Underwater Acoustic Printable Version
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Seabed Mapping • Sound Waves • Mapping Techniques • Uses • INSS
Seabed Mapping

Acoustic and seismic techniques are widely used in mapping the seabed in three dimensions (3-D). Both methods are principally based on the relection and refraction of sound waves by the water column, seabed surface and sub-seabed strata. All of these contribute to the recorded signal and hence affect the final interpretation.
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Sound Waves

Sound travels in the form of waves whose basic components are amplitude, wavelength, and frequency. The frequency of a sound wave is the rate of undulation of the wave, and is measured in cycles/sec or Hertz (Hz). Humans can only hear sound waves with frequencies between 20-20,000Hz. Sounds below 20Hz are referred to as infrasonic, and above 20,000Hz as ultrasonic. The speed of sound in the water column is around 1500m/s and in the air is around 340m/s.
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Mapping Techniques

The two main techniques for mapping the seabed can be considered under the headings of acoustic and seismic.

Acoustic
side-scan sonar
GeoAcoustic dual frequency side-scan sonar system depth rated to 2000m.
Acoustic mapping techniques are mainly used for mapping the depth of the seabed and to give information on the type of seabed and its morphology. These techniques principally measure the time it takes for a sound wave emitted by the instrument to be reflected from the seabed, in order to determine the depth. Characteristics of the signal returned such as signal strength are used to determine differences in seabed properties. Different types of seabed will have different characteristics such as hardness, roughness, density and porosity, each of which will modify the reflected acoustic signal.

The most commonly used acoustic mapping techniques include side-scan sonar, multibeam and single beam echosounders. In addition to seabed mapping, some echosounders allow acoustic evaluation of the water column and therefore are used for identifying fish shoals (e.g. fishing echosounders).
Seismic
There are certain similarities between acoustic and seismic techniques in that both emit sound pulses and collect the echoes. Seismic mapping, however, provides information on the structures below the seabed. In other words, seismic techniques are able to identify the geological structure of sub-surface seabed sediments and underlying rocks. To do this they use lower frequencies than those used in seabed surface mapping. The signal generated by specialised equipment penetrates the seabed and reflects back with a speed and characteristics dependant upon the sound velocity properties of particular geological layers. Seismic techniques vary by spatial resolution and depth of penetration into the seabed. They are commonly used for general geological mapping (to produce maps of solid geology) and for identifying hydrocarbon reservoirs (e.g. gas and oil).

TOBI
TOBI side-scan sonar vehicle onboard RV Pelagia M2002 cruise (June-July 2002).
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Uses of Acoustic Maps

Generally speaking, there are three main uses of underwater acoustic maps:
  1. General seabed mapping (e.g. habitat mapping; determination of sediment type and bedforms distribution; seabed processes).

  2. Underwater Archaeology (e.g. mapping shipwrecks).

  3. Evaluation of the seabed and sub-seabed resource potential (e.g. sand & gravel, oil & gas).
Side-scan Sonar Mapping of the Irish Coral Reefs
corals
These corals were photographed to the west of Ireland by a deep water video camera at a depth of approximately 800m.


Researchers from UCC have performed detailed mapping of Irish cold-water coral habitats mainly located on the continental slope of the Porcupine Seabight and Porcupine Bank located west of Ireland. Cold-water coral reefs in Irish waters occur in the form of carbonate mounds, which vary in shape and size. The main reef-forming coral species are Lophelia pertusa and Madrepora oculata, which are associated with numerous epifauna. Mapping with high-resolution side-scan sonar supported with underwater video truthing allows a detailed assessment of the current status of these reefs.
Large area seabed mapping is normally performed with multibeam or low frequency side-scan sonar. Bathymetry or depth is the main information that can be derived from multibeam surveys. This allows the construction of three-dimensional terrain models of the seabed. The integration of the multibeam data with other datasets (e.g. side-scan sonar, underwater video) allows the assessment of seabed processes from a three-dimensional perspective. The large-scale multibeam mapping of Irish territorial waters (525,000 km2) has recently been undertaken by the Irish National Seabed Survey (INSS) team managed by the Geological Survey of Ireland (GSI) and the Marine Institute.

Side-scan sonar mapping allows imaging of various aspects of the seabed and provides details on the bedforms distribution, topography and seabed processes. Side-scan sonar is used in habitat mapping and in underwater archaeology. Particularly, side-scan sonar has been extensively used for mapping cold-water coral habitats. Operating at a frequency of 100-400kHz the side-scan sonar is able to detect coral populations and identify sediment transport and near-bottom current patterns.

The side-scan sonar imagery obtained allows detailed planning for higher resolution ground-truthing surveys, which can involve the collection of physical samples from the seabed or underwater video. These surveys are usually carried out by a Remote Operated Vehicle (ROV) or submersible, capable of operating in very deep water. Collective interpretation of all datasets obtained allows the construction of maps of the surveyed areas of seabed. This helps in the assessment of the seabed resource potential and provides information that can aid habitat conservation and sustainable exploitation of these resources.

carbonate mounds

This three dimensional view shows an area of coral colonised carbonate mounds and sedimentary processes on the continental slope of the eastern Porcupine Seabight, in the North East Atlantic,at a depth of approximately 700 to 1000m. The white lines indicate depth as determined using a multi-beam, while the different grey colours represent the image captured with a side-scan sonar operating at 100 kHz.
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Irish National Seabed Survey

Only very general maps of the seabed within Ireland's offshore territory existed until the recent completion of the Irish National Seabed Survey (INSS). The survey was funded by the Irish Government at the cost of €32 million and was managed in partnership by the Geological Survey of Ireland (GSI) and the Marine Institute. The project which ran from 1999-2005 has mapped all waters deeper than 200m, which represents over 87% of the offshore territory. The survey has returned detailed bathymetry (depth) data and information on the seabed's characteristics.
Click here to view a map of the area surveyed in the INSS.

A number of ships took part in the survey including the Celtic Explorer and Celtic Voyager national research vessels. Multibeam sonar was the primary acoustic technique used to carry out the survey, although additional equipment including seismic sub-bottom profilers were also used to collect ancillary information. The data collected is proving indispensable for the update of navigation charts, offshore activities, research, and policy formulation regarding development of Ireland's marine resources. The successor programme to the INSS is known as INFOMAR and its primary focus is on mapping inshore waters. In addition to acoustic techniques, airborne sensors, such as LIDAR, will be used to complete this mapping.


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