Long-range Infrasound Propagation Research Articles

Direct numerical simulations of long-range infrasound propagation: Implications for source spectra estimation.

The evolution of observed dominant frequencies from a high-intensity infrasonic pulse with receiver range and stratospheric temperature is investigated using direct numerical simulations of the two-dimensional unsteady compressible Navier-Stokes equations. There is a high level of uncertainty in estimating source dominant frequencies based on received signals at sparse points on the ground. Nonlinear propagation effects in the ground-level thermospheric arrivals are found to significantly alter dominant frequency measurements compared to stratospheric arrivals with smaller amplitude sources. With a larger amplitude source, variations in observations are minimized as a result of nonlinear effects being ubiquitous across all atmospheric components of received signals but have a greater offset to the source dominant frequency. An approach to determine the source dominant frequency and minimize atmospheric variability is presented by calculating a source-to-receiver spectral transfer function averaged across the atmospheric states. This method reduces atmospheric variability in source frequency estimates within the pseudo-linear propagation regime and the average error to the known source frequency with a large amplitude source. The reduction of errors in source frequency estimates demonstrates the feasibility of using remote infrasound measurements as an indicator of source frequency and, in turn, the explosive yield of clandestine nuclear weapon test explosions.

A simplified semi-empirical model for long-range low-frequency noise propagation in the turbulent atmosphere

We present a semi-empirical long-range low-frequency acoustic propagation model, which accounts for atmospheric turbulence. Ostashev and Wilson’s scattering model is combined with a ray-theory based refraction model to account for turbulent scattering and refraction via a turbulent absorption coefficient. The coefficient is ascertained via integration of scattered energy. The model is formulated, calibrated, and validated via corresponding experiments conducted within the National Science Foundation Boundary Layer Wind Tunnel. The predictions of the newly proposed ‘bridging model’ match the wind tunnel experimental data with an average error of 11.9%. Example predictions are shown to quantify the effect of turbulent kinetic energy and turbulent integral length scale on long-range infrasound propagation. To demonstrate the approach, we present predictions of the propagation of noise from a tornado and a nonlinear wave.

The State of the Stratosphere Throughout the Seasons: How Well Can Atmospheric Models Explain Infrasound Observations at Regional Distances?

The stratosphere is the atmospheric layer with the strongest impact on long-range infrasound propagation. Natural and anthropogenic infrasound signals are efficiently ducted between refraction altitudes of 30 to 60 km and reflections on the ground and are thus propagated to infrasound receivers over long distances. The direction of favorable stratospheric ducting depends on the state of the atmosphere, primarily driven by the seasonal variation of stratospheric winds. This study uses a dataset of ground-truth infrasound events over two decades and all seasons to assess the station detectability and atmospheric model performance to correctly estimate according station observations and propagation conditions. From 2000 to 2019, the German Aerospace Center facility in Lampoldshausen has conducted ignition tests of the Ariane 5 main rocket engine. Out of the 159 engine tests considered in this study, 71 were observed at the infrasound array IS26 in the Bavarian forest, located eastward at 320 km distance. Observations were mostly made during wintertime, whereas reversed stratospheric wind patterns during summertime inhibited signal detections. A significant portion of wintertime non-detections however corresponded to stratospheric profiles that should enable signal observations. Using European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model analyses and infrasound ray tracing only two-thirds of the non-detections could be explained by the existence of a near-station acoustic shadow zone. It must thus be concluded that the applied atmospheric model is more often than expected unable to correctly explain infrasound propagation and observation at regional distances.

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.

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).

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.]

Prediction of long-range infrasound propagation from tornadoes based on new atmospheric boundary layer wind tunnel experiments

Tornadoes generate infrasonic noise that propagates over long distances. NOAA is developing tornado early warning systems via infrasound microphone arrays. Accurate detection must be coupled with a propagation algorithm that accounts for infrasound distortion due to the effects of atmospheric turbulence. Numerical solutions of the generalized Burgers' equation and the acoustic ray equations are used to predict changes to infrasonic tornado noise within the turbulent atmosphere. The effects of nonlinearity, refraction, attenuation, dispersion, and scattering are captured by the solver. We define a new parameter to model the alteration of infrasound that depends on atmospheric turbulence. Turbulent kinetic energy, integral length scale, integral time scale, and local speed of sound are arguments of the model and vary along each ray path. A series of tests are designed and conducted at the University of Florida Boundary Layer Wind Tunnel to calibrate and validate the model at scale. Predictions show satisfactory agreement with measurements with varying wind speed, turbulence intensity, humidity, and temperature. Finally, we present a parametric study for propagation of sinusoidal signals at 90 dB and 7 Hz in regions of the United States using realistic weather.

Probabilistic inversion for submerged source depth and strength from infrasound observations.

In seismology, the depth of a near-surface source is hard to estimate in the absence of local stations. The depth-yield trade-off leads to significant uncertainties in the source's depth and strength estimations. Long-range infrasound propagation from an underwater or underground source is very sensitive to variations in the source's depth and strength. This characteristic is employed in an infrasound based inversion for the submerged source parameters. First, a Bayesian inversion scheme is tested under the variations of the number of stations, the signal's frequency band, and the signal-to-noise ratio (SNR). Second, an ensemble of realistic perturbed atmospheric profiles is used to investigate the effect of atmospheric uncertainties on the inversion results. Results show that long-range infrasound signals can be used to estimate the depth and strength of an underwater source. Using a broadband signal proved to be a fundamental element to obtain the real source parameters, whereas the SNR was secondary. Multiple station inversions perform better than one-station inversions; however, variations in their position can lead to source strength estimations with uncertainties up to 50%. Regardless of the number of stations, their positions, and SNRs, all of the estimated depths were within 10% from the real source depth.

Long-range atmospheric infrasound propagation from subsurface sources.

In seismology and ocean acoustics, the interface with the atmosphere is typically represented as a free surface. Similarly, these interfaces are considered as a rigid surface for infrasound propagation. This implies that seismic or acoustic waves are not transmitted into the atmosphere from subsurface sources, and vice versa. Nevertheless, infrasound generated by subsurface sources has been observed. In this work, seismo-acoustic modeling of infrasound propagation from underwater and underground sources will be presented. The fast field program (FFP) is used to model the seismo-acoustic coupling between the solid earth, the ocean, and the atmosphere under the variation of source and media parameters. The FFP model allows for a detailed analysis of the seismo-acoustic coupling mechanisms in frequency-wavenumber space. A thorough analysis of the coupling mechanisms reveals that evanescent wave coupling and leaky surface waves are the main energy contributors to long-range infrasound propagation. Moreover, it is found that source depth affects the relative amplitude of the tropospheric and stratospheric phases, which allows for source depth estimation in the future.

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.

Infrasound scattering from stochastic gravity wave packets

Long-range infrasound propagation problems are characterized by a large number of length scales and a large number of propagating modes. In the atmosphere, these modes are confined within waveguides causing the sound to propagate through multiple paths to the receiver. In most infrasound modeling studies, the small scale fluctuations are represented as a “frozen” gravity wave field that is superimposed on a given average background state, and the normal modes are obtained using a single calculation. Direct observations in the lower stratosphere show, however, that the gravity wave field is very intermittent, and is often dominated by rather well defined large-amplitude wave packets. In the present work, we use a few proper modes to describe both the gravity wave field and the acoustic field. Owing to the disparity of the gravity and acoustic length scales, the acoustic field can be constructed in terms of asymptotic expansions using the method of multiple scales. The amplitude evolution equation involves random terms that can be related to vertically distributed gravity wave sources. To test the validity of the theory, numerical results are compared with recorded signals. It is shown that the present stochastic theory offers significant improvements over current semi-empirical approaches.

A study of infrasound propagation based on high-order finite difference solutions of the Navier-Stokes equations

The feasibility of using numerical simulation of fluid dynamics equations for the detailed description of long-range infrasound propagation in the atmosphere is investigated. The two dimensional (2D) Navier Stokes equations are solved via high fidelity spatial finite differences and Runge-Kutta time integration, coupled with a shock-capturing filter procedure allowing large amplitudes to be studied. The accuracy of acoustic prediction over long distances with this approach is first assessed in the linear regime thanks to two test cases featuring an acoustic source placed above a reflective ground in a homogeneous and weakly inhomogeneous medium, solved for a range of grid resolutions. An atmospheric model which can account for realistic features affecting acoustic propagation is then described. A 2D study of the effect of source amplitude on signals recorded at ground level at varying distances from the source is carried out. Modifications both in terms of waveforms and arrival times are described.

Infrasound detection of the Chelyabinsk meteor at the USArray

On February 15, 2013 a small asteroid entered Earth's atmosphere near Chelyabinsk, Russia. This extremely rare event was recorded by the 400-station USArray deployed in the continental United States and Alaska. These stations recorded infrasound signals from the event at distances from 6000 to 10 000 km across a sector spanning 55° that encompassed the North Pole. This dense, extensive network permitted a detailed study of long-range infrasound propagation and source characteristics. We observe long wavetrains at all stations (ranging to over 100 min) but clear variations in the character of the wavetrains across the network. Ray-tracing through a spatially and temporally varying atmospheric model indicates the source excited resonance in the thermospheric duct to all stations. Resonance was also excited in a persistent stratospheric duct between the source and stations in Alaska and along the west coast of the United States due to favorable winds at those azimuths, leading to higher group velocities and frequency content at these stations than those to the east. An attenuation formula derived from parabolic equation simulations is used to estimate infrasound transmission losses at all stations, using simplified models of the effective sound speed along each source-receiver path. Observed variations in signal energies from higher than expected at stations in the thermospheric duct in the eastern United States, to lower than expected in Alaska, at azimuths nearly orthogonal to the asteroid's Mach cone, lead us to conclude that (1) the source was dominantly isotropic and (2) the model overestimates attenuation in the thermospheric duct.

Scattering of infrasound waves by anisotropic inhomogeneities of the atmosphere.

The shaping mechanism for anisotropic wind velocity and temperature fluctuations of the atmosphere and their effect on the long-range infrasound propagation are studied. Based on the model developed for the wind velocity and temperature fluctuations induced by atmospheric gravity waves in the middle and upper atmosphere, the acoustic fields from pulsed infrasound sources (such as ground-surface explosions and volcanoes) are calculated by a parabolic equation method at different ranges from the sources (up to 300 km). The scattering of infrasound signals by highly anisotropic wind velocity and temperature inhomogeneities existing in the stratosphere and lower thermosphere is shown to significantly contribute to the stratospheric and thermospheric arrivals of the signal.