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SUMMARY The detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals that conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e. following the Bartlett or the Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a ‘clean’ spectrum. A data-driven stopping criterion for CLEAN is proposed, which relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data set and is applied to data from multiple International Monitoring System infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic that would have remained unidentified if conventional methods had been applied.

SUMMARYMeasurements of seismo-acoustic events by collocated seismic and infrasound arrays allow for studying the two wavefields that were produced by the same event. However, some of the scientific and technical constraints on the building of the two technologies are different and may be contradicting. For the case of a new station, an optimal design that will satisfy the constraints of the two technologies can be found. However, in the case of upgrading an existing array by adding the complementing technology, the situation is different. The site location, the array configuration and physical constraints are fixed and may not be optimal for the complementing technology, which may lead to rejection of the upgrade. The International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes 37 seismic arrays and 51 infrasound arrays. Although the CTBT verification regime is fixed in the treaty, an upgrade of the existing arrays by adding more technologies is possible.The Mount Meron seismic array (MMAI), which is part of the IMS, is composed of 16 sites. Microbarometers were installed at five MMAI sites to form the Mount Meron infrasound array. Due to regulation and physical constraints, it was not possible to relocate the sites nor to install analogue noise reduction filters (i.e. a pipe array). In this study, it is demonstrated that the installation of the MMAI infrasound array is beneficial despite the non-optimal conditions. It is shown that the noise levels of the individual array sites are between the high and median global noise levels. However, we claim that the more indicative measures are the noise levels of the beams of interest, as demonstrated by analysing the microbaroms originated from the Mediterranean Sea. Moreover, the ability to detect events relevant to the CTBT is demonstrated by analysing man-made events during 2011 from the Libya region.

Conditions for the occurrence of the “voice of the sea” within the infrasonic range are studied and its parameters are determined from measurements performed in the waters of the Black Sea (2011 and 2016) and the Sea of Okhotsk (2017). Different parameters (mean correlations, acoustic-arrival spectra, directions, and phase velocities) of high-frequency infrasounds (1–10 Hz) recorded during the 2011 and 2016 experiments carried out in Katsiveli (Crimea) are compared. Wind conditions over the Black Sea waters and the conditions of propagation of acoustic waves along the direction of their arrivals during these measurements have been studied in detail. In both cases, atmospheric vortices are observed in the direction of infrasound arrivals, which cause the wind to change its direction over the sea surface. The interaction between two oppositely directed atmospheric vortices (according to data obtained in 2011) and a vortex observed to the west of the recording point (according to data obtained in 2016) result in the generation of infrasounds. The generation of microbaroms and the voice of the sea due to wind-direction variations, which cause the nonlinear interaction of surface waves propagating in opposite directions and the formation of their 2nd harmonics in the form of standing surface waves, is discussed. The most probable regions of infrasound generation are determined from an analysis of the profiles of wind velocity and direction along the path of infrasound arrivals and acoustic-pressure fields calculated using the parabolic-equation method according to the $${{C}_{{{\text{eff}}}}}$$ profiles in the direction of infrasound propagation. In both cases, these regions coincide with zones in which the wind velocity had dropped to zero and the wind direction had reversed. The infrasound within a microbarom frequency range of 0.2–0.3 Hz and the (higher frequency) sea voice with a mean frequency of 5.5 Hz, which were simultaneously recorded from the same direction, are given as an example.

Microseisms in the dominant double-frequency band, around 5 s period and their ubiquitous presence makes them an interesting signal for exploring the solid Earth and associated natural hazards (e.g. Olivier et al. 2019). These microseisms are generated by opposing ocean waves of equal frequencies (Hasselmann 1963) that generally arise within the locally generated sea state at high frequencies (class I, Ardhuin et al. 2011), due to coastal reflection (class II) or when swell from a distant storm collides with another wave system, which generally corresponds to the strongest microseism sources (class III). Improving on solid Earth knowledge and natural hazard monitoring can benefit from a better quantitative knowledge of these sources. Similar applications to the study of the stratosphere can use atmospheric infrasound that are generated by the same opposing ocean waves, the microbaroms (Brekhovskikh et al. 1973, De Carlo et al. 2020). So far, very few direct measurements of wave properties have been able to quantify the presence of waves in opposite directions, and the magnitude of microseism sources has relied on numerical simulations. Here we use sun glitter measurements from the Copernicus-Sentinel 2 satellites, as processed in the SARONG project (http://www.sarong.global/). We show that the presence of opposing waves gives a strong anomaly in the phase of co-spectra of optical images from Sentinel 2 (S2). When using only 2 time-lagged images this feature generally limits the possibility to measure surface currents from waves shorter than about 25 m, that always have a significant energy in opposing directions (class I microseism sources).Caption: Processing from Sentinel 2 Level-1c images to phase speeds. Top: data from Copernicus Sentinel 2 on 29 April 2016 off California (See Figs. 3-9 in Kudryavtsev et al. 2017). Bottom: simulated S2 data based on in situ wave spectrum determined from directional moments using the Maximum Entropy Method. The phase speed anomalies, highlighted with the dashed magenta circle near the Nyquist wavelength L = 20 m, disappear when no energy propagates in opposing directions.The same also happens for longer components when strong (class III) microseism sources are present. However this signature is also an opportunity to directly measure the sources of microseisms and quantify the energy in opposing directions using 2 or more different time lags (Ardhuin et al., in 2021). Given its coastal coverage, we find that S2 is particularly well suited for estimating reflection coefficients of waves off the coast, which is a major source of uncertainty for microseism and microbarom source modelling. Ardhuin, F., Stutzmann, E., Schimmel, M., & Mangeney, A. (2011). Ocean wave sources of seismic noise. Journal of Geophysical Research, 116(C9). doi:10.1029/2011jc006952Kudryavtsev, V., Yurovskaya, M., Chapron, B., Collard, F., & Donlon, C. (2017). Sun glitter imagery of ocean surface waves. Part 1: Directional spectrum retrieval and validation. Journal of Geophysical Research: Oceans, 122(2), 1369–1383. doi:10.1002/2016jc012425 Olivier, G., Brenguier, F., Carey, R., Okubo, P., & Donaldson, C. (2019). Decrease in seismic velocity observed prior to the 2018 eruption of Kīlauea volcano with ambient seismic noise interferometry. Geophysical Research Letters. doi:10.1029/2018gl081609

Microbarom signals are generated by wind‐waves at the ocean surface and propagate all around the globe through the stratosphere and ionosphere. Microbaroms dominate the coherent infrasound ambient noise measured worldwide, with a peak around 0.2 Hz. Monitoring these signals allows characterizing the source activity and probing the properties of their propagation medium, the middle atmosphere. Here we show the first quantitative validation of global microbarom modeling based on ocean wave models, a new source model and atmospheric attenuation. For evaluating these parameters’ impact, we compare the modeling results with a global reference database of microbaroms detected by the infrasound International Monitoring System over seven years. This study demonstrates that the new source model improves the prediction rate of observations by around 20 percent points against previous models. The performance is enhanced when the new model is combined with a wind‐dependent attenuation and an ocean wave model that includes coastal reflection. Plain Language Summary Microbaroms are atmospheric ambient noise below the human hearing threshold. They are generated by ocean waves and can be detected by infrasound sensors worldwide. A better understanding is important because microbaroms could hide signals of interest in the context of the Comprehensive Nuclear‐Test‐Ban Treaty, established to unveil clandestine nuclear explosions worldwide. Furthermore, as microbaroms propagate over long‐ranges through the middle atmosphere, a better knowledge of the received signals provides new insights of middle atmosphere dynamics features that are unresolved in global circulation models. In this study, we use a historical database of microbarom detections to evaluate state‐of‐the‐art models and propose a methodology to simulate microbaroms worldwide.

A method is introduced to reconstruct microbarom soundscapes in absolute values. The soundscapes are compared to remote infrasound recordings from infrasound array I23FR (Kerguelen Island) and in situ recordings by the INFRA-EAR, a biologger deployed near the Crozet Islands. The reconstruction method accounts for all-acoustic contributions, divided into evanescent microbaroms (detectable directly above the source) and propagating microbaroms (detectable over long ranges). It is computed by integrating acoustic intensities over the ocean surface, convolved with the transfer function quantifying the propagation losses and propagation time. The reconstructed soundscapes are found within 2.7 dB for (Formula presented.) of the measurements in the microbarom band of 0.1–0.3 Hz. Infrasonic soundscapes are essential for understanding the ambient infrasonic noise field and are a basic need for applications, such as atmospheric remote sensing, natural hazard monitoring, and verification of the Comprehensive Nuclear-Test-Ban Treaty.

International audience; The study of infrasound (acoustic) and gravity waves sources and propagation in the atmosphere of a planet gives us precious insight on atmosphere dynamics, climate, and even internal structure. The implementation of modern pressure sensors with high rate sampling on stratospheric balloons is improving their study. We analyzed the data from the National Aeronautics and Space Administration Ultra Long Duration Balloon mission (16 May to 30 June 2016). Here, we focus on the balloon's transit of the Andes Mountains. We detected gravity waves that are associated to troposphere convective activity and mountain waves. An increase of the horizontal wavelengths from 50 to 70 km with increasing distance to the mountains is favoring the presence of mountain waves. We also report on the detection of infrasounds generated by the mountains in the 0.01–0.1 Hz range with a pressure amplitude increase by a factor 2 relative background signal. Besides, we characterized the decrease of microbaroms power when the balloon was flying away from the ocean coast. These observations suggest, in a way similar to microseisms for seismometers, that microbaroms are the main background noise sources recorded in the stratosphere even far from the ocean sources. Finally, we observed a broadband signal above the Andes, between 0.45 and 2 Hz, probably associated with a thunderstorm. The diversity of geophysical phenomena captured in less than a day of observation stresses the interest of high rate pressure sensors on board long-duration balloon missions.

著者人数: 11名, Accepted: 2017-10-06, 資料番号: SA1170126000

The Central and Eastern European Infrasound Network (CEEIN) detects significant irregularities in microbarom arrivals between 15 and October 18, 2017. The processes driving the irregular microbarom arrivals are searched in the microbarom source region in the North Atlantic and in the stratospheric waveguide. Generation of microbaroms is simulated using ocean WAVEWATCHIII wave-action model and an updated source theory which combines the effects of both finite depth ocean and source directivity. Signal propagation in a uniform range independent atmosphere is considered. In the studied time interval, a dominant moving microbarom source occurs at the tail of the post-tropical storm Ophelia. The storm Ophelia provides an opportunity to study the development of an intense microbarom source on the open ocean and particularly in coastal waters. Discrepancies between observations and modelling results are identified and discussed. This study shows that the state-of-the-art wave-action models are underestimated in coastal areas during storms which can pose a problem for civil security in coastal areas. The capability of the CEEIN stations to monitor microbaroms is proved. Measurement biases and uncertainties associated with the configurations of the CEEIN stations and current limitations of the processing method are discussed and improvements are suggested.

The network of infrasound stations (I34MN) in Mongolia daily registers set of infrasound from various sources besides explosions. The data from explosions in mines in region and from other sources detected since 2000 to 2009 in seismic and infrasound stations is analyzed. The analysis these signals dependence of speed distribution of sound from seasonal, wind forces and direction moreover on short distances. From detected in infrasound stations (I34MN) in year 80-90 % of signals make microbaroms, the wide range of their sources is visible from the frequency analysis. From the general analysis registered seismo and acoustic signals of explosions on the seismic and infrasound networks stations miscalculate not only speeds of distribution of sounds on close distances (50-500 km), and also the speed model of atmosphere is made.

Data on observations of acoustic gravity waves and variations in the electric field strength in the surface layer of the atmosphere are presented. Analysis of the obtained data shows that synchronous variations in the pressure and electric field strength appear with the passage of a weather front, solar terminator, and in some other cases. It is seen that the amplitude of electric field perturbations is approximately proportional to the amplitude of variation in the pressure. A possible mechanism of generating electric field perturbations during the passage of microbaroms has been considered.

Characteristic features of infrasound waves observed in the Antarctic represent a physical interaction relating surface environment in the continental margin and surrounding Southern Ocean. Source location of several infrasound events is demonstrated by using combination of two array deployments along a coast of the Lutzow-Holm Bay (LHB), East Antarctica, for data retrieving period in January-June 2015. These infrasound arrays being established in January 2013 clearly detected temporal variations in frequency content and propagation direction of the identified seven large events. Many of these sources are assumed to have cryoseismic origins; the ice-quakes associated with calving of glaciers, discharge of sea-ice, collision between sea-ice and icebergs around the LHB. Detail and continuous measurements of infrasound waves in the Antarctic are a proxy for monitoring regional environment as well as climate change in high southern latitude.

Long-term Infrasound data at Syowa Station (SYO; 39E, 69S), in the Lutzow-Holm Bay (LHB), East Antarctica was analyzed during the period from 2008 to 2014. Seasonal variations in microbaroms and high-frequency harmonic tremors were especially investigated. Infrasound data were strongly involved in local dynamics of surface environments. The microbaroms have relatively low amplitudes in austral winters by extending area of sea-ice around LHB, with decreasing oceanic swell loading effects. The other reasons of seasonal variations in microbaroms amplitudes were caused by the affections of a number of storms during whole year and snow accumulation over the porous hoses on the infrasound station at SYO. In contrast, non-linear high-frequency harmonic tremors were considered to be caused by the katabatic winds from Antarctic continent flowing in northeast dominant orientation. The high-frequency tremors had characteristics of daily variations in particular in austral summer. It is required to continue more than a few years of observation in order to identify relationships with climate change and global warming effects in the Antarctic. Continuous measurement of infrasound in the coastal margin of Antarctica is a proxy for monitoring multi-sphere interaction between the continent and surrounding Southern Ocean.

SUMMARYThe nonlinear interaction of ocean surface waves produces coherent infrasound noise—microbaroms—between 0.1 and 0.5 Hz. Microbaroms propagate through the atmosphere over thousands of kilometres due to low absorption and efficient ducting between the ground and the stratopause. These signals are globally and permanently detected by the International Monitoring System (IMS) infrasound network, which has been established to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty. At the International Data Centre (IDC) in Vienna, where IMS data are routinely processed, microbarom detections appear in overlapping frequency bands, and are treated as false alarms. Therefore, understanding the variability in microbarom detections is essential to support the IDC in the reduction of the false alarm rate. In this study, microbarom amplitudes and the direction of arrivals at the German infrasound station IS26 were modelled. For the simulations, the source was described by an operational ocean wave interaction model, and the signal amplitude was modelled using a semi-empirical attenuation relation. This relation strongly depends on middle atmosphere (MA; i.e. 15–90 km altitude) dynamics; however, vertical temperature and wind profiles, provided by numerical weather prediction (NWP) models, exhibit significant biases and differences when compared with high-resolution light detection and ranging instrument (lidar) soundings in altitudes where infrasound signals propagate. To estimate uncertainties in the modelled amplitude, a fully autonomous lidar for MA temperature measurements was installed at IS26. Temperature and wind perturbations, considering observed biases and deviations, were added to the operational high-resolution atmospheric model analysis produced by the European Centre for Medium-Range Weather Forecasts. Such uncertainties in horizontal winds and temperature strongly impact propagation conditions, explaining almost 97 per cent of the actual detections, compared to 77 per cent when using the direct output of the NWP model only. Incorporating realistic wind and temperature uncertainties in NWP models can thus significantly improve the understanding of microbarom detections as well as the detection capability of a single station throughout the year.

Infrasound environments in the Czech microbarograph network were studied. Reference Fourier amplitude spectra were calculated from data measured at three microbarograph sites of the network in May 2011-April 2012; directional analysis of infrasound arrivals in the frequency band 0.15-0.4 Hz was performed for the microbarograph array at Panska Ves in May 2014-April 2015. Diurnal, seasonal and site-to-site variability of the reference spectra was evaluated. Site-to-site variability is influenced by the location of the respective sensors in the open air and inside the observatory buildings and by local noise phenomena like wind turbines. Diurnal variability is well developed in summer with maximum ambient noise levels during the daytime and minima at night. Seasonal variability is observed at night with maxima in winter and minima in summer. Wind and wind eddies seem to be an important source of ambient noise in measurements in the Czech microbarograph network. A distinct spectral peak occurs near 0.2 Hz with amplitudes by about one order of magnitude higher in winter than in summer. Its seasonal variability is related to seasonal propagation of microbaroms from the source region in the Northern Atlantic.

Some data on a high-frequency infrasound recorded within a range of 2–16 Hz (voice of the sea) in the water area of the Black Sea are given. Different parameters of the recorded infrasonic signal—the direction and phase velocity of arriving infrasonic waves, spectral composition, and coherence—have been studied. In the course of measurements, both wind and wave conditions in the water area of the Black Sea have been studied in detail. The collision of two atmospheric vortices was observed a few hours before the first arrivals of infrasonic waves, and the collision of differently directed sea waves was observed during infrasound recording. The direction of the arrivals of infrasonic waves coincides with the direction between the zone of collision of sea waves and the point of infrasound recording. The assumption was made that, in order to explain the observed infrasonic waves, it is necessary to use the mechanism responsible for the emission of infrasound into the atmosphere by standing surface waves formed due to the nonlinear interaction of surface waves propagating in opposite directions and to take into account the frequency-filtering properties of both wind-velocity and temperature stratifications of the atmosphere itself along the path of infrasound propagation. This assumption calls for additional verification.

Infrasound microphones on free flying balloons experience very little wind noise, can cross regions that lack ground station coverage, and may capture signals that seldom reach the Earth’s surface. Despite the promise of this technique, until recently very few studies had been performed on balloon-borne acoustic sensors. We summarize the history of free flying infrasound stations from the late 1940s to 2014 and report on results from a series of studies spanning 2014–2016. These include the first efforts to record infrasound in the stratosphere in half a century, the presence of a persistent ocean microbarom peak that is not always visible on the ground, and the detection of distant ground explosions. We discuss the unique operational aspects of deploying infrasound sensors on free flying balloons, the types of signals detected at altitude, and the changes to sensor response with height. Finally, we outline the applications of free flying infrasound sensing systems, including treaty verification, bolide detection, upper atmosphere monitoring, and seismoacoustic exploration of the planet Venus.

The ability of the International Monitoring System (IMS) global infrasound network to detect atmospheric explosions and other events of interest depends strongly on station-specific ambient incoherent noise and clutter (real but unwanted infrasound waves, coherent on an infrasound array). Characterization of coherent infrasound is important for quantifying the recording environment at each station and for assessing the detection probability of specific signals of interest. We systematically characterize coherent infrasound recorded by the IMS network over 10 years on 41 stations over a broad frequency range (0.01–5 Hz). This multiyear processing emphasizes continuous signals such as mountain associated waves and microbaroms, as well as persistent transient signals such as repetitive volcanic, surf, thunder, or anthropogenic activity. We estimate the primary source regions of continuous coherent infrasound using a global cross-bearings approach. For most IMS arrays, the detection of persistent sources is controlled by the dynamics of the stratospheric wind circulation from daily to seasonal scales. Systematic and continuous characterization of multiyear array detections helps to refine knowledge of the source of ambient ocean noise and provides additional constraints on the dynamics of the middle atmosphere where data coverage is sparse.

Infrasonic waves continue to be a staple of threat identification due to their presence in a variety of natural and man-made events, along with their low-frequency characteristics supporting detection over great distances. Considering the large set of phenomena that produce infrasound, it is critical to develop methodologies that exploit the unique signatures generated by such events to aid in threat identification. In this work, we propose a new infrasonic time-series classification technique based on the recently introduced Wavelet Scattering Transform (WST). Leveraging concepts from wavelet theory and signal processing, the WST induces a deep feature mapping on time series that is locally time invariant and stable to time-warping deformations through cascades of signal filtering and modulus operators. We demonstrate that the WST features can be utilized with a variety of classification methods to gain better discrimination. Experimental validation on the Library of Typical Infrasonic Signals (LOTIS)—containing infrasound events from mountain associated waves, microbaroms, internal atmospheric gravity waves and volcanic eruptions—illustrates the effectiveness of our approach and demonstrate it to be competitive with other state-of-the-art classification techniques.

Infrasound propagation through various atmospheric conditions and interaction with environmental factors in- duce highly non-linear and non-stationary effects that make it difficult to extract reliable attributes for classi- fication. We present featureless classification results on the Library of Typical Infrasonic Signals using several deep learning techniques, including long short-term memory, self-normalizing, and fully convolutional neural net- works with statistical analysis to establish significantly superior models. In general, the deep classifiers achieve near-perfect classification accuracies on the four classes of infrasonic events including mountain associated waves, microbaroms, auroral infrasonic waves, and volcanic eruptions. Our results provide evidence that deep neural network architectures be considered the leading candidate for classifying infrasound waveforms which can directly benefit applications that seek to identify infrasonic events such as severe weather forecasting, natural disaster early warning systems, and nuclear weapons monitoring.

Continuous infrasonic signals produced by the ocean surface interacting with the atmosphere, termed microbaroms, are known to be generated by a number of phenomena including large maritime storms. Storm generated microbaroms exhibit axial asymmetry when observed at locations far from the storm due to the source location being offset from the storm center. Because of this offset, a portion of the microbarom energy will radiate towards the storm center and interact with the winds in the region. Detailed here are predictions for the propagation of microbaroms through an axisymmetric, three-dimensional model storm. Geometric propagation methods have been utilized and the predicted horizontal refraction is found to produce signals that appear to emanate from a virtual source near the storm center when observed far from the storm. This virtual source near the storm center is expected to be observed only from a limited arc around the storm system with increased extent associated with more intense wind fields. This result implies that identifying the extent of the arc observing signal from the virtual source could provide a means to estimate the wind structure using infrasonic observations far from the storm system.

The characteristic features of infrasound waves observed in Antarctica reveal a physical interaction involving surface environmental variations in the continent and the surrounding Southern Ocean. A single infrasound sensor has been making continuous recordings since 2008 at Syowa Station (SYO; 69.0S, 39.6E) in the Lutzow-Holm Bay (LHB) of East Antarctica. The continuously recorded data clearly show the contamination of background oceanic signals (microbaroms) throughout all seasons. In austral summer 2013, several field stations with infrasound sensors were established along the coast of the LHB. Two infrasound arrays of different diameters were set up: one at SYO (with a 100-m spacing triangle) and one in the S16 area on the continental ice sheet (with a 1000-m spacing triangle). In addition to these arrays, isolated single stations were deployed at two outcrops in the LHB. These newly established arrays clearly detected the propagation direction and frequency content of microbaroms from the Southern Ocean. Microbarom measurements are a useful tool for characterizing ocean wave climates, complementing other oceanographic and geophysical data from the Antarctic. In addition to the microbaroms, several other remarkable infrasound signals were detected, including regional earthquakes, and airburst shock waves emanating from a meteoroid entering the atmosphere over the Russian Republic on 15 February 2013. Detailed and continuous measurements of infrasound waves in Antarctica could prove to be a new proxy for monitoring regional environmental change as well as temporal climate variations in high southern latitudes.

This paper presents a contemporary review of vertical coupling in the atmosphere and ionosphere system induced by internal waves of lower atmospheric origin. Atmospheric waves are primarily generated by meteorological processes, possess a broad range of spatial and temporal scales, and can propagate to the upper atmosphere. A brief summary of internal wave theory is given, focusing on gravity waves, solar tides, planetary Rossby and Kelvin waves. Observations of wave signatures in the upper atmosphere, their relationship with the direct propagation of waves into the upper atmosphere, dynamical and thermal impacts as well as concepts, approaches, and numerical modeling techniques are outlined. Recent progress in studies of sudden stratospheric warming and upper atmospheric variability are discussed in the context of wave-induced vertical coupling between the lower and upper atmosphere., Accepted for publication in Advances in Space Research

Accepted: 2017-06-13, 資料番号: SA1170206000

We present the results of infrasonic interferometry applied to microbaroms, obtained from ambient noise. For this purpose the “Large Aperture Infrasound Array” (LAIA) was used, which has been installed in the Netherlands. Preprocessing appeared to be an essential step in enhancing the microbarom signals from ambient noise that strongly influences the results of the interferometry. Both the state of the atmosphere and the noise characteristics are taken into account to assess the strength of the cross correlation. The delay time of the microbaroms between two stations is determined through cross correlating the recordings. By calculating the cross correlations between all 55 station pairs of LAIA, we are able to find the delay time of microbaroms up to a interstation distance of 40.6 km. Using the strength of the cross correlations, we are able to show that the coherence of the microbaroms along the direction of arrival is higher than orthogonal to it. A comparison of the atmospheric state, with a cross correlation, over a period of 10 days, reveals that the infrasound propagation over the array is correlated with the tropospheric temperature and wind. Based on the cross correlations between the three closest stations, we are able to passively estimate the effective sound speed and the wind speed as a function of time.

Microbaroms are a continuous infrasonic signal in the 0.15 to 0.3 Hz band caused by the collision of oceanic surface waves of equal period. Such signals are often generated by large maritime storms. Current formulation of the generation mechanism predicts that the microbarom source location due to a large maritime storm in the open ocean is generally located several hundreds of kilometers from the eye of the storm. Assuming such a source location to be correct, propagation of the microbaroms along paths which pass near the storm center as well as those which propagate away from the storm structure have been examined using geometric acoustics. Microbarom propagation paths which pass near the storm center are refracted by the storm winds and are found to have back azimuths directed toward a virtual source around the storm center. Microbarom propagation paths which do not pass near the storm center are found to have back azimuths directed toward the actual source region. To validate these predictions, data from microbarom signals generated by hurricanes in the Atlantic Ocean have been collected along the east coast of the United States during the 2010 and 2011 Atlantic hurricane seasons. Data from several storm events are presented here for comparison with model predictions. In general, the observations are in agreement with the predictions of the propagation model.

The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicted signals with the observed ones at all IMS stations shows good agreements for both directional and amplitude information. Building a global reference database of oceanic noise sources opens new perspectives for providing additional integrated constraints on middle atmosphere dynamics and disturbances.The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicte...

The atmospheric wind and temperature can be estimated through the traveltimes of infrasound between pairs of receivers. The traveltimes can be obtained by infrasonic interferometry. In this study, the theory of infrasonic interferometry is verified and applied to modeled stratospherically refracted waves. Synthetic barograms are generated using a raytracing model and taking into account atmospheric attenuation, geometrical spreading, and phase shifts due to caustics. Two types of source wavelets are implemented for the experiments: blast waves and microbaroms. In both numerical experiments, the traveltimes between the receivers are accurately retrieved by applying interferometry to the synthetic barograms. It is shown that microbaroms can be used in practice to obtain the traveltimes of infrasound through the stratosphere, which forms the basis for retrieving the wind and temperature profiles.

Infrasound displays a special capacity to affect human health and adaptation because its frequencies and amplitudes converge with those generated by the human body. Muscle sounds and whole-body vibrations are predominately within the 5- to 40-Hz range. The typical amplitudes of the oscillations are within 1–50 μm, which is equivalent to the pressures of about 1 Pa and energies in the order of 10−11 W m−2. Infrasound sources from the natural environment originate from winds, microbaroms, geomagnetic activity, and microseisms and can propagate for millions of meters. Cultural sources originate from air moving through duct systems within buildings, large machinery, and more recently, wind turbines. There are also unknown sources of infrasound. It is important to differentiate the effects of infrasound from the awareness or experience of its presence. Moderate strength correlations occur between the incidences of infrasound and reports of nausea, malaise, fatigue, aversion to the area, non-specific pain, and sleep disturbances when pressure levels exceed about 50 db for protracted periods. Experimental studies have verified these effects. Their validity is supported by convergent quantitative biophysical solutions. Because cells interact through the exchange of minute quanta of energy that corresponds with remarkably low levels of sound pressure produced by natural phenomena and wind turbines upon the body and its cavities, traditional standards for safety and quality of living might not be optimal.

[1] The ability of the International Monitoring System (IMS) infrasound network to detect atmospheric nuclear explosions and other signals of interest is strongly dependent on station-specific ambient noise. This ambient noise includes both incoherent wind noise and real coherent infrasonic waves. Previous ambient infrasound noise models have not distinguished between incoherent and coherent components. We present a first attempt at statistically and systematically characterizing coherent infrasound recorded by the IMS. We perform broadband (0.01–5 Hz) array processing with the IMS continuous waveform archive (39 stations from 1 April 2005 to 31 December 2010) using an implementation of the Progressive Multi-Channel Correlation algorithm in log-frequency space. From these results, we estimate multi-year 5th, 50th, and 95th percentiles of the RMS pressure of coherent signals in 15 frequency bands for each station. We compare the resulting coherent infrasound models with raw power spectral density noise models, which inherently include both incoherent and coherent components. Our results indicate that IMS arrays consistently record coherent ambient infrasound across the broad frequency range from 0.01 to 5 Hz when wind noise levels permit. The multi-year averaging emphasizes continuous signals such as oceanic microbaroms, as well as persistent transient signals such as repetitive volcanic, surf, thunder, or anthropogenic activity. Systematic characterization of coherent infrasound detection is important for quantifying a station's recording environment, signal-to-noise ratio as a function of frequency and direction, and overall performance, which all influence the detection probability of specific signals of interest.

International audience; Several characteristic waves detected by seismographs in Antarctic stations have been recognized as originating from the physical interaction between the solid earth and the atmosphere-ocean-cryosphere system surrounding the Antarctic and may be used as a proxy for characterizing ocean wave climate. A Chaparral-type infrasound sensor was installed at Syowa Station (SYO; 39.6E, 69.0S), East Antarctica, in April 2008 during the International Polar Year (IPY2007-2008). Matching data are also available for this time period from the existing broadband seismic recorder located close by. Continuous infrasound data for 2008-2009 include background signals (microbaroms) with a broad peak in the wave period between the values of 4 and 10 s. Signals with the same period are recorded by the broadband seismograph at SYO (microseisms). This period band is identified as double-frequency microseisms/baroms (DFM). The DFM have relatively lower amplitudes during winter. We suggest that this is due to the sea-ice extent around the coast causing a decreased ocean loading effect. In contrast, the single frequency microseisms/baroms with a peak in period between 12 and 30 s are observed under storm conditions, particularly in winter. On the infrasound data, stationary signals are identified with harmonic overtones at a few Hertz to lowermost human audible band, which we suggest is due to local effects such as sea-ice cracking and vibration. Microseism measurements are a useful proxy for characterizing ocean wave climate, complementing other oceanographic and geophysical data. At SYO, continuous monitoring by both broadband seismograph and infrasound contributes to the Federation of Digital Seismographic Networks, the Comprehensive Nuclear-Test-Ban Treaty in the high southern latitudes and the Pan-Antarctic Observations System under the Scientific Committee on Antarctic Research.

Characteristic infrasound waves are clearly recorded at Syowa Station (SYO), East Antarctica, involving physical interaction in surrounding environments at the continent and SouthernOcean. A Chaparral microphone type infrasound sensor is deployed at SYO during the International Polar Year (IPY2007-2008), the most diverse international science program held recently. Continuous recorded data in 2008-2010 indicate a contamination of background oceanic signals (microbaroms). The characteristic signals are identified as the “microbaroms” with peaks between 4 and 10 s in the records. The peak amplitudes of microbaroms may be enhanced by the extratropical cyclonic storms and wind noises in Southern Ocean. The microbaroms has relatively lower amplitudes during austral winters, which may be caused by the larger amount of the sea-ice extent around theLutzow-HolmBaynear SYO, with decreasing the ocean wave loading effects. In addition, the large energy with intrinsic periods between 12 and 30 s are observable under excellent storm conditions, particularly in local winter. The oceanic effects appearing on infrasound data are modulated by the presence of sea-ice and explained by a relationship between the atmosphere-ocean-cryosphere systems. Microbaroms measurements could be a useful tool for characterizing ocean wave climate, as well as a new proxy for monitoring a regional environmental variation inAntarctica.

Repeated releases of experienced homing pigeons from single sites were conducted between 1972 and 1974 near Cornell University in upstate New York and between 1982 and 1983 near the University of Pittsburgh in western Pennsylvania, USA. No annual variation in homing performance was observed at these sites in eastern North America, in contrast to results from a number of similar experiments in Europe. Assuming pigeons home using low-frequency infrasonic signals (~0.1-0.3 Hz), as has been previously proposed, the annual and geographic variability in homing performance within the northern hemisphere might be explained, to a first order, by seasonal changes in low-frequency atmospheric background noise levels related to storm activity in the North Atlantic Ocean, and by acoustic waveguides formed between the surface and seasonally reversing stratospheric winds. In addition, increased dispersion among departure bearings of test birds on some North American release days was possibly caused by infrasonic noise from severe weather events during tornado and Atlantic hurricane seasons.

Atmospheric infrasonic signals were classified on the basis of data obtained in the United States (University of Alaska, Fairbanks) and in the Antarctic region (Windless Bight) from 1980 to 1983. The data archive included five classes of signals from different sources: explosions, mountain associated waves, microbaroms, volcanic infrasound, and auroral infrasonic waves. This classification was based on the theory of testing statistical hypotheses. The possibilities of separating these classes were studied. It was shown that the signals (from the archive used) that are characteristic of explosions and volcanic activity can be rather easily separated from those characteristic of mountain associated waves, microbaroms, and auroral infrasonic waves.

The ability of the International Monitoring System (IMS) global infrasound network to detect atmospheric explosions and events of interest strongly depends on station specific ambient noise which includes both incoherent wind noise and real coherent infrasonic waves. To characterize the coherent ambient noise, a broadband array processing was performed on 10 years of continuous recordings at IMS stations. Multi-year comparisons between the observed and modeled directional microbarom amplitude variations at several IMS stations using two-dimensional wave energy spectrum ocean wave products are performed to build of a reference database of infrasound oceanic sources. Microseisms are attributed the same source processes as microbaroms, involving the interaction of standing ocean waves. To further evaluate oceanic wave action models, the infrasound analysis will be supplemented with several other approaches including microseisms collected at seismic instrumentation (single stations and arrays). The expected b...

The first observations of microbaroms with single absolute barometers are presented and discussed. Microbaroms are pulses of atmospheric infrasound emitted by ocean surface waves. They can propagate over thousands of kilometers through the atmosphere, and they can reach altitudes well into the upper atmosphere before they are refracted down to the earth’s surface. Typical microbarom periods are 5 s, typical wavelengths are 1.5 km, and typical surface amplitudes are 100 mPa (1 μbar). The data presented here were collected during the 2-week period from 26 February through 10 March 2008 in Amherst, Massachusetts, which is located about 150 km away from the Atlantic Ocean. The authors report for the first time, to the best of their knowledge, an f−5 microbarom frequency spectrum, which is consistent with Phillips’s f−5 ocean surface wave equilibrium spectrum.

Summary For four years continuous recording of infrasonic signals in the frequency range 0.1 to 1 Hz, known as microbaroms, has been conducted at Palisades, New York. The microbaroms we recorded are radiated into the atmosphere by interfering ocean waves in the North Atlantic as far as 2000 km away. A characteristic diurnal variation in the amplitude of the received signal has been noted, independent of any variation in the source. We conclude that the variation is due to variations of the factors affecting atmospheric sound propagation, namely wind and temperature. In winter a semidiurnal variation in signal amplitude is observed, with maximum reception around 11 : 00 and 22; 00 local time. Reference to wind and temperature observations in the literature shows that at these times the lowest level of reflection of the vertically propagating signal occurs between 100 and 110 km due to the presence of strong east winds. At 18 : 00, time of minimum amplitudes, the reflection level rises to about 115 km because of a change in tidal wind phase. Viscous dissipation associated with the changed reflection height can account for the observed signal weakening. A third maximum, a less regular effect, is found to be related to more variable winds between 95 and 105 km. In summer, reflection is found to occur from about 50 km due to the presence of stratospheric easterlies. The summer diurnal variation, different from that of the winter, exhibits only a weak minimum about 20 : 00. This appears to result from a diurnal temperature variation superimposed on a diurnal wind variation. Abnormally high microbaroms were recorded at times that can be related to an atmospheric event known as a stratospheric warming. Microbaroms thus provide a continuously available natural mechanism for probing the upper atmosphere. We conclude that the establishment of microbarom observation systems could give a comprehensive technique for monitoring several upper atmospheric parameters.

Previous investigations [Frazier, ASA Pittsburg (2015); ASA San Francisco (2013); ASA Kansas City (2012)] have demonstrated that exploiting the correlated statistical properties of wind noise on short spatiotemporal scales with a compact array permits enhanced detection and reduced estimation error of infrasound transients resulting from explosions. The approach was shown to be effective in real-time using non-parametric wind noise models. More recently [Frazier, ASA Boston (2017); ASA Salt Lake City (2016)], the basic ideas of the approach were extended to estimation of quasi-stationary infrasound signals such as microbaroms using parametric wind noise models and was demonstrated to be effective, but the computational requirements were too demanding for real-time implementation. This presentation will highlight modifications that enable real-time implementation for quasi-stationary signal estimation.

This paper discusses the technique for assessing the state of atmospheric acoustic channels (AACs) for the long-distance propagation of microbaroms. We calculated two possible microbarom propagation paths to station “Badary” from the sources located in (1) the North Atlantic at an azimuth of 320° and (2) the Northwestern Pacific at an azimuth of 60°. We investigate the spatio-temporal structure of the AAC. The experimental data are compared with modeling results.

A two-fluid model, air over seawater, is used to investigate the radiation of infrasound by ocean waves. The acoustic radiation which results from the motion of the air/water interface is known to be a nonlinear effect. The second-order nonlinear contribution to the acoustic radiation is computed and the statistical properties of the received microbarom signals are related to the statistical properties of the ocean wave system. The physical mechanisms and source strengths for radiation into the atmosphere and ocean are compared. The observed ratio of atmospheric to oceanic microbarom peak pressure levels (approximately 1 to 1000) is explained.

SUMMARY In an eventual aim to detect background long-period acoustic waves in the atmosphere that have to be excited incessantly by the same sources as for background free oscillations of the solid Earth, we installed a cross array of barometers in a 10-km-wide university forest in central Honshu. The array has 28 microbarometers employing quartz crystal resonator technology with a station spacing of about 500 m. We analysed 1-s continuous sampling records in a time period from 2002 March to 2003 November to obtain 1-D frequency–wavenumber spectra and 2-D frequency–slowness spectra to detect three kinds of waves in the seismic band from 1 mHz to 0.5 Hz. We measured the dispersion curves of these waves with an assumption of stochastic stationary plane waves. The waves we detected are (1) background internal gravity waves travelling from northwest at frequencies from 1 to 5 mHz with a phase velocity of about 50 m s−1, (2) background acoustic waves traveling from northwest at frequencies from 0.01 to 0.1 Hz with a frequency-dependent phase velocity of about 400 m s−1 at 0.1 Hz and about 800 m s−1 at 0.01 Hz and (3) microbaroms travelling from southeast at frequencies from 0.1 to 0.5 Hz with a phase velocity of about 350 m s−1. Internal gravity waves (1) dominate over the random component of atmospheric turbulence in the relevant frequency range. At higher frequencies the random component with coherent length shorter than 500 m is dominant. Background acoustic waves (2) show clear annual variation with maxima in winter. The annual variation and their incident azimuths suggest their possible origins in mountain regions. Microbaroms (3) are most likely to be excited by standing ocean waves in nearby coastal regions. The power spectra of these acoustic waves were compared to the seismic spectra obtained in the same university forest. Their spectral shapes are mutually similar and change commonly with variable weather conditions, indicating that the microbaroms and microseisms have their common origins.

This work reports on a study of the distribution of wave packet sizes contained in intervals of continuous microbarom activity. Microbaroms are a class of atmospheric infrasound that is characterized by narrow-band, nearly sinusoidal, waveforms with periods near 5 s. They are known to be generated by marine storms, presumably through a nonlinear interaction of surface waves, however the detailed analysis of the process is still incomplete. The data analyzed were obtained using the University of Alaska infrasound array of four microphones located in central Alaska. Because of the narrow-band feature of the microbarom signals, the Hilbert transform is applicable as a method for finding phase breaks in the signal. The phase breaks are interpreted as the demarcation of the boundaries of wave packets. When applied to long sequences of microbaroms a broad distribution of packet lengths is found that diminishes monotonically with length and has a mean near 10 cycles and a variance nearly as large. The distribution function decreases exponentially with packet length. The distribution of packet sizes is influenced by the presence of multiple sources and multiple propagation paths between the sources and the sensor array. Identification of individual packets should open the way to a more detailed analysis of microbarom wave trains. After separating the wave train into individual wavelets the intermicrophone correlation is estimated as a function of microphone separation. As has been observed in earlier microbarom studies, a decrease in correlation was observed for microphone pairs orthogonal to the direction of propagation when compared to correlations between microphones spaced along the direction of wave propagation.

This paper describes new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes. A basis for estimating atmospheric absorption up to 160 km is described. The estimated values at altitudes above 90 km must be considered as only approximate due to uncertainties about the composition of the atmosphere above 90 km and simplifying assumptions. At high altitudes, classical and rotational relaxation absorption are dominant, as opposed to absorption by molecular vibrational relaxation that is the principle atmospheric absorption loss mechanism for primary sonic booms propagating downward from a cruising supersonic aircraft. Classical and rotational relaxation absorption varies inversely with atmospheric pressure, thus increasing in magnitude at high altitudes as atmospheric pressure falls. However, classical and rotational losses also relax at the high values of frequency/pressure reached at high altitudes and thus, for audio and infrasonic frequencies, begin to dec...

A four-element, 1-km-aperture seismo-acoustic array has been designed and installed northeast of Seoul, Korea. Each element of the array consists of a GS-13 vertical seismometer (1 Hz) in a shallow borehole (∼10 m) and a low-frequency acoustic gauge connected to an 11-element hose array (7.6-m hoses) at the surface. The array is being used to assess the importance of colocated seismic and acoustic sensors for the purposes of (1) quantifying wind as a source of seismic and acoustic noise, (2) constraining propagation path effects in the atmosphere and solid earth, (3) locating the sources of the waves, and (4) characterizing the source type. Seismic noise estimates illustrate a level that is only slightly above the low-noise model on average. Acoustic noise levels resolve the microbaroms during low-noise times but document a nearly 50-dB increase in noise during the windiest periods. Infrasonic noise in the 0.01- to 5-Hz band increases rapidly with wind velocity. The seismic noise shows little or no dependence on wind velocity. Analysis of the data from a 2-month time period suggests that there are many more acoustic signals than seismic (4–10 times as many). Approximately 1/4 of all seismic signals are associated with an acoustic arrival. The vast majority of seismo-acoustic observations come from sources in the 30- to 200-km range and occur during working hours, local time. The 30- to 200-km observation distance is surprising in that average atmospheric velocity models predict no acoustic returns in this range. Average atmospheric models modified by meteorological data for the troposphere indicate the possibility of ducting in the troposphere as an explanation for these arrivals. Event location is based upon regional seismic phase identification ( P n, P g, P m P, L g, R g) using the array and backazimuth estimates from both the seismic and acoustic data. Many of the infrasound signals have good signal-to-noise ratios from 1 to beyond 4 Hz. Despite the small size of the array, event clusters are identified at regional distances. Events associated with acoustic signals are presumed to be from mining regions. The existence of Rg arrivals and dominance of P energy at high frequency are consistent with this interpretation. Ground truth in the form of in-mine observations has validated that two of the clusters come from construction and mine blasts.

A method for real-time estimation of stationary infrasound signals such as microbaroms in wind noise at low signal-to-noise ratios using a compact infrasound sensor array is presented. A compact array is defined as a sensor array that has an aperture that is much smaller than the shortest infrasound wavelengths of interest and is unsuitable for estimation of direction-of-arrival. In this application, the spacing between sensors results in the measured wind noise being highly correlated, and therefore, simple averaging cannot be used to obtain a good estimate of the infrasound signal. However, by adequately modeling the spatiotemporal wind noise process, array gain can be realized. The method is based on using a Kalman Filter that is designed with the assumption that the measured wind noise can be adequately modeled as a dynamic Gaussian random field with a Matern covariance function (demonstrated previously at the ASA Meeting in Salt Lake City, Utah, May, 2016). The presentation describes how to design the Kalman Filter in order to estimate the infrasound signal of interest, demonstrates the filter performance using synthetic and measured data from a compact array, and describes how to extend the method to support changing wind conditions and non-stationary infrasound signals.

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

Low frequency sound (infrasound) measurements have been selected within the Comprehensive Nuclear-Test-Ban Treaty (CTBT) as a technique to detect and identify possible nuclear explosions. The Seismology Division of the Royal Netherlands Meteorological Institute (KNMI) operates since 1999 an experimental infrasound array of 16 micro-barometers. Here we show the rare detection and identification of an exploding meteor above Northern Germany on November 8th, 1999 with data from the Deelen Infrasound Array (DIA). At the same time, sound was radiated from the Atlantic Ocean, South of Iceland, due to the atmospheric coupling of standing ocean waves, called microbaroms. Occurring with only 0.04 Hz difference in dominant frequency, DIA proved to be able to discriminate between the physically different sources of infrasound through its unique lay-out and instruments. The explosive power of the meteor being 1.5 kT TNT is in the range of nuclear explosions and therefore relevant to the CTBT.

Infrasonic waves propagate at long range through atmospheric ducts resulting from the stratification of atmospheric properties. These ducts are characterized by their spatio-temporal variability. Hence, infrasonic waves integrate information upon the atmosphere along their propagation paths. In order to study infrasonic wave propagation, we resort to atmospheric specification combining numerical weather prediction and climatological models. However, due to the lack of observations these models fail to describe small scale variability such as perturbations associated to the presence of internal gravity waves. These waves play an important role in the atmospheric dynamic by transferring momentum to the mean flow at critical levels and at wave-breaking altitudes. In this study, we intend to describe the interaction of infrasonic waves with internal gravity waves in order to understand the long-tail behavior observed in infrasound broadband signals. We developed a model for the propagation of internal waves used to generate realistic perturbations of the background atmospheric states. By using a linear full-wave model of infrasound propagation, our goal is to ultimately relate infrasound characteristics to internal waves properties. We apply those numerical models to infrasound observations recorded during the 2010 deployment of the Utah Test and Training Range (UTTR) experiment.