First Pan-American/Iberian Meeting on Acoustics, Cancun

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A Global Network of Hydroacoustic Stations
for Monitoring the Comprehensive Nuclear-Test-Ban Treaty

Martin Lawrence -
Marta Galindo Arranz, Patrick Grenard, John Newton
Comprehensive Nuclear-Test-Ban Treaty Organization
Vienna International Centre
A-1400 Vienna, Austria
Mobile: +43-664 544 7327

Popular version of paper 3pUWa2
Presented Wednesday afternoon, December 04, 2002
First Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico

A hydroacoustic network to monitor the major world oceans is being constructed and operated. The development of this unique resource, which will be based on eleven, widely separated stations, currently has three stations complete, with work on the other eight in various stages of progress. Every station will be operated continuously, into the indefinite future, with high levels of data availability. Already, very interesting results are being observed on the completed stations. The location of all eleven hydroacoustic stations comprising the IMS hydroacoustic network is shown in the figure.

Monitoring of Compliance with the CTBT

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans all nuclear explosion tests. The CTBT provides for verification of compliance with the Treaty, by means of a global network of sensors. This network, known as the International Monitoring System (IMS), is capable of detecting, locating, and identifying signals generated by any nuclear explosion.

Four different technologies are being used for the IMS, allowing monitoring of tests conducted in any terrestrial environment. Worldwide networks of sensors for each of the four technologies are currently being deployed. The data from each station is transmitted in real time to the CTBT headquarters in Vienna, Austria.

The hydroacoustic network, discussed here, monitors signals generated by an explosion located either in the ocean, above the ocean but in the low atmosphere, or underground near the ocean shore.

Hydroacoustic Network

The most effective way to monitor underwater explosions is to make use of the exceptionally efficient transmission of sound through the ocean. Because sound propagates over long distances in the ocean, it is possible to design a global monitoring network to cover the major world oceans, which only requires a small number of stations. When complete, the eleven hydroacoustic stations will be located with an emphasis on the vast ocean areas of the Southern Hemisphere.

Each individual hydroacoustic station comprises a number of sensing elements. This enables each station to have a built-in direction finding capability. When combined with other stations, a triangulation process allows for even better locating ability.

The characteristics of hydroacoustic signals are especially effective at identifying the nature of the event that created the signal. This is very valuable, in particular, in determining whether an event was an explosion or not.

The stations that make up the hydroacoustic network consist of two different types. Six stations are based upon hydrophone sensors. A hydrophone station, shown to the right, is essentially an underwater microphone. Some key features for these hydrophone-based stations are as follows. The stations will use hydrophones in deep water, floated from the sea-floor to mid-water depth. A long cable (tens to hundreds of kilometers) brings the signal back to a shore station. From here the signal is sent by satellite to CTBT headquarters in Vienna. When these shore stations are on small islands (which is the usual situation) there are hydrophones deployed on opposite sides of the island, in order to prevent blocking of the signal by the island. The hydrophone stations monitor at the low frequencies (1 to 100 Hz) suitable for recording explosions at large distances.

The other type of station used in the hydroacoustic network is based on use of seismometers located on small islands, as shown in the figure to the left. These five hydroacoustic stations, called T-phase stations, are located on steep-sloped islands in order to make use of seismic equipment to detect hydroacoustic waves. A hydroacoustic wave travelling through the water and striking the flank of the island is converted to a seismic wave in the ground. These T-phase stations are significantly cheaper than the hydrophone stations, but their performance is not as good (due to losses and complexities in the conversion process). The make-up of the hydroacoustic network is a balance between performance and cost.


All of the eleven IMS hydroacoustic stations are being built as new facilities for CTBT monitoring purposes. Although there were pre-existing stations at three of the locations, the CTBT requirements have necessitated the complete replacement of these stations.

Prior to installation of a station, a site survey is undertaken. The site survey determines such things, for a hydrophone-based station, as where to best locate hydrophones and what is the optimum cable route. The site survey program for the hydroacoustic network is essentially complete.

As of late 2002, there are three complete and operational stations, two of these are hydrophone based and one is a T-Phase station. Work has begun at all of the remaining stations. Several are scheduled for completion during 2003, with the total network is expected to be complete by the end of 2005.

Operation of the stations to CTBT standards leads to very high levels of data availability. The data from all stations will be sent to the CTBT headquarters 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 in principle be expected to provide a unique resource for scientific investigation of various ocean phenomena.

Hydroacoustic Signals

Signals recorded by the hydroacoustic stations are caused by many different events. In the absence of signals from nuclear tests, some of the strongest signals detected come from earthquakes (including undersea earthquakes).

During the time since completing the first stations, many interesting hydroacoustic phenomena have been observed in the data.

The great sensitivity of the hydrophone-based stations has even allowed clear detection of signals that are reflected off continental boundaries. An example of such a reflected path that has been observed in the data is shown here. The event generating the signal is located to the right in the figure, marked with a yellow dot. Paths of detected signals are shown, including one reflecting off Antarctica.

Signals have been detected from unexpected sources. Apparently, icebergs generate low frequency tones. The frequencies are well below those than can be detected by human hearing. These iceberg generated tones vary slowly in pitch and may last for several minutes. The location of generation of the sounds has been correlated with the position of large icebergs. The mechanism of production of the signal is not clear at the present time.

Whales and other biologic sources of sound are detected on the hydroacoustic stations. The daily number of such detections shows seasonal patterns, as the whales undertake their annual migration cycle.

These are just some of the many sources of sound that lead to signals detected by this new global network.

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