advection, or nonlinearity is explicitly included. However, infrasound propagation through a one-dimensional, range-averaged, atmospheric model yields waveforms with lower amplitudes and frequencies, suggesting that small-scale atmospheric variability causes significant scatter within the troposphere, leading to enhanced infrasound penetration into shadow zones. Thus, unresolved fine-scale atmospheric dynamics are not required to explain infrasound propagation into shadow zones.

Infrasound phenomenology, propagation, and detection

Infrasound is generally understood to refer to acoustic disturbances in the atmosphere in frequency bands below the threshold of human hearing, but above the frequencies at which internal gravity waves propagate. The infrasonic band is nominally taken to be 0.05 Hz up to about 20 Hz. Infrasonic signals tend to be generated by large, violent events and propagate efficiently in sound ducts formed by wind jets and temperature gradients in the middle and upper atmosphere. Infrasonic signals can be detected at very large distances from the source, sometimes even globally. This tutorial will present an overview of the generation of infrasound by both natural and anthropomorphic sources and of the subsequent propagation and detection of infrasonic signals. Two particular sources will be discussed in detail: the signal generated by a large explosion and the so-called microbarom signal generated by colliding ocean waves. Signal propagation through the atmosphere will then be discussed. Available open source infrasound propagation packages and their use will be introduced, the significant atmospheric sound ducts will be identified, and the difficulties inherent in modeling propagation through a dynamic atmosphere will be emphasized. The current state and availability of atmospheric specification will be touched upon. Finally, the use of array processing for the extraction of infrasonic signals from the pressure fluctuations inherent to a turbulent atmosphere will be discussed.

Effects of simplified atmospheric profiles on short range infrasound propagation

Meteorological profiles strongly influence the propagation of infrasound signals. Much of the infrasound literature has focused on long-range (>250 km) propagation; however, there is interest in short-range (<150 km) propagation from an infrastructure monitoring standpoint. Therefore, understanding the effects of meteorological profiles is crucial. This paper focuses on simplified vertical temperature and wind profiles up to 20 km altitude and their effect on an infrasonic signal emanating from an arbitrary point source. A wide-angle, finite-element PE model that correctly handles discontinuities in wavenumber is used for calculating transmission loss. A large number of simulations were performed to investigate the effect that temperature and wind profiles in different layers of the atmosphere have on surface transmission loss and to assess the sensitivity of varying these profiles. A discussion of the results of these simulations are presented as well as an overview of temperature profiles used in this study.

Influence of low-altitude meteorological conditions on local infrasound propagation investigated by 3-D full-waveform modeling

Influence of low-altitude meteorological conditions on local infrasound propagation investigated by 3-D full-waveform modeling

Infrasound propagation in the atmosphere with mesoscale fluctuations induced by internal gravity waves

The influence of the mesoscale wind velocity and temperature fluctuations induced by internal gravity waves (IGWs) on infrasound propagation in the atmosphere is studied. The statistical characteristics of the fluctuations in the parameters of infrasonic signals (such as variances and temporal spectra of the fluctuations in travel time and angle of arrival, amplitude and time duration) caused by gravity wave-associated fluctuations are studied based on the nonlinear model of shaping of the 3-D spatial spectrum of the fluctuations. The nonlinear shaping mechanism for the 3-D spectrum is associated with both the non-resonance interactions between IGWs and wave breaking processes caused by the wave-induced shear or convective instabilities. The 1-D wave number spectra (vertical and horizontal) of the mesoscale fluctuations obtained from the 3-D model are compared with the observed spectra derived from the radar, lidar and airplane temperature and wind measurements in the middle atmosphere. The results of theory and numerical modeling of infrasound scattering from gravity wave-associated fluctuations are presented. The vertical profiles of the wind velocity fluctuations in the stratosphere and lower thermosphere up to the altitudes of 130 km are retrieved from the infrasound scattered from the mesoscale wind velocity and temperature fluctuations. The results of acoustic probing of the stably stratified atmospheric boundary layer using detonation source of acoustic pulses are discussed.

The study of sudden stratospheric warmings using infrasound

Infrasound has a long history in monitoring SSWs. Several pioneering studies have focused on the various effects of a major warming on the propagation of infrasound, described throughout this chapter. A clear transition can be denoted from observing anomalous signatures towards the use of these signals to study anomalies in upper atmospheric specifications. First studies describe the various infrasonic signatures of a major warming. In general, the significant change in observed infrasound characteristics correspond to summer-like conditions in midwinter. More subtle changes are denoted during a minor warming, recognizable by the presence of a bidirectional stratospheric duct. A combined analysis of all signal characteristic unravels the general stratospheric structure throughout the life cycle of the warming. From then on, infrasound observations are used to evaluate the state of the atmosphere as represented by various NWP models. A new methodology, comparing regional volcano infrasound with simulations...

Mapping nonlinear infrasound penetration into a shadow zone: Results from rocket motor blasts at the Utah Test and Training Range

Each summer, large-scale detonations are carried out at the Utah Test and Training Range (UTTR) west of Salt Lake City. In 2016, acoustic sensors were placed at ranges up to 90 km east of the UTTR test site. The frequencies of the recorded signals indicate that they are infrasonic waves. The travel times imply that they are direct arrivals although the sensors lie within a shadow zone. Finally, the signal amplitudes indicate that acoustic propagation is nonlinear near the source. Consequently, numerical simulations that account for diffraction, and the effects of strong shocks are required to accurately map infrasound propagation into this region. A finite difference time-domain infrasound propagation method is applied to these signals. The algorithm relies on the assumption that the environmental model is azimuthally symmetric about the source location, allowing for efficient numerical computation of acoustic propagation from a spherical source. For each detonation, numerical computations are performed along a series of azimuths from the shot position, using accurate weather and topography along each path. The results show that infrasound penetration into the shadow zone is accurately predicted. The synthesized over-pressures are positive correlated with observed pressure amplitudes, to an accuracy of 8 dB.

Local infrasound propagation in three dimensions simulated with in situ atmospheric measurements

Local infrasound propagation is influenced by atmospheric conditions. The vertical gradients of local ambient temperatures and winds can alter the effective sound speed profiles in the atmosphere and dramatically change the focusing and defocusing behaviors of acoustic waves at local distances. Accurate prediction of local infrasound amplitude is critical to estimating explosion energies of natural and/or man-made explosions, and physics-based numerical simulation that accounts for three-dimensional propagation effects should be required for that purpose. The accuracy of a numerical modeling is, however, often compromised by the uncertainty of atmospheric parameters that are used for the modeling. In this study, we investigate the impacts of local atmospheric conditions on infrasound propagation using the data from chemical explosion experiments. In situ atmospheric conditions during the experiments are measured by a combination of (1) local radiosonde soundings, (2) Atmospheric Sounder Spectrometer for Infrared Spectral Technology (ASSIST), (3) surface weather stations, and (4) a wind LIDAR profiler, which can complement atmospheric profiles for numerical simulations and capture local atmospheric variability. We simulate three-dimensional local infrasound propagation using a finite-difference method with the local atmospheric measurements, and the accuracy of the numerical simulations are evaluated by the comparison with the field observations.

Estimating the effects of an ellipsoidal earth and topography on infrasonic propagation

Infrasonic signals often propagate significant horizontal distances so that predictions obtained using a flat ground approximation can introduce inaccuracies. Simulations of propagation in an atmospheric layer around a spherical globe have shown non-negligible deviations from flat ground predictions for both arrival locations and propagation times. Simulation predictions for flat-ground and spherical earth models will be discussed and the additional challenges of implementing a non-spherical globe model and the inclusion of topography will be discussed using the approximation of geometric acoustics. A non-spherical globe model, such as the WGS84 ellipsoid, is found to produce range dependence in the propagation medium, even in the case that a stratified local atmosphere is assumed. Further, although scattering and diffraction effects are not included in the geometric limit, variations in the ground surface level can be included in ray path computation to more accurately model propagation of infrasound. Propagation effects will be detailed in the case of a tropospheric waveguides for which interaction with the ground surface is significant as well as the case that source and receiver locations have differences in elevation.