Introduction

The Comprehensive Nuclear-Test Ban Treaty (CTBT) was adopted by the General Assembly of the United Nations in September 1996. It prohibits nuclear explosions in the atmosphere, underwater and underground; explosions in outer space are covered by an existing treaty. The CTBT will enter into force 180 days after ratification by 44 named states. By 1 April 1999, the treaty had been signed by 152 states and ratified by 33, with 41 signatures and 17 ratifications from the required 44. The states will hold a review meeting in late 1999 to discuss progress towards entry-into-force.

The signatory states established the Preparatory Commission for the Comprehensive Nuclear-Test-Ban-Treaty Organisation (CTBTO PrepCom) to establish the global verification system required for the CTBT. CTBTO PrepCom is made up of a plenary body of all the state signatories and a Provisional Technical Secretariat (PTS). The plenary body and its working groups meet regularly in Vienna; Working Group B discussing technical issues. The PTS is based in Vienna and works under the direction of the plenary body. The PTS has a staff of almost 200 and an annual budget approaching US$80 million.

The PTS has three main technical tasks. It has to establish the International Monitoring System (IMS), the worldwide network of monitoring stations. It has to set up the International Data Centre (IDC) in Vienna to process the data from these stations and to distribute it, and derived data products, to state signatories. Finally, the PTS has to implement a system for on-site inspections in the event of any suspected violation of the treaty.

The IMS, communications and data integrity

There are a total of 321 stations in the IMS, using four different methods to monitor nuclear explosions; radionuclide (80 stations), hydroacoustic (11), infrasound (60) and seismic (170). The three "waveform technologies" continuously record data in their respective environments; water pressure in the oceans, atmospheric air pressure and ground motion. In radionuclide monitoring, the atmosphere is regularly sampled for the presence of nuclear decay products.

Data from all the IMS stations will be transmitted to the IDC in Vienna. The PTS is establishing a Global Communications Infrastructure (GCI) based on the Very Small Aperture Terminal (VSAT) satellite communications system. Data flow, and station control, will be over either a direct link to Vienna or a link through the host state's National Data Centre (NDC). If the latter, the state can opt to use either the GCI or their own communications links to connect the station and the NDC. The specifications of the GCI are very high, with an up-time of greater than 99.8%.

The CTBTO will provide raw IMS data and IDC data products that will enable states to make their own assessment of events. It is vital that the states can be sure of the integrity of this data; that it has not been altered. CTBTO PrepCom intend to ensure this using a combination of data authentication, with hardware devices at the stations digitally signing the data, and tamper-detection devices on infrastructure at the stations.

CTBTO data will be available to interested parties through their NDC.

IMS seismic stations

Of the four IMS technologies, seismology is the most mature, with a large number of existing stations in the network. This has the advantage that not all stations will have to be built from scratch and existing equipment can be upgraded. The disadvantage is that it will result in a very inhomogenous network.

There are two distinct seismic networks in the IMS, the primary and auxiliary networks, designed by a group of experts convened during the treaty negotiations in Geneva. Both include seismic arrays and 3-component stations. The distribution of stations is shown below and the stations are listed in Table 1. (There are five stations not included in the figure because their locations have not yet been decided.)

In order to accommodate the wide range of existing, upgraded and new stations in the networks, the experts specified minimum technical requirements for IMS seismic stations. These are given in Table 2 (pdf).

The primary seismic network provides even coverage of the Earth's land surface. There is no need to monitor the oceans or even small islands as explosions in these environments will be detected by the hydroacoustic network. The primary network includes 30 seismic arrays; mainly regional arrays with apertures of a few kilometres. These stations will transmit continuous data to the IDC.

The auxiliary seismic network consists of 120 stations, all 3-component except for 7 seismic arrays. Continuous data will be stored in a local buffer - either at the station or the NDC - for a minimum of 7 days. The IDC will then access this data as it is required.

The auxiliary network has two purposes:

- To provide data to augment that from the primary network to improve location accuracy, particularly depth, and to help identify the source.

- To act as backup to stations in the primary network stations in the event of a problem with a primary station.

Consequently, the distribution of auxiliary stations is not even, concentrating on areas of high seismicity, including mining seismicity. The design of this network used as many existing seismic stations as possible; there are only 11 new auxiliary stations and one new array, in Israel. However, current estimates are that at least 10 of the stations will need complete replacement of equipment to fulfill IMS requirements.

IMS and network operators

The majority of the stations in the IMS seismic network already exist, and many of these are part of international networks such as IRIS/IDA, IRIS/USGS, GEOFONE, GEOSCOPE, OHP and MEDNET.

A good relationship between the PTS and the network operators is vital, especially for the auxiliary stations. The network operators were not involved in the negotiations and therefore view some proposals with scepticism. Some of the issues of concern are:

- Upgrading selected stations in a network will result in an inhomogeneous network which has a variety of implications, including increased costs, for operating the network.

- The addition of GCI and authentication equipment could have a major impact on the homogeneity of the network. They will also require more power, which could be a problem at remote sites.

- The operational requirements of the IMS network, which are still to be established, will probably be more strict than existing arrangements, with more frequent calibration and reporting and required responses to breakdowns. This will also have cost, and maybe other, implications.

The PTS is committed to cooperating with the network operators to resolve these issues.

The PTS at work

The CTBTO has a mandate, and a budget, to install and operate the IMS networks. The PTS will therefore pay for any upgrading or new equipment required at primary seismic stations and for the installation of that equipment. The IMS will also pay for the operation of these stations, although in the case of dual-use stations, where the data is also used for national purposes, these costs are to be shared.

The situation is slightly different for auxiliary seismic stations. The PTS will fund equipment and its installation, upgrading stations to the desired specifications and installing new stations. The PTS will also bear all the costs associated with the installation of the GCI and authentication devices. It was however the intent of the negotiators in Geneva that CTBTO will not pay operational costs for auxiliary stations. This may not be possible and it seems likely that the IMS will have to bear at least some of the costs of operating the new auxiliary stations.

The PTS is required to be cost effective in all its work, including establishing the IMS. Wherever possible, work that is contracted out is awarded to the lowest tender. The minimum station specifications are very important in establishing Terms of Reference against which bids for work are placed.

For seismic stations, there are a number of distinct stages in the work carried out by the PTS. These are not always done separately and can be combined if convenient.

1. Site Survey

The site survey establishes the requirements for a particular station. For existing, well-established stations this is just a matter of documenting equipment and procedures, a task which can often be carried out by the parent network. Some existing sites have equipment that falls well short of the IMS requirements and also need independent noise measurements. For new sites the site survey involves a rigorous site selection procedure as well as comprehensive noise measurements; this can be done by either by the PTS or a contractor, or both together.

2. Station Design

Station design varies greatly in its scope. Upgrades may only need an identification of needs as part of the site survey. Much more formal design work, including buildings and communications systems, may be needed for a new station or an array. It is important that all stations are designed to satisfy the minimum specifications, not just exact equipment requirements but also to ensure the required data availability; 98% for primary and 90% for auxiliary stations. The station therefore has to have adequate provisions for power, security and manning.

3. Equipment Procurement

Equipment procurement is carried out under a strict set of financial rules. Competitive tendering is the norm, although this is often not appropriate when carrying out an upgrade on existing equipment.

4. Site Preparation and Installation

The preparation and installation of the station can involve a lot of construction work, and the PTS encourages the use of local contractors. Contracts for installing equipment will also include providing training for operators.

5. Testing and Evaluation

Not all stations will require extensive testing and evaluation, particularly the existing ones. All new equipment will be tested in situ before certification.

6. Certification

Certification marks the acceptance of a station into the IMS, that the station is providing reliable data that can be used safely by states. Certification requires full documentation of the station equipment and procedures. The amount of work required to achieve certification will vary; stations built by the PTS will require much less documentation effort than existing stations.

There is a lot of work to be done in establishing the IMS seismic network. It is expected that almost all of the site surveys will be completed by the end of 1999. Design, procurement and installation are underway for a growing number of stations, most notably the upgrade of the Warramunga array in Australia and the installation of two new borehole stations in Iran. The process of certification has been started at two stations with more to follow this year.

The future

Current efforts concentrate on establishing the IMS. By the time of entry-into-force however, the focus should have changed towards running the network in a manner that ensures that timely, authenticated data is available to states. This will require continual checking of the data and the performance of the stations.

Procedures for the running of the network are being established. The most important documents are probably the Operational Manuals, being developed by Working Group B, which will set out how stations have to be operated.

Training of station staff is also very important to ensure good network operations. In 1998, the PTS held its first IMS Technical Training Programme (TTP) for seismic station operators in Vienna and at the NORSAR array. A second TTP will be held later this year, with further courses at regular intervals.