Junghyun Park

Structural Evolution of Poly(ether-b-amide12) Elastomers During the Uniaxial Stretching: An In situ Wide-Angle X-Ray Scattering Study

AbstractThe structural evolution of poly(ether-b-amide12) (PEBAX®) elastomers was studied by using wide angle X-ray scattering (WAXS) at a synchrotron radiation source during the uniaxial stretching. The changes in the crystal structure of the nylon 12 were tracked during the stretching of the P6333 and P2533 compressed films, which represent a hard rubber and a soft elastomer, respectively. The nylon 12 chains of both samples favored the γ-phase in the unstrained state but their responses to the external drawing force were different. For the P6333 film containing small amounts of the soft segments, the applied force dilated the 001 crystalline lattice plane of nylon 12 which is perpendicular to its main chain and the γ form was transformed into the α″ form when the applied force was high enough. However, for the P2533 containing large amounts of the soft segments, the lattice dimensions of the nylon 12 crystals did not change with the external drawing force and the transient α″ form was not observed. These results might suggest that the transfer of the external force to the crystal was difficult for P2533 due to the chain relaxation of the soft segments.

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.

Detection of regional infrasound signals using array data: Testing, tuning, and physical interpretation

This work quantifies the physical characteristics of infrasound signal and noise, assesses their temporal variations, and determines the degree to which these effects can be predicted by time-varying atmospheric models to estimate array and network performance. An automated detector that accounts for both correlated and uncorrelated noise is applied to infrasound data from three seismo-acoustic arrays in South Korea (BRDAR, CHNAR, and KSGAR), cooperatively operated by Korea Institute of Geoscience and Mineral Resources (KIGAM) and Southern Methodist University (SMU). Arrays located on an island and near the coast have higher noise power, consistent with both higher wind speeds and seasonably variable ocean wave contributions. On the basis of the adaptive F-detector quantification of time variable environmental effects, the time-dependent scaling variable is shown to be dependent on both weather conditions and local site effects. Significant seasonal variations in infrasound detections including daily time of occurrence, detection numbers, and phase velocity/azimuth estimates are documented. These time-dependent effects are strongly correlated with atmospheric winds and temperatures and are predicted by available atmospheric specifications. This suggests that commonly available atmospheric specifications can be used to predict both station and network detection performance, and an appropriate forward model improves location capabilities as a function of time.

Seasonal variations of infrasound detections and their characteristics in the western US

Automatic infrasound detection is analyzed with infrasonic data from the western US for the time period of November 2010 to October 2012. Data from nine University of Utah Seismograph Stations (UUSS) infrasonic arrays are supplemented by three additional arrays in Nevada, operated by Southern Methodist University (SMU). In this study, the detection procedure is based on an adaptive F-detector (AFD) that accounts for both correlated and uncorrelated noise by capturing the time-varying background noise conditions. The adaptation of the detector, characterized by time varying linear remapping of the F-distribution (C-value), depends on the background noise level at the arrays including weather conditions with seasonal variations and local site effects. The infrasound detection catalog consists of 580,177 signals, depending on time and space as well as data availability at individual stations. Based on the Ground-to-Space (G2S) specifications, the number of detections is shown to be correlated with seasonal variations in the stratospheric winds, illustrating the time varying nature of infrasound propagation. These seasonal variations of infrasound detections include the number of detections, correlation value, occurrences in time, phase velocity, and azimuth. The majority of events occur during working hours, Monday through Friday, suggesting a human cause. Based on the automatic detection results, we reviewed the waveforms generated from the known repeated sources such as Dugway Testing Ground (DTG) and Utah Test and Training Range (UTTR) in Utah, and New Bomb (NB) in Nevada. In order to validate the automatic infrasound detection catalog, we document the celerities of automatic infrasound arrivals during the summer and winter using the seismic origin times of NB explosions. Infrasound propagation is dependent on the seasonal variation of wind conditions as well as source location and array distribution.

On the Use of a Dense Network of Seismo-Acoustic Arrays for Near-Regional Environmental Monitoring

A dense network of eight, seismo-acoustic arrays operates in the southern Korean Peninsula, and since the first array installation in 1999, has provided data for monitoring local and regional seismic and infrasound signals from natural and anthropogenic phenomena. The main operational purpose of the network is to discriminate man-made seismic events from natural earthquakes to produce a clean earthquake catalog, and to ensure that seismic and infrasonic data are appropriately used for analyzing and characterizing various sources using the seismo-acoustic wave fields. This chapter summarizes results of several studies that used the network dataset to; (i) Compare seasonal variations in infrasound detections with local surface weather measurements and stratospheric wind dynamics, (ii) Develop seismic and acoustic data fusion methods that enhance source discrimination synergy, (iii) Understand the characteristic of local and regional infrasound propagation using repetitive surface explosion sources, and (iv) Review infrasound observations from earthquakes and underground nuclear tests. Finally, this chapter illustrates the usefulness of dense regional networks to characterize various seismo-acoustic sources and enhance detection capability in regions of interest in the context of future verification of the Comprehensive Nuclear-Test-Ban Treaty.

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.

Analysis of explosive sources using a distributed network of infrasound arrays

Analysis of signatures observed on a distributed network requires a model describing the propagation of the energy between network nodes as well as a statistical framework to quantify confidence in analysis conclusions. For the case of infrasound analysis, the propagation model must account for the inhomogeneous, dynamic nature of the atmosphere and the resulting temporal and spatial variations of propagation effects as well as the anisotropic nature of infrasonic propagation due to the dependence on the directionality of the winds. Association and localization methods have been developed utilizing a Bayesian framework to identify explosive sources from distant, infrasonic observations with possible applications to other infrasonic sources. The confidence of the analysis conclusions is dependent on the accuracy and realism of the likelihood definitions utilized in the method, which are therefore an area of ongoing research. An overview of the association and localization methods will be presented along wi...