Infrasound Propagation Research Articles

Experiments in a Large Boundary Layer Wind Tunnel: Propagation of Noise through the Turbulent Boundary Layer

A series of acoustic propagation experiments was conducted in the University of Florida Boundary Layer Wind Tunnel (UFBLWT) to investigate tornadic infrasound propagation in the turbulent atmosphere. A synthetic acoustic wave was generated within a boundary layer wind field using a speaker, and distorted by the surrounding turbulent atmosphere as it propagated along the UFBLWT fetch where it was measured by an acoustic microphone 22 m downwind of the speaker. The wind tunnel operating speed and the configuration of the mechanical turbulence generating roughness elements were varied between experiments to alter the turbulence characteristics and thus the distortion of the acoustic wave. The wind field turbulence data were obtained by three Cobra probes mounted on an actuated and automated gantry. The turbulence-induced distortion of the acoustic wave was captured by analyzing the signal at its source and as-measured downwind. Data were characterized, organized, and curated into four stages.

Infrasound propagation through the atmosphere with mesoscale wind velocity and temperature fluctuations

Some results on modeling and observation of infrasound propagation in the atmosphere in a presence of mesoscale and anisotropic wind velocity and temperature fluctuations are presented. The theoretical model of infrasound scattering from anisotropic wind velocity and temperature inhomogeneities of the atmosphere is developed. With this model, the appearance of the stratospheric, mesospheric, and thermospheric arrivals of the infrasound signals in the acoustic shadow zones is explained. The analytic relation between the wave field of the scattered infrasound signal and the vertical profile of the effective sound speed fluctuations is obtained. Using this relation, the vertical profiles of the fluctuations within the upper stratosphere (25–55 km) and the lower thermosphere (105–140 km) were retrieved from the waveforms and travel times of the signals recorded in the acoustic shadow zones. The theoretical frequency spectrum of the infrasound wavefield reflected from the fine-scale layered structure of the atmosphere is obtained and compared with the spectra of the observed stratospheric arrivals.

On the possibility of electrostatically generated infrasound during thunderstorms

Acoustic emissions from lightning discharges are usually attributed to two mechanisms. The audible part of their spectrum mainly results from the shock wave generated by the heating of the lightning channel. On the other hand, the infrasonic component is associated with the conversion to sound of the electrostatic energy stored in the thundercloud. While there is a broad scientific consensus on the former mechanism, the latter process is still controversial. The electrostatic mechanism was proposed by C. T. R. Wilson in 1921 and can be summarized as follows. Due to the electrostatic repulsion of the charged particles, the pressure within a thundercloud is lower than outside. At discharge, the electrostatic field collapses, and the sudden air volume contraction produces an acoustic pulse. This work examines the existing theoretical models of the electrostatic mechanism with the data observed for thundercloud dimensions, charge densities, and total charges. Our results show that, although possible, this mechanism, as currently described, cannot explain observations. Our findings might support the hypothesis that the heating of the lightning channel is responsible for the generation of both infrasound and audible sound. However, a definitive answer remains precluded and requires a better understanding of cloud formation and electrification processes.

How does knowledge of acoustics guide the parameterizations of gravity waves?

Describing the statistics of gravity wave (GW) fields represents a major motivation for both current research on atmospheric GWs and long-range infrasound propagation. In practice, the probability density functions (PDF) of the momentum fluxes are estimated combining observations, numerical modelling, and theory. Numerical models (such as WRF) show that the PDFs vary in a robust way relative to the background local wind speed. For non-orographic GWs, these PDFs are approximated as lognormal distributions, with characteristics found to depend on the background wind speed. Studies show that some trends are not observed using a state-of-the-art stochastic parameterization of GWs, unless the phase velocities of GW sources (essentially tropospheric) are dramatically changed. As the vertical wavelength and the phase velocity are related to each other, such changes also affect the interaction between infrasound and lower-stratospheric GWs. Consequently, significant efforts have been made to use ground-based acoustic sensors for characterizing the GW sources, including the use of neural networks. This approach provides a promising way to describe the statistics of GW sources from ground-truth infrasound events and an additional constraint to tune stochastic parameterizations of GWs.

Microbarometer measurements to probe turbulent kinetic energy in the atmospheric boundary layer

Microbarometer networks are typically in place for the detection of atmospheric infrasound waves. The monitoring of such waves can be of interest for source characterization, which include a wide array of geophysical and man-made sources, as well as atmospheric infrasound propagation studies. In such studies, the presence of turbulence and other wind-induced effects are considered a nuisance. For this reason, wind noise filters are typically in place for suppression. As Cuxart et al. [BLM (2013)] pointed out, microbarometer measurements indeed have a value for detection of local and remote turbulence and measurements can be used to estimate turbulent kinetic energy (TKE). In this study, we compare microbarometer observations at the Cabauw atmospheric research site to co-located in-situ TKE measurements. Moreover, we compare turbulence timeseries to predictions from the high-resolution HARMONIE weather model in which this quantity is parameterized. This approach has two foreseen applications: (1) modeled TKE fields could possibly help in identifying regions that are most appropriate for infrasound monitoring due to low TKE values and (2) microbarometer observations could possibly be of use in the further refining of the modeled TKE parameter.

Topographically Scattered Infrasound Waves Observed on Microbarometer Arrays in the Lower Stratosphere

AbstractWhen an acoustic wave strikes a topographic feature, some of its energy is scattered. Sensors on the ground cannot capture these scattered signals when they propagate at high angles. We report observations of upwardly‐scattered acoustic waves prior to refraction back to the ground, intercepting them with a set of balloon‐borne infrasound microbarometers in the lower stratosphere over northern Sweden. We show that these scattered waves generate a coda whose presence can be related to topography beneath balloons and low‐altitude acoustic ducts. The inclination of the coda signals changes systematically with time, as expected from waves arriving from scatterers successively closer to receivers. The codas are present when a temperature inversion channels infrasound from a set of ground chemical explosions along the ground, but are absent following the inversion's dissipation. Since scattering partitions energy away from the main arrival, these observations imply a mechanism of amplitude loss that had previously been inaccessible to measurement. As such, these data and results allow for a better comprehension of interactions between atmospheric infrasound propagation and the solid earth.

Evidence for near-source nonlinear propagation of volcano infrasound from Strombolian explosions at Yasur Volcano, Vanuatu

Volcanic eruption source parameters may be estimated from acoustic pressure recordings dominant at infrasonic frequencies (< 20 Hz), yet uncertainties may be high due in part to poorly understood propagation dynamics. Linear acoustic propagation of volcano infrasound is commonly assumed, but nonlinear processes such as wave steepening may distort waveforms and obscure the sourcing process in recorded waveforms. Here we use a previously developed frequency-domain nonlinearity indicator to quantify spectral changes due to nonlinear propagation primarily in 80 signals from explosions at Yasur Volcano, Vanuatu. We find evidence for le 10−3 dB/m spectral energy transfer in the band 3–9 Hz for signals with amplitude on the order of several hundred Pa at 200–400 m range. The clarity of the nonlinear spectral signature increases with waveform amplitude, suggesting stronger nonlinear changes for greater source pressures. We observe similar results in application to synthetics generated through finite-difference wavefield simulations of nonlinear propagation, although limitations of the model complicate direct comparison to the observations. Our results provide quantitative evidence for nonlinear propagation that confirms previous interpretations made on the basis of qualitative observations of asymmetric waveforms.

Evidence for Short Temporal Atmospheric Variations Observed by Infrasonic Signals: 1. The Troposphere

AbstractInfrasound monitoring is used in the forensic analysis of events, studying the physical processes of sources of interest, and probing the atmosphere. The dynamical nature of the atmosphere and the use of infrasound as a forensic tool lead to the following questions; (1) what is the timescale of atmospheric variability that affects infrasonic signals? (2) how do infrasound signals vary as a function of time? This study addresses these questions by monitoring a repetitive infrasound source and its corresponding tropospheric returns 54 km away. Source‐receiver empirical Green's functions are obtained every 20 s and used to demonstrate the effect of atmospheric temporal variability on infrasound propagation. In addition, observations are compared to predicted simulated signals based on realistic atmospheric conditions. Based on 127 events over 3 days, it is shown that infrasound properties change within tens of seconds. Particularly, phases can appear and disappear, the propagation time varies, and the signals' energy fluctuates. Such variations are attributed to changes in temperatures and winds. Furthermore, atmospheric models can partly explain the observed changes. Therefore, this study highlights the potential of high temporal infrasound‐based atmospheric sounding.

Investigation of near-surface chemical explosions effects using seismo-acoustic and synthetic aperture radar analyses

Chemical explosions are ground truth events that provide data, which, in turn, can enhance the understanding of wave propagation, damage assessment, and yield estimation. On 4 August 2020, Beirut, Lebanon was shocked by a catastrophic explosion, which caused devastating damage to the Mediterranean city. A second strong chemical explosion took place at the Xiangshui, China chemical plant on 21 March 2019. Both events generated shock waves that transitioned to infrasound waves, seismic waves, as well as hydroacoustic signals with accompanying T-phases in the case of the Beirut event. In this work, the seismo-acoustic signatures, yields, and associated damage of the two events are investigated. The differentiainterferometry synthetic aperture radar analysis quantified the surface damage and the estimated yield range, equivalent to 2,4,6-trinitrotoluene [C7H5(NO2)3] (TNT), through a "boom" relation of the peak overpressure was evaluated. Infrasound propagation modeling identified a strong duct in the stratosphere with the propagation to the west in the case of the Beirut-Port explosion. In the case of the Xiangshui explosion, the modeling supports the tropospheric propagation toward the Kochi University of Technology (KUT) sensor network in Japan. Although the Beirut yield (202-270 ± 100 tons) was slightly larger than the Xiangshui yield (201 ± 83.5 tons), the near-source damage areas are almost the same based on the distribution of damaged buildings surrounding the explosions.

Trends in volcano seismology: 2010 to 2020 and beyond

Volcano seismology has been fundamental to our current understanding of crustal magma migration and eruption. The increasing availability of portable seismic networks with the creative use of seismic sources and ambient noise has led to a better understanding of the volcanic structure of many volcanoes and is producing increasingly detailed images of the volcanic subsurface. The past decade (2010-2020) has seen advances in our understanding of seismic sources under and surrounding volcanoes through precise locations, and through analysis of source mechanisms from seismic signals that are more varied and smaller in magnitude, reaching beyond traditional techniques. In tandem with continued research on fundamental physics-based understanding of volcano-seismic sources, new advances in computational analyses including machine learning methods will push our understanding of volcanic processes into the future. Incorporation of multidisciplinary geophysical observations (especially infrasound) has become commonplace, and our understanding of infrasound propagation and sources will feed back into our ability to monitor ongoing eruptions and surficial mass movements. Open-source codes will permit widespread evaluation and adoption of new methodologies for volcano-seismic analysis and inversion. Combined with quantitative and conceptual source models using improved structural constraints, these new methodologies will better characterize the range of volcano-seismic signal evolution scenarios and hold promise for creating better short-term forecasts. Finally, permanent instrumentation is available on an expanding range of volcanoes, and open data policies are increasingly making these data available to the scientific community in near real time.

Spectral Element Modeling of Acoustic to Seismic Coupling Over Topography

AbstractAcoustic waves in the atmosphere are commonly recorded on seismometers as they couple into the ground. These signals, here called ground coupled airwaves, are not commonly considered in numerical modeling of infrasound propagation, which often assumes a rigid unmeshed boundary. Starting from an analytically‐tractable spherical wave model, we analyze how the coupling of an acoustic wave into a planar elastic halfspace changes over a wide range of scenarios. We use energy admittance to quantify acoustic to seismic coupling over both a planar elastic halfspace and meshed topography. Our spectral element and analytic calculations have different maxima as a function of incidence angle, with very high admittance values for near‐vertical incidence (maximum ≈78%). Energy admittance calculations at shallow incidence angles are much smaller (less than 1%). In simulations over the complex topography of Sakurajima Volcano, we attribute the variable spatial pattern of energy admittance to changes in earth parameters between each model. The observed pressure difference over the simulated 15 km region appears to be . While this value is relatively small, the cumulative addition over 100s of km and multiple acoustic bounce points may be significant. Acoustic to seismic coupling along the propagation path may bias long distance yield estimates, particularly when infrasound propagates over regions with steep topography or particularly slow seismic velocities, such as alluvial planes.

Numerical modeling of mesoscale infrasound propagation in the Arctic.

The impacts of characteristic weather events and seasonal patterns on infrasound propagation in the Arctic region are simulated numerically. The methodology utilizes wide-angle parabolic equation methods for a windy atmosphere with inputs provided by radiosonde observations and a high-resolution reanalysis of Arctic weather. The calculations involve horizontal distances up to 200 km for which interactions with the troposphere and lower stratosphere dominate. Among the events examined are two sudden stratospheric warmings, which are found to weaken upward refraction by temperature gradients while creating strongly asymmetric refraction from disturbances to the circumpolar winds. Also examined are polar low events, which are found to enhance negative temperature gradients in the troposphere and thus lead to strong upward refraction. Smaller-scale and topographically driven phenomena, such as low-level jets, katabatic winds, and surface-based temperature inversions, are found to create frequent surface-based ducting out to 100 km. The simulations suggest that horizontal variations in the atmospheric profiles, in response to changing topography and surface property transitions, such as ice boundaries, play an important role in the propagation.

Design and Experimental Study of Bellows Type Standard Infrasound Generator

Infrasound is widely used in nature and human activities, such as earthquakes, volcanic eruptions, mudslides, nuclear explosions, supersonic flight, weapons targeting, etc. Infrasound has strong penetrating ability, long propagation distance, low attenuation, and is widely used in environmental monitoring, military fields, industrial production and other fields. The monitoring of infrasound sound pressure signal mainly depends on the infrasound sensor, and the accurate measurement of infrasound sound pressure mainly depends on the calibration of the infrasound sensor, and the accurate measurement of the infrasound sensor is the premise and guarantee of the application of the infrasound sensor. In this paper, we propose a bellows type standard infrasound generator solution for the piston leakage problem of the most widely used gas cavity pressure method in the calibration of infrasound sensors, obtain the expression of sound field distribution in the bellows by theoretical derivation, and design a calibration device containing an infrasound sound pressure generator, feedback control and infrasound excitation system, data acquisition conditioning system and laser measurement calculation system. The above device was physically built and experiments were conducted on the bellows type standard infrasound generator, and the infrasound time-domain waveform maps from 0.01Hz to 20Hz were obtained. The experimental results show that the bellows type standard infrasound generator designed in this paper can generate standard infrasound sound pressure signals.

Bolide Energetics and Infrasound Propagation: Exploring the 18 December 2018 Bering Sea Event to Identify Limitations of Empirical and Numerical Models

Abstract Infrasound observations are an important tool in assessing the energetics of bolides and can help quantify the flux of meteoroids through Earth’s atmosphere. Bolides are also important atmospheric sources for assessing long-range infrasound propagation models and can be used as benchmark events for validating the International Monitoring System (IMS) infrasound network, which is designed to detect nuclear tests in the atmosphere. This article exploits unique infrasound observations from a large bolide recorded on IMS infrasound arrays and high-density infrasound deployments in the United States to assess limitations in infrasound source scaling relationships. The observations provide an unprecedented sampling of infrasound propagation along a transect at an azimuth of 60° from the source to a distance of ∼8000 km. Widely used empirical laws for assessing bolide energetics and state-of-the-art numerical models for simulating infrasound propagation are assessed to quantify important discrepancies with the observations. In particular, empirical laws for equivalent yield, which are based on signal period and are assumed to be relatively unaffected by propagation effects, can be heavily contaminated by site noise. In addition, by modeling infrasound propagation over a range of ∼8000 km, we show that state-of-the-art models do not reproduce the observed amplitude decay over this long range (which decays by a rate of at least 2 higher than can be modeled).

Three-dimensional parabolic equation for infrasound propagation above topography

The parabolic equation (PE) is one of the most popular methods for the modeling of low-frequency sound in the atmosphere. Over the past decades, considerable efforts have been dedicated to the inclusion of medium inhomogeneities, such as wind and turbulence, extending the PE to more realistic atmospheric conditions. On the other hand, few models take topography into account, while its effects on infrasound propagation are important in some cases. A new development has enabled the inclusion of irregular terrain in the narrow-angle 3DPE for a moving inhomogeneous medium [Khodr et al., JASA (2020)]. A coordinate transform similar to the Beilis-Tappert mapping used in 2D [Parakkal et al., JASA (2012)] is introduced to account for the variation in topography. The numerical solution relies on a Crank–Nicolson implicit scheme along the propagating direction. The pressure field at every step is computed with an iterative fixed-point algorithm which consists of a succession of tridiagonal systems that can be efficiently solved. A validation is performed against 3D Boundary Element simulations for propagation above a Gaussian hill in a homogeneous atmosphere. The existence of transversal scattering in the shadow zone, not represented by 2D models, is highlighted. Further developments include the extension of the method to a split-step wide-angle formulation. [This work was conducted at the University of Bristol in partnership with AWE Blacknest.]

Evidence for short temporal atmospheric (tropospheric) variations observed by infrasonic signals

Infrasound monitoring is used in the forensic analysis of events, to study the physical processes of sources of interest and to probe the atmosphere. The dynamical nature of the atmosphere and the use of infrasound as a forensic tool lead to the following questions: (1) what is the time-scale of atmospheric variability that affects infrasonic signals? (2) can we link variations of infrasound signals to specific atmospheric phenomena? This study addresses these questions by monitoring a repetitive infrasound source and its corresponding tropospheric returns 54 km away. Source-receiver empirical Green's functions are obtained every 20 s and used to demonstrate the effect of atmospheric temporal variability on infrasound propagation. In addition, observations are compared to predicted simulated signals based on realistic atmospheric conditions. It is shown that infrasound properties change within tens of seconds. Particularly, phases can appear and disappear, the propagation time decreases, and the signals' energy fluctuates. Furthermore, the similarity between the predicted and observed signals varies significantly within a few minutes. The observed changes are related to variations in temperature and wind, which are coupled to dynamic processes such as radiation, gravity waves, and turbulence. Therefore, this study highlights the potential of high temporal infrasound-based atmospheric sounding.

On cataloguing the possible waveforms for impulse propagation along ground to ground thermospheric paths

Infrasound propagation in the thermosphere is characterized by very low mean density, a very steep mean temperature gradient, and tidal winds. The steep temperature gradient means that there is always a thermospheric duct and this always provides ground to ground propagation paths that pass through the thermosphere. These paths are modulated by the atmospheric tides, while the very low density in the thermosphere causes severe attenuation and non-linear distortion. For an impulsive signal initially in the form of an N-like wave, such as the signal produced by a large explosion, non-linear propagation effects cause the positive pressure phases to propagate more rapidly than the negative pressure phases, in turn causing the duration of the impulse to increase. This effect can be quite significant in the thermosphere and continues until a caustic is encountered transforming the N-wave into a U-wave. The severe attenuation causes the high frequency components of the signal to attenuate resulting in a long period and fairly clean waveform that propagates back to the ground. The combination of steep temperature gradient and atmospheric tides produce a canonical thermospheric propagation environment with a diurnal cycle. In this presentation, it is shown that non-linear ray theory (weak shock theory) can be used to catalogue the waveform shapes of ground returns from explosive signals traveling along thermospheric paths. Qualitative comparisons to observed thermospheric returns are made.

Atmospheric parabolic equation with Galerkin discretization and boundary fitted grid for modeling infrasound propagation over topography

Studies utilizing the parabolic equation (PE) to model infrasound propagation over topography have traditionally implemented them as 2D or Nx2D PE models. These methods, to one extent or another, trade-off accuracy for enhanced computational speed. Three-dimensional parabolic equation (3DPE) models are seldom seen in atmospheric acoustic propagation studies compared to their 2D and Nx2D counterparts, despite the fact that theta coupling and proper modeling of horizontal refraction and diffraction in a 3DPE model should enable greater accuracy. In this work a 3DPE in cylindrical coordinates is developed under the alternating direction implicit scheme, with a Galerkin discretization and boundary fitted grid, such that the PE is capable of handling arbitrary and irregular topography features. Finally, 2D, Nx2D, and 3D implementations of the aforementioned PE model are examined for cases of propagation over a simple hill in homogeneous, downward refracting, and upward refracting atmospheres. Insights are drawn with regards to the accuracy at a variety of positions and ranges, and the computational speeds of the various methods.

PLetma—Parallel software package for intensive infrasound simulation

The Plateform of the Laboratory of Studies and Modeling in Acoustics (PLetma) is a suite of scientific softwares developed by the partner institutions of LETMA since 2015. Currently under development, it contains four different models, namely 3D ray-tracing, 3D non-linear one-way equation, 3D normal modes and 2D direct Navier–Stokes, as well as user interfaces for model execution and data visualization. The aim of this package is to provide a set of complementary tools for the modeling of broadband infrasound signals in a moving inhomogeneous atmosphere. It allows to cover a large panel of low-frequency propagation problems arising in geophysics or engineering. Implemented with MPI and OpenMP, the package relies on parallelism to improve computational efficiency of simulations. PLetma provides the infrastructure for incorporation of existing propagation and variability modeling capabilities. Comparisons between the four methods have been successfully undertaken on a test case [Sabatini et al., JASA (2016)]. Sensitivity analyses have been conducted to understand the effect of environmental parameters on modeling of long-range infrasound propagation. [This work was conducted within the framework of LETMA (Laboratoire ETudes et Modélisation Acoustique), a Contractual Research Laboratory shared between CEA, CNRS, Ecole Centrale de Lyon, C-Innov, and Sorbonne Université.]

Simulations of local and regional scale infrasound propagation in the Arctic

High-resolution, three-dimensional reconstructions of the Arctic atmosphere (the Arctic System Reanalysis version 2) are employed as input to wide-angle parabolic equation calculations to simulate infrasound propagation through Arctic weather conditions. The calculations involve horizontal distances out to 200 km (i.e., local and short regional scales), for which interactions with the troposphere and lower stratosphere dominate. The atmospheric phenomena examined include sudden stratospheric warmings (SSWs), strong polar lows, and boundary-layer phenomena such as low-level jets (LLJs), katabatic winds, and surface-based temperature inversions. The SSWs and polar lows are found to be opposites from an acoustic refraction standpoint: the former weaken upward refraction by temperature gradients in the troposphere, thus creating strongly asymmetric wind-dominated refraction, whereas the latter enhance negative temperature gradients in the troposphere, thus leading to prevailing upward refraction conditions. The LLJs and surface-based inversions are found to create particularly strong surface ducting conditions. Horizontal variations in the atmospheric profiles, in response to changes in topography and surface property transitions such as ice boundaries, significantly impact the propagation.