National Academies Press: OpenBook

The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States (2012)

Chapter: 2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES

« Previous: 1 SAFETY, SECURITY, AND RELIABILITY OF THE U.S. NUCLEAR WEAPONS STOCKPILE
Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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2

TECHNICAL MONITORING CAPABILITIES AND CHALLENGES

Nuclear explosions generate large amounts of energy and radioactive debris in a small fraction of a second. The rapid release of energy affects the surroundings and is propagated in ways that can be detected and separated from other phenomena. Monitoring such signals and radiological sampling are important tools in understanding the capabilities and threats presented by a State that conducts nuclear-explosion tests. The formal goals for U.S. monitoring are established by a classified presidential directive based on considerations of available monitoring technology and risks associated with detected and undetected tests. The requirements, techniques, and capabilities of the U.S. system are classified and are described only in the classified version of this report.

This chapter provides an overview of how nuclear-explosion monitoring works, with different technologies serving as elements of an overall system. U.S. national technical means (NTM) and the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) International Monitoring System (IMS) are summarized, followed by an assessment of the capabilities of monitoring technologies (seismic, radionuclide, hydroacoustic, infrasound, and satellite-based techniques). Phenomena associated with nuclear explosions, from the subsurface to outer space, are illustrated in Figure 2-1. The chapter closes with a summary of CTBTO operational capabilities, on-site inspections, and a discussion of transparency and confidence-building measures.

OVERVIEW AND 2002 REPORT FINDINGS

To monitor for compliance with the CTBT, it is essential to be able to put potential violators at risk of detection through vigilant monitoring for nuclear explosions. The possibility of nuclear-explosion testing must be considered in four environments—underground, underwater, in the atmosphere, and in space. The Limited Test Ban Treaty of 1963 banned signatories (the United States, the United Kingdom, and the Soviet Union1) from nuclear-explosion testing underwater, in the atmosphere, and in space. All known nuclear-explosion tests by other countries (China, France, India, Pakistan, and the Democratic Peoples’ Republic of Korea) have been underground since the last Chinese atmospheric explosion in 1980. Underground nuclear-explosion testing may be attractive to an evader because it offers the possibility of hiding details of the test and containing radioactive debris.

Each of the four environments requires different monitoring methods, each with different capabilities. All monitoring methods are based on physical signatures that are associated with nuclear explosions. These signatures are the basis for (1) concluding that an event has occurred (detection); (2) determining the location of the event (location); (3) discriminating the event from non-explosive phenomena (identification); and (4) in the case of a suspected explosion, evaluating the yield, its nuclear or non-nuclear nature, and the source of the event (characterization and attribution). A full definition of these terms is given in the glossary in Appendix K. As described in the 2002 report, and in more detail below, there are three types of sensor networks used for monitoring purposes: U.S. NTM networks, the international network

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1 A number of other states have since signed the LTBT and the Russian Federation is bound by the Soviet Union’s treaty commitments.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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defined in the CTBT, and sensor networks deployed primarily for purposes unrelated to nuclear-explosion monitoring (for example, to monitor earthquakes). By design, these networks are separate; in practice, they are often complementary. U.S. NTM provides detection, location, identification, characterization, and attribution capabilities that are primarily classified. The CTBT IMS data is available to all Member States, and the CTBTO uses it for detection and location, but the role of identification and further analysis is defined in the Treaty as one for the Member States alone. All member states’ NTM, including the U.S. NTM, can make use of other sensor network data for nuclear explosion monitoring purposes.

Throughout this report IMS network capabilities are given in terms of detection thresholds. These determine the events the CTBTO reports on and determine the data available to the States Parties, including data such as IMS auxiliary stations, which are retrieved upon formation of an event. These thresholds define the IMS data available for States Parties to use in their role of event identification, characterization and attribution. Differences between detection and identification threshold are discussed on pages 48-49, and as noted, they can be the same when calibrated regional discriminants are used.

Figure 2-1 shows the various regions of nuclear explosion phenomenology—subsurface, atmosphere (low altitude), transition (high altitude), and space (monitored by the satellite nuclear detonation detection system).

Nuclear Detonation Phenomena

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FIGURE 2-1: Phenomenology of nuclear explosions, from the subsurface to outer space. SOURCE: U.S. DOE, 2004

Table 2-1 summarizes the signals that originate from nuclear explosions in different media, how they propagate through the environment, and what technologies are used to detect the signals today.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

TABLE 2-1: Phenomena Associated with Nuclear Explosions, and Technologies Used for Monitoring Them.

Phenomena Primary Monitoring Environments Propagation Technology Used
Seismic Waves Underground, underwater Through the Earth and water Seismometers
Radionuclides— Particulate and Gases Atmosphenc, underground, underwater and space Through air; through water; through rock fractures; through space (trapped in the Earth's magnetic field) Ground-based and airborne collectors: satellite-based detectors
Hydroacoustic Waves Underwater Through water H yd rophon es—T-phase seismic stations
Infrasound Waves Atmosphenc Through air Infrasound detectors
Electromagnetic Pulse (EMP)2 Atmosphenc Through air and space Satellites—EMP burst detectors*
Optical Flash Atmosphenc, space Through air and space Satellites—Optical flash detectors*
Nuclear Radiation Space Through space Satellites—Radiation detectors*

* Not included in the IMS but available through NTM.
SOURCE: Committee

In cases where multiple detection technologies complement one another for the same event, the data from these disparate detection technologies can be brought together or “fused” in order to improve the capabilities for detection, location, and identification. It is this data fusion that determines the ultimate monitoring capabilities relevant to the CTBT. Based on developments over the past decade, each of the technologies that can be used to monitor compliance with the CTBT is reviewed below and updated from the 2002 Report. For each technology, the committee considers U.S. NTM, the IMS, and other capabilities. Augmenting the technologies summarized in Table 2-1 is information derived from confidence-building measures (CBMs) and on-site inspections (OSI), both of which are also described in this chapter.

The 2002 Report assessed the capabilities proposed under the CTBTO IMS and stated that, when fully implemented, the IMS would detect and identify explosions with a yield of at least one kiloton (kt) with high confidence in all environments, assuming no efforts at evasion. For underground explosions, it said that a yield of 10-100 tons would be detectable, although explosions of less than a few kilotons might require an on-site inspection to confirm the explosions as nuclear. Tests above 500-1,000 tons for atmospheric explosions could be characterized as nuclear, and nuclear-explosion tests as low as one ton would be detected if

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2 EMP is an intense pulse of electromagnetic radiation resulting from electric currents produced by energetic radiation (neutrons, gamma rays and x-rays) from a nuclear explosion. High-altitude EMP (HEMP) observed at the ground from a space nuclear explosion has a very fast component (E1) produced by the gamma rays interacting with the atmosphere and a slow component (E3) caused by the expansion of the debris “bubble” in the Earth’s magnetic field.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

they were conducted underwater. Technical advances since the 2002 Report are discussed in sections dealing with specific monitoring technologies later in this chapter.

Several proposed evasion scenarios were also considered in the 2002 Report, which concluded that the only plausible techniques were cavity decoupling (a potential method for reducing the size of the seismic signal created by an explosion by muffling the explosion in a large underground cavity) and mine masking (concealing a nuclear explosion by conducting a nuclear test in a region that has frequent, large chemical explosions associated with mining operations). The report also concluded that either of these techniques would be difficult to employ successfully and that they would have a significant chance of being detected by the IMS for all but very low yields. Evasion techniques are discussed further in Chapter 4 and Appendix E.

U.S. NATIONAL TECHNICAL MEANS

The United States developed and maintained the capacity to monitor nuclear-explosion tests long before any test limitation treaties existed. Today that work is carried out by the Air Force Technical Applications Center (AFTAC). AFTAC’s mission is “the detection of nuclear detonations…anywhere in the world: below ground, in water, surface blasts, free-air, and in space.” It operates and maintains the U.S. Atomic Energy Detection System (USAEDS), which is used to monitor treaty compliance. AFTAC collects and analyzes data from a variety of sources and is home to the U.S. National Data Center (U.S. NDC), which engages with the IMS and the International Data Centre (IDC) of the CTBTO in the exchange of data and data products as specified in the text and protocols of the CTBT. Other government agencies, notably NNSA and the DOE national laboratories, carry out research on new and improved technologies to monitor nuclear explosions and AFTAC draws on the expertise of scientists in the DOD and DOE laboratories, the U.S. Geological Survey, U.S. academic institutions, and private contractors. New technologies are transferred to and beta-tested by AFTAC operators.3 In contrast with the CTBTO monitoring system, whose monitoring networks, technologies, and station placements are defined by the Treaty, U.S. national technical means may take advantage of technologies not part of the IMS (for example, space-based monitoring) and may focus its monitoring efforts on areas of particular interest to the United States.

NTM give the United States significant additional information beyond what is available to other countries that do not have a robust NTM program. U.S. NTM can focus on monitoring countries of concern to the U.S. The United States global monitoring capabilities are generally better than those of the CTBTO because they can go beyond data available to the CTBTO with classified capabilities. However, the inclusion of classified means and data limits the extent to which analyses and even results may be shared and used openly. Drawing on all available assets is important because there are CTBTO installations in locations where the United States cannot readily deploy stations, as well as thousands of stations that operate independently of U.S. NTM and the IMS.

Finding 2-1: U.S. National Technical Means provide monitoring capability that is superior to that of the CTBTO, but the use of U.S. NTM for diplomatic purposes may be constrained due to its largely classified nature.

Finding 2-2: The International Monitoring System provides valuable data to the United States, both as an augmentation to the U.S. NTM and as a common baseline for

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3 See: http://www.tt.aftac.gov/WRT/U.S.IMS/Index.html.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

international assessment and discussion of potential violations when the United States does not wish to share NTM data.

THE CTBTO INTERNATIONAL MONITORING SYSTEM

The CTBTO is based in Vienna, Austria. Its key elements are (1) the IMS, which generates data from its radionuclide, seismic, infrasound, and hydroacoustic networks, and (2) the International Data Centre, which collects and processes the IMS data. Because the threat of data manipulation or denial using cyber attacks is a possibility, the IDC takes precautions to reduce this risk, including the use of a dedicated global communication infrastructure, data authentication and encryption, station intrusion-detection devices, and regular data back-up.

In addition, if the Treaty enters into force, there will be the possibility for conducting on-site inspections (OSI). IMS data are transmitted in near-real time to the CTBTO and to those national data centers (NDC) that request them. Figures 2-2 through 2-6 locate and give the status of the more than 300 stations (whether certified, installed, under construction, or not yet started) of the IMS (the 50-station primary seismic network, the 120-station auxiliary seismic network,4 and 80-, 11-, and 60-station networks for radionuclide, hydroacoustic, and infrasound technologies, respectively) as of mid-2010. The IMS monitoring station locations were determined to maximize global coverage without focusing on any particular countries.5

A major advance in the last 10 years is that the number of certified6 IMS stations grew from three in October 2000 to 264 in February of 2011, with an additional 17 stations installed and undergoing evaluation. By February of 2011, about 83 percent of the full network was implemented (certified stations plus those undergoing testing) and will approach 90 percent by the end of 2011. Different stations address different components of the IMS mission, as shown in Figures 2-2 through 2-6.

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4 Many of the primary stations, which operate continuously for the IMS, are seismic arrays that enable measurement of the direction to an event of interest and that enhance capability to detect small signals and features, such as the surface-reflected waves that help determine the depth of the event. The auxiliary seismic network records continuously but currently its data are used only when deemed necessary to augment analysis of an event that has already been detected by the primary network

5 In assessing the capabilities of the IMS, it is important to understand that the treaty specifies that event identification, characterization, and the attribution of a nuclear explosion to a particular country are the responsibility of the member states, not the CTBTO. Thus the CTBTO provides data on all detected events, as well as subsets for which events characterized (with high confidence) as earthquake-like have been screened out, but the United States does not rely on the CTBTO for screening to decide whether a particular event was a nuclear explosion. CTBTO is not responsible for identifying and characterizing seismic events as nuclear explosions, earthquakes, or chemical explosions; nor does it deal with assessing evasive testing such as decoupling (see Chapter 4).

6 A certified station in the context of the CTBT refers to a station that has been judged to substantially meet a set of technical and operational requirements specified in the operational manuals. Certification is essentially the last step in the process of establishing a station before data from that station is trusted.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

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FIGURE 2-2: Location and status of 49 stations of the IMS Primary Network, as of mid-2010 (the location of an additional station was undecided). Certification is the final step in preparing to send data in real time via satellite to Vienna, and 42 stations are shown here as having passed this stage. SOURCE: Modified from CTBTO

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FIGURE 2-3: Location and status of 120 stations of the IMS Auxiliary Network, as of mid-2010. Auxiliary stations are supported by the hosting state and not the CTBTO. SOURCE: Modified from CTBTO

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

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FIGURE 2-4: Location and status of 80 stations of the IMS Radionuclide Network, as of mid-2010. All of these stations will monitor for particulates, with 40 of the stations to monitor for xenon (gas) isotopes that are diagnostic of nuclear explosions. As of February 2011, 26 of these noble-gas stations were transmitting data to the IDC in Vienna. SOURCE: Modified from CTBTO

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FIGURE 2-5: Location and status of 11 stations of the IMS Hydroacoustic Network, as of mid-2010, when damage at two stations was under review for repairs (Crozet Island, shown in green; and a station off-shore from Chile which was destroyed by a tsunami early in 2010). SOURCE: Modified from CTBTO

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

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FIGURE 2-6: Location and status of 60 stations of the IMS Infrasound Network, as of mid-2010. SOURCE: Modified from CTBTO

The Treaty defines mechanisms for non-IMS data to be used for the purposes of “consultation and clarification” and for consideration of an OSI request. Member States may also request particular technical analyses from the CTBTO that can incorporate non-IMS data, resulting in a special product that is attributed to the requesting State but made available to all Member States. Finally, States may do their own processing, using any data they wish, including NTM, and present their results to other Member States for Treaty purposes.

One of the CTBTO’s main products is the IDC Reviewed Event Bulletin (REB) for waveform-technology-based events (the waveform-based REB merges data from seismic, hydroacoustic, and infrasound stations). The REB, along with other bulletins, keeps States informed of events detected by the IMS. The IDC’s event identification role is that of “Assisting individual States Parties…with expert technical advice…in order to help the State Party concerned to identify the source of specific events.”Even though the monitoring capability of the United States through its NTM is generally superior to that of the IMS, the data collected by the CTBTO is a trusted, valuable and reliable source of relevant information for U.S. monitoring purposes. USAEDS and IMS share 22 seismic stations and 3 hydroacoustic stations.

The raw data from the IMS is both protected and authenticated in multiple ways to decrease the likelihood that data collected and transmitted could be tampered with. In addition, there are multiple methods used to back up the IMS data so that accidental or intentional erasure is also very unlikely. Great efforts have been made to provide data that is both well characterized and secure. Electronic data encryption, intrusion detection, and power backups protect data at the source; data is then transmitted through the Global Communications Infrastructure (GCI), a dedicated communications channel designed and enabled specifically for the CTBT.7 In addition, any state party is free to keep its own back-up of IMS data and/or IDC products if it chooses to do so.

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7 These IMS stations have a satellite link to the IDC in Vienna. Some send data continuously (e.g., the primary seismic network). Others send data only on request or at regular intervals.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

The CTBTO plays an important role for the U.S. by providing a common baseline of data to the world scientific community, as well as providing data from areas that the United States has previously had difficulty accessing. CTBTO analysis, software, and training/outreach activities help to create and maintain a level of technical capability and common interpretation of data. The CTBT provides additional verification tools beyond the IMS through an on-site inspection mechanism and means to address events of concern through consultation and clarification with Member States. The IDC event bulletins provide screened information to help Member States make their own compliance decisions. A common misconception is that the United States and its allies will rely solely on the IMS for verification of compliance with the CTBT. “The treaty mandates that each State Party maintain a National Authority to serve as the national focal point for liaison with the CTBTO and with other signatories” (NRC, 2002, pp. 38).

Recommendation 2-1: The United States should support both the completion of the IMS and its operations, training, and maintenance, whether or not the CTBT enters into force. If the CTBT were to enter into force, the resulting expertise would also aid in gaining consensus on compliance issues, including, for example, authorizing an on-site inspection.

OTHER CAPABILITIES

There are instrument networks that have other primary uses but that have value for detection of nuclear explosions. Examples are the numerous regional networks of seismometers used for earthquake detection and the international tsunami warning system. Other networks, such as those that monitor radionuclides from nuclear power plants, could also provide potentially relevant data for analysis in the event of a suspected atmospheric detonation.

MONITORING TECHNOLOGIES8

Seismic

Seismology is the most effective technology for monitoring underground nuclear-explosion testing, the one environment that was not precluded from nuclear-explosion testing by the Limited Test Ban Treaty of 1963. Seismic monitoring for nuclear explosions is complicated by the great variety and number of earthquakes, chemical explosions, and other non-nuclear phenomena generating seismic signals every day. More than 600 earthquakes per day are regularly documented in an international summary report, and mining operations use several million tons of chemical explosives each year. Programs to sort out and identify signals from underground nuclear explosions in the midst of signals from these other phenomena have made great progress since they commenced in the 1950s, with notable improvements in the past ten years.

Changes since the 2002 Report

Substantial improvements in the U.S. and international ability to monitor underground nuclear-explosion testing have been made since the 2002 Report in three areas:

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8 In this report, we use the term monitoring to refer to all aspects of detection and characterization of an event (e.g., nuclear vs. non-nuclear, geolocation, yield estimates, etc.).

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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1. Verification research and development efforts have resulted in the implementation of new regional distance (<1,600 km/1,000 miles) seismic methods to detect, locate and identify events.

2. Many more high-quality, broader bandwidth, digital seismic stations and arrays have been deployed globally and many of these stations transmit data in near-real time.

3. Increase sin computing power and affordable, online, digital storage of waveforms on the terabyte scale have led to improvements in all aspects of seismic monitoring.

Two important events since the 2002 Report were the announced underground nuclear-explosion tests by North Korea in 2006 and 2009. These events were readily detected, located, and identified as described in Box 2-3,9 demonstrating how the technological advances have improved both U.S. NTM and CTBTO IMS monitoring. Figure 2-7 indicates the extent of improvements in seismic monitoring achieved over the last 20 years.

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FIGURE 2-7: Improvement in seismic monitoring over the last 20 years. Threshold values indicate statistically significant confidence of detection. Note that yield and seismic magnitude scales are logarithmic; each unit of improvement is a factor of ten. Seismic sensitivity to nuclear explosions has improved significantly due to increased deployment of seismometers and improved data analysis. For locations of interest, this allows regional monitoring at distances less than about 1,600 km (1,000 miles), which has a much lower threshold (~ 20 tons or 0.020 kilotons explosive yield) than does global monitoring (~ 200 tons or 0.20 kilotons) recorded at distances typically greater than 2,000 miles. Monitoring at test sites (e.g., through transparency

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9 See, for example, the U.S. Director of National Intelligence (DNI) statements in Box 2-3 and the CTBTO web site pages on these events. Examples of regional seismic techniques for these events are given in Appendix D.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

measures) can bring the sensitivity down to about 5 tons, or 0.005 kilotons.10 SOURCE: Seismology Subcommittee

Seismic Signals

Seismic signals can propagate as “body waves” through the solid body of the Earth, including its deep interior, or as “surface waves,” which travel along the Earth’s surface (analogous to the ripples on the surface of a pond). Body waves can further be divided into P-waves (the P standing for “primary”—these waves travel the fastest of all seismic waves and hence are the first to arrive) and S-waves (S standing for “secondary”—their speed is about 60 percent that of P-waves). P-waves (which entail a longitudinal motion pushing and pulling in the same direction in which they travel) are efficiently excited by explosions, whereas typical earthquakes are efficient producers of S-waves (which entail a transverse motion, or shearing, perpendicular to the direction in which they travel) and surface waves.

Seismic signals are traditionally grouped into categories of “teleseismic” or “regional” depending upon the distance at which they are observed.11 Teleseismic waves do not greatly diminish with distance in the range from about 2,500 to 9,000 km (1,563-5,625 mi), so they are suited to monitoring a large country from stations deployed outside that country’s borders. Teleseismic waves were the basis of most nuclear-explosion test monitoring prior to the 1990s. For sub-kiloton explosions, teleseismic monitoring can often still help with event detections but is likely to be inadequate for event identification and therefore monitoring benefits from regional-distance signals.

Regional waves, though often useful because they can carry more information than teleseismic waves, are often harder to interpret because they exhibit greater variability. The characteristics of these waves, and the ways they vary for stable and tectonic regions as well as for continents and oceans, have been extensively researched, beginning in the late 1980s. Under the Threshold Test Ban Treaty, the primary monitoring concern was nuclear-explosion testing over 150 kt. This monitoring requirement could be easily handled at teleseismic distances; under the CTBT, however, the challenge is to identify and characterize all nuclear-explosion tests, regardless of size. Stations at closer distances can see smaller-sized events, but the waves travel through the most complex part of the Earth—the crust and upper mantle—which can distort the seismic signals. While research into regional seismic methods was underway in the 1990’s, research products were just beginning to be applied to real-time monitoring. Today, many regional monitoring methods have proven viable for detection/association, location, identification and event characterization in real time. For many continental areas, the use of regional data has improved monitoring sensitivity by as much as a factor of ten compared to purely teleseismic methods. As more stations and arrays are deployed

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10 Approximate fully-coupled yields are shown in Figure 2-7, as described in the 2002 Report (NRC, 2002, p. 41), with global monitoring sensitive down to magnitudes mb = 3.8 (corresponding to fully-coupled yields of about 135-250 tons, depending on geology, shown here as 200 tons) and regional monitoring sensitive down to mb = 2.8 (fully coupled yields of about 6-25 tons, depending on geology, shown here as 20 tons). Sensitivity for monitoring at test sites goes down to mb = 2.2, or ~ 5 tons fully coupled yield. Yields for Regional and Test Site thresholds refer to unclassified capabilities for monitoring countries of concern to the U.S. for well-coupled explosions. The mention of a local threshold, lower right in figure, refers to the future possibility of bilateral monitoring agreements, distinctly separate from the CTBT. The magnitude estimates listed on this figure are from the IDC and other public sources. See Figure D-1 and additional detail in Appendix D.

11Regional and teleseismic refer to event-station distances of less than 1,600 km (1,000 miles) and greater than 1,600 km, respectively.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

and more signals are transferred in near-real time, these current and newly emerging techniques will continue to lower monitoring thresholds.

Seismic Event Detection, Association, and Location

A high-quality station may be expected to detect tens or even hundreds of seismic signals per day, many of them from nearby or “local” sources. With many different events each day, seismic waves from different events may be superimposed at any particular station. The work of association is to identify the sets of signals, from different stations, which all originate from the same seismic event such as an earthquake or an explosion.

A refined estimate of the location of the seismic source is obtained by iterating to find a point in the Earth (latitude, longitude, depth), and an origin time, from which the seismic waves arrived at the set of observed times at different stations. The accuracy of seismic event location depends on measurement and model errors. These errors lead to seismic event location uncertainty, usually quantified as an area, such as an ellipse (within which there is a specified degree of confidence that the event must lie), rather than as a point. Accurate seismic locations are important for attribution, to help with identification (for example, if the event is definitely deeper than, say, 10 km [6.2 miles], it is unlikely to be an explosion) and because the CTBT limits an OSI area to no larger than 1,000 km2 (for example a circle with a radius of about 18 km, or 11 miles).

Location measurement error is related to the uncertainty in timing the arrival of the seismic signals, which can vary with event size and distance. Larger events with simple paths through deep Earth can be timed more accurately than those with weak signals or complex paths. Increases in computing power and the online storage of large amounts of seismic

BOX 2-1 Estimation of the Yields of
Underground Nuclear Explosions from
Seismic Magnitudes

To assess the size of a detected event in terms of nuclear yield, yield typically must be derived from seismic magnitude. A single relationship between magnitude and yield does not exist. This is because explosions of a given yield generate different amplitudes of seismic waves (and hence different magnitudes) depending upon 1) the efficiency of seismic wave propagation from source to recording stations, 2) the rock type at the source, 3) depth of the explosion, and 4) whether the explosion is well coupled or decoupled. Here we examine the first three factors in the calculation of yield from seismic measurements for well-coupled explosions in either hard rock or below the water table (See Appendix E for details about decoupling).

Formulas relating the body-wave magnitude, mb, to the yield, Y, based on data from past underground nuclear explosions are of the form

mb = A + B log (Y),

where A and B are constants that depend on features 1–4.

Most past tests of yield greater than about 1 kiloton were detonated at greater depths as yield was increased so as to ensure containment. Their data are well fit by B = 0.75 (Murphy, 1996). Nuclear explosions at eastern Kazakhstan, Lop Nor China and northern India are characterized by efficient propagation of P waves such that

mb = 4.45 + 0.75 log (Y),

where Y is in kilotons. Explosions in Nevada are characterized by poorer propagation of P waves such that the constant A is smaller

mb = 4.05 + 0.75 log (Y),

Hence, for a given mb the yields calculated for explosions at Lop Nor are smaller than those at the Nevada Test Site. Propagation of P waves from th main Russian test site at Novaya Zemlya is somewhat less efficient than that from eastern Kazakhstan, resulting in A = 4.30. Nuclear explosions in hard rock, in salt or below the water table are characterized by magnitudes that differ very little (± 0.1 mb units) once corrections are applied for differences in the propagation of P waves (Murphy, 1996). Explosions in water and saturated clay produce seismic waves that are substantially larger (Murphy, 1996). For explosions of varying yield at the same depth B =1.0. For explosions with very small magnitudes, i.e. those less than mb = 4, we calculate yields using B = 1.0 because such small nuclear tests are not likely to be conducted at the depths that B = 0.75 would imply. For a given mb, use of B = 1.0 leads to more conservative (larger) estimates of yield for very small explosions than does B = 0.75.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

data have allowed much greater use of signal processing techniques that greatly reduce measurement errors. This is an area for which there are opportunities for substantial future improvement.

Location error also arises from inadequate models of the geologic variability in the Earth, which is not fully known. Seismic waves travel at different speeds through different rock types. If this is not fully accounted for in the Earth models used for seismic location, the event area will not be centered on the true location. Regional wave signals, which travel through the most complex part of the Earth, can be misinterpreted more easily than teleseismic wave signals if not properly calibrated. Since the 2002 Report there has been significant progress in performing continental scale calibrations. These calibrations have been done using well-located reference events to derive travel time corrections for all future nearby events. Most recently, increases in computing power, data storage, and regional R&D have resulted in new models for Eurasia that allow accurate location of events well away from reference events. These models are derived using large-scale tomography, similar to medical imaging, but where the regional seismic waves from many earthquakes are used to image the Earth. They reduce the regional model errors to be of similar size to teleseismic ones allowing easy mixing of teleseismic and regional data and can achieve location accuracies of 1,000 km2 (390 mi2) area or better for small events (Myers et al., 2010). As our knowledge of Earth structure improves, location model error can be expected to decrease.

Seismic Event Magnitude and Identification

Event identification is done by comparing the amplitude characteristics of different types of seismic waves. For teleseismic waves, the relative size of body waves and surface waves is an effective discriminant. As noted in Box 2-1 the strength of a seismic source as determined from the amplitude of its body waves is conventionally reported as the body wave magnitude, symbolized as mb.12 Correspondingly, the size of surface waves is reported as the surface wave magnitude, Ms. Shallow earthquakes have a larger relative surface-wave magnitude than do underground nuclear explosions having the same body-wave magnitude. For small events, it can become difficult to measure the surface-wave magnitude above the background noise. However, new regional identification methods have proven very effective at identifying small explosions, extending seismic identification capabilities (as an explosion) down to the smallest events.

During the past decade, many new research products were implemented into the routine operational systems that continue today. Regional seismic waves travel through the Earth’s crust and uppermost mantle (the top few tens of kilometers of the Earth’s interior) at high frequencies. The high frequencies enable new methods for distinguishing the seismic signals produced from a small underground explosion as compared with naturally occurring earthquakes. In Appendix D, examples are shown of how regional high-frequency signals can be used to identify explosions for broad regions of the world and for explosions as small as a few tons.

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12 The signal amplitude is measured at each station with a detectable P-wave, a correction for the effect of the distance between source and station and for the source depth is made, and the magnitudes obtained for each station are averaged to obtain the network magnitude. Details can differ in the way that different institutions assign m b. Noise levels at stations that do not report a detection can be incorporated into the process to obtain more accurate magnitudes of small events (e.g., maximum likelihood methods).

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Seismic Monitoring in Tectonically Active Regions

Magnitude scales in seismology are logarithmic, so signal amplitudes are ten times smaller for a source with mb = 3, as compared with one with mb = 4. On average, about 21 seismic events worldwide above magnitude 4.0 occur daily, and the number goes up by a factor of about 10 for each drop of one unit in magnitude (say, from 21 to 210 events a day for a drop in magnitude from 4.0 to 3.0).13 The majority of the small earthquakes each day occur in tectonically active regions where the Earth is undergoing active deformation (e.g., California, Japan, Iran, or Italy). Such areas pose challenges both because of the number of events that need to be correctly processed and because the Earth’s complex structure distorts and attenuates the seismic waves. On the other hand, areas with large numbers of seismic events generate a great deal of data that can be used to calibrate them.

Initially, the Middle East presented a serious example of the challenges of monitoring a seismically active area. The overall burden to identify seismic events as earthquakes in this region (and thus not of concern under the CTBT) was significant. Monitoring seismic activity in the Middle East became the focus of much research and development effort, which used the large number of earthquakes to calibrate the region. Identification of explosions in the Middle East can now be accomplished by the usual methods once the complex Earth structure in the region is taken into account.

Seismic Monitoring Sensitivity

The relationship between explosive yield and event magnitudes depends on the geology in the region of the event and the strength of coupling to surrounding media. The mb-yield relations applicable to Semipalatinsk (a former Soviet test site in Kazakhstan) and Nevada Test Site (NTS) explosions represent the lower and upper bounds for non-evasively-tested underground explosions in good coupling media. Comparison of the resulting Semipalatinsk and NTS detection thresholds indicates that the latter are about a factor of 4 larger in terms of yield than the former (see Box 2-1). The NTS mb-Y assessment will be valid for seismically attenuating regions (e.g., Iran), whereas the Semipalatinsk assessment will be valid for regions of more efficient wave propagation (e.g., Lop Nor in China and North Korea).

Detection and identification sensitivities are governed by having an adequate number of sensors to record the higher frequency regional signals.14 Accurate location and identification will depend upon whether a sustained calibration effort has been undertaken.

In teleseismic monitoring, signal levels needed for event identification are often given as higher than the levels for event detection. That is, signals need to be available at higher signal-to-noise ratios for identification than for detection and location. However, with regional monitoring, small explosion (<~1 kt) identification may be done at one or two of the closest stations. In cases where P-and S-waves propagate efficiently, this leads to an identification threshold at or below the three-or-more-station detection/location threshold. In active tectonic areas such as the Middle East, however, the S-waves can be more strongly attenuated than P-waves, leading to a higher threshold for identification than for detection/location when high-

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13 This number is based on the CTBTO’s IDC defined maximum likelihood mb scale, which was set up for explosions and which takes into account stations where the signal is below the noise level (e.g., Ringdal, 1986). For comparison, the U.S. Geological Survey mb scale is similar to that of the IDC for explosions, but for earthquakes (for complex reasons) mb (USGS) ~ mb (IDC) + 0.45 (see Granville et al., 2005) leading to about 35 events per day above mb = 4. See: http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php.

14 Throughout this report (unless otherwise qualified), by “detection threshold” we mean detection at 90 percent confidence and at enough stations to provide a location estimate.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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frequency P/S identification methods are used. On average, in many regions of the world, the regional detection/location and identification thresholds are expected to be similar, but more work is needed to quantify the identification threshold in regions of strong S-wave attenuation. Examples of good identification capability, albeit with a regional network that is sparse, are given in Appendix D.

In the following sections, seismic maps based on both empirical data and computer simulations are used to estimate the minimum detectable monitoring levels for both the U.S. NTM and CTBT IMS networks in terms of seismic magnitude mb at a fixed confidence level (e.g., 90 percent).

Finding 2-3: Independent of the CTBT, the national security interests of the United States and its allies require the seismic monitoring of foreign nuclear-explosion tests.

Finding 2-4: Technical capabilities for seismic monitoring have improved substantially in the past decade, allowing much more sensitive detection, identification, and location of nuclear events. More work is needed to better quantify regional monitoring identification thresholds, particularly in regions where seismic waves are strongly attenuated.

U.S. Seismic NTM

Over several decades the United States has built a sophisticated national monitoring system to detect, identify, and characterize nuclear-explosion tests. From 1999 to 2009, this system has improved significantly.

Recommendation 2-2: AFTAC should study the extent to which detection thresholds could be improved by making fuller use of the authenticated data from the IMS as well as targeted use of calibrated non-IMS seismic stations to help characterize special events of high concern.

Recommendation 2-3: To meet its national security needs, the United States should continue to enhance and sustain its NTM seismic monitoring capabilities.

IMS Seismic Monitoring

One of the major advances of the last 10 years is that 84 percent of the planned primary seismic stations are operating and certified for data quality (including calibration) and integrity (with respect to tampering and data authenticity), as well as 83 percent of the planned auxiliary stations (as of February 2011). Many of the primary stations are seismic arrays, which, unlike single stations, have the capability to determine the azimuth from which a seismic wave arrived and the distance to its source. Many arrays are very good at detecting small events and seismic waves following the P-wave, which allow an event’s depth to be determined. Several of the IMS seismic stations and arrays, such as the array in Niger in west-central Africa, are among the most sensitive in the world.

Figure 2-8 shows the detection capability of the IMS seismic network as of 2007.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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FIGURE 2-8: Detection Capability of the IMS Primary Seismic Network in late 2007, with 38 stations sending data to the IDC. Contours, indicate the magnitude of the smallest seismic event that would be detected with a 90 percent probability at three or more stations; that is, at enough stations to enable a location estimate. Red circles are seismic arrays, and triangles are single seismic stations. Completion of this network (to 50 stations) would reduce these magnitudes by about 0.1 or 0.2 units for Asia, much of Africa, and the Indian Ocean. The resulting capability, based extensively on operational experience, is quite similar to that described in the 2002 Report, which was a calculation of how well the then far-from-complete primary network would eventually operate (The magnitude yield relationship comes from Box 2-1). SOURCE: Capability map prepared by Tormod Kværna and Frode Ringdal, NORSAR

The detection capability of approximately magnitude mb = 4.0 or ~1 kiloton (kt) for well-coupled nuclear explosions is being significantly exceeded by existing IMS primary and auxiliary stations today. Globally, the IMS seismic network provides complete coverage at magnitude 3.8, with about 80 percent of stations operational. For Europe, Asia, North Africa, and North America, the 90 percent probability is better: mb = 3.4. As described in Appendices C and D, the detection capabilities shown for the Russian test site at Novaya Zemlya are even better than

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

indicated in Figure 2-8, which shows only the result for primary IMS seismic stations, because it does not include the auxiliary IMS station in Spitsbergen.

The least sensitive seismic detection capabilities in Figure 2-8 are found in the southern hemisphere, particularly the southern oceans. Hydroacoustic capabilities (see corresponding section below) complement those of seismic for most of the southern oceans and lead to much better detection capabilities than do those of the IMS seismic stations alone. Hydroacoustic data are incorporated into approximately 20-30 percent of the IMS analyses of seismic events, where they can help improve location and screening (see section below on hydroacoustic monitoring).

For the purposes of detection, location, and especially event identification, the combination of seismic and infrasound techniques (see section below on infrasound) has also been expanding.

In Table 2-2, detection capabilities are converted into yields of nuclear explosions that would be detected by the IMS primary stations alone (see Box 2-1 above). All of North Korea, most of Russia, much of Saudi Arabia, and large areas of China consist of older rocks and are regions of efficient propagation of seismic waves. Hence the yields in column 3 are appropriate for them. Iran, much of Turkey, and other parts of the Middle East are regions of poorer propagation of seismic waves. Hence the yields in column 4 are appropriate for them. All of these capabilities are well below one kiloton. A new primary station started operation in Turkmenistan in late 2009. Its added capabilities are not considered in Table 2-2.

The capabilities shown in this table represent the minimum detectable seismic events likely to be included in IMS bulletins sent to national data centers. To conclusively confirm that a reportable event is the result of a nuclear explosion at such low yields would likely require additional evidence; for example, the collection of radioactive debris or possibly even an on-site inspection.

TABLE 2-2: Event Detection Capabilities Using IMS Primary Stations in 2007.

Probability of Detection Primary IMS Stations Magnitude (mb) Yield Hard Rock, Regions of Better Propagation Yield Hard Rock, Regions of Poorer Propagation
90 percent, entire world 3.8 0.22 kt 0.56 kt
90 percent, Asia, Europe, N.Africa 3.4 0.09 kt 0.22 kt

Note: The formulas used to calculate these figures are mb = 4.45 + 1.0 log (yield Y in kt) (kilotons) for event yield in hard rock, regions of better propagation and mb = 4.05 + 1.0 log (yield Y in kt) for event yield (kilotons) in hard rock, regions of poorer propagation.
SOURCE: Committee

Finding 2-5: One of the major advances in monitoring in the last 10 years is that most of the IMS seismic stations are operating now, and most of those have been certified for data quality (including calibration) and integrity (with respect to tampering and data authenticity). The threshold levels for IMS seismic detection are now well below 1 kt worldwide for fully coupled explosions. (See Chapters 2 and 4 for further discussion)

Other Seismic Monitoring

Seismic stations that are supplementary to those operated as part of NTM and CTBTO fall into four categories:

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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•  Those that are part of the international Federation of Digital Seismographic Networks, which are open stations with data often available in near-real time to any interested user;

•  National networks, from which data may be available upon request on an ad hoc basis;

•  Stations operated by numerous smaller institutions or research groups (typically on a temporary basis) that in the past have acquired excellent data including signals from events of interest, and that allow tomographic methods to provide detailed models of 3-D Earth structure.

•  In addition, Member States may establish stations as cooperating national facilities (CNFs), and use the data from those stations to supplement data from the IMS. These stations must be certified for operation and the data authenticated. The International Data Centre may use the data from these CNFs for the purposes of “consultation and clarification,” as well as the consideration of on-site inspection requests.

These supplementary stations provide useful data; for instance, many non-IMS and non-NTM stations have been used to reveal new details and better locations of the 2006 and 2009 North Korean declared nuclear-explosion tests (e.g., Chun and Henderson, 2009; Kim et al., 2009; Ford et al., 2009; and Wen and Long, 2010). Some of the seismic stations used in these studies were available in near real-time.

Summary

Seismic technologies for nuclear explosion monitoring have improved significantly over the past decade. Much of the improvement is due to the use of regional-distance (< 1,600 km, or 1,000 mi) seismic recordings of broader bandwidth signals. Though the seismic technologies for monitoring nuclear-explosion tests are highly developed, they continue to evolve. In general, there is the potential to improve event detection, location, and identification substantially over the next years to decades.

•  Continued development of high-frequency regional and local seismic methodologies will lower thresholds for the detection, location, identification and characterization of small events.

•  Continued development of improved models of the Earth’s crust and upper mantle to provide 3-D velocity and attenuation models will improve event location and identification accuracy.

•  Continued development of seismic source models will allow prediction of potential explosion signals in untested emplacement geometries and geologies and would enhance monitoring capabilities.

•  Continued development of numerical modeling capabilities tying together source and propagation models would enhance monitoring capabilities.

•  U.S. capability to characterize and refine assessments of nuclear explosions would be enhanced by utilizing high-quality data from seismic stations that are becoming increasingly available in real or near-real time, especially those in and those adjacent to countries of concern to the United States.

•  The use of better models to integrate seismic with other monitoring data have the potential to enhance monitoring.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Finding 2-6: Seismic technologies for nuclear monitoring have the potential to improve event detection, location, and identification substantially over the next years to decades.

Recommendation 2-4: The United States should renew and sustain investment in seismic R&D efforts to reap the rewards of new methodologies, source models, Earth models, and data streams to enhance underground nuclear explosion monitoring, regardless of the status of CTBT ratification.

Finding 2-7: Closer collaboration among the U.S. monitoring, NTM, national laboratory, and academic communities would help the United States keep up with new developments and technologies for seismic nuclear-explosion test monitoring.

Radionuclide Monitoring

Radionuclides are produced in nuclear explosions in the form of fission products of the nuclear explosive material in the device or by nuclear reactions with material in and surrounding the device, such as metal, dirt, or water (activation products). Radionuclides from a nuclear explosion are produced in high quantities relative to natural processes and can be detected when they are either released (or vented) into the atmosphere or deposited on the ground near the detonation point.

Radionuclides are detected as particulate matter or as noble gases, some of which, such as xenon and argon, are important indicators of a nuclear explosion. For atmospheric nuclear detonations, a large fraction of the radionuclides will be carried by wind and can be detected remotely.

Some radionuclides can reach the surface from underwater and underground detonations. In addition to radionuclides that enter the atmosphere or the surrounding medium immediately following detonation, certain radioactive gases from an underground nuclear explosion can be released slowly by seeping out through fractures that can occur over periods of weeks to months or more (Box 2-2).15

Radionuclide detection is an extremely sensitive technique. For example, the radioactive material liberated into the atmosphere by an atmospheric nuclear explosion would be detectable by samplers for more than a week, even if it were diluted by the entire earth’s atmosphere. Conventional means, such as those used in the IMS, can

BOX 2-2 Radioactive Noble Gases

Radioactive noble gases can reach the surface and can be liberated into the atmosphere following a nuclear test, because these elements are nonreactive and therefore difficult to contain even when there is a concerted effort to do so. There are several radioactive isotopes of xenon noble gas (131mXe, 133Xe, 133mXe, and 135Xe) that are produced in high enough quantities from a nuclear explosion that they can be detected a few days to weeks after the explosion. In addition, 37Ar is also produced in relatively large quantities via the 40Ca + n→37Ar + α reaction when neutrons from a nuclear explosion react with calcium in the dirt surrounding a surface or underground explosion. Only radioactive xenon is used by the IMS for remote detection, because of the higher amounts of these isotopes produced and the ease of the collection of xenon. For on-site inspections, both radioactive xenon and radioactive argon are targeted. Long-term seepage exceeding 300 days following an explosion is theoretically possible under some conditions via the detection of the 11.9-day half-life of 131mXe, the 5 day half-life of 133Xe, and the 35-day half life of 37Ar. The emission of noble gases from a cavity is more likely under scenarios in which cavity decoupling technology is used, because of the absence of shock heating that would take place in well coupled media, as discussed in Appendix E. This makes monitoring of radioactive noble gases an important part of the CTBT verification regime.

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15 Seeping is caused by a process known as barometric pumping, which is the same mechanism that releases radon gas from the ground. See Appendix F for additional information on seeping and venting.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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easily detect atmospheric detonations from nuclear explosions down to 0.001 kiloton.

Radionuclides are carried by weather systems and therefore over periods of a week or two tend to stay in plumes. Depending on the direction of the winds, these plumes could miss emplaced stations, such as those deployed by the IMS, but could be detected by mobile detection systems, such as those that are operated by some nations as part of their national technical means.

Nevertheless, it is very likely that in most places on Earth, radioactive particulate stations in the IMS will allow detection of atmospheric detonations below 1 ton of nuclear explosive yield. For other locations, detection may be accomplished through non-IMS technical means and detonations pinpointed through infrasonic or satellite technologies, as well as atmospheric transport backtracking.

Radionuclide detection functions synergistically with other detection methods, as indicated in Table 2-1. Radionuclides can be detected by sensors either in stationary or mobile platforms, and the samples can be either measured at the collection location or taken to a laboratory for processing and measurement. Because there are other processes that release fission and activation products into the atmosphere (i.e., nuclear fuel-cycle operations), radionuclide detection equipment commonly keys on specific signatures that are unique to nuclear explosions through the use of specific isotopes or ratios of activities of one isotope to another. The ratio of isotopes within a sample can also be used to determine the approximate detonation time of the explosion based on the rate of decay of the radionuclides in the sample.

Atmospheric transport modeling (ATM) is an important tool used to calculate the origin of detected radionuclides, as well as the probable future location of air masses. First, radionuclide concentrations that are collected at specific locations (or the lack of the presence of radionuclides), and the corresponding times are used to backtrack a plume to determine probable source locations consistent with the measurements. Second, ATM is used to predict the future location of a plume of radioactivity so that actions can be taken to collect airborne gases or particulates that may be present or, for example, to check the consistency between seismic and previously-emplaced radionuclide detection systems. ATM will in most cases be combined with other techniques such as seismic and infrasound technologies to accurately determine detonation locations. By combining these technologies, a more definitive determination regarding possible nuclear explosions can be made.

Changes Since the 2002 Report

The most significant improvement in radionuclide detection since 2002 has been the development of radioactive xenon noble-gas detection. The concept of this type of monitoring as part of the IMS was considered new during the drafting of the Treaty, and therefore only 40 of the 80 Treaty-defined monitoring stations were specified as noble gas stations. The CTBTO decided in 1999 to conduct an International Noble Gas Experiment (INGE) to test aspects of the detection of radioactive isotopes of xenon (“radioxenon”) for the IMS.

The results of these experiments showed that sensitivities of the equipment generally exceeded all of the specifics laid out by the Provisional Technical Secretariat (PTS) for noble gas measurements (Auer et al., 2004). Shortly after the second phase of the INGE experiment began, commercial partners were identified, and now there are three commercial entities that have successfully produced radioactive xenon noble gas equipment that meet or exceed requirements for the IMS. Currently, 31 of the 40 noble gas systems planned are either installed or under contract.

One of the outcomes of the INGE experiment was that the radioxenon equipment that was proposed for use in the IMS was ready for certification and provided a useful tool for verification. At the start of the experiment, there was a single prototype technology available for

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

radioactive xenon noble gas measurements, whereas today a working network of samplers collect and send data to the International Data Centre (IDC) in Vienna on a daily basis.

In addition to improvements in radioactive xenon noble gas monitoring, there have also been significant advances in particulate radionuclide detection, including more reliable mechanical cooling for high-resolution gamma ray spectrometers (Upp et al., 2005), better data analysis methods used by the International Data Centre to detect ever smaller quantities of radionuclides (Zähringer and Kirchner, 2008; Plenteda, 2002; Zähringer et al., 2009), and measurements of background levels of radioactivity at many locations around the world.

U.S. National Technical Means for Radionuclide Monitoring

Radionuclide Signal Detectability

The United States Air Force Technical Applications Center (AFTAC) operates WC-135 aircraft for detecting radioactive debris that could accompany nuclear explosions. AFTAC calculates the probable and accessible locations of the plume that could be released from either an underground, underwater, or atmospheric detonation and flies an aircraft to the location of the plume. The aircraft is equipped with external flow-through devices to collect particulates on filter paper and radioactive xenon gas in pressurized holding spheres. The radioactive xenon and the particulates collected on the filter paper are sent to a laboratory for analysis and evaluation for the fission products indicative of a nuclear-explosion test.

Radionuclide Event Location Accuracy

The detection of radionuclides is normally not the first sign of a possible nuclear explosion. Although seismic or other signals may be detected first, a capable radionuclide monitoring network may provide the key measurements that will confirm a potential event was nuclear. Meteorological conditions play a large role in identifying the possible source of a nuclear explosion. Once a potential event location has been identified using other technologies, ATM can be used to predict how an atmospheric plume will travel, providing opportunities to obtain samples for analysis. Using atmospheric transport calculations to backtrack from detections allows a rough determination of the location of emitted radionuclides, and much more accurate determinations of the specific location of a detonation can be made with other technologies (infrasound for atmospheric tests and seismic monitoring for underground tests). For the United States, if an airborne sampler can be flown close to the release point, more accurate estimates of event location are generally possible than would be the case using fixed distant samplers.

Radionuclide Event Classification

Fission and activation products can be produced by processes other than nuclear explosions. Atmospheric detonations will release millions of times more debris than any other process, with the exception of a serious nuclear reactor accident. In an accident scenario, however, it will be quite straightforward to discriminate the isotopes released from a reactor and those from a nuclear detonation based on the types and relative amounts of isotopes detected. The more challenging case involves discriminating underground tests that only release only a small amount compared to other man-made phenomena. Understanding the background of fission products that are in the atmosphere is key. A number of studies have been performed in the last 10 years to allow very good discrimination between nuclear explosion isotopes and those arising from other sources.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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The types and relative amounts of isotopes that are emitted from a nuclear detonation are precisely calculable and well known for any type of fissionable material. These are the isotopes that are targeted for detection of nuclear detonations (see Box 2-2), and other sources that also produce fission isotopes are separated from these sources by comparing the detected isotope ratios with those expected from a nuclear detonation.

In the past decade, there have been significant improvements in the U.S. NTM for detection of radionuclides associated with nuclear-explosion testing. These include the development of new techniques to collect and measure radioactive xenon, substantial improvements in gamma ray spectrometry capabilities, new abilities to discriminate xenon backgrounds from man-made sources, nuclear detection algorithm development, improved ATM, and other technologies.

The NNSA Office of Nuclear Detonation Detection’s Ground-Based Systems Nuclear Explosion Monitoring Research and Development (GNEMRD) program began in November 1993 in anticipation of the CTBT negotiations. The GNEMRD program has made significant advances toward technologies to both lower the background of radiation detection systems used for detection of airborne debris and develop more selective techniques to measure debris, while taking into account specific operational requirements. Specifically, since 2002 NNSA has developed:

•  Automated, robust samplers and analysis algorithms for collection of both gases and particulates from nuclear explosions at levels much lower than the levels possible in 2002.

•  Fundamental new ways to detect airborne debris, including data analysis algorithms and a deployable analysis system for measurements of short-lived xenon isotopes.

•  Better understanding of global backgrounds of fission products that cause background in radionuclide detection systems.

Finding 2-8: AFTAC has demonstrated notable achievements over the past decade, including major enhancements in all aspects of radionuclide monitoring.

Recommendation 2-5: The United States should continue to actively support radionuclide collection, including R&D activities to better discriminate nuclear-test signature radionuclides from background, thus improving the ability to detect well-contained and lower-yield nuclear-explosion tests.

IMS Capabilities for Radionuclide Monitoring

Radionuclide data is handled completely separately from the waveform technologies (seismic, infrasound, and hydroacoustic). IMS station data are reviewed by radionuclide analysts and, depending upon the characteristics, are ranked from 1 to 5, with 5 being the most consistent with a possible nuclear source. ATM tools are then used to correlate multiple station detections together, tie them to a source region, and to any potentially associated waveform event. The CTBTO has developed software tools16 that allow Member States to perform their own ATM using multiple meteorological models. Typically there are a few Level-5 detections each year arising from non-nuclear-explosion sources such as medical isotope reactors.

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16 For example, the CTBTO has developed a software tool called “webGrape” that allows Member States to determine the probable location of emission of an air parcel, if radionuclides were detected at an IMS location.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Radionuclide results are reported in the Reviewed Radionuclide Event Bulletin (RREB), which is analogous to the REB for seismic monitoring.

Radionuclide Signal Detection

Based on the current coverage of the radionuclide detection stations listed in the Treaty, there is very little chance that an atmospheric detonation of even a modest size would go undetected by the IMS, largely due to the high sensitivity of radionuclide samplers in the network. Figure 2-9 illustrates a calculation17 of the probability of detection of a 1-kiloton atmospheric nuclear detonation after 14 days by the IMS, based on a 79-station network and detection of a single species of isotope. The figure illustrates that the global coverage for atmospheric detonations is almost 100 percent, except for a few locations in the oceans near the equator where the probability is in the range of 30-90 percent. The coverage for xenon gas from underground detonations (Figure 2-10) is not as complete as for detection of particulates from an atmospheric detonation.

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FIGURE 2-9: IMS coverage map for detection of radioactive particulates by one or more stations for atmospheric nuclear-explosion tests, showing the probability of detection (expressed in percent) of a 1-kiloton atmospheric nuclear detonation by the 79-station IMS particulate detection network (75 percent of stations certified as of February 2011) within 14 days. A global average detection probability for such an event is approximately 97 percent. The 80th particulate station, the location of which has not yet been decided, would presumably be located on the Indian subcontinent and would improve detection capabilities in that region overall. SOURCE: CTBTO

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17 This calculation was performed by Andreas Becker from the CTBTO.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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FIGURE 2-10: IMS coverage map for noble gas detection by one or more stations for underground nuclear-explosion tests, showing the probability of detection (expressed in percent) of a 1-kiloton underground nuclear detonation by the 39 station IMS noble gas detection network (65 percent of stations operational as of February 2011) within 14 days, assuming 10 percent of the radioactive xenon inventory is promptly vented from the detonation. A global average detection probability for such an event is approximately 88 percent. The 40th noble gas station, the location of which has not yet been decided, which would presumably be located on the Indian subcontinent, would improve detection probabilities in that region and overall. The Treaty allows for the entire 80-station network to eventually be populated with noble gas systems and under that scenario, a global detection probability for underground detonations would then reach near 97 percent. SOURCE: CTBTO

Radionuclide Event Location Accuracy and Classification

The same physical principles identified in the section on Radionuclide Event Location Accuracy above apply to the IMS as well.

Finding 2-9: In the past 10 years, the IMS radionuclide network has gone from being essentially non-existent to a nearly fully functional and robust network with new technology that has surpassed most expectations.

Finding 2-10: The IMS has made significant improvements in data processing.

Other Radionuclide Monitoring Capabilities

There are a number of national efforts in other countries to detect airborne radionuclides that would indicate a nuclear-explosion test, but these efforts are essentially NTM, and the number of open sources is relatively low. However, some national programs make atmospheric measurements of radionuclides for health and safety purposes—especially for remote monitoring for accidental releases from nuclear reactors—that could have some application to the detection of nuclear explosions, especially atmospheric testing. One such example is a network of radioactivity measurement systems throughout Germany, run by the Bundesamt für

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

Strahlenschutz (BfS), which makes continuous measurements of airborne fission products in Europe (Bieringer et al., 2009).

Finding 2-11: Ongoing measurement and understanding of global backgrounds of radionuclides relevant to nuclear-explosion monitoring are critical for improving radionuclide detection.

Recommendation 2-6: The United States should support research needed to understand the global background of radionuclides.

In addition, based on the discussion in Box 2-2 and Appendix F, we find the following:

Finding 2-12: In at least 50 percent of underground nuclear-explosion tests near 1 kt or larger, even those carried out by experienced testers, xenon noble gases may be detectable offsite above the detection limits of the IMS (0.1 to 0.2 mBq/m3) from prompt venting of nuclear-explosion tests; also, long-term seepage of appreciable noble gases would be expected that could be detectable, both offsite and onsite.

Detection of such noble gases at fixed sampling sites would provide evidence that a test had taken place and, with the help of atmospheric models, could also suggest the source of the event. A CTBT cheater would have to take into account the significant probability that his test would be detected by this means.

Hydroacoustic Monitoring

Hydroacoustic monitoring for nuclear explosions in or near bodies of water has been utilized by the United States for many decades. Hydroacoustic signals can be generated by both in-water events and in-earth seismic events (e.g., earthquakes). An in-water source generates hydroacoustic waves (H-phase). A hydroacoustic wave can also convert to a seismic wave at a steep land—ocean boundary (called a T-phase), and similarly, a seismic wave can convert to a hydroacoustic signal.18 In this sense, the hydroacoustic and seismic monitoring networks are complementary, and both play a role in monitoring the underground and underwater environment.

The CTBT specified a new hydroacoustic network as part of the IMS designed for monitoring Treaty compliance. The CTBTO IMS hydroacoustic network consists of six underwater stations and five seismic stations on islands.

Changes Since the 2002 Report

Fundamental improvements in the U.S. ability to monitor underwater nuclear-explosion testing have occurred since the 2002 Report in two areas:

•  The near completion of the IMS hydroacoustic network has improved global underwater monitoring capabilities. This network also enhances underground monitoring in and around the ocean basins.

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18 In addition, we will refer to seismic waves that convert to acoustic waves just below the monitoring station as H-phases.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

•  Verification research and development has greatly improved the fusion of hydroacoustic data with seismic data, lowering detection thresholds in some regions by combining the two technologies.

Whereas the ability to model hydroacoustic signals continues to improve, the basic technology has not changed much since the 2002 Report, where it is well described. Here, for completeness, we give a very brief review of hydroacoustic processing.

Hydroacoustic Signal Detectability

Hydroacoustic stations are designed to exploit efficient propagation in the sound fixing and ranging (SOFAR) channel in the deep ocean. For in-water events, when a hydroacoustic station has a clear SOFAR channel view to the source, the detection threshold is very low. If the SOFAR channel is blocked by land or does not exist due to low temperatures at high latitudes, detection capability is worse.19Figure 2-11 gives a map showing the CTBT IMS detection threshold for in-water explosions around the world using both seismic and hydroacoustic data. Thresholds in the open ocean are generally less than 1 metric ton TNT (and often less than 100 kg). Detection thresholds in coastal areas or inland bodies of water are generally around 10 tons, where the sensitivity in some cases is augmented by seismic networks.

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FIGURE 2-11: Map showing the IMS detection threshold in equivalent mass of TNT for in-water explosions detected on either the hydroacoustic or seismic IMS networks. Note that 1000 kg = 0.001 kt. Areas not covered by the hydroacoustic network—such as the Mediterranean Sea—are covered by the seismic network to a threshold of around 0.01 kt. SOURCE: CTBTO

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19 Of direct relevance to hydroacoustic monitoring is the changing background noise field in the oceans. Deep-ocean ambient noise in the northeast Pacific has increased 10-12 dB in the band from 30-50 Hz in the last 40 years due to increased shipping traffic (McDonald et al., 2006). The Ocean Studies Board of the National Research Council convened in 2003 quotes 15 dB as the average increase in deep ocean noise since 1950 due to shipping (NRC, 2003). The bulk of noise radiated from modern cargo ships is also within the hydroacoustic monitoring band (Arveson, 2000). Observations of increased ice melt during the northern and southern summers, attributed to global climate change, could also result in increased seasonal ambient noise. Studies are underway to quantify this phenomenon, which could reduce detection capabilities.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

Event Location Accuracy in Hydroacoustic Monitoring

In-water events can be located with relatively high accuracy using H-phases, depending on the number of stations and reflections used and proximity to the event, whereas in-earth event location using T-phases has fundamental and unpredictable accuracy limitations. Hydroacoustic location accuracy for in-water events is controlled by:

•  The network coverage. For example, the IMS network coverage is sparse, with only 6 hydroacoustic and 5 T-phase island stations to monitor all of the earth’s oceans. Coverage is best in the Indian Ocean.

•  Signal measurement error. This is the error in measuring phase arrival time and back-azimuth bearing. Signal measurement error is much smaller for the H-phases than for the T-phases. For example, the IMS locates in-water events using as few as 2 H-phase stations, whereas T-phases are not used in location because of their large measurement error.

•  Model error. This is the error in the seasonally dependent hydroacoustic propagation velocity model.

Location accuracy can be improved when reflected phase arrivals (e.g., from continents or island margins) are present and can be associated with known reflectors. However, hydroacoustic tracking of events on land or below the ocean floor yields degraded location accuracy.

Hydroacoustic Event Classification

There is no way using only hydroacoustic monitoring to distinguish between a large in-water chemical explosion and an in-water nuclear explosion. However, large in-water chemical explosions are extremely rare. For example, the IMS locates only a handful of purely hydroacoustic events each year. Any large in-water explosion event would merit careful and comprehensive analysis. In addition, in-water nuclear explosions would also release massive amounts of radionuclides into the marine environment and into the atmosphere, which might be detected using radionuclide detection technology.

To distinguish between an in-water explosion and some other kind of in-water or in-earth event, two methods are utilized by the International Data Centre for event screening: cepstral analysis (to indicate the presence of a bubble pulse) and the frequency content in the signal. Cepstral analysis measures the degree to which the signal has a repeating pattern. Such repeating patterns are characteristic of an in-water explosion, because the explosion gas bubble expands and contracts as it moves to the surface. It should be noted that shallow in-water explosions that vent to the atmosphere on detonation do not have a bubble pulse.

Most earthquakes that convert to a T-phase are depleted in high frequency energy, with little signal above 30 Hz. In contrast, in-water explosions show significant energy across the 1–100 Hz monitoring band. Consequently, events are classified based on their relative high-frequency energy content. This measure will effectively group in-water explosions in one population; however, that population will also include Antarctic ice events, submarine volcanic events, and some earthquakes that produce T-phases with significant energy across the monitoring band. Further screening based on location can identify ice and volcanic events.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

U.S. NTM Hydroacoustic Monitoring

The U.S. maintains a hydroacoustic monitoring capability that meets the U.S. requirements for in-water event detection. U.S. monitoring is generally better than IMS hydroacoustic monitoring due to the great number of hydroacoustic sensors available to the U.S. NTM network.

IMS Hydroacoustic Monitoring

The CTBT IMS hydroacoustic network consists of an 11-station network consisting of 6 hydroacoustic (H-phase) stations and 5 T-phase stations (see Figure 2-12 below, where yellow marks hydroacoustic stations, red marks T-phase stations). The hydroacoustic stations were designed to exploit efficient propagation in the SOFAR channel in the deep southern oceans.

The IMS hydroacoustic monitoring network, with the exception of two stations, is operational and certified. One non-operational station is Crozet Island in the south Indian Ocean. The station is operated by France and has presented great difficulties over the years due to remoteness and the site environment. Options for the Crozet Island station are currently in active discussion at the IMS. The other non-operational station is Juan Fernandez Island, which was destroyed in March 2010 by the tsunami induced by the large Chile earthquake.

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FIGURE 2-12: The IMS 11-station hydroacoustic network consists of 6 hydroacoustic stations underwater in the SOFAR channel (yellow symbols) and 5 T-phase seismic stations on land (red symbols). SOURCE: CTBTO

Finding 2-13: The IMS detection threshold for in-water explosions is 10 tons (0.01 kt) or below worldwide and below 1 ton (<0.001 kt) throughout the majority of the world’s oceans.

Finding 2-14: As of 2010, two of the six hydroacoustic stations of the IMS were damaged and became non-operational after installation and certification; one will be restored.

Recommendation 2-7: The U.S. should assess the need for data from the damaged hydroacoustic stations and, if appropriate, work with the CTBTO to restore these stations to operational capability.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

Hydroacoustic Research and Development Needs

Developments needed in hydroacoustic monitoring fall into two general areas: network assessment and event processing. Network assessment is the collection of prediction models backed with data that, where possible, allow prediction of network event detection, location, and identification capability. Network assessment software tools and databases need to be updated and further developed. Hydrodynamic source calculations need to be revisited and updated using a more realistic model for interface energy losses. Noise models for the hydroacoustic stations need to be developed based on the archived data to date.

Event processing encompasses the utilization of the network recordings to detect, locate, and identify explosive events. Recent research using back-azimuths and reflections could be exploited in routine hydroacoustic data processing to provide one-station or two-station event location and improve overall location and identification. Also, of high importance, is the further development of an explosion event database using past data where available and new events that may occur. Such a database will be important in developing effective identification techniques and testing IDC screening methods. Research is also needed to improve processing approaches that use hydroacoustic data in synergy with seismic data for event detection, location, and identification.

Infrasound Monitoring

Infrasound waves are sound waves with frequencies between 0.01 and 10 Hz, below the sensitivity range of the human ear. They are produced by explosions in the atmosphere and can be detected at great distances. They are also produced by ground motion from underground explosions and provide a complementary source of data to detect and discriminate underground nuclear-explosion tests. The IMS uses its infrasound stations to detect and locate atmospheric nuclear-explosion tests.

Changes Since the 2002 Report

To establish an independent atmospheric detection/location capability for the parties to the CTBT, the IMS began in 2001 to establish an infrasound network consisting of 60 geographically distributed stations (see Figure 2-6). At the end of 2000, one infrasound station was transmitting data to the IDC. As of February 2011, 43 stations (72 percent of the planned infrasound network) have been certified and are contributing to the IDC. The first atmospheric non-nuclear event built only from infrasound arrivals was reported in 2003.

Traditionally, infrasonic detection and location methods have been borrowed from the seismological community and have not accounted for the complex effects of the atmosphere. In some instances, the IDC has had to shut down its automated infrasound system because it produced too many false alarms. Within the last few years, however, new methods for detection and location that are tailored for infrasound have been developed and show great potential for meeting the IDC operational needs (Arrowsmith et al., 2008; Modrak et al., 2010). In addition, high-resolution models of the state of the atmosphere in near real-time have been developed to realistically simulate the time/regional dependence of infrasound wave propagation in the context of local weather and wind patterns.

The resulting performance of the system with the present 42-station network allows explosions with a yield of 1 kt and greater to be detected across 80 percent of the Earth’s surface. When the full proposed IMS network is operational, 1 kt explosions will be detectable across 90–95 percent of the Earth’s surface. In some areas of the earth, much better detection levels will be possible. Recent studies (Le Pichon et al., 2008; 2009) have shown that where

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

station coverage is favorable, as in the northern hemisphere, the infrasound network is capable of detecting and locating atmospheric explosions down to a level of tens of tons of TNT. The best performance is predicted around January and July when stratospheric winds are stable and strong. On the other hand, the best location of the event is in the spring and fall, when detection at a larger range of azimuths is more likely.

Finding 2-15: Infrasound detection is a valuable approach for monitoring atmospheric nuclear explosions.

Although the IDC infrasound network is ostensibly for monitoring nuclear-explosion tests in the atmosphere, it has great potential to be used in parallel with seismic techniques to detect, locate, and identify underground or near-surface explosions.

Finding 2-16: Integration of infrasound with seismic data and analysis will provide better detection, location, and identification of explosions.

Infrasound Research and Development Needs

Infrasound detection research has generally received lower levels of funding due to the U.S. reliance on satellites and radionuclide-based techniques instead of infrasound for detecting atmospheric explosions. However, U.S. infrasound research has resulted in the discovery of the operational usefulness of seismo-acoustic measurements (the combination of local infrasound measurements with seismic signals for event discrimination). Due to high cost, the nuclear explosion monitoring community relies on other fields for R&D progress on meteorology, and field tests are limited to collecting measurements from tests conducted by others for different missions.

Significant improvements in capability are possible via validation of real-time atmospheric models and acoustic propagation algorithms. These models have been tested only on a limited number of ground-truth events. The fusion of acoustic propagation algorithms with detection and location methods will ultimately enhance capability. For instance, event discrimination is a new area of development within the infrasound community and requires much further research. Even so, the ability to produce coverage maps (see Figure 2-13) with confidence is a major development of the past decade.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

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FIGURE 2-13: The explosive energy detectable by the full IMS infrasound network. The scale, in tons of TNT, runs from 10 to 1,000. SOURCE: J. Vergoz, CEA-DASE, 2009

Satellite-Based Monitoring for NTM

Satellite-based nuclear explosion detonation detection adds an important capability of the USAEDS for detecting nuclear explosions in the atmosphere and serves as the USAEDS means for detecting nuclear explosions in space (see Figure 2-1). The objective of satellite-borne nuclear detonation detection instruments is to provide timely and accurate information that a nuclear explosion has occurred in the atmosphere or space environments, including its time and location of occurrence and an estimate of the yield. In the case of monitoring the CTBT, satellite nuclear detonation detection systems, if they continue to be deployed and maintained for this mission, will play an important role in conjunction with other elements of the USAEDS. Monitoring for nuclear explosions using satellites is not part of the CTBTO IMS system.

Satellite Detonation Detection Capability

The technical basis for detecting nuclear explosions in the atmosphere or in space derives from the U.S. atmospheric test experience, which has been studied extensively. Different phenomena are detectable at various altitudes depending on where the nuclear explosion takes place. Based on this knowledge and experience starting with the Vela satellites in the early 1960s, the United States has produced and maintained an impressive satellite nuclear detonation detection capability for continuous coverage of Earth and space. Modern coverage is provided by nuclear detonation detection payloads carried on multi-mission satellites (Global Positioning System [GPS] and, at geosynchronous altitudes, the Defense Support Program [DSP] satellites), which are procured and operated by the U.S. Air Force. The nuclear detonation detection sensor payloads are provided by DOE NNSA, whereas AFTAC receives the data and interprets it as part of the USAEDS.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

Satellite Event Location and Classification

In the past 10 years, research sponsored by DOE/NNSA at the national labs has resulted in improved optical detection with better sensitivity and coverage, even in the presence of obscuration due to clouds, debris, etc. In addition, modern satellite electro-magnetic pulse (EMP) detectors are able to operate independently of the optical detectors and can now provide credible confirmation of an atmospheric nuclear explosion. The significance of this is improved timeliness and confidence in the detection and identification of atmospheric nuclear explosions. In the case of a high-altitude explosion occurring in the transition region, this information would also aid civil authorities in assessing the potential damage to ground-based infrastructure caused by EMP effects. Finally, advances over the past decade due to incorporating modern data processing and communications allow this vitally important information (i.e., the occurrence of a nuclear explosion) to be made available to the highest levels of U.S. authorities quickly and with very high confidence. For example, in the case of a nuclear explosion in space, the results of satellite detection of nuclear radiation would immediately be available to the military authorities in assessing the potential effects of an exoatmospheric nuclear explosion on vital U.S. space assets.

The Future of Satellite-Based Nuclear Detonation Detection Monitoring

Today there is uncertainty regarding the future of the U.S. satellite nuclear detonation detection capability. Modernization of the detector payloads for both upgraded GPS (GPS Block IIF and Block III) and the follow-on to the geosynchronous DSP satellites Space-Based Infrared System (SBIRS) has been accomplished. DOE/NNSA has continued to fund the national labs to develop the technically complex satellite detonation detection systems, including data processing and display systems. AFTAC continues to operate the satellite nuclear detonation detection portion of the USAEDS, including the anticipation of upgrades and improvements in the detection capabilities. The uncertainty in the future satellite nuclear detonation detection monitoring capability arises because the lead time for planning future Air Force satellite programs can be a decade or more and, once the planning and programming decisions are made, these satellite systems are very expensive to acquire and operate. Changes come slowly. It is not unusual for a transition to require a decade of time. Thus, during the planning stages, competition for scarce resources (e.g., size, weight, and power) on future satellites can be fierce. In this environment, all requirements receive intense scrutiny, including the nuclear detonation detection mission—in other words, whether the requirement for nuclear detection is still a sufficiently high priority relative to other military requirements to merit inclusion, or perhaps the nuclear detonation detection mission might be accommodated on other platforms, become important issues. Concerns with the latter option are that the alternatives potentially available are usually far in the future, are more costly, do not meet coverage requirements, or some combination of these. More detail on the potential loss of capability is given in Appendix G, which notes that satellite monitoring capability is needed for warfighting and space control missions, as well as for treaty monitoring.

Finding 2-17: Sustainment of the U.S. satellite monitoring capability to detect any nuclear explosion in the atmosphere or space, whatever its origin, will continue to be in the interest of the United States and its allies, regardless of whether the CTBT enters into force.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

Recommendation 2-8: Enhanced satellite nuclear detonation detection systems should be deployed in upgrades to GPS (GPS Block IIF and Block III) and the follow-on to DSP, the Space-Based Infrared System (SBIRS). Provision for adequate ground-based data processing is also essential. Decisions regarding whether and at what level to maintain the satellite nuclear detonation detection capability should be made as part of high-level national security policy and acquisition assessments.

OPERATIONAL CAPABILITIES OF THE CTBTO

The committee was asked to assess what commitments are required to sustain an adequate international verification regime, including on-site inspection. The views below were developed following interactions with the staff of the Preparatory Commission of the CTBTO and a visit by a sub-group of the committee to the CTBTO in Vienna, Austria.

The budget available to the CTBTO is determined by the Member States to the Treaty. In principle, the States could agree to any level of budget needed to establish and operate the CTBTO monitoring network. The total annual operating budget for the CTBTO for 2009 was $100M, which funded all of the activities of the CTBTO such as installation and certification of the stations in the IMS, funding of the Global Communications Infrastructure (GCI), preparation activities for on-site inspections, post certification activities of the stations such as operations and maintenance of stations, reporting, analysis of data from the IMS, preparedness exercises, management of the organization, etc. (The U.S. contribution to the annual operating budget was $25M in 2009 and $30M in 2010.) In addition to the agreed budget allocated to the CTBTO, States also can make voluntary contributions to the budget as well as provide support to the verification regime, via contributions-in-kind. Although the States hosting primary and/or auxiliary stations may and typically do make voluntary contributions, including contributions-in-kind to defray these costs, the CTBTO must be in a position to cover the costs fully, if necessary, because there is no requirement for States that host monitoring stations to contribute. In addition, the CTBTO bears the cost of preparing and transmitting reports requested by the States and must train inspectors for on-site inspection.

In 2004–2005, the CTBTO performed a System-Wide Performance Test 1 (SPT1) to establish a performance baseline for the IDC and IMS. “Drawing upon SPT1, narrower focused exercises to test individual components of the system were conducted in 2007 and 2008. These exercises might lead to another system-wide performance test at a later time” (Dahlman et al., 2009; Zerbo, 2006).

The CTBTO has a competent and dedicated staff that is operating well. It has managed the significant increase in the number of certified IMS stations from three in October 2000 to 264 in February 2011, with an additional 17 stations installed and undergoing evaluation prior to certification.20 These installed stations represent about 83 percent of the full network of 337 stations (321 plus 16 laboratories) and should approach 90 percent by the end of 2011. The CTBTO staff level, however, has changed very little historically, despite the very large increase in operating stations, data flow, and analysis. As a result, there are some signs of strain, primarily due to the long hours worked. To move to full-time operations after entry into force, the CTBTO will need a significant increase in staff.

In addition to collecting raw data, the CTBTO also generates a series of waveform-based (seismic, hydroacoustic, and infrasound) events lists (REB) and a series of radionuclide detections lists (RREB). The CTBTO also provides software, training to Member States and the potential for State-directed additional analysis, which could include additional non-Treaty station data.

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20 See map at: http://www.ctbto.org/map.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

To meet Treaty obligations at entry into force, all of these components will have to be fully operational. Sustaining the monitoring capabilities into the future will require further steps. For example, the CTBTO is subject to a number of political and funding restrictions outside its control. Examples of these include:

•  a cap on the total number of employees (about 280), now limited by the provisional status of the organization;

•  a slightly lower pay scale than their neighboring agency, the IAEA due to the CTBTO’s provisional status;

•  a number of limitations tied to the Treaty itself (e.g., confidentiality of data, auxiliary station data “by request” only, and the need to operate by consensus); and

•  CTBTO employees’ are currently being limited to a 7-year tenure. Exceptions are granted, but only for a few years. This is a potential problem for maintaining a core group of competent analysts, because it takes several years for analysts to become proficient.

Finding 2-18: Although the IMS is operating effectively, meeting the needs of CTBT entry into force will require more staff and easing of budgetary constraints.

Recommendation 2-9: The United States and others should ensure that priorities and funds are sufficient for IMS to meet ongoing needs, including after entry into force.

Table 2-3 summarizes the CTBTO’s current provisional operations compared with what will be required when the Treaty enters into force. In many areas, the CTBTO already meets its Treaty requirements, but this is not true in all cases.

TABLE 2-3: Comparison of the CTBTO’s Current Provisional Operations with Those That Will Be Required When the CTBT Enters into Force.

Function Provisional operation Entry into Force
Hours of operation People on duty only for business hours {Monday-Friday) as directed by Working Group B Constant operation
Analyst training Up to a year of training needed to reach high proficiency. The CTBTO conducts training classes in Vienna using the software to create a pool of potential analysts for future hires and to help staff Member States National Data Centers (NDCs).
Reviewed Event Bulletins ~ 10 days ~ 48 hours*
Standard Event List (SEL) Automated bulletins are generated in 1,4, and 6 hours after an event Level of automation and timelines currently meets EIF requirement
Screened Event Bulletin (SEB) Lists events that have earthquake-like signals and that therefore should not be of concern as potential Treaty violations. Screening algorithms may need updating as experience with the full IMS is gained**
Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×
Radionuclide processing 80-station particulate network, 40-station noble-gas network A plan for an 80-station noblegas network will be considered
Sensor Operation 24 hours a day 24 hours a day

Note: * Automatic reviewed event bulletins (REBs) will be produced less than 48 hours from the end of the day on which the event occurred, 98 percent of the time. This will necessitate a significant increase in staff, particularly analysts.
** The majority of events not processed have mb < 4.5 Because the 2006 and 2009 North Korean declared nuclear-explosion tests came close to being screened out by the Ms:mb criteria, the Waveform Expert Group under Work Group B recommended modifying the rules, and the new Ms:Mb screening criteria were adopted in 2010.
SOURCE: Committee

IMS Construction and Operations

The CTBTO has done a good job of building and operating the IMS. The CTBTO appears to be on a solid path to have more than 90 percent of the network built, certified and close to meeting entry-into-force operational objectives by the end of 2010. Budgetary constraints have caused slower than optimal construction of the IMS, and fewer analysts are currently employed than are needed for 24-hour operation. The CTBTO has started to build maintenance and replacement costs into its budget estimates. The seismic and infrasound stations seem to have well-understood failure rates and sustainment budgets. The radionuclide stations have higher failure rates than desired and may need further failure analysis and hardware development both to meet entry-into-force operational objectives and reliable sustainment cost estimates.

Opportunities for Technical Improvements to CTBTO Capabilities

The CTBTO seismic monitoring system could be improved if auxiliary station data were available and incorporated into the automated system on a continuous basis. This would be straightforward to do technically, but because the Treaty states that auxiliary data is “upon request” and auxiliary stations are supported by the hosting state and not the CTBTO, making such a change is a political issue and does not seem to be under consideration at present.21

Finding 2-19: A technical exercise that tests the advantages of incorporating auxiliary seismic station data into the CTBTO’s automated system would be useful to demonstrate the feasibility of this proposed improvement.

Finding 2-20: Location accuracy of events identified with waveform signals (seismic, hydroacoustic or infrasound) can be improved technically by better calibration to reduce the size of error ellipses and to improve detection and location accuracy. A technical review that evaluates calibration efforts, such as station tuning, phase labeling, and location accuracy, could identify ways to improve absolute location accuracy.

The CTBTO’s International Scientific Studies Conference (ISS), which occurred in 2009, provided a way to bring CTBTO staff into contact with the broader community. These contacts

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21 Note, however, that two points need to be appreciated:

(1) Lowering the magnitude level at which events are detected has the potential to increase the number of events that remain unidentified.

(2) If the auxiliary network is incorporated into a detection network, then the CTBTO could have a different type of obligation for maintenance of auxiliary stations.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

should be encouraged and continued as a way to provide ideas, resources, and peer review needed to improve the CTBTO’s performance. In addition, the CTBTO could make greater use of scientific community catalogs and bulletins (e.g., International Seismological Center [ISC], National Earthquake Information Center [NEIC], local bulletins) to evaluate its REB.

Finding 2-21: The CTBTO benefits from systematic, sustained interaction with the broader scientific communities involved in areas relevant to its mission.

On-Site Inspections

Article IV.D. of the CTBT provides that each State Party has the right to request an on-site inspection (OSI) for “the sole purpose… to clarify whether a nuclear weapon test explosion or any other nuclear explosion has been carried out in violation of Article I and, to the extent possible, to gather any facts which might assist in identifying any possible violator” (CTBT Art.IV.D.35). Inspection requests must be based on data collected by the IMS, National Technical Means, or both.22 The OSI Request must be approved by the CTBTO Executive Council.

After entry into force, once an inspection request is submitted, which could occur after a lengthy political process, the location where an OSI may occur is determined from information from either the IMS or Member States NTM. The inspection area is limited by the Treaty to 1000 km2 (390 mi2), a location uncertainty that can usually be achieved with data from the IMS. Within the inspection area the Inspected State Party can declare restricted-access sites that can be no larger than 4 km2 (1.6 mi2) each. The Inspected State Party can declare up to a total of 50 km2 (20 mi2) of restricted-access sites. The restricted-access sites must be separated by a minimum of 20 m (66 ft). Unless stopped by the CTBTO Executive Council following the required 25-day report from the Inspection Team Leader, the OSI continues up to the allowed 60 days. A request for continuation to perform specified activities can extend the OSI for an additional 70 days. A request to conduct drilling to obtain radioactive samples can be submitted at any time during the OSI.

The conduct of an OSI consists of a continuing process to focus the search on the location of a possible nuclear-explosion test. For underground nuclear-explosion tests, the goal is to ultimately find the location of the explosion test and recover radiological evidence of a recent nuclear-explosion test by means of gas, liquid, or solid sampling. Technologies are used in three ways in this process: (1) to narrow the search area to one or more subareas, (2) to identify specific sites within these subareas for application of very localized OSI measures such as the detection of relevant radionuclides emitted by the detonation, and (3) to find direct evidence of the nuclear character of the suspect event.

The Treaty’s OSI provisions provide for the use of a broad range of technologies, including visual inspection from the ground and during overflight, seismic aftershock monitoring, radionuclide measurements, multispectral imaging, environmental sampling and analysis, geophysical technologies, and drilling. Within the list of inspection technologies, drilling requires a special request to the CTBTO Executive Council, a majority of which must approve the request.

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22 Based on experience with other treaties, some doubt it will prove possible to garner the necessary 30 votes to authorize an inspection from the 51-member Executive Council. The 2002 Report discussed this point and concluded that on-site inspection “constitutes a deterrent to treaty violation whether or not an inspection actually takes place, and it provides a mechanism for the innocent to clear the record” (pp. 55-56). The committee agrees with this conclusion; therefore this section is limited to assessing what could be gained from an OSI if it were to occur.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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When the ratification of the CTBT was considered by the U.S. Senate in 1999, there was limited experience with how the OSI process would work. Since then, the CTBTO mounted a significant Integrated Field Exercise in 2008 (IFE08), during which a mock inspection took place at the former Soviet nuclear-explosion test site at Semipalatinsk, Kazakhstan. Planning and performance of the exercise did not mimic the timeline of a real inspection, but the CTBTO was able to work through the challenges of fielding large amounts of technical equipment; negotiating access by the mock inspection team with the mock inspected State; dealing with Treaty constraints; and identifying a number of logistical, scientific, and technical areas that need further development, such as communications, deep penetration geophysics, noble gas sampling and analysis, multi-spectral imaging, and active seismic surveys (CTBTO, 2008).

The Effectiveness of an OSI

Several factors can influence the effectiveness of an OSI: the size and placement of the supposed nuclear-explosion test; constraints such as Treaty-allowed managed access areas; the expertise of inspectors; time since the detonation; evasive actions that are taken before, during or after the presumed nuclear-explosion test, such as containment or shrouding of equipment; and the precision and accuracy of location by monitoring.

Given the difficulty and uncertainty associated with the containment of nuclear explosions, a nuclear-explosion test of sufficient size to be detectable is likely to result in radionuclide evidence (depending on the time since detonation) that could be detected during an OSI, and in fact radionuclide evidence detected remotely in one or more IMS stations may be one of the factors on which an OSI request is made. The extent of radionuclide evidence will depend on whether the explosion is successfully contained, and it may be necessary to obtain a fairly accurate event location (within a few km for the case of gas migration and within ~100 m for the case of drilling) in order to find such evidence. In addition to radionuclide evidence, other evidence associated with nuclear-explosion testing, such as detection of a borehole casing or other nuclear-explosion test artifacts, evidence of containment measures, disturbed ground, and/or location of the explosion cavity or rubble zone, may be considered sufficient.

Some of the radionuclides associated with a nuclear-explosion test are long-lived radioactive noble gases, which are known to seep slowly from a nuclear explosion site (Carrigan et al., 1996), and can be present at the surface at detectable levels for hundreds of days following a nuclear-explosion test, even at yields of 1 kiloton and less. As Figure 2-14 shows, detections under an OSI are probable with noble gases alone if conducted within 150 days of the event, and longer if detection thresholds are improved.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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img

FIGURE 2-14: Plot of the approximate surface concentrations of the noble gases expected after a 1-kt nuclear detonation. The horizontal lines represent the detection limits that are expected from instrumentation that will be used during an OSI. The plot shows two curves for xenon (Xe-131m and Xe-133) and a range of values (shaded in grey) for argon (Ar-37). The latter corresponds to the concentration of Ar-37 that would be expected, over the range of values of calcium content that could be encountered during an OSI. SCM: Standard cubic meter of air. SOURCE: Adapted from Carrigan et al., 1996

In the case where an Inspected State Party (ISP) has violated the CTBT and attempts to thwart an OSI through the use of restricted-access areas allowed in the Treaty, it could actually make the job of the inspectors easier because the restricted-access areas may provide a map of the most interesting locations to perform environmental sampling. Even with restricted-access areas, inspectors could still operate equipment as close as 1,000 m to ground zero. This distance is expected to be close enough for both gaseous noble gas radionuclide measurements and geophysical techniques to be effective, as well as for slant drilling to reach the cavity.

Although it is somewhat scenario dependent, it is likely that in the 0.1 to 1 kiloton range of yields, there will be significant aftershocks; possible surface disturbances, such as visible and detectable cratering and fracture zones; human-made artifacts; and/or radionuclides at the surface that have vented from the nuclear explosion. Also, the Treaty allows the use of magnetic, electrical, and gravimetric techniques to assist in the detection of borehole casings and other underground structures and artifacts that will aid in pinpointing the location of a supposed nuclear-explosion test.

Unless other factors not guaranteed were used or detected, such as human intelligence or other evidence associated with nuclear-explosion testing as discussed on the previous page, it is likely that the effectiveness of an OSI to detect a test much below about 0.1 kilotons of yield would be low.

The requirements on evidence needed for the Executive Council to call an OSI may be so high that an OSI is never called. Moreover, if an OSI is called, some believe that treaty

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

provisions for managing access to the site could limit the effectiveness of an OSI. However, our assessment is that if the United States had enough evidence to call for an OSI, it would likely have enough information from NTM and other sources to determine whether a nuclear explosion had taken place.

Finding 2-22: A CTBTO on-site inspection (OSI) would have a high likelihood of detecting evidence of a nuclear explosion with yield greater than about 0.1 kilotons, provided that the event could be located with sufficient precision in advance and that the OSI was conducted without hindrance.

Transparency/Confidence Building Measures

Any monitoring system intended to support verification of Treaty compliance—whether the CTBT’s IMS, National Technical Means or other systems—has to be considered potentially imperfect. The system will have sensitivity, as well as resolution in space and time, below which events are unreliably detected; or parts of the system may occasionally break down. Such eventualities call for additional mechanisms, including confidence-building measures, to enhance Treaty verification.

There are many opportunities for confidence-building measures to support CTBT monitoring. For example, China, Russia, the U.S., France and the UK all claim to sustain nuclear arsenals through stockpile stewardship programs involving no nuclear explosions, and there are many aspects of these programs that can be discussed openly and in a manner enhancing mutual confidence.

Several areas that could be considered are:

1.   Plutonium Science: There is an ongoing series of conferences at which researchers from the United States, the United Kingdom, France, Russia and (most recently) China discuss the basic science of plutonium. Though sensitive topics are avoided, the exchange of information is useful for these nations’ technical communities. In addition to the scientific information that is communicated, simply evaluating the level of expertise available in each country provides significant insight about competencies and capabilities.

3.   Subcritical Tests: Advanced nuclear-weapons states conduct subcritical tests, often at past nuclear-explosion test sites; these are by definition compliant with the CTBT and are below the sensitivity of any remote-monitoring system. How does the testing nation assure sub-criticality, however? What procedures are in place, and what is the combination of computational simulation and experimental measurement used to document subcriticality? Although details are likely to be too sensitive to be discussed, many aspects of the procedures used can be shared among individual nations. As demonstrated during the U.S.-Soviet Joint Verification Experiments of the late 1980s, one could even allow close-in monitoring of one nation’s subcritical tests by another nation, with appropriate protection of classified information.

Other opportunities involve collaboration in closely related scientific disciplines, such as inertial confinement fusion (ICF) for producing energy or the study of warm dense matter (WDM) at conditions existing deep inside planets. These involve major experimental facilities that can be made available to users worldwide, with appropriate controls, including non-nuclear as well as nuclear-weapons states.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

In addition to such activities in basic research, cooperative efforts in monitoring can significantly enhance confidence by engaging scientists and engineers from all nations around the world. This is already well documented in seismology, a field largely devoted to mitigating the hazards of earthquakes, as well as documenting the structure and dynamics of our planet. Similarly, atmospheric monitoring—and global environmental monitoring more generally—could be greatly enhanced through cooperative measurement and modeling efforts if, for instance, it is applied to data sharing on issues related to radiological incidents and emergency response, and documenting spatial-temporal patterns in abundances of carbon dioxide, other greenhouse gases, dust, or industrial pollutants.

Confidence-building measures are best established on a bilateral (or, possibly, limited-multilateral) basis in order to ensure protection of sensitive information. For example, certain technical details that can be shared between advanced nuclear-weapons States should not be revealed more widely, for fear of proliferating weapons-relevant knowledge. Also, there are many examples of information considered classified by one nation but not by another, thereby limiting the extent of cooperation. This means that confidence building should be pursued distinct from and in parallel with the CTBT, thereby complementing the monitoring and other capabilities associated with the Treaty regime. The cooperative measurement of radionuclides in the atmosphere is an example of an area that could lead to confidence-building and data exchange in the future.

Finding 2-23: There are many opportunities for confidence-building measures to support nuclear explosion monitoring, particularly through engaging scientists and engineers in cooperative efforts.

Recommendation 2-10: The United States should pursue bilateral (and, to the extent justified and politically feasible, limited multilateral) programs of scientific cooperation for purposes of confidence building in support of monitoring nuclear explosions. These programs should be periodically reviewed for effectiveness and for appropriate controls on information.

Test Site Transparency

Because of the detection limits at extremely low yields and the lack of a clear definition of “nuclear test explosion,” transparency measures at known test sites could become an important adjunct to the Treaty and merit special consideration.

One of the actions that a country, and especially a nuclear weapon state, can take to show that it is acting in good faith in complying with the CTBT is to grant access to its nuclear-explosion test site, including allowing some types of non-sensitive measurements to take place at the test site. Allowing continuous measurements at a test site would also decrease the detection threshold significantly for that location.

The U.S. Department of Energy/National Nuclear Security Administration (DOE/NNSA) recently conducted a series of experiments to test technologies that could be used under a test site transparency regime. Technologies evaluated included geophysical, ground-based visual, remote monitoring, overflight, and radiological technologies. In addition, a technical evaluation matrix was introduced to demonstrate how a set of criteria could be used to prioritize possible monitoring technologies. Those criteria included relevance, intrusiveness, detection sensitivity, measure confidence, equipment factors, personnel factors, and composite (an overall assessment).

The subcritical experiment carried out at the test site contained on the order of a few kilograms of explosive, and this amount was easily detected by seismometers approximately a

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
×

kilometer away from ground zero. In addition, radioactivity sensors were placed near the detonation point to verify that no radioactivity was emitted, such as xenon noble gases. These measurements showed that it is possible to conduct a series of null-measurements at a nuclear-explosion test site and give some level of confidence that the activities were not consistent with a nuclear-explosion test. NNSA concluded that, among other technologies, passive seismic, acoustic, radioactive xenon noble gases, and video would be effective for test site transparency applications.

The technologies demonstrated during this experiment could have applications to a long-term monitoring strategy at a test site, which might be an option for policy makers to have confidence that activities below the detection limits of the IMS or NTM are consistent with the CTBT.

Although in principle it would be desirable to improve test site transparency with all States possessing nuclear weapons, the United States should give priority to Russia and China. They are the States whose weapons programs are of the greatest strategic concern. They are also the States most capable of benefiting from very-low-yield testing (yields up to about 1 ton, fully coupled; see Chapter 4). Transparency measures with nuclear-capable States outside the NPT would be more difficult to agree on politically and less important strategically.

Finding 2-24: Test-site transparency agreements can provide a mechanism for mitigating concerns about very-low-yield testing (yields up to about 1 ton).

MONITORING AND THE NORTH KOREA NUCLEAR-EXPLOSION TESTS

The two announced Democratic People’s Republic of Korea (DPRK) nuclear-explosion tests of 2006 and 2009 provided practical opportunities to exercise the monitoring capabilities of the technologies discussed in this chapter. The results are discussed in Box 2-3.

BOX 2-3 The 2006 and 2009 DPRK Nuclear-explosion Tests

DPRK Status: The DPRK has not signed the CTBT; it withdrew from the NPT in 2003. In 2005, it declared that it possessed nuclear weapons.

2006 Test: On October 9, 2006, the DPRK declared that it had conducted a nuclear-explosion test.

•  Seismic signals: registered magnitude 4.1 as reported by the CTBTO IDC Reviewed Event Bulletin (REB) (22 stations reported—14 primary; 8 auxiliary).

•  Radionuclides: Xe-133 was detected approximately 14 days following the detonation by the IMS station in Yellowknife, Canada; Xe-133 and Xe-133m were detected by Swedish researchers working in South Korea.

•  Infrasound: none reported.

•  U.S. Assessment: On October 16, 2006 the U.S. Director of National Intelligence (DNI) released a statement: “Analysis of air samples collected on October 11, 2006, detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion in the vicinity of P’unggye on October 9, 2006. The explosion yield was less than a kiloton” (DNI news release, 2006).

2009 Test: On May 25, 2009, North Korea declared it had conducted a second nuclear-explosion test.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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•  Seismic signals: Registered a magnitude 4.5 as reported by the CTBTO IDC REB (61 stations reported—31 primary; 30 auxiliary)

•  Radionuclides: none reported.

•  Infrasound: Very small signal detected (e.g., Che et al., 2009).

•  U.S. Assessment: On June 15th, the U.S. DNI released a statement that “North Korea probably conducted an underground nuclear-explosion test… (t)he explosion yield was approximately a few kilotons” (DNI news release, 2009).

Background: The DPRK has relatively few natural earthquakes per year compared to other countries in the region (e.g., China, Japan and South Korea). Natural earthquakes in the DPRK above magnitude 4 occur only once every few years, so the seismic signal stood out as unusual even before the DPRK government made its announcements. There are several IMS primary and auxiliary seismic stations within 1,200 km (750 mi) of the DPRK. Within the same distance there are additional non-IMS seismic stations that produce publicly available data and that were used by seismic researchers to analyze the tests in the days immediately afterward. In addition, China, Japan and South Korea each maintain dense networks of seismic stations to monitor earthquake hazards in the region. Although the data from these stations are not all publicly available, researchers in these countries have used them in analysis of the DPRK tests (e.g., Hong et al., 2008). In the region, earthquakes are routinely reported at levels of magnitude 3 and below; mine blasts and industrial chemical explosions are usually less than magnitude 3.5.

Observations:

•   The seismic signals from the two events clearly indicate that they were explosions and not earthquakes. The 2006 nuclear explosion was an excellent real world test of empirical seismic methods for a sub-kiloton explosion in a new region and the discrimination methods worked very well.

•   A question surrounding the two DPRK tests is why radionuclides were reported following the 2006 test but not following the 2009 test. Many, if not most, tests in the range of 1 kt have resulted in the release of detectable levels of radioactive noble gases. Containment of radionuclides following a test is complex, but it is thought that containment may be harder for a smaller test than for a larger one because it may be more straightforward to establish a “stress containment cage” for larger tests (see Appendix D). No system is perfect and the failure to detect in one case does not invalidate the utility of such a detection network.

The two DPRK tests made it clear that if multiple nuclear-explosion tests occur in the same region, then relative (“differential”) methods of detection, location, discrimination and yield estimation can be brought to bear on the verification problem.

Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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Suggested Citation:"2 TECHNICAL MONITORING CAPABILITIES AND CHALLENGES." National Research Council. 2012. The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States. Washington, DC: The National Academies Press. doi: 10.17226/12849.
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The Comprehensive Nuclear Test Ban Treaty: Technical Issues for the United States Get This Book
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This report reviews and updates the 2002 National Research Council report, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty (CTBT). This report also assesses various topics, including:

  • the plans to maintain the safety and reliability of the U.S. nuclear stockpile without nuclear-explosion testing;
  • the U.S. capability to detect, locate, and identify nuclear explosions;
  • commitments necessary to sustain the stockpile and the U.S. and international monitoring systems; and
  • potential technical advances countries could achieve through evasive testing and unconstrained testing.

Sustaining these technical capabilities will require action by the National Nuclear Security Administration, with the support of others, on a strong scientific and engineering base maintained through a continuing dynamic of experiments linked with analysis, a vigorous surveillance program, adequate ratio of performance margins to uncertainties. This report also emphasizes the use of modernized production facilities and a competent and capable workforce with a broad base of nuclear security expertise.

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