On 23 December 1999 the Minister for Foreign Affairs, Mr Downer, issued
a press release welcoming the conclusion of a contract valued in excess of $10
million to establish a hydroacoustic monitoring station off Cape Leeuwin for the Comprehensive
Nuclear-Test-Ban Treaty (CTBT). This
station will be one of only eleven CTBT stations around the world listening for signs of an underwater nuclear
explosion. Its location off the
south-west corner of Australia is critical to effective monitoring of
significant parts of both the Indian and Southern Oceans.

The heart of the Cape Leeuwin station will be an undersea
triplet of hydrophones designed to detect the sounds generated by explosions,
and in particular by any nuclear explosion, that may be carried out at or below
the ocean surface. Overall, the station comprises three main elements:

• a triplet of
  hydrophones located approximately 114 km south-west of Cape Leeuwin in 1590
  metres of water. The hydrophones will
  be at a depth of about 1100 m and will be suspended from a cable attached to
  the sea floor;
• a shore facility
  which powers the hydrophone array and records and transmits data from it via a
  satellite link (and possibly by land-line) to analysis centres in Vienna and
  Canberra; and
• a seabed cable (2-3 cm diameter) to carry power
  and data between the hydrophone array and shore facility. The cable would be laid on the sea floor,
  anchored at various points (and buried where possible). The few kilometres closest to shore would be
  laid in a split pipe fixed to rock.
  This should minimise the risk for accidental damage from marine traffic.

Hydrophone arrays such as this are extremely sensitive listening
tools. Sound travels very efficiently
through water, but this is especially so through part of the ocean called the
SOFAR channel:

• the change in the
  temperature and pressure of water with depth works to refract sound waves into
  a particular channelwhich in the deep oceans is about 1km down;
• by placing hydrophones in this channel, it is
  possible to pick up sounds at very large distances.

The hydroacoustic
signal generated by an underwater nuclear explosion has certain characteristics
which assists its identification. The
appearance of a bubble pulse (from expansion and then contraction of the gas
bubble formed by the explosion) is a strong indicator. The frequency mix of sound generated by an
explosion is a good indicator also, as is a rapid rise time when the signal
first arrives.

Establishment and
operation of the CTBT's International Monitoring System (IMS) is being
co-ordinated and financed internationally by the Provisional Technical
Secretariat of the CTBT Organisation (CTBTO-PTS)based in Vienna. Co-ordination of the development of
Australian IMS facilities is the responsibility of ASNO. In 1998 the CTBTO-PTS commissioned CSIRO to
carry out a survey of the site for the Cape Leeuwin station. The
contract to establish the station has been concluded with the US firm Science
Applications International Corporation (SAIC), with Nautronix Ltd of Perth (WA)
as their major sub-contractor.
Installation of the station is planned during 2001.

The potential for data
from the IMS to contribute to scientific and humanitarian activities is an
important element of the CTBT. Data
from the Cape Leeuwin station could contribute to monitoring of global warming
through a CSIRO project to accurately measure the speed of sound in the Indian
Oceanand thereby ocean temperature.

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