Synergy and the International Monitoring System

Peter D Marshall, O.B.E.

The International Monitoring System (IMS) was designed by the Group of Experts (GSE) during the CTBT negotiations in Geneva. During the negotiations the experts were instructed to take full account of the potential synergy of the technologies to be deployed to monitor compliance with the provisions of the treaty in order to maximise the cost-effectiveness of the IMS. The Experts' proposals and design were accepted by the diplomatic representatives of the states negotiating the treaty.

Of the four approved technologies deployed in the IMS only the detection by the radionuclide system of specific radionuclides can uniquely identify a source as a nuclear explosion. Data from the three waveform technologies: seismic, hydroacoustic and infrasound sensors are used to detect, locate and identify explosions in the atmosphere, underground and underwater but it is not possible using data from these systems to determine whether an explosion source is nuclear or not. In the absence of diagnostic radionuclide evidence a state party may wish to request an on-site-inspection (OSI). The synergy of the techniques deployed for the detection of nuclear explosions was taken into account when the Expert Group considered what technologies should be employed during an OSI.

To consider the synergy within the IMS it is convenient to consider the role of each technology in monitoring a particular environment. However, it should be noted that the synergy between the various technologies deployed within the IMS remains the same whether the IMS is complete or not. Furthermore, any additional monitoring system operated as a national facility or by independent non-government bodies will operate synergistically with the IMS.

1. Underground Nuclear Explosion

To detect and locate underground nuclear explosions, the seismic network of primary and auxiliary stations is fundamental. However, for source identification purposes, seismology is only a complementary, not a definitive technique. I It is not possible through seismological means to identify a source as being a nuclear or conventional explosion; for this task the detection of radionuclides is essential. Radionuclides from an underground nuclear explosion may leak to the surface through fissures or fractures surrounding the cavity created by the explosion.

Detection of specific radionuclides during an OSI is vital evidence of a breach of the provisions of the treaty. Detection may be achieved by using drilling techniques to obtain samples from debris in or around the explosion cavity which may have been located using geophysical, and in particular seismic, methods.

The hydroacoustic system may detect signals from underground explosions, particularly from those detonated on small islands in oceanic basins. The technique is itself only complementary to the seismic and radionuclide networks for detection, location and source identification. However, it has a significant role in the identification of earthquakes which occur in the sub-oceanic crust or upper mantle, thus ruling out the possibility that such phenomena are explosions. The detection and analysis of hydroacoustic T-phase signals will prove of significant value to the event-screening process required by the treaty, which is being developed for use by the International Data Centre (IDC) in Vienna. The detection of a T-phase signal in the hydroacoustic system data can also be used to improve source location when used in conjunction with the seismic system.

Infrasound is of minor value for the detection and location of fully contained underground explosions and no value for source identification. However, an underground explosion which breaks the surface may be detected by the infrasound system, and if the source is nuclear, radionuclides may be detected by the radionuclide system and identified as nuclear.

It is important that the IMS and IDC provide high quality and timely data to enable states parties to discriminate between natural phenomena and nuclear explosions. However, at low magnitudes (below mb4 equivalent to a fully contained nuclear explosion of about 1 kt) many conventional explosions used for mining and quarrying purposes will be detected by elements of the IMS. A synergy exists between the various detection technologies which is of value in identifying such events as non-nuclear. The ability to correctly identify such explosions builds confidence in adherence to the treaty by states in which large mining explosions are routinely conducted for economic purposes.

Summary:

1.1. Underground Nuclear Explosions Technology Detection Location Identification Radionuclides Complementary Little value Fundamental (if detected) Seismic Fundamental Fundamental Complementary Hydroacoustic Complementary Complementary Complementary Infrasound Little value Little value - 1.2. Earthquakes Radionuclides - - - Seismic Fundamental Fundamental Fundamental Hydroacoustic Complementary Complementary Complementary Infrasound - - - 1.3. Conventional Mining and Quarrying Explosions1 Radionuclides - - Fundamental2 Seismic Fundamental Fundamental Fundamental Hydroacoustic - - - Infrasound Fundamental Complementary Fundamental

1In areas of extensive mining, national co-operating facilities may be installed by a state party to demonstrate its compliance with the treaty. This table indicates the synergy that exists between the technologies to monitor mining explosions which are not contained, in which the surface above the shot point is severely fractured, thus releasing shock wave energy into the atmosphere.

2 The absence of radionuclides from an explosion that has clearly vented to the atmosphere and detected by the infrasound system would indicate that the explosion is non-nuclear and hence not a treaty violation.

2. Underwater Nuclear Explosions

Explosions detonated underwater or on small islands in oceanic basins may be detected by the hydroacoustic network. If the explosion is not contained, radionuclides may be deposited into the atmosphere and carried by the prevailing winds to radionuclide detectors. Submarine volcanoes and geophysical surveys may also generate hydroacoustic signals and it is important that such events are not misidentified as possible nuclear explosions.

The presence of a bubble-pulse oscillation in, and the high-frequency content of, a hydroacoustic signal is clear evidence of an underwater explosion. But again only the detection of specific radionuclides can identify the source as nuclear. The detection of an infrasound signal will depend on whether or not the explosion was fully contained within the water.

Summary:

2.1. Underwater Nuclear Explosions Technology Detection Location Identification Radionuclides - - Fundamental (if detected) Seismic Complementary Complementary Complementary Hydroacoustic Fundamental Fundamental Fundamental (Identification of explosion) Infrasound Little value1 Little value1 Little value1 2.2. Underwater Volcanoes and Conventional Explosions Radionuclide - - Fundamental1 Seismic Complementary Complementary Complementary Hydroacoustic Fundamental Fundamental Fundamental Infrasound Little value1 Little value1 Little value1 1 Only if venting to the air occurs.

3. Atmospheric Nuclear Explosions

3.1 Detonated over land The principal methods deployed to detect nuclear explosions detonated in the atmosphere are radionuclide and infrasound techniques and a synergy between these two systems is of significant value for treaty monitoring purposes. Again the unique identifier of a nuclear explosion is the presence of specific radionuclides. However the back-tracking technique used to locate the epicentre of the radionuclide release is not very accurate, making it very difficult to identify the state responsible in areas such as Europe, where numerous states are located in a relatively small area. To improve the location capacity of the IMS for atmospheric explosions, the infrasound system is deployed, illustrating the significant synergy between the radionuclide and infrasound systems. The seismic network and the hydroacoustic system may detect an atmospheric explosion if large enough, but will contribute little to verification. The major source of signals detected by the infrasound system is from explosive volcanic eruptions, the passage of weather fronts, sonic booms and signals from venting quarrying explosions. The contribution that the IMS data can make in identifying natural or man-made non-nuclear phenomena are summarised following the discussion on atmospheric explosions detonated over water.

Summary: 3.1. Atmospheric Nuclear Explosions Over Land Technology Detection Location Identification Radionuclides Fundamental Complementary Fundamental Seismic Little value Little value Little value Hydroacoustic - - - Infrasound Fundamental Fundamental Complementary

3.2 Detonated over oceanic basins The major difference between the detection of explosions over land and over water is the contribution that the hydroacoustic network can make. This is illustrated by a comparison of the Summary below (Table 3.2.1) with that given in 3.1.

A nuclear explosion detonated over an oceanic basin in which the shock-wave strikes the water may be detected by any of the four technologies within the IMS. As can be seen in Table 3.2.1 below, the synergy between the infrasound and hydroacoustic system can be used together with the seismic system to provide a very accurate estimate of the location of the explosion. Heavy rain or cooling vapourised water resulting from the explosion may cause the radionuclide particulates to be 'washed-out' in the immediate area of the epicentre, with the result that particulate radionuclides may not propagate to the particulate detectors so that the essential evidence to uniquely identify the source as a nuclear explosion will not be gathered. However, an accurate location would make it possible to go to the area and collect water samples for subsequent analysis to identify the source as a nuclear explosion.

The deployment of noble gas detectors as part of the IMS could be vital for the detection of radioactive noble gases which are not 'washed-out' and are distributed by the prevailing winds to the radionuclide detectors. Thus the synergy between the four technologies is maintained and contributes significantly to the overall cost-effectiveness of the IMS.

Summary: 3.2.1. Atmospheric Nuclear Explosions Over Water Technology Detection Location Identification Radionuclides Fundamental Complementary Fundamental Seismic Little value Little value Little value Hydroacoustic Complementary Complementary Some value Infrasound Fundamental Fundamental Complementary 3.2.2 Non-nuclear Atmospheric Sources, e.g. Volcanoes Radionuclides - - - Seismic Complementary Complementary Some value Hydroacoustic - - - Infrasound Fundamental Fundamental Fundamental

4. On-Site-Inspection: Post-Explosion Activities

The IMS may provide data which indicates that a detected and located event may have been a nuclear explosion and such data, together with non-IMS data may be used to request an on-site-inspection. Of the four IMS technologies, only two have a role in OSI: (a) seismic to detect post-shot tectonic seismicity and seismic activity associated with the decay of the explosion-generated cavity and the redistribution of stress within the hypocentre region and (b) the detection of radionuclides in or around the hypocentre to produce the evidence that the event was indeed nuclear. The location of the cavity and the presence of specific radionuclides will only be detected by seismic and radionuclide detectors taken into the search area by an OSI team. A synergy exists between the deployment of seismic and radionuclide technologies to make an OSI an effective verification process, as well as being a possible deterrent to a potential violator.

5. Conclusion

To prove that a source detected by the IMS is indeed a nuclear explosion and as such is a violation of the provisions of the treaty it is essential to detect and identify specific radionuclides either as particulates or as noble gases. Thus the radionuclide network is a vital element of the IMS. To provide maximum cost-effectiveness and to ensure adherence to the provisions of the CTBT down to a very low level, it is imperative that the radionuclide network works in synergy with the three waveform technologies to provide data to ensure detection (and hence deterrence), location and identification of nuclear explosions conducted in the atmosphere, underwater or underground.

The numbers of stations within the IMS was determined by the group of Experts in Geneva working within the consensus guidelines provided by the negotiating delegations in Geneva. To improve the verification regimes of the IMS, IDC and OSI, states parties were encouraged to deploy national facilities. Such systems will operate in a synergistic way with the IMS to improve treaty monitoring in areas where additional technical systems are deployed to demonstrate adherence to the provisions of the treaty.

Summary Table of the Synergy of the IMS for Detection of Nuclear Explosions

Technology Detection Location Identification OSI Radionuclides1 Fundamental Complementary Fundamental Fundamental Seismic Fundamental Fundamental Complementary Fundamental Hydroacoustic Fundamental Fundamental Complementary No value Infrasound Fundamental Fundamental Complementary No value 1Particulates and noble gases.