Search

Coal and gas desorption, as a major form of gas energy release, is a key factor in triggering coal and gas outbursts. Therefore, studying the physical characteristics during coal and gas desorption is essential for understanding the development process of coal and gas outbursts. Based on gas dynamics during coal particle gas desorption, this study established a connection between gas desorption and infrasound signals, elaborating on the generation mechanism of infrasound signals during coal particle gas desorption and validating the feasibility of the theory through experimental data, thereby demonstrating the spontaneous occurrence of subsonic tremors during coal particle gas desorption. Combining observational data, it was found that the peak value of infrasound signals generated during desorption experiments is correlated positively with the initial pressure; while, the dominant frequency of infrasound signals is influenced by the proportion of intergranular pores and fractures within the experimental vessel. To further validate the theory of subsonic generation, a mathematical model describing pressure oscillations within intergranular pores, thereby explaining the mechanism of subsonic tremors, was established. The model confirms that the generation and characteristics of infrasound signals are controlled by the parameters of intergranular pores in coal samples. The model effectively simulates changes in the characteristics of infrasound signal tremors during desorption under different conditions, confirming that the physical properties of intergranular pores are crucial factors influencing the generation of infrasound signals and their characteristics during coal and gas desorption. [ABSTRACT FROM AUTHOR]

AbstractThis article addresses the insufficient application of infrasound in the detection of nonmetallic pipeline leaks. Through a combination of numerical simulations and experimental verification, we analyze the sound field distribution, spectral characteristics, and attenuation laws of infrasound leakage signals under varying pressures and leakage apertures. The results indicate that the amplitude of the sound pressure level of the leakage infrasound signal consistently exceeds that of the non-leakage sound pressure level. Additionally, the sound pressure level exhibits rapid attenuation with increasing frequency, pressure, and leakage aperture, particularly in proximity to the leakage hole. [ABSTRACT FROM AUTHOR]

The paper describes a planetary laser interferometric seismoacoustic observatory consisting of six stationary unequal arm laser strainmeters. Based on the triangulation method, the fundamentals of direction finding of various infrasound disturbances at any planetary distance have been developed. The authors show that in addition to determining locations of the occurrence of the recorded disturbance, using data from spatially separated laser strainmeters, it is possible to determine the nature of these signals' divergence and, also, the loss of their energy in the propagation medium. The creation of the planetary laser interferometric seismoacoustic observatory, consisting of five stationary single-coordinate laser strainmeters and one two-coordinate laser strainmeter, united into a single measuring network with an accurate time clock TRIMBLE 5700 that is capable of recording displacements on their bases with an accuracy of 10 pm in the frequency range from 0 (conventionally) to 1000 Hz and two auxiliary laser strainmeters, will allow us to determine, at any planetary distance, the primary source of deformation infrasound disturbances with primary amplitudes from 100 nm. [ABSTRACT FROM AUTHOR]

We investigated the relation between high-frequency seismic signals and eruption size and duration using seismic data of eruption tremor and explosion events generated during sub-Plinian and Vulcanian eruptions, respectively, at various volcanoes. We estimated source amplitude functions from seismic envelope seismograms in the 5–10 Hz band, in which S waves are assumed to radiate isotropically. Because seismic data associated with explosive eruptions can be contaminated by infrasound signals, we confirmed that contamination did not significantly affect the source amplitude functions quantified from our analysed waveforms. We approximated the source amplitude functions of eruption tremor and explosion events by quadrilateral and triangular shapes. For eruption tremor, the durations of the source amplitude functions increased with decreasing slope of the initial phase, that is between onset and maximum amplitude. For explosion events, both the maximum and cumulative amplitudes of the source amplitude functions increased with increasing slope of the initial phase, but the overall durations clustered around a typical value. Moreover, the initial phase durations of eruption tremor were longer than those of explosion events. Based on eruption models proposed by previous studies, Vulcanian and sub-Plinian eruptions have been thought to be triggered by accumulation of magma at a shallow part in a conduit and mixing of cool mushy magma with hot fresh magma in a reservoir, respectively. The above differences between the source amplitude functions of eruption tremor and explosion events can be explained by the distinct eruption triggering processes of sub-Plinian and Vulcanian eruptions. Our results suggest that source amplitude functions are useful for investigating eruption processes and estimating eruption sizes and durations for seismic eruption monitoring. [ABSTRACT FROM AUTHOR]

We present the results of synchronous observations of microbaric oscillations and variations of electric and magnetic fields during large fires of man-made origin in Moscow. The main instrumental observations were carried out at the Center for Geophysical Monitoring in Moscow at the Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences (CGM IDG RAS). Using an analysis of the data, we show that large fires in a megalopolis are manifested in microbaric variations and variations of the electric and geomagnetic fields in the surface atmosphere. Under certain conditions (the absence of intense natural movements of air masses and sufficiently strong heating of the atmosphere in the combustion source), fires can lead to an increase in wind speed and direction in an area up to 25 km. The emergence of a dynamic region with altered thermodynamic characteristics above the fire zone causes the formation of infrasonic waves during the period of the most intense combustion. Strong fires lead to an increase in the concentration of fine particles in the surface atmosphere. [ABSTRACT FROM AUTHOR]

The verification of nuclear test ban necessitates the classification and identification of infrasound events. The accurate and effective classification of seismic and chemical explosion infrasounds can promote the classification and identification of infrasound events. However, overfitting of the signals of seismic and chemical explosion infrasounds easily occurs during training due to the limited amount of data. Thus, to solve this problem, this paper proposes a classification method based on the mixed virtual infrasound data augmentation (MVIDA) algorithm and multiscale squeeze-and-excitation ResNet (MS-SE-ResNet). In this study, the effectiveness of the proposed method is verified through simulation and comparison experiments. The simulation results reveal that the MS-SE-ResNet network can effectively determine the separability of chemical explosion and seismic infrasounds in the frequency domain, and the average classification accuracy on the dataset enhanced by the MVIDA algorithm reaches 81.12%. This value is higher than those of the other four types of comparative classification methods. This work also demonstrates the effectiveness and stability of the augmentation algorithm and classification network in the classification of few-shot infrasound events. [ABSTRACT FROM AUTHOR]

Deploying acoustic sensors on free-flying, long-living balloons helps to reach the areas not accessible with the traditional ground-based sensors, reduce flow noise, and improve characterization of various infrasound sources. Instrumented balloons can potentially increase the infrasonic detection range and early warning lead time for natural hazards. Balloons are also considered as platforms for planetary exploration. When assessing the capabilities of balloon-borne infrasonic sensors and interpreting the measurements, it is imperative to recognize that the balloon inevitably distorts the signals and background infrasound field by scattering the incoming sound. This paper quantifies the effects of hot-air and helium balloons on acoustic pressure and particle acceleration and the role of balloon skin in infrasound diffraction. It is found that balloon-borne vector sensors are more susceptible to distortions than pressure sensors, leading to major differences between the apparent and true source bearing and directionality. [ABSTRACT FROM AUTHOR]

Infrasound generated during the formation and movement of debris flows exhibits long propagation distance and slow energy attenuation, rendering it ideal for remote monitoring of debris flows. However, accurately identifying the infrasound signals of debris flows is challenging because of the complexity of background noise interference and variations in signal characteristics linked to the event's specific characteristics. In this study, the infrasound signal is preprocessed using high and low-pass filters, wavelet soft threshold denoising to mitigate noise interference, and then time-frequency transformed into a two-dimensional image, which is then input into a deep learning model fused with ResNet18 and Vision Transformer for training. The fusion model offers a potent feature extraction capability and the ability to generalize models, which leads to a better understanding of the details of the infrasound signals from debris flows. The experimental results show that the recognition accuracy of the proposed method is up to 88.60%, which is able to effectively predict and warn about the upcoming debris flow events. [ABSTRACT FROM AUTHOR]

Explosion monitoring is performed by infrasound and seismoacoustic sensor networks that are distributed globally, regionally, and locally. However, these networks are unevenly and sparsely distributed, especially at the local scale, as maintaining and deploying networks is costly. With increasing interest in smaller-yield explosions, the need for more dense networks has increased. To address this issue, we propose using smartphone sensors for explosion detection as they are cost-effective and easy to deploy. Although there are studies using smartphone sensors for explosion detection, the field is still in its infancy and new technologies need to be developed. We applied a machine learning model for explosion detection using smartphone microphones. The data used were from the Smartphone High-explosive Audio Recordings Dataset (SHAReD), a collection of 326 waveforms from 70 high-explosive (HE) events recorded on smartphones, and the ESC-50 dataset, a benchmarking dataset commonly used for environmental sound classification. Two machine learning models were trained and combined into an ensemble model for explosion detection. The resulting ensemble model classified audio signals as either "explosion", "ambient", or "other" with true positive rates (recall) greater than 96% for all three categories. [ABSTRACT FROM AUTHOR]

The article "Tsunamis, volcanoes, and landslides. The recent landslide in Greenland and other complex phenomena" analyzes a study published in the journal Science about a massive landslide in Greenland in 2023 that generated a locally tsunami. It highlights the importance of combining records from different observation networks, such as seismological, tide gauge, and infrasound, to understand complex phenomena such as tsunamis and seiches. Historical and recent cases of tsunamis generated by landslides and volcanic eruptions are mentioned, emphasizing the need to work together to study these events. [Extracted from the article]

The Korean infrasound catalogue (KIC) covers 1999–2022 and characterizes a rich variety of source types as well as document the effects of the time-varying atmosphere on event detection and location across the Korean Peninsula. The KIC is produced using data from six South Korean infrasound arrays that are cooperatively operated by Southern Methodist University and Korea Institute of Geoscience and Mineral Resources. Signal detection relies on an Adaptive F-Detector that estimates arrival time and backazimuth, which draws a distinction between detection and parameter estimation. Detections and associated parameters are input into a Bayesian Infrasonic Source Location procedure. The resulting KIC contains 38   455 infrasound events and documents repeated events from several locations. The catalogue includes many anthropogenic sources such as an industrial chemical explosion, explosions at limestone open-pit mines and quarries, North Korean underground nuclear explosions and other atmospheric or underwater events of unknown origin. Most events in the KIC occur during working hours and days, suggesting a dominance of human-related signals. The expansion of infrasound arrays over the years in South Korea and the inclusion of data from the International Monitoring System infrasound stations in Russia and Japan increase the number of infrasound events and improve location accuracy because of the increase in azimuthal station coverage. A review of selected events and associated signals at multiple arrays provides a location quality assessment. We quantify infrasound events that have accompanying seismic arrivals (seismoacoustic events) to support the source type assessment. Ray tracing using the Ground-to-Space (G2S) atmospheric model generally predicts observed arrivals when strong stratospheric winds exist, although the predicted arrival times have significant discrepancies. In some cases, local atmospheric data better captures small-scale variations in the wind velocity of the shallow atmosphere and can improve arrival time predictions that are not well matched by the G2S model. The analysis of selected events also illustrates the importance of topographic effects on tropospheric infrasound propagation at local distances. The KIC is the first infrasound catalogue compiled in this region, and it can serve as a valuable data set in developing more robust infrasound source localization and characterization methods. [ABSTRACT FROM AUTHOR]

Infrasound sensing plays a critical role in the detection and analysis of bolides, offering passive, cost-effective global monitoring capabilities. Key objectives include determining the timing, location, and yield of these events. Achieving these goals requires a robust approach to detect, analyze, and interpret rapidly moving elevated sources such as bolides (also re-entry). In light of advancements in infrasonic methodologies, there is a need for a comprehensive overview of the characteristics that distinguish bolides from other infrasound sources and methodologies for bolide infrasound analysis. This paper provides a focused review of key considerations and presents a unified framework to enhance infrasound processing approaches specifically tailored for bolides. Three representative case studies are presented to demonstrate the practical application of infrasound processing methodologies and deriving source parameters while exploring challenges associated with bolide-generated infrasound. These case studies underscore the effectiveness of infrasound in determining source parameters and highlight interpretative challenges, such as variations in signal period measurements across different studies. Future research should place emphasis on improving geolocation and yield accuracy. This can be achieved through rigorous and systematic analyses of large, statistically significant samples of such events, aiming to resolve interpretative inconsistencies and explore the causes for variability in signal periods and back azimuths. The topic described here is also relevant to space exploration involving planetary bodies with atmospheres, such as Venus, Mars, and Titan. [ABSTRACT FROM AUTHOR]

The classification of infrasound events has considerable importance in improving the capability to identify the types of natural disasters. The traditional infrasound classification mainly relies on machine learning algorithms after artificial feature extraction. However, guaranteeing the effectiveness of the extracted features is difficult. The current trend focuses on using a convolution neural network to automatically extract features for classification. This method can be used to extract signal spatial features automatically through a convolution kernel; however, infrasound signals contain not only spatial information but also temporal information when used as a time series. These extracted temporal features are also crucial. If only a convolution neural network is used, then the time dependence of the infrasound sequence will be missed. Using long short-term memory networks can compensate for the missing time-series features but induces spatial feature information loss of the infrasound signal. A multiscale squeeze excitation-convolution neural network-bidirectional long short-term memory network infrasound event classification fusion model is proposed in this study to address these problems. This model automatically extracted temporal and spatial features, adaptively selected features, and also realized the fusion of the two types of features. Experimental results showed that the classification accuracy of the model was more than 98%, thus verifying the effectiveness and superiority of the proposed model. [ABSTRACT FROM AUTHOR]

For prompt detection of large (>1 kt) above-ground explosions, infrasound microphone networks and arrays are deployed at surveyed locations across the world. Denser regional and local networks are deployed for smaller explosions, however, they are limited in number and are often deployed temporarily for experiments. With the expanded interest in smaller yield explosions targeted at vulnerable areas such as population centers and key infrastructures, the need for more dense microphone networks has increased. An "attritable" (affordable, reusable, and replaceable) and flexible alternative can be provided by smartphone networks. Explosion signals from a fuel air explosive (thermobaric bomb) and a high explosive with trinitrotoluene equivalent yields of 6.35 and 3.63 kg, respectively, were captured on both an infrasound microphone and a network of smartphones. The resulting waveforms were compared in time, frequency, and time-frequency domains. The acoustic waveforms collected on smartphones produced a filtered explosion pulse due to the smartphone's diminishing frequency response at infrasound frequencies (<20 Hz) and was found difficult to be used with explosion characterization methods utilizing waveform features (peak overpressure, impulse, etc.). However, the similarities in time frequency representations and additional sensor inputs are promising for other explosion signal identification and analysis. As an example, a method utilizing the relative acoustic amplitudes for source localization using the smartphone sensor network is presented. [ABSTRACT FROM AUTHOR]

One of the problems present when measuring noise generated in outdoor environments is the appropriate microphone placement, and there are different approaches and techniques applied within this scope. For measurements of noise generated by wind turbines, standard EN 61400 specifies installing the microphone on a mounting board at ground level, though practical demands, especially for long-term monitoring, suggest other microphone positions, above the ground level. The consequences of different microphone placements are a key issue that should be analysed and taken into consideration. Another problem specific to measurements of noise generated in outdoor environments is the influence of wind on the obtained results and the potential correction required on account of wind speed. Studies were undertaken with those issues in mind, and a comparison of different mounting positions and methods was performed. Certain other aspects, such as the influence of wind speed and the use of different windscreens, are discussed as well. This is only a preliminary stage intended to provide a perspective on the differences that may be expected when performing measurements of infrasound noise sources, and related studies should be continued in the future. The work presented in this paper concerns three aspects: the first is the dependence of the measured noise signal on the microphone position above the ground level, the second is how wind influences the measured signal quality, and the third is related to microphone positioning on a ground board. Results for a windscreen modification are presented as well. [ABSTRACT FROM AUTHOR]

A method for recognizing infrasound acoustic signals for two types of sources based on the analysis of the shape of their wavelet spectra is proposed. The idea of constructing this form is based on the principal component method. Morphological image analysis methods are used to search for characteristic areas. The proposed method makes it possible to effectively solve the problem of multiclass classification of acoustic signals. [ABSTRACT FROM AUTHOR]

SUMMARY: We conduct a thorough analysis of seismic and acoustic data purported to be from the so-called 'interstellar meteor' which entered the Earth's atmosphere off the coast of Papua New Guinea on 2014 January 08. Previous work had suggested that this meteor may have been caused by an alien spacecraft burning up in the atmosphere. We conclude that both previously reported seismic signals are spurious—one has characteristics suggesting a local vehicular-traffic-based origin; whilst the other is statistically indistinguishable from the background noise. As such, previously reported localizations based on this data are unreliable. Analysis of acoustic data provides a best-fitting location estimate which is very far ($\sim$ 170 km) from the reported fireball location. Accordingly, we conclude that material recovered from the seafloor and purported to be from this event is almost certainly unrelated to it, and is likely of more mundane (non-interstellar) origin. [ABSTRACT FROM AUTHOR]

Previous field experiments support the claim that a tornado can radiate discernible infrasound between 0.5 and 10 Hz. The physical mechanisms of tornado sound generation are still not fully understood, although several potential mechanisms have been proposed. In this paper, the theory of vortex sound is applied to the sound radiation from two numerical tornado simulations based on large eddy simulations. There are two different vortex-related mechanisms in distinct frequency regimes. It is found that rotation of a non-axisymmetric vorticity field produces low-frequency infrasound less than 2.0 Hz. High-frequency tornado infrasound can be attributed to more complex vortex dynamics such as vortex merging. [ABSTRACT FROM AUTHOR]

Recent geophysical studies have highlighted the potential utility of integrating both seismic and infrasound data to improve source characterization and event discrimination efforts. However, the influence of each of these data types within an integrated framework is not yet well‐understood by the geophysical community. To help elucidate the role of each data type within a merged structure, we develop a neural network which fuses seismic and infrasound array data via a gated multimodal unit for earthquake‐explosion discrimination within the Korean Peninsula. Model performance is compared before and after adding the infrasound branch. We find that the seismoacoustic model outperforms the seismic model, with the majority of the improvements stemming from the explosions class. The influence of infrasound is quantified by analyzing gated multimodal activations. Results indicate that the model relies comparatively more on the infrasound branch to correct seismic predictions. Plain Language Summary: Earthquakes and explosions can produce energy that travel as waves through the ground, seismic, and the air, infrasound. As these waves travel to the station where they are detected, they can be changed so drastically by the medium that it makes it difficult to determine what caused them. In these instances, it has been shown that using both seismic and infrasound data works better to characterize an event than using them independent of one another. However, due to the differences in how the air and ground influence the movement of energy, it is not well‐known how these types of data work in unison to give us more information about an event. In this study, we use a machine learning model trained on both seismic and infrasound data to help us better understand how they can be used together to determine their source. Key Points: Discrimination performance within the Korean Peninsula is improved after fusing seismoacoustic data within a deep learning architectureNeural network framework provides insight into how information in multimodal data combine to distinguish between different event types [ABSTRACT FROM AUTHOR]

There are sparse opportunities for direct measurement of upper stratospheric winds, yet improving their representation in subseasonal‐to‐seasonal prediction models can have significant benefits. There is solid evidence from previous research that global atmospheric infrasound waves are sensitive to stratospheric dynamics. However, there is a lack of results providing a direct mapping between infrasound recordings and polar‐cap upper stratospheric winds. The global International Monitoring System (IMS), which monitors compliance with the Comprehensive Nuclear‐Test‐Ban Treaty, includes ground‐based stations that can be used to characterize the infrasound soundscape continuously. In this study, multi‐station IMS infrasound data were utilized along with a machine‐learning supported stochastic model, Delay‐SDE‐net, to demonstrate how a near‐real‐time estimate of the polar‐cap averaged zonal wind at 1‐hPa pressure level can be found from infrasound data. The infrasound was filtered to a temporal low‐frequency regime dominated by microbaroms, which are ambient‐noise infrasonic waves continuously radiated into the atmosphere from nonlinear interaction between counter‐propagating ocean surface waves. Delay‐SDE‐net was trained on 5 years (2014–2018) of infrasound data from three stations and the ERA5 reanalysis 1‐hPa polar‐cap averaged zonal wind. Using infrasound in 2019–2020 for validation, we demonstrate a prediction of the polar‐cap averaged zonal wind, with an error standard deviation of around 12 m·s −1$$ {}^{-1} $$ compared with ERA5. These findings highlight the potential of using infrasound data for near‐real‐time measurements of upper stratospheric dynamics. A long‐term goal is to improve high‐top atmospheric model accuracy, which can have significant implications for weather and climate prediction. [ABSTRACT FROM AUTHOR]

A minequake of magnitude ML 4.1 occurred on 18 May 2020 early in the morning at the LKAB underground iron ore mine in Kiruna, Sweden. This is the largest mining‐induced earthquake in Scandinavia. It generated acoustic signals observed at three infrasound arrays at 9.3 (KRIS, Sweden), 155 (IS37, Norway), and 286 km (ARCI, Norway) distance. We perform full‐waveform focal mechanism inversion based on regional seismic data and local infrasound data. These independently highlight that this event was dominated by a shallow‐depth collapse in agreement with in‐mine seismic station data. However, regional infrasound data cannot inform the inversion process without an accurate model of atmospheric winds and temperatures. Yet, our numerical simulations demonstrate a potential of using local and regional infrasound data to constrain an event's focal mechanism and depth. Plain Language Summary: The largest mining‐induced earthquake in Scandinavia (ML 4.1) occurred on 18 May 2020 early in the morning at the LKAB underground iron ore mine in Kiruna, Sweden. The seismic waves coupled to the atmosphere and propagated large distances as sound waves which were observed at three infrasound arrays at 9.3 (KRIS, Sweden), 155 (IS37, Norway), and 286 km (ARCI, Norway) distance. Our seismic and acoustic modeling results highlight a strong collapse event within the northern section of the mine. The modeling of acoustic and seismic waves across the Earth‐atmosphere suggests that sound wave data can help when determining the location and properties of a seismic source. Key Points: Seismic and acoustic records indicate a strong collapse at shallow depth of 1 ± 0.5 km for the 2020 Kiruna minequakeFocal mechanisms and depths of shallow seismic sources can be retrieved from local infrasound recordsInversions using regional infrasound data is possible when accurate weather models are available [ABSTRACT FROM AUTHOR]

Acoustic waves generated by seismic waves contain information on the internal structure of planets, and can be sensed by pressure sensors onboard high-altitude balloons. To identify the various contributions (infrasound signal, noise, balloon response, etc.) in such pressure records, a full waveform modeling is implemented and completed by infrasound ray tracing and additional data analysis. Here, we analyze the Stratéole-2 pressure data associated with two earthquakes (Garcia et al. Geophys. Res. Lett. 49(15):e98844, 2022) and compared these to full waveform simulations by SPECFEM2D-DG-LNS software. Even if our simulations do not precisely reproduce the waveform observed in the frequency range [0.05, 0.3] Hz, we show that the waveform presents more sensitivity to quake and internal structure parameters than to atmospheric structure, and that seismic surface wave dispersion is observed in balloon pressure records. The long-duration pressure oscillations observed after the main infrasonic signal cannot be fully reproduced by our one-dimensional input model even when source time function complexity and aftershocks are considered. These features are ascribed mainly to the complex vertical ground movements below the balloon and partly to late secondary infrasound arrivals excited by the interactions of seismic waves with the topography. These results enhance the advantages and limitations of quake-related infrasound observations on board terrestrial and planetary balloon platforms. [ABSTRACT FROM AUTHOR]

Wind turbines emit infrasound and low-frequency noise (ILFN), which can be annoying for people living near wind farms. To assess the acoustic impact of wind turbines on the environment, it is essential to model ILFN propagation during the forecasting stage. This study assesses the effectiveness of three commonly used sound propagation models (ISO 9613-2, CNOSSOS-EU for favourable propagation conditions, Nord2000) in predicting ILFN generated by wind turbines. The performance of these models in modelling ILFN is generally not validated or guaranteed. The analysis covers octave frequency bands ranging from 4 Hz to 250 Hz, and comparisons are made against measurements conducted at a wind farm in Poland. Non-parametric statistical tests were used with a significance level of α = 0.05 to determine significant differences between measured and predicted results. The results show that the Nord2000 method provides accurate calculations, while the ISO 9613-2 method can be used for simplified assessments of ILFN generated by wind turbines during the investment preparation phase. [ABSTRACT FROM AUTHOR]

The elemental composition of chemical elements can vary between healthy and diseased tissues, providing essential insights into metabolic processes in physiological and diseased states. This study aimed to evaluate the calcium (Ca) and phosphorus (P) levels in the bones of rats with/without streptozotocin-induced diabetes and/or exposure to infrasound. X-ray fluorescence spectroscopy was used to determine the concentrations of Ca and P in Wistar rat tibiae samples. The results showed a significant decrease in bone P concentration in streptozotocin-induced diabetic rats compared to untreated animals. Similarly, the Ca/P ratio was higher in the streptozotocin-induced diabetic group. No significant differences were observed in bone Ca concentration between the studied groups or between animals exposed and not exposed to infrasound. Moreover, streptozotocin-induced diabetic rats had lower bone P concentration but unaltered bone Ca concentration compared to untreated rats. Infrasound exposure did not impact bone Ca or P levels. The reduced bone P concentration may be associated with an increased risk of bone fractures in diabetes. [ABSTRACT FROM AUTHOR]

We consider the problem of the effect of small plasma density variations on the propagation and reflection of ELF (extremely-low-frequency) electromagnetic wave incident on the morning ionosphere from above. Modifications of the electromagnetic wave field near the Earth surface and coefficient of wave energy reflection from ionosphere caused by infrasonic wave are studied. The greatest modifications of the field on the Earth surface and reflection coefficient are related to the density perturbations at altitudes of strong electromagnetic wave attenuation (80–110 km). Reflection of electromagnetic waves is especially sensitive to the plasma density perturbations if the value of horizontal refractive index is close to a unit. In this case, modulation percentage of reflection coefficient can exceed the modulation percentage of density in the infrasonic wave by an order of magnitude. The parametric influence of plasma density perturbations in an infrasonic wave on the ELF electromagnetic field is analyzed. If the values of the horizontal wave numbers of the electromagnetic and infrasonic waves are close to each other, the field of the electromagnetic wave near the Earth surface can increase several times. The obtained results are important for understanding the interconnection of ionospheric wave processes of different nature. The study of the modulation of coefficient of ELF reflection from the ionosphere is relevant for explaining the operation modes of a plasma magnetospheric maser. [ABSTRACT FROM AUTHOR]

The geomagnetic field (GMF) is a worldwide source of compass cues used by animals and humans alike. The inclination of GMF flux lines also provides information on geomagnetic latitude. A long-disputed question, however, is whether horizontal gradients in GMF intensity, in combination with changes in inclination, provide bicoordinate "map" information. Multiple sources contribute to the total GMF, the largest of which is the core field. The ubiquitous crustal field is much less intense, but in both land and marine settings is strong enough at low altitudes (< 700 m; sea level) to mask the core field's weak N–S intensity gradient (~ 3–5 nT/km) over 10 s to 100 s of km. Non-orthogonal geomagnetic gradients, the lack of consistent E–W gradients, and the local masking of core-field intensity gradients by the crustal field, therefore, are grounds for rejection of the bicoordinate geomagnetic "map" hypothesis. In addition, the alternative infrasound direction-finding hypothesis is briefly reviewed. The GMF's diurnal variation has long been suggested as a possible Zeitgeber (timekeeper) for circadian rhythms and could explain the GMF's non-compass role in the avian navigational system. Requirements for detection of this weaker diurnal signal (~ 20–50 nT) might explain the magnetic alignment of resting and grazing animals. [ABSTRACT FROM AUTHOR]

An attempt was made to assess the quality of the International Data Centre (IDC) Reviewed Event Bulletin (REB) by comparing it with the Reviewed bulletin of the International Seismological Centre (ISC). To accomplish the task of comparison, the ISC Reviewed Bulletin events as downloaded from the ISC Web page for the month of October 2020 were used. The corresponding REB events as listed in the ISC bulletin were considered. During this period, a total of 3431 and 2652 (excluding 39 Infrasound events) events were considered for ISC and IDC bulletins, respectively. The comparison was performed using the information given in the header lines of the ISC bulletin from which matched and unmatched events could be extracted. The results showed that a total of 2315 events could be matched between the two bulletins. The percentage of matched events with relative location difference (D) < 1° was about 95.3% while the percentage of events with D ≥ 5o was about 0.35%. There were two potential events with IDC magnitude (mb) ≥ 4.0 and D ≥ 5°. The percentage of matched events with intersecting error ellipses was about 75.5%. Some results obtained from earlier studies which were produced by using similar approaches were also used in this study. In general, the results obtained in this study show that there was a slight difference when compared to the earlier comparison results. [ABSTRACT FROM AUTHOR]

Comprehensive simulation of a number of main processes induced in all geospheres by the fall and explosion of the Kyiv meteoroid on April 19, 2023, have been conducted. Magnetic, electrical, electromagnetic, ionospheric, and seismic effects and the effects of acoustic gravity waves have been assessed. The magnetic effect of the ionospheric currents and the current in the wake of the meteoroid could be considerably large (approximately 0.4–0.7 nT). Owing to the capture of electrons in the atmospheric gravity wave field, the magnetic effect can reach the levels of 0.6–6 nT. Under the influence of an external electric field, a transient current pulse with a current strength of up to approximately 102–103 A can arise. The electrostatic effect can be accompanied by the accumulation of an electric charge of approximately 1–6 mC, which produces the electric field strength of approximately 10 MV/m. The flow of the electric current in the meteoroid wake can give rise to generation of an electromagnetic pulse in the 10–100 kHz band with an electric field strength in the range of 1–10 V/m. The electromagnetic effect of infrasound could be substantial (approximately 0.6–6 V/m and approximately 2–20 nT). The absorption of the shock wave at the ionospheric dynamo region altitudes (approximately 100–150 km) can generate secondary atmospheric gravity waves with a relative amplitude of approximately 0.1%. The fall of the meteoroid produced a plasma wake not only in the lower atmosphere but also in the upper atmosphere at altitudes of not less than 1000 km. The possibility of the appearance of an electrophonic effect is unlikely. The possibilities of generating the ion and magnetic sound by infrasound and generating gradient drift and drift dissipative instabilities seem to be unlikely as well. The magnetic, electrical, and electromagnetic effects dealt with in this study partially fill up gaps in the theory of physical effects produced by meteoroids in the Earth–atmosphere–ionosphere–magnetosphere system. The magnitudes of magnetic, electrical, electromagnetic, ionospheric, and acoustic effects are significant. The magnitude of the earthquake caused by the meteoroid explosion did not exceed one. The mean rate of recurrence of the fall of celestial bodies similar to the Kyiv meteoroid equals 32.3 yr–1, i.e., one event in 11 days. [ABSTRACT FROM AUTHOR]

Behavioral hearing thresholds and noise localization acuity were determined using a conditioned avoidance/suppression procedure for three Helmeted guineafowl (Numida meleagris). The guineafowl responded to frequencies as low as 2 Hz at 82.5 dB SPL, and as high as 8 kHz at 84.5 dB SPL. At a level of 60 dB SPL, their hearing range spanned 8.12 octaves (24.6 Hz–6.86 kHz). Like most birds, they do not hear sounds above 8 kHz. However, the guineafowl demonstrated good low-frequency hearing (frequencies below 32 Hz), showing thresholds that are more sensitive than both the peafowl and pigeon, both of which hear infrasound. It thus appears that infrasound perception may be more common than previously thought and may have implications for species that inhabit areas with wind energy facilities. The guineafowls' minimum audible angle for a 100-ms broadband noise burst was 13.8 °, at the median for birds and near the mean for mammals. Unlike in mammals, the small sample of bird species and limited representation of lifestyles do not yet allow for meaningful interpretations of the selective pressures or mechanisms that underlie their abilities to locate sound sources. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Infrasonic waves have been observed to propagate to regional (greater than 15 km) distances through the troposphere. Infrasound propagation in the geometric acoustics approximation has shown that realistic terrain can scatter acoustic energy from tropospheric ducts; however, ray methods cannot intrinsically capture finite-frequency behavior such as diffraction. A two-dimensional finite-difference time-domain (FDTD) method has been developed to solve linearized equations for infrasound propagation with realistic terrain. Acoustic wave propagation over 100 km with both flat terrain and a Gaussian hill was first simulated in order to compare finite-frequency propagation with ray predictions. The effects of realistic terrain and atmospheres on infrasound signals from a 2012 surface explosion at the Utah Testing and Training Range are then investigated. Propagation through the troposphere is suggested by array processing results, but eigenrays are not predicted due to weak to nonexistent ducting conditions. FDTD modeling suggests that the inclusion of terrain and finite frequency effects helps explain much of the observed signal in a realistic scenario. These results suggest that geometric acoustics may underestimate propagation through the troposphere, and that recorded waveforms at regional distances may be noticeably affected by terrain. [ABSTRACT FROM AUTHOR]

Ambient infrasound noise contains an abundance of information that is typically overlooked due to limitations of typical infrasound arrays. To evaluate the ability of large‐N infrasound arrays to identify weak signals hidden in background noise, we examine data from a 22‐element array in central Idaho, USA, spanning 58 days using a standard beamforming method. Our results include nearly continuous detections of diverse weak signals from infrasonic radiators, sometimes at surprising distances. We observe infrasound from both local (8 km) and distant (195 km) waterfalls. Thunderstorms and earthquakes are also notable sources, with distant thunderstorm infrasound observed from ∼800 to 900 km away. Our findings show that large‐N infrasound arrays can detect very weak signals below instrument and environmental noise floors, including from multiple simultaneous sources, enabling new infrasound monitoring applications and helping map the composition of background noise wavefields. Plain Language Summary: Infrasound, or sound with a frequency below what humans can generally hear, is often produced by various earth processes. Compared to audible sound, infrasound can travel longer distances while retaining more energy, making it especially useful for remote monitoring purposes. Infrasound arrays are stations that consist of multiple infrasonic microphones, which use time‐delays between sensors to determine the direction of a wave's arrival. Using a greater number of microphones allows for the improved detection of weak signals, a technique referred to as "large‐N." Our study uses an arrangement of 22 microphones to locate sources of infrasound, such as waterfalls, thunderstorms, and earthquakes. Infrasound from waterfalls was identified at both nearby and distant locations, reaching almost 200 km. Thunderstorms and earthquakes were also recorded, with thunder infrasound coming from distances of nearly 900 km. Our results show that large‐N infrasound can reveal earth processes that may otherwise go unnoticed. Key Points: A 22‐element infrasound array is used to identify weak acoustic sources that could otherwise be hidden in background noiseInfrasound from thunderstorms, earthquakes, and both local and distant waterfalls is observedLarge‐N infrasound monitoring enables detection of distant waterfall infrasound, with potential applications in hydrologic monitoring [ABSTRACT FROM AUTHOR]

Low-frequency noise, the frequency range from approximately 10 Hz to 200 Hz, has been recognized as a special environmental noise problem. The World Health Organization recognizes the special place of low-frequency noise as an environmental problem. Noise can damage hearing, and it affects the whole body. Low-frequency sound is heard by humans, but infrasound is not audible. Low-frequency sound is most often measured based on a weighting function of the frequency. The A-weighted level underestimates the effects of low-frequency noise. For the detrimental effects of low-frequency sound, it would be appropriate to apply measurements using Z-weighting. The aim of this paper was to propose a comprehensive method of acoustic risk assessment (CMARA) that implements the effects of low-frequency values of noise exposure. The proposed methodology has been applied in practice at four workplaces for seven work activities. A risk assessment using the proposed CMARA method for individual activities shows that the noise exposure time may pose a health risk in the occupational and environmental process due to exposure to low-frequency noise at the limit of audibility. A high risk was assessed for activities WA2 (machining) and WA3 (spot welding). This paper highlights the need to measure low-frequency noise using Z-filter weighting. [ABSTRACT FROM AUTHOR]

Abstract—During a series of destructive earthquakes in Turkey on February 6, 2023, in particular, two strong earthquakes with magnitudes 7.8 and 6.7 immediately following each other, the results of instrumental observations of microbaric and geomagnetic variations made in the surface layer of the atmosphere, as well as variations of the electric field and the critical frequency of the regular F2 layer of the ionosphere are presented. It is shown that the earthquakes were accompanied by variations of magnetic and electric fields, generation of infrasound waves recorded at a considerable distance from the sources, as well as variations of the critical frequency f0F2. [ABSTRACT FROM AUTHOR]

The paper discusses one of the aspects of calibration of the equipment for measuring acoustic pressure in air. To measure complex sensitivity of LS-type microphones during primary pressure calibration (by using a reciprocity or a pistonphone method), it is necessary to theoretically determine the complex acoustic impedance of air within a cylindrical closed volume with absolutely rigid walls. The adiabatic approximation was used as the first step in determining the acoustic impedance of air inside the specified closed volume. However, according to various experimental and theoretical studies, adiabatic approximation is only applicable within a relatively narrow frequency range, while the calibration results can also be affected by other significant factors. Such factors include heat exchange between air inside the cylindrical closed volume and ambient air (through the walls of this volume), as well as reflected waves that appear at high frequencies (depending on how the sound wavelength compares to the length of the closed volume). To study these factors, numerical simulation is proposed. The numerical algorithm is based on the regularized Navier-Stokes equations with quasi-gas-dynamic closure, and accounts for the viscosity, thermal conductivity, and compressibility of air. The phase of the complex acoustic impedance of air in a closed volume with heat-conducting and heat-insulated walls has been characterized. The study results are relevant for both the calibration of measurement microphones at low and infrasound frequencies by using the pressure reciprocity and pistonphone methods, and for studying acoustic processes in liquid and gaseous media using numerical modeling, since these results show the applicability of the model used for numerical calculation. Measurement devices that receive the unit of acoustic pressure in air from the measurement microphones, calibrated by the primary method, are used, for example, to monitor noise from various sources (industrial activity, transport), to monitor noise inside residential and industrial buildings, and to study geophysical phenomena (low-frequency sound oscillations in the atmosphere caused by daily and semi-daily variations in atmospheric pressure, atmospheric currents, tsunamis, volcanic eruptions, etc.). [ABSTRACT FROM AUTHOR]

The Golden (British Columbia, Canada) meteorite fall occurred on October 4, 2021 at 0534 UT with the first recovered fragment (1.3 kg) landing on an occupied bed. The associated fireball was recorded by numerous cameras permitting reconstruction of its trajectory and orbit. The fireball entered the atmosphere at a 54° angle from the horizontal at a speed of 18 km s−1. The fireball reached a peak brightness of −14, having first become luminous at a height of >84 km and ending at 18 km altitude. Analysis of the infrasonic record of the bolide produced an estimated mass of 78−65+157 kg while modeling of the fireball light curve suggests an initial mass near 70 kg. The fireball experienced a major flare near 31 km altitude where more than half its mass was lost in the form of dust and gram‐sized fragments under a dynamic pressure of 3.3 MPa. The strength and fragmentation behavior of the fireball were similar to those reported for other meteorite‐producing fireballs (Borovička et al., 2020). Seven days after the fireball occurred, an additional 0.9 kg fragment was recovered during the second day of dedicated searching guided by initial trajectory and dark flight calculations. Additional searching in the fall and spring of 2021–2022 located no additional fragments. The meteorite is an unbrecciated, low‐shock (S2) ordinary chondrite of intermediate composition, typed as an L/LL5 with a grain density of ~3530 k gm−3, an average bulk density of 3150 kg m−3 and calculated porosity of ~10%. From noble gas measurements, the cosmic ray exposure age is 25 ± 4 Ma while gas retention ages are all >2 Ga. Short‐lived radionuclides and noble gas measurements of the pre‐atmospheric size overlap with estimates from infrasound and light curve modeling producing a preferred pre‐atmospheric mass of 70–200 kg. The orbit of Golden has a high inclination (23.5°) and is consistent with delivery from the inner main belt. The highest probability (60%) of an origin is from the Hungaria group. We propose that Golden may originate among the background S‐type asteroids found interspersed in the Hungaria region. The current collection of 18 L/LL—chondrite orbits shows a strong preference for origins in the inner main belt, suggesting multiple parent bodies may be required to explain the diversity in CRE ages and shock states. [ABSTRACT FROM AUTHOR]

On March 5, 2022, a 12 kg meteoroid crossed the sky above Central Italy and was observed by three different observational systems: the PRISMA all-sky camera network (10 stations), the Italian national seismic network (61 stations), and a 4-element infrasound array. The corresponding datasets, each with its own resolution, provided three independent assessments of the trajectory, size and speed of the meteoroid. The bolide traveled across central Italy with an azimuth of 102 degrees, becoming visible at about 91 km above sea level with a velocity of about 15.4 km/s. Its visible trajectory lasted about 15 s. Reasonably, the residual portion of the ablated bolide terminated its path in the Adriatic Sea and could not be recovered. Seismic and infrasound data well match optical observations detecting the bolide Mach cone at 68 km above sea level with a back azimuth of 25 degrees with respect to the array. By comparing results from the three different systems, discrepancies are within the estimated uncertainties, thus confirming the mutual consistency of the adopted methodologies. Therefore, this study shows that different approaches can be integrated to improve the detection capability for bolide crossing the sky in monitored regions. [ABSTRACT FROM AUTHOR]

Global empirical models for infrasound signal celerity (where celerity is defined as the epicentral distance divided by the total traveltime) and backazimuth deviation (the difference between the measured and predicted backazimuth assuming great circle propagation), can be used for the association of infrasound automatic detections, for event location and for the validation of acoustic propagation simulations. Using software developed to consistently analyse a global ground truth database, we observe 296 detections in the 0.32–1.28 Hz passband; predominantly stratospheric arrivals, with a smaller number of tropospheric and thermospheric arrivals, in agreement with previous studies. We develop an updated global celerity-range model and introduce a global backazimuth deviation model. These new models suggest that the variation in backazimuth deviation is not range-dependent; 93 per cent of the 296 detections studied have a deviation magnitude ≤5°. The maximum deviation observed is 8.9°. The variation in celerity, however, is range-dependent. An exponential range-dependent celerity model with bounds calculated using a quantile regression fit to the traveltime residuals is determined. This traveltime residual model, with uncertainties that increase with range, produces bounds on the celerity consistent with both the observations and current understanding of infrasound propagation. The traveltime residual model derived celerity bounds are not symmetric around the celerity model, which further reflects the physical processes. At long ranges (>3000 km), maximum peak-to-trough amplitude arrivals are not observed with celerities <280 ms−1, but even at long ranges, we occasionally observe celerities (>320 −1) more usually consistent with tropospheric arrivals. [ABSTRACT FROM AUTHOR]

Infrasonic waves are influenced by variations in the density, pressure and temperature as well as the ambient winds. Modelling infrasonic propagation can be challenging due to the dynamic nature of the atmosphere as well as the sparseness of measurements which result in variability and notable uncertainty. A framework is presented to quantify spatial and seasonal trends in atmospheric structure via analysis of the effective sound speed profile and identification of temporal trends in the middle atmospheric waveguide produced by the circumpolar vortex winds. Seasonal definitions identifying typical atmospheric structures during the summer, winter and spring/fall transition periods are identified using atmospheric data from 2010 through 2020. Seasonal trend analysis is conducted for a number of locations across the contiguous United States to quantify spatial variations in atmospheric structure that impact infrasonic propagation. For each season and location, empirical orthogonal function analysis is used to reduce the historical archive of atmospheric data into a smaller representative set that can be analysed using numerical tools more efficiently. Infrasonic ray tracing and finite-frequency modal propagation analyses are applied to construct propagation path geometry and transmission loss statistics which are useful in localization and yield estimation for infrasonic sources, respectively. An example application is detailed in which transmission loss statistics are combined with an explosive source model and noise statistics to quantify the capability of a network to detect nearby sources. [ABSTRACT FROM AUTHOR]

Infrasound—acoustic waves in the atmosphere below 20 Hz—is a useful monitoring tool. Topography and atmospheric structure strongly control infrasound propagation, and at common source–receiver distances neither of these effects can be ignored when quantitative source constraints are sought. Detailed spatial measurements of the infrasound wavefield would inform propagation models and improve source estimates. However, the "large‐N" deployment strategy now well‐known in seismology has not yet been realized for infrasound studies. Here, we use the 900‐node seismic array from the 2014 Imaging Magma Under St. Helens (iMUSH) experiment as a proxy for a large‐N infrasound network, by leveraging acoustic–seismic coupled arrivals. The active‐source component of iMUSH consisted of 23 shallowly buried explosions around Mount Saint Helens volcano; these explosions produced epicentral infrasound recorded on the nodes. We find that the bulk presence of ground‐coupled infrasound on the nodes is controlled by wind noise and source–receiver distance, with observed arrivals for eight explosions. Explosions with the most extensive coupling produce complex spatial waveform patterns across the array. These patterns are related to both topographic and atmospheric propagation effects, as well as spatially variable site (coupling) effects. We compare our observations to simple topographic diffraction and high‐resolution wind advection models, and full‐wave numerical simulations. We find strong spatial correlations between (a) coupled arrival strength and modeled topographic obstruction and (b) coupled arrival time and along‐path winds. Our seismoacoustic analyses and results are applicable to other existing and future nodal seismic data sets and can expand the utility of such deployments. Plain Language Summary: Infrasound is sound in the atmosphere too low‐pitched for humans to hear. Hazards like volcanoes and avalanches produce infrasound, making it a useful monitoring tool. However, infrasound bounces off of and bends around topography, and winds and temperature changes in the atmosphere control where infrasound can be "heard" by sensors. To better characterize hazards, we need to understand how infrasound travels, but we don't usually have the spatial density of sensors required to capture the relevant effects. However, seismometers can record infrasound as it interacts with the ground nearby, and explosions buried near Earth's surface can generate infrasound by moving the ground. Here, we leverage this "coupling" to study infrasound—at a high spatial density previously unavailable—by analyzing infrasound from buried explosions recorded on a spatially dense seismic network. We use simple models to account for topographic and atmospheric effects on infrasonic waves and compare these with the recorded infrasound arrivals. We capture fine‐scale features such as louder infrasound on exposed ridges versus sheltered valleys. We demonstrate that seismic networks can be used as spatially dense infrasound networks for some applications. Future work is required to better understand how infrasound travels in the atmosphere and couples into the Earth. Key Points: We use a dense (200 m spacing) "large‐N" nodal seismic array as a proxy for a large‐N infrasound networkWe observe and model topographic and atmospheric effects on infrasound propagation from active sources at atypically‐small spatial scalesDespite challenges related to variable acoustic–seismic transfer functions, coupled infrasound on dense seismic networks has great utility [ABSTRACT FROM AUTHOR]

The ways in which seabirds navigate over very large spatial scales remain poorly understood. While olfactory and visual information can provide guidance over short distances, their range is often limited to 100s km, far below the navigational capacity of wide-ranging animals such as albatrosses. Infrasound is a form of low-frequency sound that propagates for 1,000s km in the atmosphere. In marine habitats, its association with storms and ocean surface waves could in effect make it a useful cue for anticipating environmental conditions that favor or hinder flight or be associated with profitable foraging patches. However, behavioral responses of wild birds to infrasound remain untested. Here, we explored whether wandering albatrosses, Diomedea exulans, respond to microbarom infrasound at sea. We used Global Positioning System tracks of 89 free-ranging albatrosses in combination with acoustic modeling to investigate whether albatrosses preferentially orientate toward areas of 'loud' microbarom infrasound on their foraging trips. We found that in addition to responding to winds encountered in situ, albatrosses moved toward source regions associated with higher sound pressure levels. These findings suggest that albatrosses may be responding to long-range infrasonic cues. As albatrosses depend on winds and waves for soaring flight, infrasonic cues may help albatrosses to identify environmental conditions that allow them to energetically optimize flight over long distances. Our results shed light on one of the great unresolved mysteries in nature, navigation in seemingly featureless ocean environments. [ABSTRACT FROM AUTHOR]

We considered three events on 4 November 2015, 22 December 2016, and 12 November 2018, when the signals travelling in the polar winter mesosphere with high horizontal velocities above 300 m/s were measured by the atmospheric radar ESRAD (Chilson et al., 1999) located at Esrange, near Kiruna in northern Sweden. We proposed four mechanisms of generation of such special cases of polar mesosphere echoes, e.g. high-speed PMWE, that involve microbaroms, i.e. infrasound waves at 0.1–––0.35 Hz frequencies created by ocean swell. These mechanisms are (i) generation of viscous waves, (ii) generation of thermal waves, (iii) direct contributions of infrasound, and (iv) generation of secondary waves at sound dissipation. These processes necessarily accompany sound propagation in inhomogeneous, thermally conducting and viscous fluid (air). The four models were theoretically analysed and their efficiency was estimated. The infrasound measurements at the IS37 station (Gibbons et al., 2019) located about 170 km north-west from the ESRAD radar, modelled maps of the microbarom sources, infrasound propagation conditions and ionospheric conditions for these three PMWE events support the proposed models. Infrasound-generated thermal waves are suggested to be the most probable specific cause of the observed high-speed, high-aspect-ratio PMWE events. However, absence of in-situ infrasound and plasma measurements did not allow us to quantify contributions of individual physical mechanisms to the fast-travelling echoes generation. [ABSTRACT FROM AUTHOR]

Advancements in space exploration and sample return technology present a unique opportunity to leverage sample return capsules (SRCs) towards studying atmospheric entry of meteoroids and asteroids. Specifically engineered for the secure transport of valuable extraterrestrial samples from interplanetary space to Earth, SRCs offer unexpected benefits that reach beyond their intended purpose. As SRCs enter the Earth's atmosphere at hypervelocity, they are analogous to naturally occurring meteoroids and thus, for all intents and purposes, can be considered artificial meteors. Furthermore, SRCs are capable of generating shockwaves upon reaching the lower transitional flow regime, and thus can be detected by strategically positioned geophysical instrumentation. NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) SRC is one of only a handful of artificial objects to re-enter the Earth's atmosphere from interplanetary space since the end of the Apollo era and it will provide an unprecedented observational opportunity. This review summarizes past infrasound and seismic observational studies of SRC re-entries since the end of the Apollo era and presents their utility towards the better characterization of meteoroid flight through the atmosphere. [ABSTRACT FROM AUTHOR]

Infrasound signals are used to investigate and monitor active volcanoes during eruptive and degassing activity. Infrasound amplitude information has been used to estimate eruptive parameters such as plume height, magma discharge rate, and lava fountain height. Active volcanoes are characterized by pronounced topography and, during eruptive activity, the topography can change rapidly, affecting the observed infrasound amplitudes. While the interaction of infrasonic signals with topography has been widely investigated over the past decade, there has been limited work on the impact of changing topography on the infrasonic amplitudes. In this work, the infrasonic signals accompanying 57 lava fountain paroxysms at Mt. Etna (Italy) during 2021 were analyzed. In particular, the temporal and spatial variations of the infrasound amplitudes were investigated. During 2021, significant changes in the topography around the most active crater (the South East Crater) took place and were reconstructed in detail using high resolution imagery from unoccupied aerial system surveys. Through analysis of the observed infrasound signals and numerical simulations of the acoustic wavefield, we demonstrate that the observed spatial and temporal variation in the infrasound signal amplitudes can largely be explained by the combined effects of changes in the location of the acoustic source and changes in the near-vent topography, together with source acoustic amplitude variations. This work demonstrates the importance of accurate source locations and high-resolution topographic information, particularly in the near-vent region where the topography is most likely to change rapidly and illustrates that changing topography should be considered when interpreting local infrasound observations over long time scales. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Sound Fixing and Ranging (SOFAR) channel in the ocean allows for low frequency sound to travel thousands of kilometers, making it particularly useful for detecting underwater nuclear explosions. Suggestions that an elevated SOFAR‐like channel should exist in the stratosphere date back over half a century and imply that sources within this region can be reliably sensed at vast distances. However, this theory has not been supported with evidence of direct observations from sound within this channel. Here we show that an infrasound sensor on a solar hot air balloon recorded the first infrasound detection of a ground truth airborne source while within this acoustic channel, which we refer to as the AtmoSOFAR channel. Our results support the existence of the AtmoSOFAR channel, demonstrate that acoustic signals can be recorded within it, and provide insight into the characteristics of recorded signals. Results also show a lack of detections on ground‐based stations, highlighting the advantages of using balloon‐borne infrasound sensors to detect impulsive sources at altitude. Plain Language Summary: A layer exists in the ocean that allows sound to travel great distances. Something like this could exist in our atmosphere, too, but until now no one had verified this. Low frequency sound, known as infrasound, travels efficiently through the atmosphere. Specialized instruments, called infrasound sensors, are microphones that can record low frequency sound. We attached an infrasound sensor to a solar hot air balloon to make the first recording of a known object within the theorized atmospheric layer. Our results show that this layer exists, sound can be recorded within it, and analysis can give us more information about what generated that sound. The rocket launch was not well recorded by infrasound sensors on the ground, highlighting the advantages of our method for detecting objects that travel through this region of the atmosphere. Key Points: First direct observation of an elevated acoustic duct using an airborne acoustic source and receiverDetections provide insight into the characteristics of signals from ascending and descending supersonic objectsLack of detections on ground‐based stations highlights the advantages of using balloon‐borne sensors to detect sources at altitude [ABSTRACT FROM AUTHOR]

Thunderstorm activity on March 25, 2023 provided a unique opportunity to study the mechanism of lightning events on changes in air pressure. In particular, this event made it possible to study changes in air pressure during thunderstorms using various instruments. This paper presented comprehensive results of infrasound, satellite data, weather radar and weather measurements at the ground during the storm. Observations of lightning events were confirmed using observational data from the International Space Station's Lightning Imaging Sensor (ISS LIS). This work estimated three spectral percentile values on infrasonic sensor data, time series interpolation of standard meteorology profiles, weather radar reflectivity and total radiant energy of lightning from ISS LIS observations during the day and night periods. As a result, during the investigation, it was seen that the recorded infrasound signal in the 0.6–0.8 Hertz (Hz) range was contaminated by background environmental noise, but in the 1–3 Hz band range it was consistent with the appearance of storms that produce high energy blows. Infrasound detection and electromagnetic lightning detection show good correlation up to a distance of 100 km from the infrasonic station. During a thunderstorm, the ISS LIS flight directly above the observation site detected more than 2,000 lightning events. In addition, the application of lightning detection from several independent instruments can provide a complete picture of the observed event. [ABSTRACT FROM AUTHOR]

The Pacific Northwest region is significantly affected by tropical cyclones, which pose a threat to humans due to strong winds and intense precipitation. Observations have shown that international meteorological agencies lower the hazard class of typhoons too early, or even stop tracking them altogether. We studied the results of in situ measurements of infrasonic microseismic oscillations in the upper frequency range, generated in the shelf area of the northwestern part of the Sea of Japan as a result of the long-term influence of strong tropical cyclones on the sea area. The measurement data were obtained using coastal laser strainmeters, which are components of the seismoacoustic hydrophysical measuring complex. The dynamics of upper-frequency-range infrasonic microseismic signals during the movement of Typhoon Chan-Hom 2015 in the Sea of Japan are shown. The same infrasonic microseismic oscillations excited by Typhoon 2022 Hinnamnor, which had no impact on the measuring station, have a difference in the width of the signal spectrum and frequency shift with maximum amplitude compared to the signals from Typhoon Chan-Hom. [ABSTRACT FROM AUTHOR]

During eruptions, volcanoes produce air‐pressure waves inaudible for the human ear called infrasound, which are very helpful for detecting early signs of magma at the surface. Compared to violent ash‐rich explosions, recording more discrete atmospheric disturbances from effusive eruptions remains a practical challenge depending on the distance to the source. At Nyiragongo volcano (D.R. Congo), towering above a 1‐million urban area, we analyzed local infrasonic records between January 2018 and April 2022. An acoustic signature from this open‐vent volcano is detected up to the volcano observatory facilities in Goma city center about 17 km from its crater. We compared infrasound signals with space‐based observations of the intra‐crater activity (SO2 emissions, thermal anomalies, crater depth/radius). We thus obtain a comprehensive picture of Nyiragongo's eruptive activity during this period, encompassing the drainage of its lava lake during its third known flank eruption on 22 May 2021. Plain Language Summary: Similar to seismic waves propagating within the solid Earth, acoustic waves propagate in the atmosphere as a result of near‐surface volcanic activity. In contrast with powerful volcanic explosions, close‐range deployment of instruments around the active vent (for instance <15 km) is generally needed for continuously monitoring long‐living lava effusion or degassing as observed at open‐vent volcanoes, such as Kīlauea (Hawaii) or Nyiragongo (D.R. Congo). Nyiragongo hosted the world's largest lava lake up to 2021, which was drained during the third known flank eruption of this volcano on 22 May 2021. While watching for possible reawakening, we detected the infrasound signature of a nascent lava lake four months later, highlighting the crucial role of infrasound monitoring. Remote observations from space (ground deformation, gas emissions, thermal anomalies) can also provide essential information about the eruptive state of a volcano and were jointly analyzed with infrasound records. Knowing that Nyiragongo towers above the 1‐million inhabitant city of Goma, such an approach has significant implications for optimizing future monitoring efforts in a harsh field environment. Key Points: Nyiragongo volcano (D.R. Congo) is a permanent emitter of infrasound that pauses during rare flank eruptionsIntra‐crater lava eruptions (lava lake, spatter cone, lava pond) are detected up to the observatory in a dense urban area ∼17 km awayJoint analysis with space‐based observations over the period 2018–2022 provides a clear picture of the persistent unrest at this volcano [ABSTRACT FROM AUTHOR]