IMS hydroacoustic station network

The global network of hydroacoustic stations consists of only eleven stations located in the oceans around the world. Although the area they cover is much larger than the area of the continents, this small number of stations is enough to detect even small underwater explosions. This is made possible by the almost unhindered propagation of the waves in the so-called SOFAR channel (SOund Fixing And Ranging) at a depth of around 1000 m below sea level. The sound waves generated by an underwater explosion spread out in this wave guide – analogous to light in a fibre-optic cable – and can still be registered very far away.

The world topographic map shows the location of the hydroacoustic stations which are marked with stars Source: BGR

The very complex and expensive hydroacoustic stations consist of several sensors (hydrophones) which are positioned in the SOFAR channel using floats. The stations are supplied with power from the nearest island or land by an underwater cable – which is also used to transmit the recorded data. Germany does not have its own station in this network but is involved in technical further development together with the Research Institute for Hydroacoustics and Geophysics (FWG) in Kiel.

Early experiments done in the 1970s already revealed the power of a global hydroacoustic network. The hydroacoustics of a test explosion in the waters off the west coast of Australia using 100 kg (0.0001 kt) TNT equivalent was registered over 20,000 km away on the Bermuda Islands. The following example, for which the IDC has digital data, shows an underwater explosion of unknown strength off the eastern coast of Sri Lanka which was registered by all three stations in the Indian Ocean.

The example of an underwater explosion of unknown strength off the coast of Sri Lanka on 27 May 2003 is shown here: the topographic map on the left shows the source and the station in the Chagos archipelago in the Indian Ocean; the direction-finding is shown in the centre; and the diagram on the right shows the hydroacoustic signals recorded in Pascal (Pa). Source: BGR

To minimise the high costs involved in constructing and maintaining hydroacoustic arrays, four of the eleven stations are what are known as tertiary stations (T-stations). These are equipped with seismometers instead of hydrophones and are located on volcanic islands such as the Azores in the Atlantic. Thanks to the almost loss-free sound propagation in the SOFAR channel, and the good coupling between the water and the volcanic island, the sound waves transmitted from the water into the rock are also registered by the highly sensitive seismometers. Although these stations are not as sensitive as the hydrophone systems, they can also satisfy the specification for detecting nuclear explosions with a strength of around 1 kt.