Nuclear Test Monitoring Research Articles

Automatic Determination of Secondary Seismic Phase Arrival Times Using Wavelet Transforms

For regional nuclear test monitoring applications, determining accurate locations for small-to-intermediate events (2.5 m b P -wave observations will be available for hypocenter location, secondary seismic phases, such as Lg and Pg, must be considered to improve location accuracy to the level required for nuclear test monitoring. For these events, single array or single station locations may be used to determine an epicenter for an event, based on estimates of back azimuth and epicentral distance. Because Lg propagates at a consistent group velocity of approximately 3.5±0.2 km/s, the distance estimates using the travel-time difference between this arrival and primary phases ( e.g., Pn, Pg ) are often quite reliable. However, Lg is a very complicated phase and identifying its onset time on seismograms remains one of the most difficult problems encountered in regional seismology. We have tested an improved version of the Lg wavelet phase-picker (Wavelet 1.0), based on a method developed by Tibuleac and Herrin (1999, 2001), and two other phase-picking methods: CUMSQ (Inclan and Tiao, 1994; Der and Shumway, 1999) and AR (Taylor et al., 1992). We tested the three methods on Lg and Pg (as secondary arrivals) from a data set of 97 shallow ( m b

CHARACTERIZATION OF MINING EXPLOSIONS AT REGIONAL DISTANCES: IMPLICATIONS WITH THE INTERNATIONAL MONITORING SYSTEM

The International Monitoring System (IMS) being constructed in support of the Comprehensive Nuclear‐Test‐Ban Treaty introduces new opportunities to nuclear test monitoring by providing open access to global data from seismic, hydroacoustic, infrasonic, and radionuclide sensors. These sensors will detect myriad natural and man‐made events and can be used to identify those that have explosive characteristics and therefore might be clandestine nuclear tests. Detection and identification of seismic events must be conducted at a lower magnitude threshold (mb = 3.5 and lower) than has been previously considered. Concomitant with the lower monitoring threshold will be an increased number of events that must be scrutinized. This collection will be largely composed of regional observations in which the seismic waves have traversed complex geological structures. High‐fidelity regional, geophysical models will be needed to support accurate location and source identification. Source identification will not be limited to the separation of single‐fired nuclear explosions from earthquakes as in previous testing treaties. The lower‐magnitude threshold and increased reliance on regional observations assures that mining explosions will be detected by the monitoring system. It is important that the signals from mining explosions are properly identified to avoid false alarms of the monitoring system. Cooperation with the mining industry, including deployment of close‐in instrumentation and extensive documentation of the explosions, provides critical information for interpreting the performance of regional discriminants. Linkage of these observations to appropriate physical models of the blasting process is also enhanced through this cooperative research effort. A number of discriminants for characterizing mining explosions have been identified including P to Lg ratios at high and low frequencies, surface wave to high‐frequency body wave amplitudes, Rg at short distances, high‐ and low‐frequency time‐independent spectral modulations, signal correlation, and temporal clustering. Variable performance of individual discriminants results from such factors as mine‐specific blasting practices and the complexity of the regional wave propagation. This variability attests to the need for a suite of region‐specific discriminants. Regional calibration with modest‐size, single‐fired contained chemical explosions can provide a basis for developing region‐specific procedures. Broadband data provide the basis for the most robust set of discriminants. The inclusion of infrasonic data as part of the IMS introduces a potential for the combined use of seismic and infrasonic data for the identification of near‐surface explosions. The generation and, to a greater extent, the propagation of mining explosion infrasonic signals is not well understood, but empirical data attest to its future utility. Evidence for the accidental, near‐simultaneous detonation of a large amount of explosives during standard delay‐fired explosions is presented. Events such as these have single‐fired characteristics and may prove to be problematic in discrimination analysis.

Source Effects on Regional Seismic Discriminant Measurements

Source effects on high-frequency regional seismic discriminant measurements involving P / S -type ratios ( Pn/Lg, Pg/Lg ) are isolated for earthquakes in southern California and southern Nevada. The scatter in earthquake population discriminant measurements for fixed propagation paths is statistically analyzed to assess the spatial and frequency-dependent contributions from source radiation pattern and source depth variations along with variable near-source scattering. This is useful because procedures for reducing the variance caused by propagation effects must be constrained by knowledge of the intrinsic scatter resulting from independent source effects, for which corrections are not available without a priori knowledge of the source type and depth. Broadband waveforms for earthquakes near the Nevada Test Site (NTS) and in southern California recorded at stations of the Livermore NTS Network (LNN), the Caltech TERRAscope Network, and the Berkeley Digital Seismic Network (BDSN) are used to obtain Pg / Lg and Pn / Lg measurements and to assess source depth and mechanism contributions to measurement variance. Pn/Lg and Pg/Lg spectral amplitude ratios for several frequency bands between 0.5 and 10 Hz from vertical velocity waveforms are computed for a total of 18 tightly located clusters of earthquakes. For each cluster, we examine the correlations between observed Pn/Lg and Pg/Lg ratios and estimated source depths or predictions based on previously determined source mechanisms. Focal mechanism predictions do not match the observations well, likely due to strong near-source scattering and directivity effects that overwhelm the point-source radiation pattern. However, the data do show significant scatter even for sources only a few kilometers apart. Source depth produces some coherent trends, which can be emphasized by averaging results from multiple stations. These trends vary from region to region, presumably due to specific local crustal layering, but most of the variance is not deterministically predictable and appears to arise from strong variations in near-source scattering. Variance estimates for logarithmic amplitude ratios normalized by mean amplitude ratios vary from 0.01 to 0.05 for different phase pairs. Frequency-dependent variations of the mean, variance, minimum, and maximum amplitude ratio parameters show strong distance dependence out to about 300 km and then appear to become independent of event-station distance. These variance parameters appear to be suitable for use in spatial interpolations of observed P/S -type ratios for a specific unidentified event path, as needed for nuclear test monitoring.

Whither Fall Meeting?

The international nuclear test monitoring system of the Comprehensive Nuclear Test Ban Treaty, or CTBT, provides a high level of confidence to verify treaty compliance and detect nuclear explosions, according to a new report.This is with or without additional U.S. national technical means of verification, the report notes. The high level of confidence assumes the full capability of the international monitoring system, which is currently under construction and will eventually include a global network of 337 sensors and laboratories to detect seismic, infrasound, hydroacoustic, and radionuclide signals.

Test Ban Treaty provides high level of verification, explosion detection capabilities, report notes

The international nuclear test monitoring system of the Comprehensive Nuclear Test Ban Treaty, or CTBT, provides a high level of confidence to verify treaty compliance and detect nuclear explosions, according to a new report.This is with or without additional U.S. national technical means of verification, the report notes. The high level of confidence assumes the full capability of the international monitoring system, which is currently under construction and will eventually include a global network of 337 sensors and laboratories to detect seismic, infrasound, hydroacoustic, and radionuclide signals.

Potential Impact of "Digital Earth" on Seismological Monitoring

“Digital Earth” is a newly proposed concept of a 3D, real-time, dynamic representation of geographical information by integrating the techniques of remote sensing, GIS, virtual reality, and computer networking, among others. The potential impact of Digital Earth on seismological monitoring is presented by discussing the fast assessment of seismic disasters, earthquake information service to the public, and the monitoring of clandestine nuclear explosion tests. It is pointed out that Digital Earth can provide the public with an opportunity to join in seismological analysis and is an ideal platform for the combination of remote sensing information and seismological information, with both positive and negative effects associated with such “transparent” seismology. For the proposed international monitoring system (IMS), which has in recent years turned its focus to the detection of low-yield explosions and the monitoring of a nonproliferation treaty, one of the challenging problems is the integration of different monitoring schemes, including seismic, hydroacoustic, infrasound, radionuclides, and satellite remote sensing. It is foreseen that Digital Earth can provide such integration with a practical technical framework. In monitoring a CTBT, on-site inspection (OSI) is a practically difficult and politically sensitive issue. Based on the implementation of Digital Earth, a new concept, “virtual onsite inspection” (VOSI) is proposed as a generalization of the concept of OSI. It seems that Digital Earth as a future technology in the post-Internet era will have a significant impact on the development of seismology.

Analysts examine implications of U.S. Senate defeat of Nuclear Test Ban Treaty

The U.S. Senate's October 14 vote against the Comprehensive Nuclear Test Ban Treaty (CTBT) came in the wake of the October 12 military coup in Pakistan—the first‐ever coup in a nuclear state. Shortly beforehand, representatives from 92 nations attending an October 6–8 conference in Vienna, Austria, had urged laggards to speed their ratification of the CTBT On October 6, AGU and the Seismological Society of America (SSA) issued a joint position statement expressing confidence that the combined worldwide nuclear test monitoring resources, when implemented, would meet the verification goals of the CTBT.In the aftermath of the Senate vote, politicians and analysts are examining the potential fallout from that action, including the international reaction, the possibility of achieving the accord's goal of halting the proliferation and testing of nuclear weapons, and the future of the treaty itself.

Putting nuclear-test monitoring to the test

On the 22 August 1998, 0.1 kilotonnes of conventional explosives was fired underground at the nuclear test site of the former Soviet Union at Degelen Mountain in eastern Kazakhstan. This explosion, which we refer to as K980822, was carried out by the Republic of Kazakhstan, in cooperation with the United States, to seal tunnels at the site. It provided an opportunity to assess the network of seismological stations being set up as part of the International Monitoring System (IMS) to verify the Comprehensive Test Ban by detecting and identifying low-magnitude disturbances. Despite its low yield, short-period signals from the explosion were recorded up to 88° away. The seismograms recorded are sufficient to identify the disturbance as suspicious and, if the test-ban treaty were in force, could have led to a demand for an on-site inspection to determine whether a nuclear test had taken place.

Global radionuclide monitoring in near-real time for verification of the Comprehensive Nuclear Test Ban Treaty

A global radionuclide monitoring system is being engineered as part of a multi-technology verification system for the Comprehensive Nuclear Test Ban Treaty. The system detects airborne radioactive aerosols and gases that can indicate nuclear weapons test debris. The backbone of the system is a network of 80 remote detection stations that utilize high-volume air sampling and high-resolution gamma spectrometry to provide in-situ assay and near-real time reporting. These stations are linked to the International Data Centre, which is a central data processing hub where raw spectral data is automatically processed, analyzed, and disseminated to the states parties. Measurements are categorized based on spectral content to determine which contain anomalous anthropogenic radionuclides that require intensive radiochemical analysis at a certified laboratory. The resulting system has the capability to measure microbecquerel concentrations of radionuclides and provide accessible data products within minutes of field measurements. During the past year of international operations, the minimum detectable concentrations and spectroscopy processing statistics were recorded as a function of geographical location and time. The results show that this system is an effective tool for nuclear test monitoring, as well as other applications such as radiological emergency response, public health monitoring, and scientific research.

Modeling of atmospheric and ionospheric disturbances from shallow seismic sources

Earthquake sources, as well as contained underground explosions and volcanic explosions, initiate atmospheric waves at the air–ground interface which propagate upward and outward. The propagating atmospheric waves produced are of two types: a high-frequency acoustic wave and a low-frequency gravity wave with horizontal wavelength much longer than its vertical wavelength. Because of the exponential decrease of atmospheric density with height, the acoustic and particularly the gravity waves can grow to significant amplitude in the upper atmosphere, where they can affect the ionosphere causing changes in the distribution of neutral and charged particles. The coherent fluctuations of electron densities and ionization layer boundaries produced by these waves can be detected by electromagnetic sounding methods and hence the occurrence and character of the disturbances can be inferred. A particular application of interest is the detection and discrimination of underground and near surface chemical explosions in a nuclear test monitoring context. Specifically, identification of the different source types is enhanced by combining seismic detection methods with detection of the ionospheric disturbances caused by explosion and earthquake sources. In this study, numerical models of non-linear gravity controlled atmospheric disturbances produced by seismic sources near the surface of the Earth are investigated in order to obtain quantitative predictions that might be used in evaluating detection methods based on gravity wave excitation. Explicit numerical integration of the non-linear finite difference equations is used to simulate the transient flows produced in a three-dimensional ARDC atmosphere. Results from the simulations agree with many results from linear theory approximations and also show non-linear characteristics similar to important gravity wave observations. Electron density changes in the ionosphere are predicted with their spatial and temporal behavior found to be particularly sensitive to the type and magnitude of the dissipative mechanisms that may occur. In the numerical examples studied, the amplitudes of the ionospheric electron density fluctuations due to the gravity waves produced by large explosions and some types of large earthquakes are predicted to be well within the range of detection using E-M ionospheric sounding methods.

Radiation of elastic waves from sources embedded in anisotropic inclusions

SUMMARY We develop an explicit formulation of the far-field Green's tensor in an anisotropic inclusion surrounded by an otherwise isotropic and homogeneous elastic medium. The technique handles all anisotropic systems without exception (21 independent elastic constants), but is restricted to cases where the size of the inclusion is small with respect to both the source-receiver distance and the wavelength. Hence, the application of the approximation is restricted to sources in localized regions of anisotropy so that errors related to geometrical spreading and propagation direction will be negligible. The predictions of our model are of interest for application to signals generated by sources in seismic exploration as well as nuclear test monitoring.

Numerical modeling for underground nuclear test monitoring

The symposium for Numerical Modeling for Underground Nuclear Test Monitoring was held March 23–25 in Durango, Colo. Funded by the DOE Office of Arms Control and Nonproliferation (OACN) and hosted by the Source Region Program at Los Alamos National Laboratory (LANL), the meetings's purpose was to discuss the state‐of‐the‐art in numerical simulations of nuclear explosion phenomenology with applications to test‐ban monitoring. In particular, we wished to focus on the uniqueness of model fits to data, the measurement and characterization of material response models, advanced modeling techniques, and applications of modeling to monitoring problems.The concept for the meeting arose through discussions with Marv Denny, who was on assignment at Department of Energy Headquarters from Lawrence Livermore National Laboratory (LLNL). In these conversations, the following question was discussed: how are numerical modeling techniques being used to understand the effects of explosion‐ source phenomenology on test‐ban treaty monitoring? Numerical studies are becoming increasingly important in the evaluation of capabilities for proliferation monitoring; this trend has accelerated with the curtailment of the nuclear testing program. During these discussions, the issue of the uniqueness and limitations of numerical models arose. It was decided to address these questions by convening a group of experts to present and discuss the problems associated with modeling of close‐in data from explosions.

Near-source contributions to teleseismic P waves and P-wave coda for underground explosions

Characterization of near-source crustal properties for underground nuclear explosions using distant seismic observations can potentially improve nuclear test monitoring capabilities. Previous analyses of the relative spectral content of direct P waves and P-wave coda have suggested that important physical parameters such as source overburden velocity and shot point velocity can be inferred from teleseismic signals. Such observations have the potential to both improve yield estimation (by indicating coupling conditions) and to shed light on the basic mechanisms of coda excitation. Reliable isolation of any near-source influences requires thorough suppression of propagation and near-receiver effects on the signals, which are shown herein to be significant for comparisons of P and P coda. A spectral factoring procedure is used to determine eventaveraged source spectra from which propagation and receiver terms have been removed for both direct P signals and early P coda. A large short-period waveform data set (2457 signals) for 71 events at the Nevada and Novaya Zemlya test sites is analyzed with the event-averaging procedure. Spectral ratios and slopes of spectral ratios for the event-averaged P and P-coda source terms are examined for dependence on event magnitude, burial depth, overburden velocity, and other known source characteristics. The slopes of P I P-coda ratios over the frequency band 0.3 to 2.5-Hz for NTS events show only weak dependence on near-source properties for the event-averaged spectra, while individual stations sometimes show strong systematic trends, as discovered by Gupta and Blandford (1987). The tendency is for larger, deeper events (with higher average overburden velocities) to have relative enrichment of high frequency P-wave energy compared to the coda. Single-frequency PIP-coda spectral amplitude ratios increase with increasing magnitude, depth, and overburden velocity for frequencies less than 0.5 Hz, with these trends being reversed near 0.8 Hz. For Novaya Zemlya, the slope of P/P-coda shows a strong variation with magnitude, but P waves from larger events have relatively depleted high-frequency content. Differences in magnitude-depth scaling may contribute to this difference between test sites. Single-frequency PIP-coda spectral ratios at frequencies less than 1.0 Hz increase with magnitude for Novaya Zemlya similarly to the NTS events, possibly as a result of enhanced coda excitation for shallower events combined with low-frequency pP interference. These variations are not yet quantitatively understood but may ultimately provide empirical procedures for characterizing the near-source environment of isolated events or additional test site explosions.

Seismic monitoring of Soviet nuclear tests to resume

Seismic monitoring of Soviet nuclear tests to resume

An evaluation of seismic decoupling and underground nuclear test monitoring using high‐frequency seismic data

An effective solution to the problem of the detection and identification of low‐yield coupled and fully decoupled underground nuclear explosions appears available via use of high‐frequency seismic data ranging up to 30 or 40 Hz. In order to evaluate detection‐identification capabilities when using such data, it is necessary to estimate (1) spectral characteristics and relative amplitudes of both P and S waves from explosions and earthquakes over the frequency band from 5 to 40 Hz, (2) signal transmission characteristics over this band through pertinent types of earth structure, and (3) recording system and ground noise characteristics over this frequency band. In this study, each of these topics is considered in turn as they relate to detection and discrimination of the signals from low‐yield coupled and decoupled explosions in the regional and teleseismic distance ranges. Estimates of the capabilities of specific hypothetical networks to detect and identify (insofar as signal‐to‐noise ratio is an important factor in identification) explosions within the USSR are then considered. These estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability. Following the assessment of detection capabilities, the problem of identification of small events is considered, with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal‐to‐noise ratio. The principal results and conclusions of this study are as follows: (1) seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30–40 Hz when using presently available hardware; (2) average amplitudes of high‐frequency noise in a variety of geological environments are very low and change little with time or season; (3) transmission of high‐frequency P and S wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 Hz, with effective Q factors in shield areas being about 9000 and 4000 for high‐frequency Pn and Sn, respectively, while the effective Q for Pn waves in tectonic areas is about 1000; (4) a properly designed and deployed network of 25 simple three‐component nonarray stations internal to the USSR and 15 similar stations surrounding the USSR is predicted to be capable of multistation detection at high signal‐to‐noise ratio of fully decoupled 1‐kt explosions located at all potential decoupling sites within the USSR; (5) by inference from the quantitative agreement of empirical observations and theoretical predictions, when using lower‐frequency data over a great range of explosion yields, we conclude that procedures based on the use of both detectable P and S waves will serve to identify explosion‐generated seismic signals at least as small as those expected from a fully decoupled 1‐kt explosion.

Emerging Technologies for Verification of Arms Control Treaties

Progress in the technical means of monitoring to verify compliance to arms control treaties is discussed in the following areas: Real-time surveillance with charge-coupled devices in the visible and infrared; image enhancement with digital 1 image processing and with adaptive optics; imaging with radars based on satellites and on the ground; seismic monitoring with high frequency discrimination and with unattended in-country seismic stations; and nuclear weapons test monitoring with the global positioning satellite system.

Applications of Optical Fibers to Analog Telemetry Delay Lines and Sensing Systems

This paper describes requirements for a broad-band analog telemetry link for nuclear test monitoring. It also describes prototype hardware developed to meet these requirements.

The collection and analysis of seismological data to detect possible violations of a nuclear test ban treaty

In an experimental study of requirements for the operational monitoring of nuclear tests, seismological data collected by a small but globally distributed network of array stations are transmitted to Blacknest Data Analysis Centre. Important features of the experiment have been the improvement of data communications links with the cooperating agencies which operate the overseas arrays and with other national data centres, together with the creation of a data-base on computer files linked to the ARPANETWORK and accessible to collaborating organisations. A brief summary is given of some results of event analyses using data mainly from single recording points at the array stations. These are preliminary results from the network, preceding an eventual improvement in detection capability of the array stations which will ensue when the installation of on-line processing systems is completed.

Principles of an experimental large aperture seismic array (LASA)

One of the most useful techniques for improving capability to observe small remote seismic events is to connect a large number of seismic sensors distributed in the horizontal plane so as to form an array. This has proved to be of great interest in seismological research and is particularly useful in the monitoring of underground nuclear tests, since the events are of small magnitude and the criteria used to distinguish source type depend on freeing the weak signal as far as possible from additive microseismic noise, reverberation components, and interfering earthquake activity that might chance to occur simultaneously. Results achieved by the use of seismic arrays of several kilometers aperture and several tens of sensors were sufficiently promising to suggest that substantial further improvements might be expected from an even larger array having more than an order of magnitude greater aperture and number of sensors. Such a Large Aperture Seismic Array (LASA) has been built and is being evaluated. The number of sensors is 525 and the aperture is 200 km. The notions underlying the LASA development are presented briefly in this paper, which serves as an introduction to three more detailed papers in this issue. After a brief historical resume, the principles of array signal processing are reviewed, and a prognosis of LASA capabilities is presented. The paper concludes with a discussion of the extent to which the results obtained to date have confirmed our expectations.

Analytical study of a new seismic sensor, gyro-seismometer

Abstract A new seismic wave detector incorporating a gyroscope is described and analyzed. The instrument can attain separation of linear and angular movements of seismic piers. High sensitivity is obtained by virtue of the gryroscopic characteristics. A further advantage lies in the facility with which the natural frequency of a gyro-seismometer can be varied by changing its spin velocity. The instrument probably is very valuable for distant seismic detections, for which the periods are large. The ability to isolate and measure angular movements may be of particular interest in applications to nuclear test monitoring. A simple, mathematical treatment of the instrument is presented in this paper.