Wind noise reduction (WNR) is important in the measurement of infrasound. Spatial averaging theory led to the development of rosette pipe arrays. The efficacy of rosettes decreases with increasing wind speed and only provides a maximum of ∼20 dB WNR due to a maximum size limitation. An Optical Fiber Infrasound Sensor (OFIS) reduces wind noise by instantaneously averaging infrasound along the sensor's length. In this study two experiments quantify the WNR achieved by rosettes and OFISs of various sizes and configurations. Specifically, it is shown that the WNR for a circular OFIS 18 m in diameter is the same as a collocated 32-inlet pipe array of the same diameter. However, linear OFISs ranging in length from 30 to 270 m provide a WNR of up to ∼30 dB in winds up to 5 m/s. The measured WNR is a logarithmic function of the OFIS length and depends on the orientation of the OFIS with respect to wind direction. OFISs oriented parallel to the wind direction achieve ∼4 dB greater WNR than those oriented perpendicular to the wind. Analytical models for the rosette and OFIS are developed that predict the general observed relationships between wind noise reduction, frequency, and wind speed.

The 2011 Mw9.0 Tohoku earthquake generated infrasound that was recorded by nine infrasonic arrays. Most arrays recorded a back azimuth variation with time due to the expanse of the source region. We use ray tracing to predict group velocities and back azimuth wind corrections. A Japan accelerometer network recorded ground shaking in unprecedented spatial resolution. We back projected infrasound from arrays IS44 (Kamchatka) and IS30 (Tokyo) to the source region and compare these results with acceleration data. IS44 illuminates the complex geometry of land areas that experienced shaking. IS30 illuminates two volcanoes and a flat area around the city of Sendai, where the maximum accelerations occurred. The arrays and epicentral region define three source‐receiver profiles. The observed broadband energy transmission loss (TL) follows an exponential decay law. The best fitting model, which has parameters that are interpreted to include the effects of geometric spreading, scattering, and the maximum ratio of the effective sound speed in the stratosphere to that at the ground (accounts for stratospheric wind speed), yields a 65% variance reduction relative to predictions from a traditional TL relationship. This model is a simplified version of the model of Le Pichon et al. (2012), which yields an 83% variance reduction for a single frequency, implying that fine‐scale atmospheric structure is required to explain the TL for stratospheric upwind propagation. Our results show that infrasonic arrays are sensitive to ground acceleration in the source region of megathrust earthquakes. The TL results may improve infrasonic amplitude scaling laws for explosive yield.

Barometers deployed as part of the USArray Transportable Array (TA) recorded large (300 Pa peak-to-peak) gravity waves, or gust fronts, that originated near a severe tornadic storm system in the southern United States on April 27, 2011. We present a new method in which the TA is divided into a large number of elemental sub-arrays. Each 3-element sub-array (triad) is sufficiently closely spaced so that the long-period gravity wave signal is coherent, but large enough to provide a robust estimate of the signal's direction and speed. The results from each triad are combined to follow the progress of gravity waves as they propagate across the TA over 2 days in late April, 2011. We observe a large, high-amplitude gravity wave, spanning a region over 200,000 km2, progressing to the NNW away from the tornadic storm region. We also observe gravity waves with lower amplitudes and smaller spatial extent along the gulf coast, propagating southward, away from the storm region. This study demonstrates the functionality of the USArray pressure sensors for analyzing gravity wave dynamics over periods greater than 40 min and across a wide region. In principle the method could be applied to study other long-period wave phenomena recorded by any dense large-scale network and is not limited to gravity waves.

We discuss the use of reverse time migration (RTM) with dense seismic networks for the detection and location of sources of atmospheric infrasound. Seismometers measure the response of the Earth's surface to infrasound through acoustic-to-seismic coupling. RTM has recently been applied to data from the USArray network to create a catalogue of infrasonic sources in the western US. Specifically, several hundred sources were detected in 2007-2008, many of which were not observed by regional infrasonic arrays. The influence of the east-west stratospheric zonal winds is clearly seen in the seismic data with most detections made downwind of the source. We study this large-scale anisotropy of infrasonic propagation, using a winter and summer source in Idaho. The bandpass-filtered (1-5 Hz) seismic waveforms reveal in detail the two-dimensional spread of the infrasonic wavefield across the Earth's surface within approximately 800 km of the source. Using three-dimensional ray tracing, we find that the stratospheric winds above 30 km altitude in the ground-to-space (G2S) atmospheric model explain well the observed anisotropy pattern. We also analyse infrasound from well-constrained explosions in northern Utah with a denser IRIS PASSCAL seismic network. The standard G2S model correctly predicts the anisotropy of the stratospheric duct, but it incorrectly predicts the dimensions of the shadow zones in the downwind direction. We show that the inclusion of finer-scale structure owing to internal gravity waves infills the shadow zones and predicts the observed time durations of the signals. From the success of this method in predicting the observations, we propose that multipathing owing to fine scale, layer-cake structure is the primary mechanism governing propagation for frequencies above approximately 1 Hz and infer that stochastic approaches incorporating internal gravity waves are a useful improvement to the standard G2S model for infrasonic propagation modelling.

Two microphone arrays were deployed in California during 2010 to record microbaroms, quasi‐continuous atmospheric pressure oscillations with a period of ∼5 s. In this paper a time‐progressive, frequency domain beamforming method is developed and used to analyze microbaroms recorded by these and 10 other infrasonic arrays along the North Pacific rim. Common pelagic microbarom sources that move around the North Pacific are observed during the boreal winter. Summertime North Pacific sources are only observed by western Pacific arrays, presumably a result of weaker microbarom radiation and westward stratospheric winds. A well‐defined source is resolved ∼2000 km off the coast of California in January 2011 that moves closer to land over several days. The source locations are corrected for deflection by horizontal winds using acoustic ray trace modeling with range‐dependent atmospheric specifications provided by ground‐to‐space models. The observed source locations do not correlate with anomalies in NOAA Wave Watch 3 (NWW3) model field data. However, application of the opposing wave, microbarom source model of Waxler and Gilbert (2006) to the NWW3 directional wave height spectra output at buoy locations within 1100 km of the western North America coastline predicts microbarom radiation in locations that correlate with observed locations. These results suggest that pelagic North Pacific microbarom radiation detected by infrasonic arrays during the boreal winter could be routinely used to validate NWW3 results in regions with poor sensor coverage.

The recently reinvigorated field of infrasonics is poised to provide insight into atmospheric structure and the physics of large atmospheric phenomena, just as seismology has shed considerable light on the workings and structure of Earth's solid interior. Although a natural tool to monitor the atmosphere and shallow Earth for nuclear explosions, it is becoming increasingly apparent that infrasound also provides another means to monitor a suite of natural hazards. The frequent observation of geophysical sources—such as the unsteady sea surface, volcanoes, and earthquakes—that radiate energy both up into the atmosphere and down into the liquid or solid Earth and transmission of energy across Earth's boundaries reminds us that Earth is an interconnected system. This review details the rich history of the unheard sound in the atmosphere and the role that infrasonics plays in helping us understand the Earth system.

[1] In this study reverse time migration is applied to signals recorded by the 2007–08 USArray, presumably due to acoustic-to-seismic coupling, to detect and locate in two-dimensional space and time 901 sources of atmospheric infrasound, defining the Western United States Infrasonic Catalog (WUSIC). The detections are visually inspected and ranked. Uncertainties are estimated using a bootstrap technique. The method correctly locates most rocket motor detonations in Utah and a bolide explosion in Oregon with an average spatial accuracy of 50 km and 25 km, respectively. The origin time statistics for 2007 and 2008 events are nearly identical and suggest a predominant human origin. The event locations illuminate repeating sources of infrasound, or “infrasonic hot spots,” in Nevada, Utah, and Idaho that are spatially associated with active military areas. The infrasonic arrivals comprise several branches that are observed to a range between 200 and 1500 km to the east and west of the epicenter in the winter and summer, respectively. The optimum group velocities are Gaussian distributed and centered at 295 m/s. A seasonal variation in optimum group velocities exhibits good correlation with atmospheric temperature. The results show that relatively dense seismic networks fill in the gaps between sparsely located infrasound arrays and provide valuable information for regional infrasonic source location and propagation studies. Specifically, the catalogs presented here can be used to statistically validate and improve propagation models, especially above the middle stratosphere where winds are not directly measured by ground-based weather stations or meteorological satellites.