UNDERWATER

Acoustic Properties of Marine Sediments

The acoustic properties of the top layers of the seabed are important predicators of acoustic propagation in shallow water such as envelopes the continental shelves around Australia. This has a critical bearing on the application of shallow water propagation models and on the operation of low and high frequency sonar systems. Direct measurement of these properties is difficult due to the remoteness of the sea floor, but core samples are often available and these may be used to help characterise the seabed.

The Laboratory can make measurements of sound speed, attenuation constant and shear velocity on sediment cores to precisions of 0.3%, 10% and 20 % respectively. These measurements at frequencies of 90 and 340 kHz, and 1.1 and 5 MHz and application of Biot/Stoll theory enable modelling of the sediments and prediction of their acoustical properties over a wide range of frequencies viz 100 Hz to 10 MHz. See references listed below.

 Water Borne Sediment Suspensions
Monitoring the pollution of water bodies by suspended sediments is a common problem for environmental and marine engineers. The usual measurement procedure to determine this turbidity relies on bottle samples or in-situ optical measurement. Remote based on the reflection of high frequency transmitted sound waves by suspended particles is available and offers the advantages of economy, speed and coverage .

The Laboratory is equipped with acoustic backscatter equipment which may be used to determine the concentration of sediment particles suspended in the water column. The computer controlled equipment, ACP-1, is portable and easily deployed off a vessel. It utilises frequencies of 0.5 and 1.5 MHz with measurement ranges up to 22 m (0.5 MHz) and 2 m (1.5 MHz) and detects particle concentrations in the range 0.01 to 10.0 kg/cu m.

A survey has recently been completed of the acoustic backscatter prevailing in Darwin Harbour and environs (November 1998). Current projects involve the monitoring of harbour dredging spoils and sewage pollution from ocean outfalls.

Geo-Profiling

The geological structure of the seabed is an important consideration in many marine engineering projects such as the siting of oil platforms or pipelines, harbour dredging etc. The upper layers of the sea bed may be explored and mapped using acoustic or seismic techniques such as geo-profiling.

The Laboratory operates equipment, GPY, for shallow water profiling of the seabed sediment structures - with bottom penetration down to 50 m depending on sediment. The high resolution pinger operates at frequencies of 5 kHz upwards and a map of the sub-structure can be produced by passes over the studied areas. The equipment is portable (240 V supply) and easily deployed from a vessel.

Harbour and estuary bottom profiling in Sydney Harbour has been carried out recently.

A narrow beam parametric sonar source (10 kHz) is currently being assessed in sea trials. This has a beam width of 3 or 5 degrees and is ideally suited to small detail mapping of the sub-bottom in shallow water.

References

J.I. Dunlop (1988) Propagation of Acoustic Waves in Marine Sediments: A Review, Exploration and Geophys., 19, 513-535.

J.I. Dunlop (1992) Measurement of acoustic attenuation in marine sediments by impedance tube, J. Acoust. Soc. Am., 91, 460-469

J.I.Dunlop, (1992) Acoustic properties of marine sediments, Acoustics Australia 20, 81-85

P.P.Thomson and J.I.Dunlop, (1995) Acoustic properties of marine sediments in the North West Shelf of Australia, J. Acoust. Soc. Am. 92, 2971-2972

J.I.Dunlop and P Philip Thomson (1996) Determination of the acoustic properties of marine sediments, . Acoust. Soc. Am. 100, 2767 - 2767