Acoustical Society of America
ICA/ASA '98 Lay Language Papers

A Global Hydroacoustic Monitoring System for the
Comprehensive Nuclear-Test-Ban Treaty:
Plans and Progress

Martin W. Lawrence and Marta Galindo Arranz- mlawrence@ctbto.org
Comprehensive Nuclear-Test-Ban Treaty Organization
Vienna International Centre
A-1400 Vienna, Austria

Popular version of paper 4pUW3
Presented Thursday afternoon, June 25, 1998
ICA/ASA '98, Seattle, WA

MONITORING OF COMPLIANCE WITH CTBT

A Comprehensive Nuclear-Test-Ban Treaty (CTBT) to ban all nuclear explosion tests was opened for signature in New York on the 24th September 1996. It is open to all States for signature and as of early 1998 there have been 149 signatures. During the CTBT negotiations many States insisted that compliance with the Treaty should be adequately and cost-effectively verified. Verification of compliance with the CTBT requires establishment of a 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 (IMS).

Previous treaties on nuclear testing have only involved a small number of States and also did not cover all testing conditions. Importantly, these treaties did not contain a verification agreement; although some States did deploy and operate nuclear detection monitoring systems. In contrast, the CTBT bans all nuclear explosion testing and is backed by a global verification system. In order to make difficult the evasion of the monitoring system by potential violators, the CTBT verification system is being established to monitor tests conducted in all terrestrial environments.

The IMS will be capable of detecting, locating and identifying explosions down to less than 1 kt detonated within the atmosphere, underwater or underground. The system is designed to provide (roughly) uniform global coverage. Four technologies are being used in the IMS: seismic, hydroacoustic, infrasound and radionuclide. networks of sensors for the four technologies are currently being deployed. The data from each station will be transmitted in real time to an International Data Centre (IDC) in Vienna. The IDC will process the data and produce a bulletin of detected events for States Parties.

HYDROACOUSTIC COMPONENT OF IMS

The most effective way to detect underwater explosions is to deploy a network of hydrophones in the SOFAR channel in order to detect the sound wave generated by the explosion. As is well known, sound propagates exceptionally well through the SOFAR channel with very little attenuation.

The IMS hydroacoustic network contains eleven hydroacoustic stations located with an emphasis on the Southern Hemisphere. The extensive seismic network is located with an emphasis on the Northern Hemisphere. The seismic network provides a good capability in the Northern Hemisphere for the detection of underwater explosions to less than 1 kt as well as providing capability to locate and identify them. However, the paucity of land mass in the Southern Hemisphere means that the large oceanic areas can be most effectively monitored by making use of a hydroacoustic system working in synergy with the seismic system.

Of the eleven stations that make up the hydroacoustic network, six will be based upon hydrophone sensors. Some key features for these hydrophone stations are as follows. The stations will utilise fixed hydrophones at the centre of the SOFAR channel, with a cable bringing the signal back to a shore station. From here the signal will be sent by satellite to the IDC. When these shore stations are on small islands (which is the common situation) there will be hydrophones deployed on opposite sides of the island in order to prevent blocking of the signal by the island. The hydrophone stations will monitor at the low frequencies (1 to 100 Hz) suitable for recording explosions at large distances.

The other five hydroacoustic stations are located on steep-sloped islands and make use of seismic equipment to detect hydroacoustic waves. In this latter method, the hydroacoustic wave is converted to a seismic wave at the boundary of the island (this type of propagation has long been known to the seismic community as T-phase propagation). These T-phase stations are included in the network in order to achieve a significant cost reduction (although with some performance loss in comparison with a fully hydrophone based network).

The location of the eleven hydroacoustic stations is shown in the following figure (the stars are hydrophone stations, the triangles are T-phase stations). The locations selected in this network were chosen to achieve maximum coverage of ocean areas and ease of construction and operation.

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 evidence for identification of earthquakes located in the crust beneath the ocean floor.

The characteristics of hydroacoustic signals are especially effective in identifying a detected event as an underwater explosion or otherwise. There are two major reasons for this. The first is that if the explosion is contained beneath the water, the signal is characterised by a clear bubble-pulse oscillation. The second is that the hydroacoustic signal from an underwater explosion, even if not fully contained, has far more energy at higher frequencies than does any signal that has propagated through the ground for some distance.

IMPLEMENTATION

Of the eleven IMS hydroacoustic stations, two of the hydrophone stations and one of the T-phase stations existed prior to the treaty. All of the other IMS hydroacoustic stations will be installed over the next few years and upgrading action will be taken at the existing sites. Prior to installation of a station, a site survey is undertaken. This site survey program commenced in 1997 and installation of stations starts this year. Certification of operation to CTBT standards will follow, leading to operation at a very high level of availability into the indefinite future. The data from all stations will be sent to the IDC in Vienna in real time, 24 hours per day every day of the year. Although this network is being installed for monitoring of nuclear explosions, it can also be expected to provide a unique resource for scientific investigation of various ocean phenomena.