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De Groot-Hedlin, CD, Hedlin MA.  In Press.  Detection of Infrasound Signals and Sources using a Dense Seismic Network. Global Continuous Infrasound Monitoring for Atmospheric Studies. : Springer Geosciences
Hedlin, MAH, Ritsema J, De Groot-Hedlin CD, Hetland EA.  2018.  A multidisciplinary study of the 17 January 2018 bolide terminal burst over southeast Michigan. Seismological Research Letters. 89:2183-2192.   10.1785/0220180157   AbstractWebsite

A meteor that burst above Michigan in early 2018 was recorded by nearby seismometers, regional infrasonic microphones, and optical sensors. The relatively large, but sparse, infrasonic network provided a location and time for the event that was consistent with ground-truth data from the optical sensors, although uncertainty regarding the infrasonic location was large. Seismic arrival times from four local seismometers constrain the location and height of the burst to within kilo-meters and agree with the optical data. A widely used period-yield relation applied to 40 high signal-to-noise recordings of infrasound signals from the event at distances from 2 degrees to 12 degrees indicates a preferred yield of 2.2 tons of trinitrotoluene (TNT) equivalent with a likely range from 0.8 to 8.1 tons. The successful recording of this relatively small meteor suggests that moderate-density infrasonic networks can be used to refine occurrence statistics of bolides, although such studies will likely be complicated by uncertain source yield estimates.

Fan, WY, De Groot-Hedlin CD, Hedlin MAH, Ma ZT.  2018.  Using surface waves recorded by a large mesh of three-element arrays to detect and locate disparate seismic sources. Geophysical Journal International. 215:942-958.   10.1093/gji/ggy316   AbstractWebsite

Surface waves recorded by global arrays have proven useful for locating tectonic earthquakes and in detecting slip events depleted in high frequency, such as glacial quakes. We develop a novel method using an aggregation of small-to continental-scale arrays to detect and locate seismic sources with Rayleigh waves at 20-50 s period. The proposed method is a hybrid approach including first dividing a large aperture aggregate array into Delaunay triangular subarrays for beamforming, and then using the resolved surface wave propagation directions and arrival times from the subarrays as data to formulate an inverse problem to locate the seismic sources and their origin times. The approach harnesses surface wave coherence and maximizes resolution of detections by combining measurements from stations spanning the whole U.S. continent. We tested the method with earthquakes, glacial quakes and landslides. The results show that the method can effectively resolve earthquakes as small as similar to M3 and exotic slip events in Greenland. We find that the resolution of the locations is non-uniform with respect to azimuth, and decays with increasing distance between the source and the array when no calibration events are available. The approach has a few advantages: the method is insensitive to seismic event type, it does not require a velocity model to locate seismic sources, and it is computationally efficient. The method can be adapted to real-time applications and can help in identifying new classes of seismic sources.

De Groot-Hedlin, CD, Hedlin MAH.  2018.  A new automated approach to detecting and locating seismic events using data from a large network. Bulletin of the Seismological Society of America. 108:2032-2045.   10.1785/0120180072   AbstractWebsite

The Automated Event Location Using a Mesh of Arrays (AELUMA) method, originally developed for detection of atmospheric sources using infrasonic data, is modified here to detect and locate seismic events. A key feature of AELUMA is that it does not require a detailed velocity model to locate events. The new method was applied to vertical-component seismic data recorded by the USArray Transportable Array to (1) test its efficacy when applied to a very large dataset, (2) test its ability to detect and accurately locate distinct event types across a geologically diverse region without analyst oversight, and (3) assess the sensitivity and accuracy of the method. Using data filtered from 1 to 8 Hz, 9996 events were detected in clusters within the central United States-with most events located near areas known for anthropogenic activity. The method was compared with three catalogs in Oklahoma-a region known for small anthropogenic events. In comparison with accurate locations from a template study, AELUMA detected all events from M-L >= 1.9 but none below M-L 1.3. The median absolute origin time and location offset were 9.5 s and 6.0 km, respectively. Comparisons of AELUMA's catalog in Oklahoma with two others (produced by Oklahoma Geological Survey [OGS] and the Array Network Facility) showed that AELUMA found more events than either catalog, including clusters of emergent events that were largely missed by the other methods. However, most of the smaller magnitude events detected by OGS were missed by AELUMA, mainly due to the sparser network used by AELUMA.

Hedlin, MAH, De Groot-Hedlin CD, Forbes JM, Drob DP.  2018.  Solar terminator waves in surface pressure observations. Geophysical Research Letters. 45:5213-5219.   10.1029/2018gl078528   AbstractWebsite

We report observations of waveforms in surface pressure made over several years by a network of ground-level barometers in the eastern United States. The waveforms can be reconstructed by superimposing the 4th through 10th subharmonics of the solar day. Some of these solar harmonics are likely generated by the temperature and pressure gradients across the solar terminators. The measurements presented here enable a wave detection analysis which indicates that some waveforms are coherent between stations with a median speed of 49.7 m/s to the southeast. We interpret these propagating signals, which are interference patterns created by internal gravity waves with periods that are subharmonics of a solar day, as a previously undiscovered type of terminator wave. The waveforms appear predominantly postsunrise during winter and postsunset in summer. Their quasi-eastward propagation direction suggests an analogy with "stern" waves left behind by the faster, westward-moving terminator.

De Groot-Hedlin, CD, Hedlin MAH, Hoffmann L, Alexander MJ, Stephan CC.  2017.  Relationships between gravity waves observed at earth's surface and in the stratosphere over the Central and Eastern United States. Journal of Geophysical Research-Atmospheres. 122:11482-11498.   10.1002/2017jd027159   AbstractWebsite

Observations of tropospheric gravity waves (GWs) made by the new and extensive USArray Transportable Array (TA) barometric network located east of the Rockies, in the central and eastern United States and of stratospheric (30-40 km above sea level) GWs made by the Atmospheric Infrared Sounder (AIRS) are compared over a 5 year time span from 2010 through 2014. GW detections in the period band from 2 to 6 h made at the Earth's surface during the thunderstorm season from May through August each year exhibit the same broad spatial and temporal patterns as observed at stratospheric altitudes. At both levels, the occurrence frequency of GWs is higher at night than during the day and is highest to the west of the Great Lakes. Statistically significant correlations between the variance of the pressure at the TA, which is a proxy for GWs at ground level, with 8.1 mu m brightness temperature measurements from AIRS and rain radar precipitation data, which are both proxies for convective activity, indicate that GWs observed at the TA are related to convective sources. There is little, if any, time lag between the two. Correlations between GWs in the stratosphere and at ground level are weaker, possibly due to the AIRS observational filter effect, but are still statistically significant at nighttime. We conclude that convective activity to the west of the Great Lakes is the dominant source of GWs both at ground level and within the stratosphere. Plain Language Summary Observations of tropospheric gravity waves made by the USArray Transportable Array (TA) barometric network located in the central and eastern United States and of stratospheric gravity waves made by the Atmospheric Infrared Sounder were compared over a 5 year time span from 2010 through 2014. Analysis of the TA data shows a gravity wave hot spot that is located on the Earth's surface west of the Great Lakes. Gravity wave occurrence rates on the ground exhibit the same broad spatial and temporal patterns as observed at stratospheric altitudes. Evidence suggests that convective activity is the dominant source of gravity waves both at the ground and in the stratosphere.

De Groot-Hedlin, CD.  2017.  Infrasound propagation in tropospheric ducts and acoustic shadow zones. Journal of the Acoustical Society of America. 142:1816-1827.   10.1121/1.5005889   AbstractWebsite

Numerical computations of the Navier-Stokes equations governing acoustic propagation are performed to investigate infrasound propagation in the troposphere and into acoustic shadow zones. An existing nonlinear finite-difference, time-domain (FDTD) solver that constrains input sound speed models to be axisymmetric is expanded to allow for advection and rigid, stair-step topography. The FDTD solver permits realistic computations along a given azimuth. It is applied to several environmental models to examine the effects of nonlinearity, topography, advection, and two-dimensional (2D) variations in wind and sound speeds on the penetration of infrasound into shadow zones. Synthesized waveforms are compared to a recording of a rocket motor fuel elimination event at the Utah Test and Training Range. Results show good agreement in the amplitude, duration, and spectra of synthesized and recorded waveforms for propagation through 2D atmospheric models whether or not topography, advection, or nonlinearity is explicitly included. However, infrasound propagation through a one-dimensional, range-averaged, atmospheric model yields waveforms with lower amplitudes and frequencies, suggesting that small-scale atmospheric variability causes significant scatter within the troposphere, leading to enhanced infrasound penetration into shadow zones. Thus, unresolved fine-scale atmospheric dynamics are not required to explain infrasound propagation into shadow zones. (C) 2017 Acoustical Society of America.

Stephan, CC, Alexander JM, Hedlin M, De Groot-Hedlin CD, Hoffmann L.  2016.  A case study on the far-field properties of propagating tropospheric gravity waves. Monthly Weather Review. 144:2947-2961.: American Meteorological Society   10.1175/MWR-D-16-0054.1   Abstract

Mesoscale gravity waves were observed by barometers deployed as part of the USArray Transportable Array on 29 June 2011 near two mesoscale convective systems in the Great Plains region of the United States. Simultaneously, AIRS satellite data indicated stratospheric gravity waves propagating away from the location of active convection. Peak perturbation pressure values associated with waves propagating outside of regions where there was precipitation reached amplitudes close to 400 Pa at the surface. Here the origins of the waves and their relationship to observed precipitation are investigated with a specialized model study. Simulations with a 4-km resolution dry numerical model reproduce the propagation characteristics and amplitudes of the observed waves with a high degree of quantitative similarity despite the absence of any boundary layer processes, surface topography, or moist physics in the model. The model is forced with a three-dimensional, time-dependent latent heating/cooling field that mimics the latent heating inside the precipitation systems. The heating is derived from the network of weather radar precipitation observations. This shows that deep, intense latent heat release within the precipitation systems is the key forcing mechanism for the waves observed at ground level by the USArray. Furthermore, the model simulations allow for a more detailed investigation of the vertical structure and propagation characteristics of the waves. It is found that the stratospheric and tropospheric waves are triggered by the same sources, but have different spectral properties. Results also suggest that the propagating tropospheric waves may potentially remotely interact with and enhance active precipitation.

De Groot-Hedlin, CD.  2016.  Long-range propagation of nonlinear infrasound waves through an absorbing atmosphere. Journal of the Acoustical Society of America. 139:1565-1577.   10.1121/1.4944759   AbstractWebsite

The Navier-Stokes equations are solved using a finite-difference, time-domain (FDTD) approach for axi-symmetric environmental models, allowing three-dimensional acoustic propagation to be simulated using a two-dimensional Cylindrical coordinate system. A method to stabilize the FDTD algorithm in a viscous medium at atmospheric densities characteristic of the lower thermosphere is described. The stabilization scheme slightly alters the governing equations but results in quantifiable dispersion characteristics. It is shown that this method leaves sound speeds and attenuation unchanged at frequencies that are well resolved by the temporal sampling rate but strongly attenuates higher frequencies. Numerical experiments are performed to assess the effect of source strength on the amplitudes and spectral content of signals recorded at ground level at a range of distances from the source. It is shown that the source amplitudes have a stronger effect on a signal's dominant frequency than on its amplitude. Applying the stabilized code to infrasound propagation through realistic atmospheric profiles shows that nonlinear propagation alters the spectral content of low amplitude thermospheric signals, demonstrating that nonlinear effects are significant for all detectable thermospheric returns. (C) 2016 Acoustical Society of America.

Tytell, J, Vernon F, Hedlin M, Hedlin CD, Reyes J, Busby B, Hafner K, Eakins J.  2016.  The USARRAY transportable array as a platform for weather observation and research. Bulletin of the American Meteorological Society. 97:603-619.   10.1175/bams-d-14-00204.1   AbstractWebsite
De Groot-Hedlin, CD, Hedlin MAH.  2015.  A method for detecting and locating geophysical events using groups of arrays. Geophysical Journal International. 203:960-971.   10.1093/gji/ggv345   AbstractWebsite

We have developed a novel method to detect and locate geophysical events that makes use of any sufficiently dense sensor network. This method is demonstrated using acoustic sensor data collected in 2013 at the USArray Transportable Array (TA). The algorithm applies Delaunay triangulation to divide the sensor network into a mesh of three-element arrays, called triads. Because infrasound waveforms are incoherent between the sensors within each triad, the data are transformed into envelopes, which are cross-correlated to find signals that satisfy a consistency criterion. The propagation azimuth, phase velocity and signal arrival time are computed for each signal. Triads with signals that are consistent with a single source are bundled as an event group. The ensemble of arrival times and azimuths of detected signals within each group are used to locate a common source in space and time. A total of 513 infrasonic stations that were active for part or all of 2013 were divided into over 2000 triads. Low (0.5-2 Hz) and high (2-8 Hz) catalogues of infrasonic events were created for the eastern USA. The low-frequency catalogue includes over 900 events and reveals several highly active source areas on land that correspond with coal mining regions. The high-frequency catalogue includes over 2000 events, with most occurring offshore. Although their cause is not certain, most events are clearly anthropogenic as almost all occur during regular working hours each week. The regions to which the TA is most sensitive vary seasonally, with the direction of reception dependent on the direction of zonal winds. The catalogue has also revealed large acoustic events that may provide useful insight into the nature of long-range infrasound propagation in the atmosphere.

De Groot-Hedlin, CD, Hedlin MAH, Walker KT.  2014.  Detection of gravity waves across the USArray: A case study. Earth and Planetary Science Letters. 402:346-352.   10.1016/j.epsl.2013.06.042   AbstractWebsite

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 km(2), progressing to the NNW away from the tomadic 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. (C) 2013 Elsevier B.V. All rights reserved.

Edwards, WN, De Groot-Hedlin CD, Hedlin MAH.  2014.  Forensic investigation of a probable meteor sighting using USArray acoustic data. Seismological Research Letters. 85:1012-1018.   10.1785/0220140056   AbstractWebsite
De Groot-Hedlin, CD, Hedlin MAH.  2014.  Infrasound detection of the Chelyabinsk meteor at the USArray. Earth and Planetary Science Letters. 402:337-345.   10.1016/j.epsl.2014.01.031   AbstractWebsite

On February 15, 2013 a small asteroid entered Earth's atmosphere near Chelyabinsk, Russia. This extremely rare event was recorded by the 400-station USArray deployed in the continental United States and Alaska. These stations recorded infrasound signals from the event at distances from 6000 to 10 000 km across a sector spanning 55 degrees that encompassed the North Pole. This dense, extensive network permitted a detailed study of long-range infrasound propagation and source characteristics. We observe long wavetrains at all stations (ranging to over 100 min) but clear variations in the character of the wavetrains across the network. Ray-tracing through a spatially and temporally varying atmospheric model indicates the source excited resonance in the thermospheric duct to all stations. Resonance was also excited in a persistent stratospheric duct between the source and stations in Alaska and along the west coast of the United States due to favorable winds at those azimuths, leading to higher group velocities and frequency content at these stations than those to the east. An attenuation formula derived from parabolic equation simulations is used to estimate infrasound transmission losses at all stations, using simplified models of the effective sound speed along each source-receiver path. Observed variations in signal energies from higher than expected at stations in the thermospheric duct in the eastern United States, to lower than expected in Alaska, at azimuths nearly orthogonal to the asteroid's Mach cone, lead us to conclude that (1) the source was dominantly isotropic and (2) the model overestimates attenuation in the thermospheric duct. (C) 2014 Elsevier B.V. All rights reserved.

Walker, KT, Le Pichon A, Kim TS, de Groot-Hedlin C, Che IY, Garces M.  2013.  An analysis of ground shaking and transmission loss from infrasound generated by the 2011 Tohoku earthquake. Journal of Geophysical Research-Atmospheres. 118:12831-12851.   10.1002/2013jd020187   AbstractWebsite

The 2011 M(w)9.0 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.

Brown, PG, Assink JD, Astiz L, Blaauw R, Boslough MB, Borovicka J, Brachet N, Brown D, Campbell-Brown M, Ceranna L, Cooke W, de Groot-Hedlin C, Drob DP, Edwards W, Evers LG, Garces M, Gill J, Hedlin M, Kingery A, Laske G, Le Pichon A, Mialle P, Moser DE, Saffer A, Silber E, Smets P, Spalding RE, Spurny P, Tagliaferri E, Uren D, Weryk RJ, Whitaker R, Krzeminski Z.  2013.  A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature. 503:238-241.   10.1038/nature12741   AbstractWebsite

Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage(1) than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects(2). Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave(3), but owing to lack of observations this is uncertain(4,5). Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (+/- 100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185x10(12) joules). We show that a widely referenced technique(4-6) of estimating airburst damage does not reproduce the observations, and that the mathematical relations(7) based on the effects of nuclear weapons-almost always used with this technique-overestimate blast damage. This suggests that earlier damage estimates(5,6) near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques(8,9). This suggests a non-equilibrium(if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.

Hedlin, MAH, de Groot-Hedlin C, Drob D.  2012.  A Study of Infrasound Propagation Using Dense Seismic Network Recordings of Surface Explosions. Bulletin of the Seismological Society of America. 102:1927-1937.   10.1785/0120110300   Abstract

We use dense seismic network recordings of accurately located surface explosions in northern Utah to shed light on the propagation of infrasound through the stratosphere. The data clearly show propagation of infrasound downwind from the source, as expected. The data also clearly show the penetration of infrasound into geometric shadow zones near the source and the spread of infrasound to a distance of 800 km from the source. The spread of infrasound both toward and away from the source is not predicted by applying either ray theory or the full-wave finite-difference technique to smooth ground-to-space (G2S) models. The mismatch between synthetics and data suggest a missing component in these models, possibly a small-scale gravity-wave structure. Comparison of the network recordings of approximately 1500 infrasound signals with travel-time predictions based on rays shows no significant average bias in the travel times. On average, recorded signals arrived 1 s earlier than predictions. Travel-time residuals are normally distributed about the mean with a standard deviation of 15 s. The small bias of the travel-time predictions indicates that despite the fact that small-scale structure is averaged out of commonly used G2S models, the large-scale structure of the atmosphere is accurately represented. The scatter of travel-time residuals is suggestive of small-scale structure missing from the models that we used to make the predictions, but firm conclusions would require a more in-depth study.

Kelbert, A, Egbert GD, de Groot-Hedlin C.  2012.  Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track. Geology. 40:447-450.   10.1130/g32655.1   AbstractWebsite

Combining long-period magnetotelluric data from the spatially uniform EarthScope USArray and higher-resolution profiles, we obtain a regional three-dimensional electrical resistivity model in the Snake River Plain and Yellowstone areas (Idaho and Wyoming, United States), and provide new constraints on the large-scale distribution of melt and fluids beneath the Yellowstone hotspot track. Contrary to what would be expected from standard mantle plume models, the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath Yellowstone caldera. Instead, low mantle resistivities (10 Omega m and below), which we infer to result from 1%-3% partial melt, are found 40-80 km beneath the eastern Snake River Plain, extending at least 200 km southwest of the caldera, beneath the area of modern basaltic magmatism. The reduced resistivities extend upward into the mid-crust primarily around the edges of the Snake River Plain, suggesting upward migration of melt and/or fluid is concentrated in these areas. The anomaly also shallows toward Yellowstone, where higher temperatures enhance permeability and allow melts to ascend into the crust. The top of the conductive layer is at its shallowest, in the upper crust, directly beneath the modern Yellowstone supervolcano.

De Groot-Hedlin, CD.  2012.  Nonlinear synthesis of infrasound propagation through an inhomogeneous, absorbing atmosphere. Journal of the Acoustical Society of America. 132:646-656.   10.1121/1.4731468   AbstractWebsite

An accurate and efficient method to predict infrasound amplitudes from large explosions in the atmosphere is required for diverse source types, including bolides, volcanic eruptions, and nuclear and chemical explosions. A finite-difference, time-domain approach is developed to solve a set of nonlinear fluid dynamic equations for total pressure, temperature, and density fields rather than acoustic perturbations. Three key features for the purpose of synthesizing nonlinear infrasound propagation in realistic media are that it includes gravitational terms, it allows for acoustic absorption, including molecular vibration losses at frequencies well below the molecular vibration frequencies, and the environmental models are constrained to have axial symmetry, allowing a three-dimensional simulation to be reduced to two dimensions. Numerical experiments are performed to assess the algorithm's accuracy and the effect of source amplitudes and atmospheric variability on infrasound waveforms and shock formation. Results show that infrasound waveforms steepen and their associated spectra are shifted to higher frequencies for nonlinear sources, leading to enhanced infrasound attenuation. Results also indicate that nonlinear infrasound amplitudes depend strongly on atmospheric temperature and pressure variations. The solution for total field variables and insertion of gravitational terms also allows for the computation of other disturbances generated by explosions, including gravity waves. (C) 2012 Acoustical Society of America. []

Hedlin, MAH, Walker K, Drob DP, De Groot-Hedlin CD.  2012.  Infrasound: Connecting the Solid Earth, Oceans, and Atmosphere. Annual Review of Earth and Planetary Sciences, Vol 40. 40( Jeanloz R, Ed.).:327-354., Palo Alto: Annual Reviews   10.1146/annurev-earth-042711-105508   Abstract

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.

Walker, KT, Shelby R, Hedlin MAH, de Groot-Hedlin C, Vernon F.  2011.  Western US Infrasonic Catalog: Illuminating infrasonic hot spots with the USArray. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008579   AbstractWebsite

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.

de Groot-Hedlin, C, Hedlin MAH, Walker K.  2011.  Finite difference synthesis of infrasound propagation through a windy, viscous atmosphere: application to a bolide explosion detected by seismic networks. Geophysical Journal International. 185:305-320.   10.1111/j.1365-246X.2010.04925.x   AbstractWebsite

A finite-difference time-domain (FDTD) algorithm has been developed to model linear infrasound propagation through a windy, viscous medium. The algorithm has been used to model signals from a large bolide that burst above a dense seismic network in the US Pacific Northwest on 2008 February 19. We compare synthetics that have been computed using a G2S-ECMWF atmospheric model to signals recorded at the seismic networks located along an azimuth of 210. from the source. The results show that the timing and the range extent of the direct, stratospherically ducted and thermospherically ducted acoustic branches are accurately predicted. However, estimates of absorption obtained from standard attenuation models (Sutherland-Bass) predict much greater attenuation for thermospheric returns at frequencies greater than 0.1 Hz than is observed. We conclude that either the standard absorption model for the thermospheric is incorrect, or that thermospheric returns undergo non-linear propagation at very high altitude. In the former case, a better understanding of atmospheric absorption at high altitudes is required; in the latter, a fully non-linear numerical method is needed to test our hypothesis that higher frequency arrivals from the thermosphere result from non-linear propagation at thermospheric altitudes.

Hedlin, MAH, Drob D, Walker K, de Groot-Hedlin C.  2010.  A study of acoustic propagation from a large bolide in the atmosphere with a dense seismic network. Journal of Geophysical Research-Solid Earth. 115   10.1029/2010jb007669   AbstractWebsite

A large meteor entered the atmosphere above northeastern Oregon on 19 February 2008 at 530 PST. Several hundreds of broadband seismic stations in the U.S. Pacific Northwest recorded acoustic-to-seismic coupled signals from this event. The travel times of the first arriving energy are consistent with a terminal explosion source model, suggesting that the large size of the explosion masked any signals associated with a continuous line source along its supersonic trajectory. Several infrasound arrays in North America also recorded this event. Both the seismic and infrasound data have been used to locate the explosion in 3-D space and time. Climatological atmospheric velocity models predict that infrasound signals from sources that occur at mid-northern latitudes in winter are usually ducted to the east due to eastward zonal winds. In this paper, we analyze travel time picks and use 3-D ray tracing to generate synthetic travel times based on various atmospheric models to show that the seismic network data instead reveal a predominant westward propagation direction. A sudden stratospheric warming event that reversed the zonal wind flow explains this westward propagation. The seismic data illuminate in unprecedented spatial detail the range and azimuthal definition of shadow zones out to a range of 500 km, suggesting that dense seismic networks can be used to study infrasound propagation at spatial resolutions that exceed that which can be done with only a handful of globally distributed infrasound arrays.

Walker, KT, Hedlin MAH, de Groot-Hedlin C, Vergoz J, Le Pichon A, Drob DP.  2010.  Source location of the 19 February 2008 Oregon bolide using seismic networks and infrasound arrays. Journal of Geophysical Research-Solid Earth. 115   10.1029/2010jb007863   AbstractWebsite

On 19 February 2008 a bolide traveled across the sky along a southern trajectory ending in a terminal burst above Oregon. The event was well recorded by the USArray, other seismic networks, four infrasound arrays, and several video cameras. We compare the results of locating the burst using these different sensor networks. Specifically, we reverse time migrate acoustic-to-seismic coupled signals recorded by the USArray out to 800 km range to image the source in 2-D space and time. We also apply a grid search over source altitude and time, minimizing the misfit between observed and predicted arrival times using 3-D ray tracing with a high-resolution atmospheric velocity model. Our seismic and video results suggest a point source rather than a line source associated with a hypersonic trajectory. We compare the seismic source locations to those obtained by using different combinations of observed infrasound array signal back azimuths and arrival times. We find that all locations are consistent. However, the seismic location is more accurate than the infrasound locations due to the larger number of seismic sensors, a more favorable seismic source-receiver geometry, and shorter ranges to the seismometers. For the infrasound array locations, correcting for the wind improved the accuracy, but implementing arrival times while increasing the precision reduced the accuracy presumably due to limitations of the source location method and/or atmospheric velocity model. We show that despite known complexities associated with acoustic-to-seismic coupling, aboveground infrasound sources can be located with dense seismic networks with remarkably high accuracy and precision.

de Groot-Hedlin, C, Blackman DK, Jenkins CS.  2009.  Effects of variability associated with the Antarctic circumpolar current on sound propagation in the ocean. Geophysical Journal International. 176:478-490.   10.1111/j.1365-246X.2008.04007.x   AbstractWebsite

A series of small depth charges was detonated along a transect from New Zealand to Antarctica over a period of 3 days in late December of 2006. The hydroacoustic signals were recorded by a hydrophone deployed near the source and at a sparse network of permanent hydrophone stations operated by the International Monitoring System (IMS), at distances up to 9600 km. Our purpose was to determine how well signal characteristics could be predicted by the World Ocean Atlas 2005 (WOA05) climatological database for sources within the Antarctic circumpolar current (ACC). Waveforms were examined in the 1-100 Hz frequency band, and it was found that for clear transmission paths, the shot signals exceeded the noise only at frequencies above 20-30 Hz. Comparisons of signal spectra for recordings near the source and at the IMS stations show that transmission loss is nearly uniform as a function of frequency. Where recorded signal-to-noise ratios are high, observed and predicted traveltimes and signal dispersion agree to within 2 s under the assumption that propagation is adiabatic and follows a geodesic path. The deflection of the transmission path by abrupt spatial variations in sound speed at the northern ACC boundary is predicted to decrease traveltimes to the IMS stations by several seconds, depending on the path. Acoustic velocities within the ACC are predicted to vary monthly, hence the accuracy of source location estimates based only on arrival times at IMS stations depends on the monthly or seasonal database used to predict traveltimes and on whether we account for path deflection. However, estimates of source locations within the ACC that are based only on observed waveforms at IMS hydrophones are highly dependent on the configuration of the IMS network; a set of shots observed only at an IMS station in the Indian Ocean and another in the South Pacific was located to within 10 km in longitude, but was poorly constrained in latitude. Several sets of shots observed only at IMS hydrophones in the Indian Ocean were constrained to within 55 km in latitude but were poorly constrained in longitude.