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The International Monitoring System (IMS) hydroacoustic network is designed to monitor nuclear explosions in the oceans and at low altitudes above the surface of the oceans. The network consists of 6 hydrophone stations and five ‘T-phase’ stations that have been strategically located (typically on small islands) to complete the coverage of ocean monitoring. Excellent coverage of the oceans is possible from so few stations as energy propagates very efficiently across the oceans by travelling in the sound fixing and ranging channel (SOFAR channel) where attenuation and absorption of energy is minimal. The hydrophone stations themselves are purposely suspended in the SOFAR channel to record very low signal levels. For example a few kilogram charge exploded in the SOFAR channel off (say) South Africa would be clearly recorded on the hydroacoustic station at Cape Leeuwin, southwest Western Australia.

A hydrophone station is comprised of three elements:
An array of three hydrophones suspended in the SOFAR channel from a cable attached to the sea floor. The hydrophones sample vibrations in the water at a high sampling rate of 250 samples/second.

A seabed cable that carries power and data between the hydrophone array and a central shore facility. The cable is laid on the sea floor, anchored at some points and buried where necessary.

A central shore facility, which houses both electronic and electrical equipment to power the hydrophone array, record the data, reformat and transmit the data via satellite link to the IDC in Vienna. Commonly the data is also provided to the station operator on a separate communications link

A ‘T-phase’ station is a three-component seismic station (refer to seismic monitoring – above) that is typically located on a small island. It is capable of monitoring explosions in the water by recording signals that have either:

  • Travelled in the SOFAR channel to the island and converted to seismic energy at the ocean-island interface
  • converted to seismic energy at the ocean floor boundary close to the source. Data is transmitted via satellite link to the IDC in Vienna.

Signals from a number of sources have been observed on the hydrophone stations. These include, inter alia, continental and oceanic earthquakes, mid-oceanic volcanic activity, underwater landslides, whales, shipping noise and offshore exploration surveys.

If a nuclear explosion signal was recorded on a hydroacoustic station, distinct signal characteristics would be observed. The recorded signal would contain energy over a wide bandwidth (up to 100Hz), and scalloping in the frequency spectrum that indicates the generation of a bubble pulse at the source. 
Schematic of a hydrophone station. Schematic of a T-phase station. Spectrogram of fin whale calls over a period of 4 minutes recorded at HA01. The calls typically begin with strong signal intensities at 23 Hz, and downsweep to 15 Hz. The pulses are separated by approximately 20 seconds.
Schematic of a hydrophone station.
© Geoscience Australia
Schematic of a T-phase station.
© Geoscience Australia
Spectrogram of fin whale calls over a period of 4 minutes recorded at HA01. The calls typically begin with strong signal intensities at 23 Hz, and downsweep to 15 Hz. The pulses are separated by approximately 20 seconds.
© Geoscience Australia
Part of spectrogram of a ship recorded at HA01. The signal intensity lies between 30 Hz and 70 Hz, and has a rhythmic pattern. Spectrogram of possible volcano-seismic activity recorded at HA01. The signals are repetitive – 80 seconds long separated by 60 second intervals. Spectrogram of the T-phase from the Ml 5.2 Burakin (WA) earthquake recorded at HA01. The signal has an emergent start, has a spindle-shaped envelope, and all energy is below 30 Hz with highest intensities between 1 Hz and 10 Hz.
Part of spectrogram of a ship recorded at HA01. The signal intensity lies between 30 Hz and 70 Hz, and has a rhythmic pattern.
© Geoscience Australia
Spectrogram of possible volcano-seismic activity recorded at HA01. The signals are repetitive – 80 seconds long separated by 60 second intervals.
© Geoscience Australia
Spectrogram of the T-phase from the Ml 5.2 Burakin (WA) earthquake recorded at HA01. The signal has an emergent start, has a spindle-shaped envelope, and all energy is below 30 Hz with highest intensities between 1 Hz and 10 Hz.
© Geoscience Australia
Spectrogram of airgun shots recorded at HA01. Note the high frequency content of the signals.

Spectrogram of airgun shots recorded at HA01. Note the high frequency content of the signals.
© Geoscience Australia



Topic contact: hazards@ga.gov.au Last updated: July 19, 2011