Infrasound Propagation Research Articles

Detection of acoustic wave generated by Baganuur mining explosion

We analyzed all available recorded infrasound data and determined the station detection capability in Mongolia. While in the winter times, we continuously detect acoustic signals from blasts at the Baganuur open-pit mine (150 km from infrasound I34MN station), which we rarely registered during the summer times. A previous study has shown that the noise level increased by 5 dB - 10 dB in the summer, which affected the detection capability of the stations. On the other hand, this could be connected to infrasound propagation model difference between winter and summer times. To verify this phenomenon, we installed a new infrasound IBHM station in a forest area at a distance of 350 km from the Baganuur mine area. When we compared the background noise level of the two stations, the noise level at the temporary station was lower than the permanent one, which allowed us to compare winter and summer time registered infra wave characteristics. With the comparison observed and modeled, apparent velocity suggested that winter and summer time detection difference could be due to the propagation model of infrasound wave itself.

A Pilot Experiment on Infrasonic Lahar Detection at Mount Adams, Cascades: Ambient Infrasound and Wind-Noise Characterization at a Quiescent Stratovolcano

AbstractErosion, hydrothermal activity, and magmatism at volcanoes can cause large and unexpected mass wasting events. Large fluidized debris flows have occurred within the past 6000 yr at Mount Adams, Washington, and present a hazard to communities downstream. In August 2017, we began a pilot experiment to investigate the potential of infrasound arrays for detecting and tracking debris flows at Mount Adams. We deployed a telemetered four-element infrasound array (BEAR, 85 m aperture), ~11 km from a geologically unstable area where mass wasting has repeatedly originated. We present a preliminary analysis of BEAR data, representing a survey of the ambient infrasound and noise environment at this quiescent stratovolcano. Array processing reveals near continuous and persistent infrasound signals arriving from the direction of Mount Adams, which we hypothesize are fluvial sounds from the steep drainages on the southwest flank. We interpret observed fluctuations in the detectability of these signals as resulting from a combination of (1) wind-noise variations at the array, (2) changes in local infrasound propagation conditions associated with atmospheric boundary layer variability, and (3) changing water flow speeds and volumes in the channels due to freezing, thawing, and precipitation events. Suspected mass movement events during the study period are small (volumes <105 m3 and durations <2 min), with one of five visually confirmed events detected infrasonically at BEAR. We locate this small event, which satellite imagery suggests was a glacial avalanche, using three additional temporary arrays operating for five days in August 2018. Events large enough to threaten downstream communities would likely produce stronger infrasonic signals detectable at BEAR. In complement to recent literature demonstrating the potential for infrasonic detection of volcano mass movements (Allstadt et al., 2018), this study highlights the practical and computational challenges involved in identifying signals of interest in the expected noisy background environment of volcanic topography and drainages.

Infrasound Generation by Meteorological Fronts and Its Propagation in the Atmosphere

AbstractInfrasound parameters (amplitudes, coherences, grazing angles, azimuths, and horizontal phase speeds) derived during the passage of warm and cold fronts through the networks of microbarometers in the cities of Dubna and Moscow are presented. The significant differences observed in the temporal variations of the parameters of infrasound from warm and cold fronts are discussed. Such differences must be taken into account when detecting infrasound precursors of atmospheric storms. A possible mechanism for the generation of infrasound by the turbulent airstream flowing around the geometric irregularities of the meteorological front is proposed. The observed effect of internal gravity waves on the parameters of infrasound and its frequency spectrum is explained.

Complex ray tracing algorithm for infrasound propagation in the atmosphere

A classical long standing challenge for the infrasound research community is explaining frequently observed infrasound arrivals into shadow zones. This arrivals can be obtained by generalising standard ray method to complex ray tracing, which takes into account diffraction effects. This method is applied to long range infrasound propagation in a stratified and moving atmosphere by a three step numerical algorithm computing both real and complex eigenrays. First, real rays shooting allows to locate caustics and shadow zones. Second, extrapolation around caustics provides real and complex rays initial guesses. Third, their optimisation is performed using an adaptive algorithm. This process provides wave arrival times, azimuths, apparent velocities and over-pressures at a set of stations. Algorithm efficiency and robustness are illustrated by numerical examples: the ground sonic boom of a slightly supersonic aircraft (Mach cut-off problem), explosion at a ground-based point source, and the sonic boom from a meteorite atmospheric entry. In this last case, a 2D model has been developed inspired by the well-documented Carancas meteorite. Finally, comparisons are performed with other numerical methods.

Snow Avalanche Detection and Source Constraints Made Using a Networked Array of Infrasound Sensors

AbstractWe studied a triggered snow avalanche (∼60 s in duration and with ∼1,100 m run‐out) using a network of infrasound arrays and time‐synced video, with the objective of understanding the relationship between infrasound generation and flow dynamics. Using standard array processing techniques, we compared the infrasound source back azimuths with the avalanche flow path identified by frame‐differenced, geo‐referenced video. Results show that infrasound records begin with direct arrivals followed by echoes from the avalanche‐triggering explosions and these decay within 35 s of the detonations. Subsequent infrasound, which lasts 20–30 s, could then be attributed exclusively to the avalanche. These infrasound detections, and their triangulated source locations, progress downhill over time and the most intense infrasound appears to originate from a steep, mid‐path cliff band, where the avalanche reached speeds in excess of 30 m/s and accelerations of more than 5 m/s2. The recorded infrasound was compared to two candidate source models extracted from video: total flow motion and advancing flow motion. Advancing source locations were compared to acoustic intensity time series using a nonnegative least squares inversion to solve for, and to quantify, time‐varying infrasound source intensity. We observed that certain portions of the flow, most notably the early stages and the end stages (when the powder cloud was expanding and settling) were infrasonically quiet.

Infrasonic Earthquake Detectability Investigated in Southern Part of Japan, 2019.

The Kochi University of Technology (KUT) Infrasound Sensor Network contains 30 infrasound sensors which are distributed all over Japan especially in Shikoku Island. At all infrasound stations installed with three-axis accelerometers to measure the peak ground acceleration (PGA). Many earthquakes were detected by our system after establishing of the network since 2016. In this study we will focus on all the possibilities for infrasound detection generated from earthquakes using KUT sensor network and International Monitoring system (IMS) stations for the earthquakes which were detected in southern part of Japan during 2019. As for earthquakes with strike-slip mechanisms the P-waves could not be detected by our sensors. In addition, The conversion from seismic to acoustic waves can be happened through the generating of the T-phase from oceanic earthquakes. On 9 May 2019, progressive multi-channel cross correlation (PMCC) method applied infrasound and hydroacoustic waves from two earthquakes happened in west of Kyushu Island as the T-phase was well-recorded at H11N station near Wake Island. Moreover, infrasound propagation modeling is applied to the reconstructed atmosphere profile by Ground to Space Model (AVO-G2S) to confirm the infrasound arrivals, furthermore the 3D ray tracing process and the calculations by using the transmission loss equation with normal modes and parabolic equation methods are investigated. The study confirmed the infrasound generation scenario from the T-phase of oceanic propagation.

Parameters of the Infrasonic Signal Generated in the Atmosphere by a Powerful Volcano Explosion

The purpose of this work is to represent the results of performing regression analysis to fit the distance and the amplitude of the infrasonic signal generated by the explosion of St. Helens volcano, and to estimate a few signal and atmospheric parameters. The pressure amplitude in the explosion wave generated at the beginning of St. Helens volcano eruption was measured at 13 stations in the 0.9 – 39-Mm distance range; based on these data, an attempt has been made to perform a regression analysis to fit amplitude and distance. The regression based on the assumption that the infrasound propagation takes place in a waveguide where it is subject to attenuation is determined to be the most preferable regression. Based on the observations of the shock from the St. Helens volcano eruption, the shock wave energy and mean power have been estimated to be ~1016 J and ~2.3 TW, respectively. Based on the observations of the amplitude and duration of the trains of the infrasonic wave generated by the St. Helens volcano eruption, the infrasonic wave energy and mean power have been estimated to be ~1016 J and ~2 TW, respectively. Both estimates are in good agreement, but they are significantly different from those found in the literature; the latter seem to be overestimated. From the regression expression obtained, the penetration depth of the infrasonic wave is obtained to be about 33 Mm, whereas at other stations this scale length is estimated to be close to 24 Mm. Based on the theoretical dependence of the attenuation coefficient due to atmospheric turbulence, the attenuation length of the infrasound wave has been estimated for infrasound with 10–300-s periods. For 20–300-s periods, this value has been shown to be significantly larger than the values determined from the observations. Other mechanisms for attenuating the infrasonic signal are discussed (the partial radiation of the infrasonic energy through and losses due to the reflection from the waveguide walls). At the same time, the wave attenuation due to their scattering by turbulent fluctuations can be significant for the periods smaller than 20–50 s, depending on the turbulence intensity. Comparison of the regression functions obtained with the corresponding regression expressions for other sources of infrasound waves propagating in the atmosphere has been made. Keywords: volcano eruption, infrasonic wave, shock wave, signal amplitude, regression, signal attenuation

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.

Modeling infrasound propagation from Tornado producing storms

Modeling infrasound propagation from Tornado producing storms

Infrasound characteristics of nontornadic and tornadic thunderstorms using high-resolution simulations

There has been increased interest in improving severe weather detection by supplementing NOAA's WSR-88D radar network with an infrasound observation network, which may be able to detect distinct sub-audible signatures from tornadic thunderstorms. While there is evidence that tornadic thunderstorms exhibit observable infrasound signals, what is not well-understood is whether these infrasound signals are unique to tornadic storms compared to nontornadic storms or whether there is any useful signal prior to tornadogenesis (which would be most relevant to NWS forecasters). In this presentation, we will present the results of high-resolution simulations of severe thunderstorms, specifically tailored to represent acoustic wave propagation with frequencies of 0.1 to 1 Hz. The spectral analysis of pressure perturbations generated by a nontornadic and tornadic supercell thunderstorm will be compared against each other. Two additional sensitivity tests that employ different microphysics schemes (i.e., the representation of precipitation within the storm) will be discussed, since prior work has indicated that the melting of precipitation is a dominant contributor to infrasound generation in thunderstorms. So far, preliminary results suggest that there are no discernible differences in infrasound observed in the vicinity of tornadic versus nontornadic supercell thunderstorms prior to tornadogenesis.

Use of Human Step Energy as an Alternative Energy Source

The work is devoted to the development of an electrogeneration system that uses kinetic energy of human pitch. Traditional methods of electricity production using thermal power plants, hydroelectric power plants and nuclear power plants are considered. Hydroelectric and nuclear power plants have the advantage of production capacity. The main disadvantage of a thermal power plant is atmospheric pollution, a nuclear power plant is likely to have radioactive contamination, and their common disadvantage is thermal water pollution. Hydroelectric power plants have less production capacity, but are environmentally better than the above-mentioned power plants. The main alternative to these stations are power plants that use wind and solar energy. The main advantage of wind power plants is the possibility of round-the-clock power generation and the ability to install almost anywhere where there is constant air movement. The main disadvantages of wind power plants are the instability of power generation due to the instability of the air flow and the generation of infrasound during operation. The advantage of solar panels is their high environmental friendliness, the ability to install anywhere. The disadvantage is the need for a large area to produce sufficient capacity and the feasibility of location in more sunny regions. These disadvantages are absent in the pavement tile Pavegen. A characteristic feature of this technology is the generation of electricity through the use of kinetic energy in human steps. The disadvantage is the inappropriate installation in places of low intensity of movement of people. The surface of the tile is made of recycled tires for durability, abrasion resistance and the prospect of further disposal of the tires. The case is made of stainless steel. When pressed, the tile surface flexes by 5-10 mm, forcing the integrated converters to generate electricity through the effects of piezoelectric effect and electromagnetic induction. One step generates 2 to 4 J of autonomous energy, or about 5 watts per step. The block diagram of the developed power generation system includes a power generating tile that generates electricity, a voltage regulator 12 V, lead-helium battery as a drive, microcontroller, sensor, communication module, stabilizer unit, power supply designed to ensure the stable operation of modules and monitoring of information on traffic intensity and amount of energy produced, and inverter. Lighting of the territory of the enterprise, premises, ventilation, etc. can be a load.

Wave and extra-wide-angle parabolic equations for sound propagation in a moving atmosphere.

The narrow-angle parabolic equation (NAPE) with the effective sound speed approximation (ESSA) is widely used for sound and infrasound propagation in a moving medium such as the atmosphere. However, it is valid only for angles less than 20° with respect to the nominal propagation direction. In this paper, the wave equation and extra-wide-angle parabolic equation (EWAPE) for high-frequency (short-wavelength) sound waves in a moving medium with arbitrary Mach numbers are derived without the ESSA. For relatively smooth variations in the medium velocity, the EWAPE is valid for propagation angles up to 90°. Using the Padé (n,n) series expansion and narrow-angle approximation, the EWAPE is reduced to the wide-angle parabolic equation (WAPE) and NAPE. Versions of these equations are then formulated for low Mach numbers, which is the case that is usually considered in the literature. The phase errors pertinent to the equations considered are studied. It is shown that the equations for low Mach numbers and the WAPE with the ESSA are applicable only under rather restrictive conditions on the medium velocity. An effective numerical implementation of the WAPE for arbitrary Mach numbers in the Padé (1,1) approximation is developed and applied to sound propagation in the atmosphere.

Atmospheric Boundary Layer as a Laboratory for Modeling Infrasound Propagation and Scattering in the Atmosphere

In this work, we show that large-scale processes of the long-range propagation and scattering of infrasound signals in stratospheric and thermospheric waveguides are to some extent similar to the smaller scale processes of waveguide propagation and scattering of acoustic signals in stably stratified ABL. Moreover, we note a resemblance between the zeroth order tropospheric mode and the Lamb mode that is observed for larger nuclear explosions. The results of physical modeling of the long-range propagation of infrasound signals in the atmosphere by studying the propagation of the acoustic pulses from detonation sources in the stably stratified atmospheric boundary layer (ABL) are presented. Such modeling became possible due to continuous measurement of the wind velocity and temperature profiles in the ABL by Doppler sodar and temperature profiler, respectively. The resemblance between the propagation of the infrasound signals from surface explosions (20–70 t TNT) in the “shallow” tropospheric wave guide and propagation of Lamb mode from nuclear explosions is found. Also, the propagation of infrasound signals in the stratospheric wave guide is modeled by studying the propagation of acoustic pulses in the ABL during morning hours (after sunrise), when an effective sound speed profile in the ABL is similar to that in the stratospheric wave guide. The instantaneous vertical profiles of the effective sound speed and wind velocity fluctuations in the thin layers of the stably stratified ABL located at different heights (up to 700 m) above the ground and at different distances from the source (up to 2.25 km) have been retrieved using the wave forms and travel times of the recorded arrivals of the acoustic pulses from the detonation sources. The effect of scattering of acoustic pulses from the fine-scale layered structure of the stably stratified ABL is studied to model the effect of scattering of infrasound signals from the fine-scale layered structure of the stratosphere.

Acoustic radiation from an infrasonic ball-valve siren.

The concept of a ball-valve siren is developed through experimentation and theoretical modeling. The ball-valve siren is a source transducer developed for the purpose of establishing the concept of infrasound generation through the modulation of compressed air flowing through a rotating ball valve and released into the atmosphere, in the context of a siren. Directivity, frequency response, and propagation experiments were performed for the fundamental frequency component, and the results compare favorably to an empirical model based on monopole and dipole radiation. The results show that a small ball-valve siren can generate useful infrasound radiation with nominal directivity at frequencies in the range 1 to 8 Hz.

Evolving infrasound detections from Bogoslof volcano, Alaska: insights from atmospheric propagation modeling

Bogoslof volcano, a back-arc volcano in Alaska’s Aleutian arc, began an eruptive sequence in mid-December 2016 that ended in late August 2017, with 70 individual eruptive episodes. Because there were no local seismic or infrasound stations on the island, the Alaska Volcano Observatory (AVO) relied on distant geophysical networks and remote sensing techniques to assess activity during the eruption. AVO maintains six infrasound arrays to monitor activity along the Aleutian arc: Adak, the Island of Four Mountains, Okmok, Akutan, Sand Point, and Dillingham. Eruption detection at infrasound arrays is subject to local as well as mesoscale meteorological conditions that vary greatly over both short and long timescales. Infrasound detections from the array nearest to Bogoslof (Okmok), with a latency of about 3 min, played a crucial role in monitoring activity during the eruption. Despite the relative proximity of the Okmok array to Bogoslof (60 km), infrasound detections were not uniformly observed with only about two-thirds of the events successfully detected. The farthest array at Dillingham (816 km) detected approximately half of the explosive events, with all other arrays detecting less than half of the events. We compare observations with infrasound propagation model predictions, using both normal mode and parabolic equation forward models, to interpret the variation in detections of the 70 explosive events across the AVO infrasound network. The forward models utilize the newly created, publicly available AVO-G2S atmospheric reconstruction using numerical weather predictions data for the lower atmosphere, coupled with upper atmosphere empirical models of wind speeds and temperature. We find that long-range detections (> 100 km) of Bogoslof events are largely aligned with seasonal variability in favorable propagation conditions, while regional detections (< 100 km) are less consistent with propagation modeling. Understanding the output of numerical models in comparison to past observations will facilitate their use in future operational settings for AVO and other observatories.

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.

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.

Atmospheric infrasound generation by ocean waves in finite depth: unified theory and application to radiation patterns

SUMMARYBetween 0.1 and 0.5 Hz, infrasound signals recorded in the atmosphere are dominated by ocean-generated noise called microbaroms. Microbaroms propagate through the atmosphere over thousands of kilometres due to low absorption and efficient ducting between the ground and the stratopause. Different theoretical models have been developed to characterize the source of microbaroms, all based on the second-order nonlinear interaction of ocean waves. While early theories considered an infinite ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have approximated the radiation pattern as a monopole, and found a considerable effect of the water depth. This paper reviews these models and extends the previous theories to the combined effects of both finite depth ocean and source directivity in both elevation and azimuth angles. It is found that the water depth has a negligible effect for the near-horizontally propagating acoustic waves that should dominate the measured microbarom records. Another important result is that the microbarom azimuthal variation can be highly directive locally, but it generally becomes isotropic when integrated over a realistic source region.

Infrasound and seismoacoustic signatures of the 28 September 2018 Sulawesi super-shear earthquake

Abstract. A magnitude 7.5 earthquake occurred on 28 September 2018 at 10:02:43 UTC near the city of Palu on the Indonesian island of Sulawesi. It was a shallow, strike-slip earthquake with a rupture extending to a length of about 150 km and reaching the surface. Moreover, this earthquake was identified as one of very few events having a super-shear rupture speed. Clear and long-lasting infrasound signatures related to this event were observed by four infrasound arrays of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization as well as by one national infrasound station in Singapore. Although these infrasound stations SING (Singapore), I39PW (Palau), I07AU (Australia), I40PG (Papua New Guinea) and I30JP (Japan) are located at large distances of between 1800 and 4500 km from the earthquake's epicentral region, the observed infrasound signals associated with this event were intense, including both seismic and acoustic arrivals. A detailed study of the event-related infrasound observations and the potential infrasound generation mechanisms is presented, covering range-dependent infrasound transmission loss and propagation modeling and characterization of the atmospheric background conditions, as well as identification of the regions of seismoacoustic activity by applying a back-projection method from the infrasound receivers to potential source regions. This back projection of infrasonic arrivals allows one to estimate that the main infrasound source region for the Sulawesi earthquake is related to the extended rupture zone and the nearby topography. This estimation and a comparison to other super-shear as well as large regional earthquakes identify no clear connection between the earthquake's super-shear nature and the strong infrasound emission.

The Use of Environmentally Friendly Vertical-Axis Wind Power Plants for Nature Reserves and National Parks in Southern Russia

The efficiency of using original highly efficient vertical-axis wind turbines (VA wind turbines) based on N-Darya-Savonius combined rotors (KR) is substantiated for the national parks of southern Russia. Based on experimental studies, the power factor of the original combined rotor with blades having zigzag flaps was first obtained. The maximum values of the power factor of the CR reaches CP = 0.6 exceeding theoretically possible values for horizontal-axis wind power plants CP = 0.59. When working in a wind turbine, there are no emissions of pollutants into the atmosphere, oxygen consumption, thermal pollution, generation of infrasound and noise. VA wind turbines with a power of 1–10 kW, it is advisable to apply for the generation of electric and thermal energy in reserves and national parks.