Sub-bottom profilers

Acoustic Sub-Bottom Profiling (SBP) systems are used to determine physical properties of the sea floor and to image and characterise geological information a few metres below the sea floor. In recent years, sub-bottom profilers have been used to measure small scale sedimentary structures and processes in high temporal and spatial resolution. The systems have been widely adopted by marine researchers because of their ability to collect data rapidly and non-intrusively.

Sub-bottom profilers are usually comprised of single channel source that sends sound pulses into the shallow sub-sea floor sediments. The sound pulses bounce off the sea floor and subsequent buried sediment layers according to differences in their acoustic impedance (hardness). Acoustic impedance is related to the density of the material and the rate at which sound travels through this material. The different times taken for this signal to be returned and recorded by the sub-bottom profiler indicate how deep the layers are below the sea floor. The surface of the different rock strata beneath the sea floor are mapped over the study area.

Figure 1. Deployment of various shallow-water sub-bottom profiling systems. Image from Stoker et al. (1997)

Sub-bottom profiler system

There are a number of shallow SBP systems which operate using various types of sound sources and frequencies (Table 1). Different SBP systems are used depending on the objectives of the survey, water depths and prior knowledge of the rock types (if known). The 'Pinger' is a high frequency system which operates on a range of single frequencies between 3.5 kHz and 7 kHz. Depending on various factors, such as the type of sediment and the sound source characteristics (frequency, power), SBPs can achieve sea floor penetration from just a few meters to more than 50 m and vertical resolution (layer thickness) down to approximately 0.3 m. SBPs are particularly useful for delineating shallow features such gas accumulations, buried channels. The non-linear parametric sub-bottom profilers simultaneously transmit two signals of slightly different high frequencies (e.g. 100 and 110 kHz). Their interaction generates by interference a new low-frequency signal (with the difference frequency). They can achieve very high vertical resolution and are particularly good to use in shallow water environments.

Table 1. Specifications of common SBP systems.

Sub-bottom profiler systems

Common Frequency Range

Penetration depth

Parametric

~100 kHz

< 100 m, vertical resolution < 0.05 m

'Chirper'

3 to 40 kHz

< 100 m, vertical resolution ~0.05 m

'Pinger'

3.5 to 7 kHz

10 m to 50 m, with vertical resolution of 0.2 m

'Boomer'

500 Hz to 5 kHz

30 to 100 m, with vertical resolution of 0.3 to 1 m

'Sparker'

50 Hz to 4 kHz

To 1,000 m in ideal conditions, with vertical resolution of >2 m

Sub-bottom profiler data

Sub-bottom profile data is used for a range of purposes including:

  • Three-dimensional imaging of the sea floor shallow sub-surface and sediment layering.
  • Assessment of environmental considerations for marine geological resource management including the identification of buried geohazards, such as underwater landslides, gas seepage.
  • Environmental management, including establishing baseline data to support environmental monitoring.
  • Classification maps for seabed habitats.
  • Sea floor sedimentary environments for developing models of benthic environments and habitats.

Figure 2. Boomer sub-bottom profiles of the sea floor around the Whitsunday Islands, Great Barrier Reef platform, Australia (after Heap, 2000). The system used was an EG & G TM Uniboom sounder, triggered every 0.5 s at 200 J, and towed 0.3 m below the surface 11 m behind an 8 element hydrophone array. (A) The reflectors reveal a range of recent, Holocene, and pre-Holocene features, showing an exposure of bedrock surrounded by recent sand accumulations. (B) Steeply NE dipping bedding structures (clinoforms) and surficial dune bedforms record the accumulation and present-day movement of sand into a depocentre.

References

Heap, A.D. (2000). Composition and dynamics of Holocene sediment next to the Whitsunday Islands on the middle shelf of the Great Barrier Reef platform, Australia.PhD Thesis, James Cook University, Townsville, Queensland. 116pp.

Stoker, M.S., Pheasant, J.B., Josenhans, H. (1997). Seismic methods and interpretation. In T.A. Davies, T. Bell, A.K. Cooper, H. Josenhans, L., Polyak, A. Solheim, M.S. Stoker, J.A. Stravers (Eds.), Glaciated Continental margins: An Atlas of Acoustic Images. Chapman and Hall, London, 1997. 315pp.

Penrose, J.D., Siwabessy, P.J.W., Gavrilov, A., Parnum, I., Hamilton, L.J., Bickers, A., Brooke, B., Ryan, D.A., Kennedy, P., 2005. Acoustic techniques for seabed classification. Cooperative Research Centre for Coastal Zone Estuary and Waterway Management, Technical Report 32.