Philip Blom

Impulse propagation in the nocturnal boundary layer: Analysis of the geometric component

On clear dry nights over flat land, a temperature inversion and stable nocturnal wind jet lead to an acoustic duct in the lowest few hundred meters of the atmosphere. An impulsive signal propagating in such a duct is received at long ranges from the source as an extended wave train consisting of a series of weakly dispersed distinct arrivals followed by a strongly dispersed low-frequency tail. The leading distinct arrivals have been previously shown to be well modeled by geometric acoustics. In this paper, the geometric acoustics approximation for the leading arrivals is investigated. Using the solutions of the eikonal and transport equations, travel times, amplitudes, and caustic structures of the distinct arrivals have been determined. The time delay between and relative amplitudes of the direct-refracted and single ground reflection arrivals have been investigated as parameters for an inversion scheme. A two parameter quadratic approximation to the effective sound speed profile has been fit and found to be in strong agreement with meteorological measurements from the time of propagation.

Automatic infrasound detection and location of sources in the western United States

A catalog that characterizes sources of regional infrasound observed in the western U.S. (1 November 2010 to 31 October 2012) is produced. Data from nine University of Utah Seismograph Stations infrasonic arrays are supplemented by three additional arrays in Nevada, operated by Southern Methodist University. The detection procedure using an adaptive F-detector provides input into the Bayesian Infrasonic Source Location procedure. The catalog consists of 1510 events with indication of repeated events from many locations such as Dugway Testing Ground, Utah Test and Training Range, and New Bomb. We analyzed the relationship between seasonal variations in the event locations and wind conditions using the Ground-to-Space specifications based on publicly available operational numerical weather prediction data analysis products supplement by empirical models above 80 km. There is significant commonality between this studys bulletin and the Western United States Infrasonic Catalog published by Walker et al. (2011). A previous study utilized infrasound signals detected on the USArray Transportable Array seismic stations (2007–2008). Both results document the vast majority of events that occur during working hours, suggesting a human cause. To illustrate the utility of the event bulletin for exploring atmospheric dynamics, propagation paths of an event detected during the equinox period, when the stratospheric wind is low, were generated using a ray-tracing algorithm. We found that the observations contain stratospheric arrivals, not predicted by ray theory, possibly due to gravity waves increasing the effective jet speed.

On infrasound generated by wind farms and its propagation in low‐altitude tropospheric waveguides

Infrasound from a 60-turbine wind farm was found to propagate to distances up to 90 km under nighttime atmospheric conditions. Four infrasound sensor arrays were deployed in central New Mexico in February 2014; three of these arrays captured infrasound from a large wind farm. The arrays were in a linear configuration oriented southeast with 13, 54, 90, and 126 km radial distance and azimuths of 166°, 119°, 113°, and 111° from the 60 1.6-MW turbine Red Mesa Wind Farm (RMWF), Laguna Pueblo, New Mexico, USA. Peaks at a fundamental frequency slightly below 0.9 Hz and its harmonics characterize the spectrum of the detected infrasound. The generation of this signal is linked to the interaction of the blades, flow gradients, and the supporting tower. The production of wind-farm sound, its propagation, and detection at long distances can be related to the characteristics of the atmospheric boundary layer. First, under stable conditions, mostly occurring at night, winds are highly stratified, which enhances the production of thickness sound (TS) and the modulation of other higher-frequency wind turbine sounds. Second, nocturnal atmospheric conditions can create low-altitude waveguides (with altitudes on the order of 100s of meters) allowing long distance propagation. Third, night and early morning hours are characterized by reduced background atmospheric noise that enhances signal detectability. This work describes the characteristics of the infrasound from a quasi-continuous source with the potential for long-range propagation that could be used to monitor the lower part of the atmospheric boundary layer.

Modeling and observations of an elevated, moving infrasonic source: Eigenray methods

The acoustic ray tracing relations are extended by the inclusion of auxiliary parameters describing variations in the spatial ray coordinates and eikonal vector due to changes in the initial conditions. Computation of these parameters allows one to define the geometric spreading factor along individual ray paths and assists in identification of caustic surfaces so that phase shifts can be easily identified. A method is developed leveraging the auxiliary parameters to identify propagation paths connecting specific source-receiver geometries, termed eigenrays. The newly introduced method is found to be highly efficient in cases where propagation is non-planar due to horizontal variations in the propagation medium or the presence of cross winds. The eigenray method is utilized in analysis of infrasonic signals produced by a multi-stage sounding rocket launch with promising results for applications of tracking aeroacoustic sources in the atmosphere and specifically to analysis of motor performance during dynamic tests.

Analysis and modeling of infrasound from a four-stage rocket launch

Infrasound from a four-stage sounding rocket was recorded by several arrays within 100 km of the launch pad. Propagation modeling methods have been applied to the known trajectory to predict infrasonic signals at the ground in order to identify what information might be obtained from such observations. There is good agreement between modeled and observed back azimuths, and predicted arrival times for motor ignition signals match those observed. The signal due to the high-altitude stage ignition is found to be low amplitude, despite predictions of weak attenuation. This lack of signal is possibly due to inefficient aeroacoustic coupling in the rarefied upper atmosphere.

Observations of the refraction of microbaroms generated by large maritime storms by the wind field of the generating storm

Microbaroms are a continuous infrasonic signal in the 0.15 to 0.3 Hz band caused by the collision of oceanic surface waves of equal period. Such signals are often generated by large maritime storms. Current formulation of the generation mechanism predicts that the microbarom source location due to a large maritime storm in the open ocean is generally located several hundreds of kilometers from the eye of the storm. Assuming such a source location to be correct, propagation of the microbaroms along paths which pass near the storm center as well as those which propagate away from the storm structure have been examined using geometric acoustics. Microbarom propagation paths which pass near the storm center are refracted by the storm winds and are found to have back azimuths directed toward a virtual source around the storm center. Microbarom propagation paths which do not pass near the storm center are found to have back azimuths directed toward the actual source region. To validate these predictions, data from microbarom signals generated by hurricanes in the Atlantic Ocean have been collected along the east coast of the United States during the 2010 and 2011 Atlantic hurricane seasons. Data from several storm events are presented here for comparison with model predictions. In general, the observations are in agreement with the predictions of the propagation model.

Modeling the refraction of microbaroms by the winds of a large maritime storm

Continuous infrasonic signals produced by the ocean surface interacting with the atmosphere, termed microbaroms, are known to be generated by a number of phenomena including large maritime storms. Storm generated microbaroms exhibit axial asymmetry when observed at locations far from the storm due to the source location being offset from the storm center. Because of this offset, a portion of the microbarom energy will radiate towards the storm center and interact with the winds in the region. Detailed here are predictions for the propagation of microbaroms through an axisymmetric, three-dimensional model storm. Geometric propagation methods have been utilized and the predicted horizontal refraction is found to produce signals that appear to emanate from a virtual source near the storm center when observed far from the storm. This virtual source near the storm center is expected to be observed only from a limited arc around the storm system with increased extent associated with more intense wind fields. This result implies that identifying the extent of the arc observing signal from the virtual source could provide a means to estimate the wind structure using infrasonic observations far from the storm system.

Estimation of explosive yield using regional distance infrasound

Infrasound signals from large above-ground explosions are known to propagate to significant distances while remaining at observable levels due to a combination of the large source energy for such events and decreased absorption for acoustic energy at lower frequencies. Previously, propagation-based, stochastic path geometry and travel time models have been used to improve estimates of location and time for infrasonic sources with promising results. A similar approach has been applied to develop stochastic transmission loss models for infrasonic signals. The resulting models have been utilized in a Bayesian framework to compute a near-source estimate of the acoustic signal, which can then be used to compute a probability distribution for possible explosive yields of the source. Details of the propagation models and yield estimation framework will be presented along with results for application to a number of explosive events.

Improvements to infrasonic detection capabilities through application of a generalized least squares beamformer

A common challenge in seismoacoustic research and analysis is detection of low signal-to-noise ratio (SNR) signals in a variable and often coherent background. Previously, the application of an adaptive F-detector to infrasonic data has successfully reduced false detection rates attributed to coherent noise across array elements; however, the detector is applied post-processing after analysis using a standard (Bartlett) beamformer, which raises the detection threshold and can lead to missed detections. Application of a generalized least squares (GLS) beamformer can enhance processing of transient infrasonic signals, particularly in the presence of correlated noise, by adaptively accounting for the background noise characteristics. The GLS beamformer enhancement leads to improved capabilities for accurately detecting low amplitude infrasonic events of interest that would not normally produce a sufficiently high F-statistic to be declared a detection. A statistical analysis of GLS beamformer performance in ...

Improving infrasonic location estimates for underground nuclear explosions Improving infrasonic location estimates for underground nuclear explosions

Infrasound data from underground nuclear explosions conducted by North Korea in 2006, 2009, 2013 and 2016 were recorded on six seismo-acoustic arrays co-operated by Southern Methodist University (SMU) and the Korean Institute of Geosciences and Mineral Resources (KIGAM). No infrasound signals were observed during the 2006 test, while signals from the others have been used to determine event locations and yield estimations. Prior location studies have demonstrated that wind corrections for back azimuth deviation improve location estimates. Additionally, recent improvements to the Bayesian Infrasonic Source Localization (BISL) methodology have shown to reduce 90% confidence contours for location by 40% through the utilization of propagation-based likelihood priors for celerity and backazimuth deviation from seven years of archival atmospheric specifications. Relocations of the 2009, 2013 and 2016 nuclear explosions will be presented to demonstrate the application of BISL to underground nuclear explosions.