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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.

Matoza, RS, Garces MA, Chouet BA, D'Auria L, Hedlin MAH, de Groot-Hedlin C, Waite GP.  2009.  The source of infrasound associated with long-period events at Mount St. Helens. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006128   AbstractWebsite

During the early stages of the 2004-2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the "valve'' sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.

De Groot-Hedlin, CD, Hedlin MAH, Drob DP.  2009.  Atmospheric variability and infrasound monitoring. Global continuous infrasound monitoring for atmospheric studies. : Springer Geosciences Abstract
De Groot-Hedlin, CD, Hedlin MAH, Walker KT, Drob DP, Zumberge MA.  2008.  Evaluation of infrasound signals from the shuttle Atlantis using a large seismic network. Journal of the Acoustical Society of America. 124:1442-1451.   10.1121/1.2956475   AbstractWebsite

Inclement weather in Florida forced the space shuttle "Atlantis" to land at Edwards Air Force Base in southern California on June 22, 2007, passing near three infrasound stations and several hundred seismic stations in northern Mexico, southern California, and Nevada. The high signal-to-noise ratio, broad receiver coverage, and Atlantis' positional information allow for the testing of infrasound propagation modeling capabilities through the atmosphere to regional distances. Shadow zones and arrival times are predicted by tracing rays that are launched at right angles to the conical shock front surrounding the shuttle through a standard climatological model as well as a global ground to space model. The predictions and observations compare favorably over much of the study area for both atmospheric specifications. To the east of the shuttle trajectory, there were no detections beyond the primary acoustic carpet. Infrasound energy was detected hundreds of kilometers to the west and northwest (NW) of the shuttle trajectory, consistent with the predictions of ducting due to the westward summer-time stratospheric jet. Both atmospheric models predict alternating regions of high and low ensonifications to the NW. However, infrasound energy was detected tens of kilometers beyond the predicted zones of ensonification, possibly due to uncertainties in stratospheric wind speeds. (C) 2008 Acoustical Society of America.

de Groot-Hedlin, C.  2008.  A Finite Difference Solution to the Helmholtz Equation in a Radially Symmetric Waveguide: Application to Near-Source Scattering in Ocean Acoustics. Journal of Computational Acoustics. 16:447-464.   10.1142/s0218396x08003683   AbstractWebsite

A finite difference (FD) method is developed and analyzed for the Helmholtz equation in a radially symmetric waveguide. The resulting algorithm can be used to solve for sound intensities in complex models that may include high material contrasts and arbitrary bathymetry. An analysis of the effect of grid discretization on the results indicates that numerical dispersion is significant within one-third of a wavelength from a point source and decreases beyond that. Numerical results are presented and compared to wide-angle parabolic equation (PE) solutions and analytic solutions, where available. Comparison with analytic results indicates that the FD method accurately solves for the acoustic wave field at all propagation angles and is more accurate than the PE method near the source. Results are also shown for models in which mode coupling occurs near the source.

de Groot-Hedlin, C.  2008.  Finite-difference time-domain synthesis of infrasound propagation through an absorbing atmosphere. Journal of the Acoustical Society of America. 124:1430-1441.   10.1121/1.2959736   AbstractWebsite

Equations applicable to finite-difference time-domain (FDTD) computation of infrasound propagation through an absorbing atmosphere are derived and examined in this paper. It is shown that over altitudes up to 160 km, and at frequencies relevant to global infrasound propagation, i.e., 0.02-5 Hz, the acoustic absorption in dB/m varies approximately as the square of the propagation frequency plus a small constant term. A second-order differential equation is presented for an atmosphere modeled as a compressible Newtonian fluid with low shear viscosity, acted on by a small external damping force. It is shown that the solution to this equation represents pressure fluctuations with the attenuation indicated above. Increased dispersion is predicted at altitudes over 100 km at infrasound frequencies. The governing propagation equation is separated into two partial differential equations that are first order in time for FDTD implementation. A numerical analysis of errors inherent to this FDTD method shows that the attenuation term imposes additional stability constraints on the FDTD algorithm. Comparison of FDTD results for models with and without attenuation shows that the predicted transmission losses for the attenuating media agree with those computed from synthesized waveforms. (C) 2008 Acoustical Society of America.

Herrin, ET, Bass HE, Andre B, Woodward RL, Drob DP, Hedlin MAH, Garces MA, Golden PW, Norris DE, de Groot-Hedlin C, Walker KT, Szuberla CAL, Whitaker RW, Shields DF.  2008.  High-Altitude Infrasound Calibration Experiments. Acoustics Today. 4:9-21.: ASA   10.1121/1.2961169   AbstractWebsite
de Groot-Hedlin, C.  2006.  Finite-difference modeling of magnetotelluric fields: Error estimates for uniform and nonuniform grids. Geophysics. 71:G97-G106.   10.1190/1.2195991   AbstractWebsite

In the finite-difference (FD) method, one solves a set of discrete approximations to continuous differential equations: thus, the solutions only approximate the true values. For the magnetotellutic (MT) method, errors in the electric and magnetic fields computed by the staggered FD) method are precisely quantifiable for a model with uniform conductivity. In this case, the errors in the electric and magnetic fields are equal in magnitude but increase with rising node separation. In this paper, I show that errors in NIT responses, which rely on ratios of the field values, depend strongly on the method used to interpolate electric field values to the surface where the magnetic field is sampled. Analytic expressions for the FD estimates of the NIT responses for a half-space are derived and compared for three different methods of electric field interpolation. The best results are achieved when the electric field values just above and below the surface are interpolated exponentially. For a half-space, the FD estimates of the NIT responses are independent of node separation and are precisely equal to the analytic values when the electric field is interpolated exponentially. For models with sharp conductivity contrasts, the errors in the responses derived using this interpolation method increase with rising node spacing but still perform better than other examined interpolation methods. Varying the vertical node separation within a half-space model degrades the solution accuracy. The magnitude of the error depends primarily on the magnitude of the change in vertical node spacing. Lateral variations in the grid spacing do not necessarily yield errors in the FD solutions to the NIT equations.

De Groot-Hedlin, CD.  2005.  Estimation of the rupture length and velocity of the Great Sumatra earthquake of Dec 26, 2004 using hydroacoustic signals. Geophysical Research Letters. 32   10.1029/2005gl022695   AbstractWebsite

Unprecedented hydroacoustic observations of the megathrust earthquake of 26 Dec, 2004 were afforded by a network of 5 small hydroacoustic arrays located in the Indian Ocean, at distances of 2800 to 7000 km from the epicenter. Each array recorded acoustic waves, called T waves, generated by this event. Analysis of a series of short time windows within the T wave coda shows that the receiver to source azimuth varies smoothly as a function of time, indicating that the apparent T wave source is not stationary. The apparent T wave source moves northward along the Sunda trench at an average velocity of 2 km/s, closely tracking event rupture. The hydroacoustic data suggest that the rupture proceeded in two distinct phases; initially it progressed northwest along the Sunda trench with a velocity of approximately 2.4 km/s. At 600 km from the epicenter the rupture slowed to approximately 1.5 km/s, as it continued to propagate to the northwest.