Michael A. H. Hedlin

A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

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 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. 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, but owing to lack of observations this is uncertain. 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 = 4.185×1012 joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates 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. 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.

An analysis of large‐scale variations in small‐scale mantle heterogeneity using Global Seismographic Network recordings of precursors to PKP

High-frequency precursors to the core phase PKP are caused by scattering off heterogeneities in the lowermost mantle and D″ regions, and they provide a unique window into the small-scale structure of the deep Earth. We study lower mantle scattering by analyzing 412 high-quality PKP precursor records at ranges between 120° and 137.5° as obtained from the global seismic networks during the last 10 years. To examine regional variations in scattering strength, we compare individual records with the globally averaged PKP precursor stack of Hedlin et al.. [1997]. We identify strong differences in apparent scattering strength among specific source-receiver paths. Inversion of these data for scattering source regions is complicated by ambiguity between source- and receiver-side scattering and the sparse and uneven data coverage. Synthetic tests, however, suggest that inversions with applied smoothness constraints can resolve large-scale differences in scattering strength over significant parts of the lower mantle. We use a conjugate gradient method based on an approximation to Rayleigh-Born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle. Our results indicate particularly strong scattering beneath central Africa, parts of North America, and just north of India, whereas weaker scattering is seen beneath South and Central America, eastern Europe, and Indonesia. Some regions of strong scattering correlate roughly with large-scale anomalies revealed by seismic tomography including the African plume and the Tethys trench. These correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source-receiver ambiguity in all areas. Further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations.

Experiments in migration processing of SS precursor data to image upper mantle discontinuity structure

Long-period SS precursors result from underside reflections off upper mantle discontinuities. By grouping and stacking global seismic data by SS bounce point location it is possible to map lateral variations in depths to the 410- and 660-km discontinuities, a process analogous to common midpoint (CMP) stacking in reflection seismology. Because this method assumes horizontal reflectors, energy arriving from dipping or intermittent reflectors may not be correctly imaged. To address this possibility, we experiment with techniques based on migration processing of shallow seismic reflection data. The problem is complicated by the uneven distribution of sources and receivers for the SS precursor observations, but the data are sufficiently dense beneath the northwest Pacific Ocean that reasonably good coverage can be obtained for this region. We parameterize the model as a grid of point scatterers in latitude, longitude, and depth (from the surface to 1000 km depth) and compute travel times from each grid point to the source and receiver locations. These times are used to construct a matrix equation that yields predicted SS precursor waveforms from the assumed scatterers. To recover the model, we experiment with both simple back projection and full inversions using a conjugate gradient method. Tests on noise-free synthetic data (generated using the same source-receiver distribution as the actual data) suggest that detailed resolution of discontinuity structure is possible, at horizontal scales much smaller than the Fresnel zone. However, the real data do not produce coherent results unless some degree of horizontal smoothing is imposed, at least partially defeating the purpose of this approach. Results for the northwest Pacific find structure on the 410- and 660-km discontinuities and hints of intermittent reflectors at other depths. Random resampling tests, however, suggest that most of these features are not reliably resolved, with the exception of a depression on the 660-km discontinuity seen in the northwest Pacific. Our experiments show that it is unlikely that small-scale structure on the 660-km discontinuity near subducting slabs causes significant bias in maps of the large-scale 660-km topography derived from long-period SS precursor observations.

Capturing the Acoustic Fingerprint of Stratospheric Ash Injection

More than 100 separate incidents of interactions between aircraft and volcanic ash were documented between 1973 and 2003. Incidents on international flight paths over remote areas have resulted in engine failures and significant damage and expense to commercial airlines. To protect aircraft from volcanic ash, pilots need rapid and reliable notification of ash- generating events. A global infrasound array network, consisting of the International Monitoring System (IMS) and other national networks, has demonstrated a capability for remote detection of Vulcanian to Plinian eruptions that can inject ash into commercial aircraft cruise altitudes (approximately 12 kilometers) near the tropopause. The identification of recurring sound signatures associated with high- altitude ash injection implies that acoustic remote sensing can improve the reliability and reduce the latency of these notifications.

Evaluation of rosette infrasonic noise-reducing spatial filters.

The spatial noise filter currently preferred for use at the new International Monitoring System (IMS) infrasound array sites consists of an array of low‐impedance inlets connected by solid tubes to a microbarometer. We present results from recent tests of ‘‘rosette’’ infrasonic noise‐reducing spatial filters at the Pinon Flat Infrasound test‐bed in southern California. At wind speeds up to 5.5 m/s, the 96 inlet 18‐m rosette filter reduces wind noise levels above 0.2 Hz by 15–20 dB. Under the same conditions, the 144 inlet 70‐m rosette filter provides noise reduction of up to 15–20 dB between 0.02 and 0.7 Hz. Standing wave resonance inside the 70‐m filter degrades the reception of acoustic signals above 0.7 Hz. Synthetics accurately reproduce the noise reduction and resonance observed in the 70‐m filter at all wind speeds above 1.25 m/s. Experiments with impedance matching capillaries indicate that internal resonance in the rosette filters can be removed. Rosette filters are tuned to vertically incident energy. Attenuation of signals by the 70‐m rosette filter at frequencies above 3.5 Hz arriving at grazing angles of <15 deg from the horizontal are predicted to range upward from 10 dB to total cancellation at 5 Hz.

Western U.S. Infrasonic Catalog: Illuminating infrasonic hot spots with the USArray

[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.

An optical fiber infrasound sensor: A new lower limit on atmospheric pressure noise between 1 and 10 Hz

A new distributed sensor for detecting pressure variations caused by distant sources has been developed. The instrument reduces noise due to air turbulence in the infrasound band by averaging pressure along a line by means of monitoring strain in a long tubular diaphragm with an optical fiber interferometer. Above 1 Hz, the optical fiber infrasound sensor (OFIS) is less noisy than sensors relying on mechanical filters. Records collected from an 89-m-long OFS indicate a new low noise limit in the band from 1 to 10 Hz. Because the OFIS integrates pressure variations at light-speed rather than the speed of sound, phase delays of the acoustical signals caused by the sensor are negligible. Very long fiber-optic sensors are feasible and hold the promise of better wind-noise reduction than can be achieved with acoustical-mechanical systems.

Atmospheric Variability and Infrasound Monitoring

The propagation of infrasound through the troposphere, stratosphere, and thermosphere is primarily controlled by the thermal structure of the atmosphere, winds, atmospheric attenuation, and reflections from the ground terrain. It is well known that the temperature and circulation of the atmosphere vary continuously, both temporally and spatially, and that solar heating drives this change. Studies show that atmospheric absorption, and thus signal attenuation, is most severe in the thermosphere. A detailed knowledge of the variations in the atmosphere - with time, altitude, and geographic location - is needed to correctly interpret infrasound waveforms to extract useful information about infrasound sources. This will provide information that could be used to correctly identify the source, to estimate its location and time, and to predict if along a particular path a signal should be detected above noise. Much progress has been made in our understanding of the atmosphere; however, methods continue to be developed with the goal of defining atmospheric structure at a fine enough scale to accurately synthesize waveforms. A key component of this development and validation of our atmospheric modeling capability is the study of ground-truthed atmospheric events. We present results from selected case studies that shed light on the temporal and spatial variations in sound propagation characteristics to provide an overview of sound transmission through our ever-changing atmosphere and our quest for accurate atmospheric models.

A Review of Wind-Noise Reduction Methodologies

The bane of infrasound signal detection and characterization is the noise generated by wind. In this chapter, we review the physics of wind, current theories on how wind can generate “noise,” and both mature and developing techniques for reducing wind noise. This subject is not completely documented in peer-reviewed journals, and we have extended our review to include nonpeer-reviewed results that appear to be robust. Specifically, we review Daniels’ filters, pipe rosettes, microporous hoses, optical fiber line sensors, distributed sensors, rigid porous media filters, and wind barriers. We discuss the advantages, disadvantages, and potential of each technology. We conclude with a summary of the state of affairs in noise-reduction research and the potential impact of these technologies on future global infrasound monitoring and research efforts.

Beam-Stack Imaging using a small aperture array

We seek to gain a fuller understanding of seismic coda generation in the continental crust, by identifying secondary (scattering) sources illuminated by a distant primary source. We have developed a migration technique to scan seismic coda recorded by a small-aperture seismic array for phases generated locally by scattering from large heterogeneities, or topographic undulations. We use a widely distributed suite of seismic events to illuminate the local crustal volume from different directions and produce an image of the local crust. Stable apparent secondary seismic sources are observed, and interpreted as scatterers excited by the primary events.