It consists of 321 stations strategically located all over the world to provide a uniform global coverage. Each of these stations must fulfil certain specifications to be part of the IMS network. Since April 1997 work has been performed to upgrade the existing stations and to start installation of the new ones. When complete, the stations transmit the data in real time via satellite to the International Data Centre (IDC) located in Vienna. The IDC analyses the data and produces a bulletin of detected events for the States Parties to the Treaty and it archives the data on behalf of the States Parties. The overall package for verification of the Test Ban will comprise an IMS, an IDC, and a process for On-Site-Inspection (OSI) to enable States Parties to investigate a suspected Treaty violation.
THE HYDROACOUSTIC NETWORK
The ocean is a remarkably efficient medium for the transmission of sound energy. The existence of the SOFAR channel allows sound energy to propagate with little attenuation over global distances. Therefore, the most effective way to detect underwater explosions is to deploy a network of hydrophones in the SOFAR channel in order to detect the acoustic signal generated by the explosion.
The IMS hydroacoustic network contains eleven hydroacoustic stations located with an emphasis on the Southern Hemisphere. The hydroacoustic network has been designed to work synergistically with the seismic network. The seismic network provides a good capability in the Northern Hemisphere for the detection of underwater explosions to less than 1 kiloton, as well as providing capability to locate and identify them. However for global coverage, the large oceanic areas of the Southern Hemisphere can be most effectively monitored by making use of a hydroacoustic system.
The hydroacoustic verification systern has been designed based on two different wave technologies: hydroacoustics and seismology. Only six of the total eleven stations in the hydroacoustic network will be equipped with hydrophones. The other five hydroacoustic stations are located on steep-sloped islands making use of seismic sensors to detect hydroacoustic waves (this method is chosen in order to effect a significant cost reduction). In this latter case, 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). T-phase stations are not as effective in detecting and identifying hydroacoustic signals from explosions, but they are considerably cheaper than hydrophone stations. The mixture of hydrophone and T-phase stations selected for the IMS hydroacoustic network was chosen as a cost-effective compromise.
The location of the eleven hydroacoustic stations is shown in Figure 1, where the asterisks mark hydrophone stations and the triangles represent T-phase stations. The stations' names are written In the table on the right side. They are written consecutively from west to east.
The locations selected in this network were chosen to achieve maximum coverage of ocean areas, after consideration of such issues as bathymetric shadowing, network configuration for location by triangulation, and availability of local station support. Selection of hydrophone station sites also took into account the desirability of short cable runs to the local axis of the SOFAR channel. Selection of T-phase sites further considered the efficient conversion from hydroacoustic waves into seismic waves.
