IMS Stations Research Articles

In-situ calibration for real-time infrasound station monitoring and improved wave parameter determination

Infrasound signals can be detected using a time-delay of arrival approach to derive the back azimuth and trace velocity of the coherent wave. For these calculations, it is necessary to have a calibrated measure of the pressure. Although the calibration of microbarometers can be performed in a laboratory setting and in the field with specific metrological means such as those developed by the CEA, it is much more difficult to determine the transfer function of the wind noise reduction systems (WNRS), designed to reduce the wind associated noise. In-situ calibration of these WNRS's can be performed using a co-located, calibrated reference sensor. System defects, such as flooded pipes or blocked inlets, have significant impacts on the response, which in turn would influence the calculated infrasonic wave parameters. These defects can be characterized using in-situ calibration measurements. Experiments were undertaken at the infrasound station IS26 as part of the EMPIR Infra-AUV project, using a temporary detector that was used to simulate defects on the WNRS. This allowed for the effects on real infrasound detections to be quantified and corrected using in-situ calibrations. Additionally, case studies at IMS stations were performed demonstrating the utility of these methods on operational infrasound stations.

CTBT seismic monitoring using coherent and incoherent array processing

The detection and location capability of the International Monitoring System for small seismic events in the continental and oceanic regions surrounding the Sea of Japan is determined mainly by three primary seismic arrays: USRK, KSRS, and MJAR. Body wave arrivals are coherent on USRK and KSRS up to frequencies of around 4 Hz and classical array processing methods can detect and extract features for most regional signals on these stations. We demonstrate how empirical matched field processing (EMFP), a generalization of frequency-wavenumber or f-k analysis, can contribute to calibrated direction estimates which mitigate bias resulting from near-station geological structure. It does this by comparing the narrowband phase shifts between the signals on different sensors, observed at a given time, with corresponding measurements on signals from historical seismic events. The EMFP detection statistic is usually evaluated as a function of source location rather than slowness space and the size of the geographical footprint valid for EMFP templates is affected by array geometry, the availablesignal bandwidth, and Earth structure over the propagation path. The MJAR arrayhas similar dimensions to KSRS but is sited in far more complex geology which results in poor parameter estimates with classical f-k analysis for all signals lacking energy at 1 Hz or below. EMFP mitigates the signal incoherence to some degree but the geographical footprint valid for a given matched field template on MJAR is very small. Spectrogram beamforming provides a robust detection algorithm for high-frequency signals at MJAR. The array aperture is large enough that f-k analysis performed on continuous AR-AIC functions, calculated from optimally bandpass-filtered signals at the different sites, can provide robust slowness estimates for regional P-waves. Given a significantly higher SNR for regional S-phases on the horizontal components of the 3-component site of MJAR, we would expect incoherent detection and estimation of S-phases to improve with 3-component sensors at all sites. Given the diversity of the IMS stations, and the diversity of the methods which provide optimal results for a given station, we advocate the development of seismic processing pipelines which can process highly heterogeneous inputs to help associate characteristics of the incoming signals with physical events.

Xenon and radon time series analysis: A new methodological approach for characterising the local scale effects at CTBT radionuclide network

A new methodology of time series analysis has been tested on 133Xe and estimated 220Rn activity concentrations in order to characterise the site response of four different CTBT/IMS monitoring stations. Seasonal variability of 133Xe and 220Rn at these IMS stations and the role played by different meteorological parameters on such variability have been quantified. As xenon and radon are both noble gases with similar physical characteristics but very different source terms, the methodology adopted in this comparative study, once coupled to analysis of radioxenon emission time series sampled at nearby NPPs or IPFs and to direct measurements of 220Rn at IMS sites location, might help assess relative influence of near and far field air on IMS radioxenon detections. Possible applications of the adopted methodology to radioxenon categorisation schemes are also discussed.

A CZT-based radioxenon detection system in support of the Comprehensive Nuclear-Test-Ban Treaty

In this work, a prototype radioxenon detection system was designed, developed and tested at Oregon State University to study the response of CdZnTe (CZT) detectors to xenon radioisotopes for monitoring nuclear explosions. The detector utilizes two coplanar CZT detectors and measures xenon radioisotopes through beta–gamma coincidence detection between the two detection elements. The CZT-based detection system offers excellent energy resolution and background count rate compared with scintillator-based beta–gamma coincidence detectors currently in operation at the IMS stations. In this paper, we briefly discuss the detector design and report our recent measurement results with 131mXe, 133mXe, and 133Xe produced in the TRIGA reactor at OSU.

Characterization of Xe‐133 global atmospheric background: Implications for the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty

AbstractMonitoring atmospheric concentrations of radioxenons is relevant to provide evidence of atmospheric or underground nuclear weapon tests. However, when the design of the International Monitoring Network (IMS) of the Comprehensive Nuclear‐Test‐Ban Treaty (CTBT) was set up, the impact of industrial releases was not perceived. It is now well known that industrial radioxenon signature can interfere with that of nuclear tests. Therefore, there is a crucial need to characterize atmospheric distributions of radioxenons from industrial sources—the so‐called atmospheric background—in the frame of the CTBT. Two years of Xe‐133 atmospheric background have been simulated using 2013 and 2014 meteorological data together with the most comprehensive emission inventory of radiopharmaceutical facilities and nuclear power plants to date. Annual average simulated activity concentrations vary from 0.01 mBq/m3 up to above 5 mBq/m3 nearby major sources. Average measured and simulated concentrations agree on most of the IMS stations, which indicates that the main sources during the time frame are properly captured. Xe‐133 atmospheric background simulated at IMS stations turn out to be a complex combination of sources. Stations most impacted are in Europe and North America and can potentially detect Xe‐133 every day. Predicted occurrences of detections of atmospheric Xe‐133 show seasonal variations, more accentuated in the Northern Hemisphere, where the maximum occurs in winter. To our knowledge, this study presents the first global maps of Xe‐133 atmospheric background from industrial sources based on two years of simulation and is a first attempt to analyze its composition in terms of origin at IMS stations.

Aerosol sample inhomogeneity with debris from the Fukushima Daiichi accident

Radionuclide aerosol sampling is a vital component in the detection of nuclear explosions, nuclear accidents, and other radiation releases. This was proven by the detection and tracking of emissions from the Fukushima Daiichi incident across the globe by IMS stations. Two separate aerosol samplers were operated in Richland, WA following the event and debris from the accident were measured at levels well above detection limits. While the atmospheric activity concentration of radionuclides generally compared well between the two stations, they did not agree within uncertainties. This paper includes a detailed study of the aerosol sample homogeneity of 134Cs and 137Cs, then relates it to the overall uncertainty of the original measurement. Our results show that sample inhomogeneity adds an additional 5−10% uncertainty to each aerosol measurement and that this uncertainty is in the same range as the discrepancies between the two aerosol sample measurements from Richland, WA.

Analysis of Signals from an Unique Ground-Truth Infrasound Source Observed at IMS Station IS26 in Southern Germany

Quantitative modeling of infrasound signals and development and verification of the corresponding atmospheric propagation models requires the use of well-calibrated sources. Numerous sources have been detected by the currently installed network of about 40 of the final 60 IMS infrasound stations. Besides non-nuclear explosions such as mining and quarry blasts and atmospheric phenomena like auroras, these sources include meteorites, volcanic eruptions and supersonic aircraft including re-entering spacecraft and rocket launches. All these sources of infrasound have one feature in common, in that their source parameters are not precisely known and the quantitative interpretation of the corresponding signals is therefore somewhat ambiguous. A source considered well-calibrated has been identified producing repeated infrasound signals at the IMS infrasound station IS26 in the Bavarian forest. The source results from propulsion tests of the ARIANE-5 rocket’s main engine at a testing facility near Heilbronn, southern Germany. The test facility is at a range of 320 km and a backazimuth of ~280° from IS26. Ground-truth information was obtained for nearly 100 tests conducted in a 5-year period. Review of the available data for IS26 revealed that at least 28 of these tests show signals above the background noise level. These signals are verified based on the consistency of various signal parameters, e.g., arrival times, durations, and estimates of propagation characteristics (backazimuth, apparent velocity). Signal levels observed are a factor of 2–8 above the noise and reach values of up to 250 mPa for peak amplitudes, and a factor of 2–3 less for RMS measurements. Furthermore, only tests conducted during the months from October to April produce observable signals, indicating a significant change in infrasound propagation conditions between summer and winter months.

Numerical acoustic wave propagation in the atmosphere

A method is presented which simulates the acoustic wave-field in the atmosphere using a Chebyshev pseudo spectral approach. The aero-acoustic equation of motion is solved in spherical coordinates. The computational time is significantly reduced by applying a rotationally symmetric approximation. Since the complete wave-field is calculated for the two-dimensional model the solution provides pictures of the pressure propagation from the source to the receiver. These images make it possible to study two effects in detail: How the sound and wind profiles affect the synthetic barograms for a fixed source, and in which way the source in space and time is reflected in the synthetic recordings for a stationary elastic model. Examples for both applications are shown where measured data are compared with synthetics. The recording at the German IMS station I26DE of the September 21, 2001, chemical explosion in Toulouse is considered as a benchmark for synthetic barograms calculated for two-dimensional profiles based on NRL-G2S and MSISE/HWM. In the second example the estimation of the source functions of super-sonic moving bodies is demonstrated. The synthetic waveforms are compared with the signals of two bolides recorded at the French IMS station I24FR at Tahiti from the December 1, 2003.

Seismic Discrimination in Southern Xinjiang: The 13 March 2003 Lop Nor Earthquake

Earthquakes that occur near known nuclear test sites are invaluable for evaluating the capabilities of the International Monitoring System (ims), currently being established to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (ctbt). On 13 March 2003 a seismic disturbance with magnitude m b (National Earthquake Information Center [neic]) 4.8 occurred near the Chinese nuclear test site at Lop Nor, southern Xinjiang. Here we attempt to identify this disturbance as an earthquake by using three types of seismic data: (1) teleseismic P waves recorded at ims stations; (2) long-period surface waves recorded at a network of stations in Eurasia that simulates the proposed ims seismic network in this region, and (3) long-period full waveform ( P, S , and surface wave) modeling at station wmq, which is approximately 250 km from the reported epicenter of the 13 March 2003 disturbance. We find that all impulsive teleseismic P -wave onsets show compressive first motion and that discrimination using the m b : M s criterion requires assumptions that cannot be justified on theoretical grounds. However, by combining the three data types, we conclude that the observations are consistent with a double-couple source with strike φ = 125 ± 10°, dip δ = 40 ± 10°, rake λ = 90 ± 10°, and moment M 5.5 ± 1 × 1015 N m. That said, consistency of the seismic wave field with a double couple does not immediately rule out an explosion source, because tectonic release accompanying an explosion can make it impossible to resolve the isotropic moment using long-period data. The strongest argument for an earthquake is that both the regional surface waves, and waveform modeling at wmq, give an estimated focal depth of 6 ± 1 km. This result reinforces the importance of focal depth determination to ctbt monitoring, in particular, for shallow earthquakes with mechanisms that are close to perfect 45° reverse-dip-slip, which are difficult to discriminate by using other methods. Fortunately, depth determination using surface waves is especially favorable for this type of earthquake, if a sufficient number of stations can be used. We therefore recommend that long-period data recorded at auxiliary seismic stations of the ims be utilized by the International Data Centre to monitor compliance with the ctbt.

Precise Relative Location of 25-ton Chemical Explosions at Balapan Using IMS Stations

Wetest how well low-magnitude(m bLg 1.8 to 2.6), 25-ton chemical explosions at Balapan, Kazakhstan, can be located using IMS stations and standard earth models, relying on precisely determined relative arrival times of nearly similar, regional and teleseismic waveforms. Three 1997 Balapan explosions were recorded by a number of currently reporting and surrogate IMS stations. Three regional stations and two teleseismic arrays yielded consistent waveforms appropriate for relative picking. Master-event locations based on the AK135 model and ground-truth information from the first, shallowest and best-recorded explosion, fell under 1 km from known locations, for depths constrained to that of the master event. The resulting 90% confidence ellipses covered 12-13 km2and contained the true locations; however, results for depth constrained to true depth were slightly less satisfactory. From predictions based on ground truth, we found a Pgcoda phase at Makanchi, Kazakhstan to be misidentified and poorly modeled. After accounting for this, 90% ellipses shrank to 2-3 km2and true-depth mislocation vectors became more consistent with confidence-ellipse orientations. These results suggest that a high level of precision could be provided by a tripartite array of calibration shots in cases where models are poorly known. We hope that the successful relocation of these small Balapan shots will support the role of calibration explosions in verification monitoring and special event studies, including on-site inspection.