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051296B.OAC + Source: ONR Asia +
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Contributory Categories: BIO,ENG,ENV
Country: Australia
From: DSTO Aeronautical and Maritime Research Laboratory
PO Box 4331
Melbourne Victoria 3001
Telephone: (03) 626 8111
Fax: (03) 626 8999
@ Commonwealth of Australia 1995
AR No. 009-213
March 1995
KEYWORDS: Australia, Spencer Bay; Acoustic survey and Range,
ambient noise, biogenic acoustic noise
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Part I/III
SITE SURVEY FOR AN OCEAN ENGINEERING PROJECT IN
SPENCER GULF NOVEMBER 1993
Ian S.F. Jones, Douglas H. Cato, L.J. Hamilton,
Sandra Tavener and B.D. Scott
Maritime Operations Division
Aeronautical and Maritime Research Laboratory
DSTO-TR-0149
ABSTRACT
Environmental surveys were made by DSTO at three sites in
Spencer Gulf in South Australia in November 1993 to assess their
suitability for an Underwater Radiated Noise Range for the Royal
Australian Navy. Acoustic ranges are required to measure the
noise radiated from ships and submarines. Salient factors for
range design and performance include ambient noise, currents,
intemal waves, topography and nature of the seafloor, water
properties, and wind and weather conditions. Measurements of
these parameters indicate that the shallow waters of Spencer Gulf
are particularly quiet compared with the open ocean around
Australia. Swell is significantly less than in the ocean to the
south where it originates, due to attenuation by passage through
shallow water, and the sheltering effect of islands at the gulf
mouth. Currents are predominantly fidal and thus predictable,
with periods of up to 5 days at neaps with speeds less than 0.4
knot. The main disadvantage of the shallow waters is the
continual noise background of snapping shrimp. Spencer Gulf
appears to be suitable for the placement of a shallow water
acoustic range. A Thistle Island site was marginally more
suitable than a site near Wedge Island, and both were
significantly better than a site near Corny Point.
RELEASE LIMITATION
Approved for public release
EXECUTIVE SUMMARY
Following DSTO's recommendations of potentially suitable
sites for an acoustic range, the RAN requested DSTO to conduct a
survey of the most promising shallow water site: the region
inside the mouth of Spencer Gulf, South Australia, water depth 40
m. An acoustic range is designed to measure the noise radiated
from ships and submarines. The site must be sufficiently quiet
to allow the signals to be adequately measured, while the
currents, the surface wave field, the nature of the sea floor,
and the topography need to be known for the engineering design of
the system. Also of importance are the prevailing weather
conditions and the water properties. The survey thus involved
the collection and interpretation of data to assess these
environmental parameters.
Three sites were surveyed, one 7 miles NE of Thistle Island,
one 5 miles NE of Wedge Island and one 5 miles @W of Comy Point.
Currents are predominantly tidal and thus predictable.
Current speeds predicted from harmonic constants are in good
agreement with values measured for short intervals of 7 to 10
days. Peak speeds in the semidiurnal cycle vary over a period of
about two weeks, from negligible to almost I knot, with periods
of up to five days at neap tides with speeds less than 0.4 knot.
Current directions near surface and near bottom were generally
similar for speeds over 0.4 knot.
Spencer Gulf is particularly quiet compared with the open
ocean around Australia. The noise of distant shipping is very
low. Biological noise is evident only for limited periods apart
from the continuous noise of snapping shrimp which is significant
above about 2 kHz. The ambient noise is dominated by wind
dependent surface generated noise. For 40% of the time the noise
is expected to be less than that indicated by the 10 knots curve
of the summary graph over page (by comparison, the wind is less
than 10 knots off Perth, which has also been considered for this
facility, for only 14% of the time). The "noise floor", the
quietest conditions, is 10 to 15 dB less than off Perth between
20 Hz and 2 kHz. For wind speeds less than 10 knots, the noise
is 7 to 20 dB less than off Perth or Sydney for frequencies
between 20 and 100 Hz, and 0 to 15 dB less between 200 Hz and 5
kHz. The effect of these lower noise levels on system
performance is comparable to or greater than the gain achievable
by a complex hydrophone array system. Sites off Thistle and
Wedge Islands show comparable ambient noise. The site off Comy
Point tends to be significantly noisier for a significant
proportion of the time.
Significant transient noise was evident at low frequencies
from movement in the vicinity of the hydrophones on the sea
floor. The transients occurred intermittently over periods of a
few hours but did not appear to be correlated with the tidal
currents. Since this is thought to be due to movement of debris
against the recording system it is not considered to be part of
the ambient noise and so not included in the summary graph. The
design of an acoustic range would need to include isolation of
the hydrophones from such motion. It is recommended that this
effect be investigated further at the site.
Swell in Spencer Gulf is significantly less than in the
ocean to the south where it originates, due to attenuation by
passage through the shallow water. The direction is
predominantly from south to south west. The Thistle Island site
is the most sheltered, and showed little evidence of swell during
the survey. The significant wave height over four weeks was
almost always less than 2 m, and less than 1 m for 35% of the
time.
The analysis of the currents, noise, and swell indicates
that the site off Thistle Island is marginally more suitable than
the site off Wedge Island, and both are significantly more
suitable than the site off Comy Point for the siting of an
acoustic range.
1 INTRODUCTION
The RAN requires an Under Way Radiated Noise Range site for
their vessels. Following an examination of existing data,
Spencer Gulf (Fig 1. 1) (overleaf) was recommended by DSTO as a
potential site for shallow ranging. The RAN then requested DSTO
to carry out a site survey of Spencer Gulf. Three sites were
chosen as potentially suitable for noise ranging in Spencer Gulf
based on the criteria (a) adequate depth (40 m) for boat
manoeuvring over an area 5 miles by 2 miles with no shoals; (b)
close to shore to minimise cable runs; (c) the presence of
conspicuous land marks for navigation. The sites, shown in Fig
1.2 (on page 9), are listed below :
Site A: BERRY. 5 miles WNW of Comy Point 34'52.6'S, 136' 55.2'E
Site B: WEDGE. 5 miles NE of Wedge Is. 35006.6'S,
1360 33.6'E
Site C: THISTLE. 7 miles NE of Thistle Is. 34056.8'S,
1360 17.2'E
The purpose of this survey is to provide the following data
to determine the suitability of each site for noise ranging, and
particularly to compare sites.
(a)Ambient Noise. This provides the main acoustic limit on
system performance. Measurements need to be related to wind
speed statistics and shipping movements to allow noise to be
forecast in terms of the long term wind statistics and projected
shipping movements.
(b)Wind Speed measurements are needed at each site for
correlation with the ambient noise measurements and the wind
measurements at weather stations.
(c)Swell And White Cap Coverage. The swell affects the handling
of the submarines and stability of acoustic systems. 'ne white
cap coverage relates to the ambient noise since the white caps
are the main source of wind generated noise for frequencies above
about 5 Hz.
(d)Currents limit the extent that acoustic arrays will maintain
their geometry and the accuracy that boats can maintain the
required path during the sound ranging runs. High currents have
the potential to damage arrays, and to cause unacceptable levels
of flow noise. It is desirable to relate currents to tide and
other sources to allow forecasts to be made.
(e)Bottom Samples provide mechanical information about the bottom
needed to design system moorings. The nature of the bottom also
affects the acoustic bottom reflectivity, which in turn affects
the acoustic propagation.
(f) Vessel Movements through the Gulf during the period of noise
recording are needed to relate to the noise measurements.
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End Part I/III
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