1
Introduction.

CTBT Goals

As of January 1998, the Comprehensive Test Ban Treaty (CTBT) banning all nuclear explosions has been negotiated and signed by 149 countries, including the United States (India, Pakistan, and North Korea were not signatories). By the Law of Treaties, the signatories are bound to abide by the provisions of the CTBT prior to its entry into force, effectively creating an immediate moratorium on nuclear weapons testing. As of May 1998, the U.S. Senate has yet to ratify the CTBT. Verification of compliance with the CTBT will be a major concern of many nations in both the short and the long term. Compliance will require a concerted effort to enhance capabilities to identify violations, minimize false alarms, and thus maintain confidence.

The technical systems put in place to monitor the CTBT will be employed to detect, locate, and identify small “events” underground, underwater, and in the atmosphere with high confidence and accuracy. As stated in a 1997 NRC report1, the present monitoring systems will not detect with high confidence very low-yield tests. President Clinton2 stated that the CTBT goal for the United States is to be able to detect “a few kilotons evasively tested” in selected areas of the world. Evasive testing involves masking or muting the signals from a nuclear explosion (detonation in a large cavity can significantly reduce the magnitude of seismic signals, for example), although other CTBT monitoring techniques might still be able to detect evasive tests.

1  

Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring, Committee on Seismology, Board on Earth Sciences and Resources, National Research Council, p. 2.

2  

White House Press Release, August 11, 1995.



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--> 1 Introduction. CTBT Goals As of January 1998, the Comprehensive Test Ban Treaty (CTBT) banning all nuclear explosions has been negotiated and signed by 149 countries, including the United States (India, Pakistan, and North Korea were not signatories). By the Law of Treaties, the signatories are bound to abide by the provisions of the CTBT prior to its entry into force, effectively creating an immediate moratorium on nuclear weapons testing. As of May 1998, the U.S. Senate has yet to ratify the CTBT. Verification of compliance with the CTBT will be a major concern of many nations in both the short and the long term. Compliance will require a concerted effort to enhance capabilities to identify violations, minimize false alarms, and thus maintain confidence. The technical systems put in place to monitor the CTBT will be employed to detect, locate, and identify small “events” underground, underwater, and in the atmosphere with high confidence and accuracy. As stated in a 1997 NRC report1, the present monitoring systems will not detect with high confidence very low-yield tests. President Clinton2 stated that the CTBT goal for the United States is to be able to detect “a few kilotons evasively tested” in selected areas of the world. Evasive testing involves masking or muting the signals from a nuclear explosion (detonation in a large cavity can significantly reduce the magnitude of seismic signals, for example), although other CTBT monitoring techniques might still be able to detect evasive tests. 1   Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring, Committee on Seismology, Board on Earth Sciences and Resources, National Research Council, p. 2. 2   White House Press Release, August 11, 1995.

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--> CTBT Monitoring Methods An International Monitoring System (IMS) will be established for monitoring compliance of the CTBT. A major component of the IMS is a worldwide network of seismic stations. Seismic signals that will be detectable by (visible to) the IMS will originate from both natural and man-made sources; an IMS goal is to be able to detect events with a magnitude of about 3.0 (Mb). Naturally occurring events include earthquakes and volcanic eruptions, most of which are deeper than man-made seismic sources. Accurate locations of such events to hypocentral depths greater than 10 kilometers makes further consideration of these events unnecessary in the context of monitoring compliance. More enigmatic from the monitoring perspective are man-made or induced seismic signals resulting from (1) applications using conventional explosives for mining and excavation, and (2) rock bursts and collapses associated with surface or underground mining operations. The seismic magnitudes of a small number of mining explosions can be similar in size to those resulting from a small or decoupled nuclear explosion. Mine collapses or rock bursts can also generate similar-sized seismic signals. Uncertainties in the seismically determined location3 of such an event may not allow assignment of an event to a specific mine unless local data are available. By itself, determination of location may not be sufficient to identify the source type, and event identification will need to rely on distinguishing features of the seismic wave forms, possibly in conjunction with other monitoring technologies. Of the other monitoring technologies to be used by the IMS, infrasonics or detection of atmospheric signals are the most germane to monitoring explosions from mining operations. Recognizing that mining blasts represent a significant source of small-magnitude seismic signals, the CTBT calls for voluntary exchanges of information on large mining explosions as part of the treaty's confidence-building measures. These include annual surveys to determine which mines in each country detonate explosives over 300 metric tons (TNT-equivalent), notifications of blasts in which 300 metric tons or more of explosives are detonated in a single explosion4, and dedicated calibration explosions. These measures, although voluntary and not mandated by the treaty, could significantly improve the performance of the monitoring system and reduce the ambiguity of some mine-related seismic signals by allowing for the identification and calibration of mine event locations and signal types. 3   A discussion of locational errors associated with seismic sources is given in Appendix C of the 1997 NRC report, Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring. 4   The section of the CTBT dealing with confidence-building measures does not explicitly differentiate between one single explosion and a delay-fired blast where hundreds of boreholes filled with explosives may be detonated in a sequence within a few seconds. The largest delayed-fired blast (Powder River Basin, Wyoming) detonated about 3,600 metric tons of explosives over a 4- or 5-second period.

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--> Mining Activities That Generate Seismic Signals Mining activities can produce a wide variety of induced ground motions, including seismicity. At the mine site and the surrounding areas, blasting produces local vibrations though mine operators and explosives engineers strive to minimize many of the effects of these vibrations by careful blasting designs. In some cases, ground motions or seismic waves, may be detected at long distances (hundreds of kilometers) from the mine site (see Figure 1). The sources of these seismic signals include (1) surface and underground mine blasting that use large quantities of explosives and (2) the planned or unplanned failure of masses of rock that release a large amount of energy (e.g., rock bursts, coal bumps, planned roof collapses, pillar collapses, and other rock failures). Seismic waves generated from mining activities are sometimes detected by regional seismograph stations that also monitor and record earthquakes. Seismograms recorded for earthquakes and nuclear blasts can have characteristics that permit discriminating the type of event (see Figure 2). A simultaneously detonated chemical explosion can generate a ground motion-time history (see Figure 3) that is similar to one generated by a nuclear blast.5 The similar characteristics include wave form amplitude, frequency, spectral content, and duration. Current seismological instrumentation and the methods used to process and analyze seismic signals are not always capable of discriminating between certain mine-induced seismic signals and explosions resembling nuclear blasts. Hence, some mining activities can produce seismic signals that could possibly be mistaken for clandestine nuclear blasts. Ambiguity And On-site Inspections Experience suggests that most seismic signals that generate suspicion will probably be either anomalous shallow earthquakes, mine collapses, or large mining blasts that occur at the surface or underground. Under the terms of the treaty, an event detected by at least three of the primary stations of the IMS should be reported in the seismic bulletin to be published by the international CTBT organization. Events generating signals that cannot be readily explained as generated by known mining activities might initiate a request for an on-site inspection (OSI) by an international team of experts seeking to verify or deny the occurrence of a nuclear explosion. It is anticipated that on-site inspections will be rare. Mining operators could help reduce false alarms under the treaty by providing information on large mining blasts they generate. The ambiguity of seismic signals generated by mining 5   Kim, Simpson, and Richards (1994, High-Frequency Spectra of Regional Phases from Earthquakes and Chemical Explosions, Bulletin of the Seismological Society of America 84, pp. 1365–1386) noted that spectra of ripple-fired shots (delayed fired shots to the blasting engineers) have systematic patterns of constructive and destructive interference, presumably caused by signals from the individual shots in the ripple-fired set interfering with each other. Also there is a strong, impulsive, P-wave arriving from the single shot that is greatly reduced for the ripple-fired shot.

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--> activities would be lessened by efforts to (1) identify the signal sources and (2) determine the seismic character of those events. Alternatively, mining operations could become less seismically visible by modifying their procedures to send out smaller amplitude seismic signals. A robust international effort to reduce ambiguity would also aid U.S. monitoring agencies in their efforts to ensure world-wide compliance with the CTBT. A strong effort by the U.S. mining industry to reduce ambiguity would put the United States in a good position to offer technical support to other nations and to the CTBT Organization (CTBTO) so that other nations may more easily follow the U.S. example. Should a nation be suspicious of a seismic signal, it may take either bilateral or “consultation and clarification” steps to resolve its suspicions before attempting to persuade the CTBTO to carry out an on-site inspection. If the requesting nation selects bilateral measures, then it may simply contact the other nation and ask for an exchange of information and perhaps of scientists and engineers. If the nation instead chooses consultation and clarification, then it may ask the CTBTO to obtain clarification from the “requested state party,” whose response is required within 48 hours. If the response is unsatisfactory to the requesting nation, it may request a meeting of the Executive Council of the CTBTO to consider appropriate action and perhaps an on-site inspection. The latter requires approval of 30 of the 51 member nations on the Executive Council. The committee believes because of the established procedures provided by the treaty leading to an OSI and the opportunities to explain ambiguous events with data, that on-site inspections will probably be rare in the United States. For an average mine, the probability of an OSI during the mine's operational life is very low. Furthermore, for a mine with no clandestine activity—presumably the case for U.S. mines—the committee believes that an OSI probably would not be overly intrusive and probably would not require shutting down mine production. Goals Of DOE Working Group Report. Because mining activities worldwide can generate seismic signals that may in rare cases appear as prohibited nuclear explosions, the seismic signals generated by mining may become visible to seismic monitoring activities that are part of the CTBT. A stated goal of the DOE Working Group's6 draft report, Reducing the Ambiguity and Visibility of Seismic Signals from Mining Activities, is to identify 6   The Working Group convened by the Department of Energy's Office of Non-Proliferation and National Security was co-chaired by François Heuze (Lawrence Livermore National Laboratory) and Brian Stump (Los Alamos National Laboratory). Other members were Frank Chiappetta (Blasting Analysis International), Robert Hopler (Powderman Consulting, Inc.), Vindell Hsu (Air Force Technical Applications Center), Bob Martin (Thunder Basin Coal Co.), Craig Pearson (Los Alamos National Laboratory), William Walter (Lawrence Livermore National Laboratory), and Karl Zipf (Mine Safety and Health Administration).

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--> Figure 1. Seismic signals generated by mining operations in the United States that are detectable by the national network for the period May through November 1997. Of the 956 events located on the map, 237 had a local or regional magnitude of 2.5 or greater (~25%), of which 55 had a magnitude of 3.0 or greater. There were two events with a magnitude greater than 3.5 (both at 3.6). As such, most signals probably would not be visible to the International Monitoring System for the CTBT (a bried discussion of magnitude measures can be found at the end of Appendix A. The map was compiled by the National Earthquake Information Center of the U.S. Geological Survey; data, bulletins, explanations, and other maps can be found at http://earthquake.usgs.gov/neis/mineblast/.

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--> Figure 2. Seismograms (recorded at the same station) generated by an earthquake and an explosion near the Nevada Test Site. Both are filtered to emphasize the high frequencies (6–8 Hz); differences in amplitude of Lg waves clearly distinguish the two events. Adapted from Walter, 1996 (Lawrence Livermore National Laboratory, UCRL-MI-123958), and from Figure 1.B.2. of the DOE Working Group draft report). Figure 3. Comparison of the P energy at regional distance (same station) from a large single chemical explosion (non-proliferation experiment) and a near-by nuclear explosion (named Hunters Trophy). From Figure 2.B.1. of the DOE Working Group draft report. Similarities are striking. However, the NPE is not necessarily representative of chemical explosions detonated by the mining industry.

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--> those mining “practices and tools which…would likely reduce the false alarms that may be registered by an International Monitoring System.” The DOE Working Group report reviews seismic sources associated with mining operations and some recent research intended to help characterize the signals generated by these sources nearby and at regional distances. The report then considers how (1) the size of the regional signals might be reduced, or (2) the resulting signals could be more easily recognizable as mining related. The DOE Working Group report states that the “U.S. mining industry is intended to be the primary beneficiary of this work. If the mining practices recommended…are adopted by the mining companies, they will likely provide intrinsic safety and economic benefits to mine operators.” The other beneficiary is the community responsible for monitoring the CTBT. For this community, the report states that “wide spread adoption of these [recommended] mining practices will probably decrease seismic visibility and ambiguity.”

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