Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring (1997) / Chapter Skim
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2 CTBT Monitoring Technical Challenges That Drive Research
2 CTBT Monitoring Technical Challenges That Drive Research
Pages 23-48
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From page 23... ...
These disturbances propagate through the Earth system obeying well-known laws of physics. By continuously monitoring the diverse media on the Earth, one can detect arrival times and characteristics of explosion signals at different positions on the surface or in space.
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From page 24... ...
CTBT monitoring capabilities. 2.1 PHYSICAL PHENOMENA: SOURCE EXCITATION, SIGNAL PROPAGATION, AND RECORDING Nuclear explosions can occur in space, the atmosphere, underwater, and underground.
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From page 25... ...
Radionuclide detectors at the surface will detect the particulates and radiogenic noble gases that are produced by atmospheric nuclear explosions, with wind patterns determining which stations "see" the event and at what time. Wind transport velocities are relatively slow (compared to sound waves)
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From page 26... ...
The specific excitation of sound waves depends on the explosion depth, the local structure of the sound velocity profile, and lateral gradients in the velocity profile. Hydrophones, typically deployed vertically in arrays across the SOFAR channel or in some cases deployed horizontally in arrays on the ocean bottom, can detect the passage of hydroacoustic waves and measure the corresponding pressures in the sound pulses and their propagation direction (if arrays are used, but not for single sensors)
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From page 27... ...
. In addition, seismic stations can be deployed on the ocean bottom or in drill holes in the ocean floor, and these can directly observe both seismic 27 and hydroacoustic signals (changes in water pressure due to sound waves in the ocean move the ocean bottom up and down and thus are recorded on the seismic ground motion sensors)
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From page 28... ...
Underground nuclear tests are typically designed to be well contained, meaning that the depth of burial, the material properties, and other operational factors are sufficient that explosion materials do not vent to the surface. However, many underground tests in the past did in fact vent radionuclides.
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From page 29... ...
, they are more complex and harder to interpret than teleseismic waves. An extensive data set of seismic signals from a given region must first be acquired and understood before regional waves recorded within it can be interpreted in detail, a process frequently referred to as calibration.
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From page 30... ...
The coupling of seismic waves into acoustic waves in the oceans and atmosphere, as well as the direct excitation of infrasonic waves by sources that disrupt the surface, allows hydroacoustic and infrasonic methods to be used to monitor underground events. Other methods that exploit energy coupling from the solid Earth to the atmosphere could also be useful.
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From page 31... ...
The seismic signal detection process at the prototype IDC combines signal tuning, a shortterm/long-term detection algorithm, and processing for array stations to detect the waves and determine the arrival times to be used in the association algorithms. Because of the large volumes of data, automatic detection processing is essential.
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From page 32... ...
This technique uses maxi RESEARCHREQUIRED TO SUPPORT CTBT MONITORING mum likelihood spectral estimates of the waveform in an autoregressive model. The experience in the prototype IDC, using autoregressive models, was that most of the arrival times were late, and manual corrections moved the automatic arrival forward in time, up to 2 seconds.
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From page 33... ...
or empirical corrections for travel times in different parts of the Earth can be tabulated. Travel time curves for direct tele seismic P waves in the Earth typically predict global arrival times to on the order of +1 second at a given distance (greater scatter is found at regional distances)
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From page 34... ...
It must be recognized that in the absence of ground truth for a given event, there are only estimates of the actual source location. Typically, this is obtained by solving a mathematical problem where the data are the arrival times of various types of signals at different sensors around the Earth's surface and the "model" is a set of relationships between travel times and RESEARCH REQUIRED TO SUPPORT CTBT MONITORING distances for various wavetypes.
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From page 35... ...
Typically some form of direct calibration is required, involving, for example, travel time corrections for paths from a source region to the monitoring network that account for errors in the standard model used. Calibration can be performed by using events with known source parameters (e.g., controlled explosions or earthquakes that rupture the surface)
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From page 36... ...
If an event is identified confidently as a nuclear explosion, it is then possible to estimate the yield of the explosion from empirical or theoretical knowledge of how explosion sources generate the observed signals. As described in Section 2.1, the propagation effects for hydroacoustic, infrasound, and seismic waves must be understood if observed signal amplitudes are to be related to source energy release, and this must be done without a priori knowledge of the source type.
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From page 37... ...
If a deep underground nuclear explosion is conducted at the center of a large spherical cavity in hard rock, seismic signals are reduced by a decoupling factor of about 70 if the radius is greater than or equal to about 25 m times the cube root of the yield in kilotons (i.e., r 2 25 x Wl/3~.
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From page 38... ...
For CTBT monitoring purposes, however, the identification process is largely one of determining which events are definitely not nuclear tests. For example, accurate determination that the depth of a source in a continental region is greater than 10 km ensures that it is not a human-induced event.
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From page 39... ...
The rapid onset time scale and compact source dimensions of underground nuclear explosions have been noted earlier. These directly affect the spectrum of seismic wave energy radiated by the source, with explosions having weak long-period body and surface wave excitation but strong highfrequency radiation.
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From page 40... ...
FIGURE 2.3 Top: Characteristic seismic signals for explosion and earthquake sources from long- and short-period instruments, illustrating measurements of amplitudes used for magnitude and yield estimates. Pn and Sn are regional P and S waves that travel just under the crust.
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From page 41... ...
For CTBT monitoring, in which small events are of interest, application of the traditional form of this discriminant is limited by the fact that small events do not excite 20-secondperiod surface waves, independent of the type of source. Thus, successful teleseismic discriminants developed for large events typically have to be modified for application to regional signals where the propagation effects and the signal frequency content may differ.
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From page 42... ...
Given the limited experience with applying regional seismic discriminants around the world, event identification for small underground explosions stands out as one of the primary areas for improvement of national monitoring capabilities. Associated research issues are addressed in Chapter 3.
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From page 43... ...
CTBTA'IONITORING TECHNICAL CHALLENGES THAT DRIVE RESEARCH Identified explosion A J J , \ Yes \ y / Site under \ / identifiable control .\ N r Attribution possible y / Location \ /associated with\ a State? \ N / Debris on\ y ~ artifacts collected?
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From page 44... ...
If the sampling time is three weeks later, the required volumes would be larger, but not prohibitively so. The exact volume depends on the nuclide of interest, the particulars of the device (e.g., its yield and special nuclear material)
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From page 45... ...
The protocol to the treaty provides that allowable techniques and equipment may include position finding; visual observation, video and still ~8 The Executive Council will consist of 51 members. 45 photography, and multispectral imaging, including infrared measurements; measurements of radioactivity using gamma-radiation monitoring and energy resolution analysis; environmental sampling and analysis of samples; passive seismological monitoring for aftershocks; resonance seismometry and active seismic surveys, magnetic and gravitational field mapping, and ground penetrating radar and electrical conductivity measurements; and drilling to obtain radioactive samples.
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From page 46... ...
vary depending on material properties, emplacement conditions, and size of the explosion. They range from craters and rubble in chimneys above the point of explosion, to radioactive gases and aftershocks, to effects on the gravity and magnetic fields, the water table, plant life, surface features, and the velocities and attenuation of the seismic waves propagating in the material.
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From page 47... ...
SOURCE: DOE, 1993. Of the time series signals recorded by seismic, infrasonic, and hydroacoustic stations to a single analysis center, which continuously processes the gigabytes of data collected on a daily basis to produce a list of events and event attributes that will serve international treaty monitoring activities.
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From page 48... ...
NDC systems is required, as is allocation of resources for deployment of the actual field data collection systems. The extent to which funding is provided for these activities will strongly influence the time required for CTBT monitoring capabilities to reach the levels described in U.S.
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