Introduction
A Comprehensive Test Ban Treaty (CTBT) to ban all nuclear explosion tests was signed in New York on the 24 September 1996. This is an important treaty, regarded by many as "The Holy Grail" of Arms Control Treaties. If not adhered to by State Parties to the Treaty it may be perceived by some States as posing a threat to their National Security. For this reason, during the negotiations many delegations insisted that compliance with the Treaty should be adequately and cost-effectively verified. To verify compliance with a CTBT requires the establishment of an precedented global network of sensors to detect, locate and identify the signals generated by a nuclear explosion.
This system is known as the International Monitoring System or IMS and is intended as an addition or supplement to a States Parties' national technical means or facilities, to enable every State Party to verify compliance with the treaty.
Tests conducted in the atmosphere, underwater, and in space are already banned under the Partial Test Ban Treaty (PTBT) signed in 1963. The CTBT extends the PTBT to include underground nuclear explosions. The PTBT does not contain a verification agreement although some States did deploy and operate nuclear detection monitoring systems to ensure that the provisions of the treaty were being complied with. The impact of the PTBT on nuclear weapon development was small as it did not impose a ban on nuclear weapon tests conducted underground enabling a State to continue developing its nuclear weapons.
However, the CTBT which bans underground testing together with an
adequate verification system imposes very severe restraints on nuclear
weapon development and on the opportunities available to a potential violator
to conduct a nuclear test. A State, keen to test a nuclear weapon,
may be tempted to conduct a test in an environment where he may feel that
he stands less chance of being detected or identified as the perpetrator
of a treaty violation. For this reason, many delegations negotiating
a CTBT
believed that a verification system should be established to monitor
tests conducted in all environments, including those covered by PTBT.
To monitor nuclear explosions down to zero yield, i.e. a truly comprehensive treaty, is impossible and would not be cost-effective. A sensible and acceptable compromise had to be made between the cost of the monitoring system and level of detection (hence deterrence) that could be achieved. The Conference on Disarmament (CD) delegations insisted that the treaty be effective in arresting both horizontal and vertical nuclear weapon proliferation which meant ensuring a detection level that imposed, as far as technically possible, a complete restraint on nuclear weapon development.
A cost-effective IMS was designed by Expert Groups, including many experts from the U.S., during the negotiations in Geneva that gives a very high probability of detecting, locating, and identifying nuclear explosions of 1 kt in the atmosphere, underwater, and underground. Detection and location of explosions down to 10 tons may be possible in some environments in many parts of the world.
The level of 1 kt, which refers primarily to underground explosions, is not a threshold but a practical measure for the purposes of cost-effective monitoring: it is assumed that the uncertainty of avoiding detection at lower levels would ensure compliance well below 1 kt. Below 1 kt the difficulty of identifying the source of a signal recorded by the seismic component of the IMS increases significantly. Many of the techniques available to detect nuclear explosions are capable of discriminating between natural occurrences and explosions but only the detection of specific radionuclides can unambiguously identify an occurrence as a nuclear event.
The radionuclide system is essential to provide the "smoking gun" evidence of a Treaty violation. The IMS may locate and identify a detected signal as originating from a suspicious disturbance but in the absence of the specific radionuclides it may be necessary to conduct an on-site-inspection (OSI) to gather the evidence to establish whether or not the detected signals originated from a nuclear explosion. The purpose of an OSI is at least two-fold: to provide an enhanced level of deterrence as part of the verification package and to determine whether or not a treaty violation has occurred. Thus, the inclusion of OSIs into the verification package is a major element which, with the IMS, is intended to ensure compliance with the CTBT and to significantly increase the level of deterrence against a potential violator.
Atmospheric Explosions
There are many technologies available to detect nuclear explosions in the atmosphere, for example radionuclides, electro-magnetic pulse (EMP), optical flash, and shock or sound wave (infrasonics). Of these the most attractive in terms of cost and its unique ability to identify nuclear explosions is the radionuclide technology. A nuclear explosion in the atmosphere will deposit radionuclides in the form of particulates (aerosols) and noble gases. The wind will distribute the radioactive debris over large distances and will ultimately be detected by sensitive radionuclide detectors. The number of stations deployed to detect an explosion will be determined by the time limit imposed for detection by the network. In general, the greater the number of sensors the shorter the detection time. Detection times may be important for the conduct of an OSI in that it would need to be conducted before time-dependent phenomena resulting from the explosion are lost.
A major drawback of the radionuclide monitoring system is that to locate the source of the radioactivity requires the use of back-tracking techniques. This requires a detailed knowledge of the meteorological conditions over a wide area so that the wind speed and direction can be used to determine where and when the release occurred. The location of the source is required to assist attribution. However, the method is not very accurate producing a large footprint - for example in Europe, an area, covering many States.
To minimise this shortcoming a network of infrasound detectors will be established. An infrasound detector is used to detect the sound wave originating from the initial shock wave generated by the nuclear explosion. At each infrasound station several microphones (or microbarographs) are deployed so that the direction of approach of the sound wave can be determined. The determination of the direction of approach is very accurate for infrasound waves. Detection at two such stations enables a location of the source to be made by the intersection of the back-bearings. Thus the synergy of the two technologies enables an atmospheric nuclear explosion very significantly below 1 kt to be detected, located, and identified.
A very important element of the IMS is the inclusion of noble gas detectors. The use of noble gas detectors as part of the radionuclide network together with the infrasound system provides a powerful deterrent to the conduct of a clandestine test in the atmosphere and indeed underground. It is possible to conduct nuclear tests on a ship on the ocean in such a way that the EMP signal, detected by the GPS Nuclear Detection System, is disguised and thus not recognised and the radionuclide particulates are "washed-out" in a very localised area and are not transported very far. However, the noble gases will not be "washed-out" and will be transported over large distances, be detected, and identified as originating from a nuclear explosion. Furthermore, the shock wave will not be suppressed and may be detected by one or more elements of the infrasound network. If the explosion generates a hydroacoustic wave as well and the data are used to conjunction with radionuclide and infrasound data the source could be located to within a very few kilometres. Of course, attribution would still be a problem.
The noble gas monitors will also act as a significant deterrent to the
use of cavities to decouple (or "muffle") the signals from an underground
explosion in an attempt to avoid detection by the seismic system.
A nuclear test conducted in an underground cavity either excavated or created
by an
explosion may well be detected by a network of noble gas detectors
if the noble gases migrate to the surface along natural cracks or fissures
that occur in the rock surrounding the cavity. A decoupled nuclear
test detonated underground may generate seismic waves which may be too
small to be detected by the seismic network but a potential violator will
need to be very confident that no noble gases will leak to the surface
to be detected and identified as originating from a nuclear explosion.
To monitor nuclear explosions in the atmosphere, a network of eighty radionuclide particulate stations will be established, 40 of which and ultimately all 80, will include noble gas detectors, around the world as part of the IMS together with a network of sixty infrasound stations to monitor nuclear explosions in the atmosphere. Such a system will detect, locate and identify nuclear explosions with yields very significantly less than 1 kt.
Underwater Explosions
The most effective way to detect underwater explosions is to deploy
a network of hydrophones in the low velocity SOFAR (Sound fixing and ranging)
channel of the oceans to detect the sound wave generated by the
explosion. Sound propagates extremely well through the SOFAR
channel with very little attenuation. In fact, explosions of only
a few kilograms can be detected and identified as explosions at distances
of thousands of kilometres. 10 kgms detonated off the Californian
coast has been detected at a distance of 10,000 kms.
During the negotiations the main concern with the deployment of a network of hydrophones (the hydroacoustic system) was that it was viewed by some delegations as posing a threat to national maritime security. However, it is possible to ensure that for CTBT monitoring the hydroacoustic network operates in such a way so that it poses no such threat.
A hydroacoustic network, working synergistically with the seismic network, has a number of advantages for monitoring compliance with a CTBT. Firstly it provides excellent detection, location, and identification of underwater explosions. Secondly, it can provide additional diagnostic evidence for source identification particularly of earthquakes located in the crust beneath the ocean floor. This is possible either by the total absence of a hydroacoustic wave (indicating that it was not an underwater explosion) or by an analysis of a detected hydroacoustic signal to demonstrate that it could not have originated from an underwater explosion.
The reason the hydroacoustic signal is so effective in identifying underwater explosions is that if the explosion is contained beneath the water, the signal is characterised by a clear bubble-pulse oscillation generated by the evaporated water expanding and then contracting under the hydrostatic pressure. Such a signal is readily recognised by analysis and is unique to underwater explosions. In addition to the bubble-pulse, the hydroacoustic signal from an underwater explosion, even if not fully contained, is very rich in high frequencies unlike signals from natural underwater sources.
The IMS network consists of eleven hydroacoustic stations located mainly in the southern hemisphere. The northern hemisphere is covered by an extensive seismic network which should detect underwater explosions with yields very significantly less than 1 kt because of the enhanced coupling and locate and identify them. However, the paucity of land mass and hence quiet seismic sites in the southern hemisphere means that the large oceanic areas can be cost-effectively monitored by making use of a hydroacoustic system working in synergy with the seismic system. In fact, only six stations of the hydroacoustic network will be equipped with hydrophones.
Five stations located on steep-sloped islands make use of relatively inexpensive seismic equipment to detect hydroacoustic signals thus effecting a significant cost reduction. In this way underwater explosions of significantly less than 1 kt can be readily detected, located, and identified as explosions. However, in the absence of any radionuclide evidence, OSIs may be required to determine whether or not the explosion was nuclear.
Finally, the hydroacoustic system provides an additional deterrence to a potential violator considering conducting a clandestine test above water in a remote ocean basin area in that he must ensure that he does not generate a hydroacoustic signal which may be detected and identified as originating from an explosion.
Underground Explosions
This is the most difficult environment to monitor. The best and only effective way of monitoring underground nuclear explosions is by detecting the seismic waves generated by the explosion. The major difficulty arises from the ever present, background noise, and the very large number of naturally occuring seismic disturbances. For example, an explosion of 1 kt fully contained in hard rock generates a seismic signal equivalent to a seismic magnitude of about 4.
There are some 8,000 earthquakes of this magnitude or greater per year or about 1 per hour. At around 0.1 kt there are about 8-10 natural events per hour to which must be added the man-made explosions detonated during mining and quarrying operations. This means that every event detected would have to be analysed to some degree to determine the nature of the source. The seismic system may be able to identify the source as an explosion, but can not indicate whether it is a nuclear or conventional explosion. An additional problem with the seismic system is that identification of low magnitude explosions is difficult because the signal amplitude is small and may be buried in the ambient (background) noise.
However, the seismic network will detect many events significantly below magnitude 4 ( EMBED Equation.3 -1 kt) but improvement in the identification of smaller magnitude disturbances will require the network to be calibrated. The seismic network will comprise a primary, or core, network of fifty seismic stations including many arrays capable of detecting and locating seismic disturbances down to a magnitude greater than about 3-3 EMBED Equation.3 for most parts of the world.
However, such a network will not be able to guarantee location to within the order of 20 km for very small seismic events. To improve the location capability of the primary network, use will be made of a network of 120 auxiliary seismic stations.
These will be accessed by the International Data Centre (IDC), as and when required, to provide additional data to improve location accuracy and possibly provide additional identification evidence. Only 2-5 auxiliary stations would be accessed by the IDC at any one time to provide additional data. EMBED Equation.3
Thus, the proposed seismic network of primary and auxiliary stations
is capable of detecting, locating, and identifying with a high confidence
underground explosions to EMBED Equation.3 kt-1 kt with a very
high level of deterrence at a significantly lower yield. It is capable
of detecting even lower yields at individual stations which may then be
used as the trigger for an OSI request.
Again putting additional restraints on a potential violator - certainly
limiting the yield of his planned explosion.
All elements of the IMS will include arrangements for system security and data authentication to give States Parties the necessary confidence in the value of the data for verification purposes.
Comment
The network described shows that the IMS is designed to provide each State Party with verification data that gives them an equal opportunity of monitoring compliance with the treaty down to a low yield level. Additional monitoring capacity can be achieved for areas of particular concern to a State Party by the deployment of nationally owned facilities close to the area of concern either deployed unilaterally or in co-operation with other concerned neighbouring States Parties.
An important element of the verification system is The International Data Centre (IDC).
The role of the IDC is to receive raw data from the various elements of the IMS and process the data to produce a bulletin of detected and located events. The intention is to make a final bulletin available to States within forty-eight hours. However, States Parties can access the IDC for a preliminary bulletin which will be available within an hour or two. Data from the primary network of seismic stations the hydroacoustic and infrasound systems will be transmitted continuously and in real time to the IDC where detection and association processing will take place. Supplementary data from auxiliary seismic stations will be accessed directly, as and when required by the IDC. The IDC will also process data for a State Party using that State?s own verification criteria - a valuable contribution to States with little experience in special event analysis.
The IDC will receive radionuclide data and report in the bulletin details of any radionuclide detections by the network. The negotiators decided that detection of radionuclides from a nuclear explosion should be made within no more than 6 days. The time-delay in detection and reporting is determined by the number of radionuclide stations deployed. The greater the number of stations the quicker the detection. The number of 80 radionuclide stations was a negotiated consensus and is a comprimise between cost and effectiveness.
It is estimated that the daily flow of data into the IDC will be over 10 billion characters of raw data.
Finally, the verification package includes the right of a State Party to request an OSI in an area where an anomalous signal occurred. Permission for an OSI will be sought from the Executive Council where a vote for the OSI will be taken and will require approval from 30 of the 51 members to proceed. The OSI search area will be limited to 1000 sq. kms that is a circle with a radius of about 35 km. Inspectors will be able to use a whole range of geophysical and video equipment together with overflight data and radionuclide systems to search for evidence of a nuclear test. Even the opportunity to drill into a suspected cavity to recover radioactive debris is permitted.
Conclusion
The verification package provided within the CTBT comprises 321 monitoring
stations in some 90 countries which will provide data to an IDC.
The IDC will produce a bulletin of detected events which will be used,
in addition to any national technical facilities, to monitor compliance
with the treaty.
Any event reported in the bulletin or detected by National Technical Means can be used to request an OSI.
Thee progress to date in establishing the CTBT verification system is impressive. The real challenge that remains is not technical but political and that is to get all of the 44 States that are required to ratify the Treaty to do so. This will ensure entry into force of the Treaty so that the IMS and its associated verification system can be used for the purpose it is intended for and that is to monitor compliance with the Treaty.
Without ratification and entry-into-force we will be faced with a moritorium without the necessary resources to monitor.
The provisions of the protocol of the Treaty do impose a severe limitation on a potential violator and should meet its prime objective of arresting vertical and horizontal proliferation thus ensuring the re-newal of the important Non-proliferation Treaty. Remember the CTBT does not ban the bomb - it bans the bang!
