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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.  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.  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.

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.

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, 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.