Technical

The technical section presents a brief description of the survey techniques used in the benthic habitat mapping process of seafloor characterisation and classification of habitats. There is a further technical section relating to fine-scale underwater measurements for ecological species analysis and modeling of seagrass, fishes etc.

The techniques are broken in to three main categories: the broad-scale acoustic sea floor characterisation techniques, airborne electro-optical near shore characterisation techniques and fine-scale sea floor sampling techniques. These are referenced to the case studies containing the research developments and results of the work done by the Coastal CRC.

Broad-scale acoustic sensors

Broad-scale acoustic sensors

Acoustic remote sensing techniques are most appropriate for benthic habitat mapping purposes where the water is too turbid or too deep for optical techniques. The choice of which acoustic system is the best for a particular application is discussed in the acoustic system selection section. Developments in sonar technology coupled with improved accuracy and precision of ancillary systems, such as GPS and motion sensors, have enabled acoustic techniques to become useful tools in contemporary marine resource management. Acoustic techniques rely on the connectivity of biological communities with physical parameters to enable the generation of broad scale maps of biological resources. Hence, as for any remote sensing technology, it is important to obtain adequate ground truth information, in order to optimize the interpretation of the sonar images. There are various acoustic systems and combinations of systems that could be employed to facilitate the production of benthic habitat maps, each with varying costs and benefits. Acoustic system selection has to take into consideration the depth of water operating in and the objectives of the project. However, it is usually a compromise between the size of area to be mapped and the time and resources available for the project. Overall, an integrated approach in mapping is best to identify all features.

  1. Single beam echo sounder
  2. acoustic sediment classification
  3. Multi-beam and swath sounders
  4. Side Scan Sonar
  5. Seismic and sub-bottom reflection
  6. List of references

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Airborne electro-optical sensors

Airborne electro-optical sensors

Electro-optical methods are used to characterise relatively large areas (that is, on the order of hundreds to many thousands of square meters). Satellite images and aerial photographs can be used to identify different habitats along the shore and in shallow water where water clarity is not an issue. This type of broad-scale data collection rapidly provides general information throughout an area of interest. The research work done by the CWHM Group does not cover the airborne techniques but focuses mainly on hydroacoustic and underwater video systems, in conjunction with the physical sampling techniques. For more on the electro-optical sensors a Remote Sensing toolkit is available that shows managers, scientists and technicians working in coastal marine environments how images collected from satellites and aircraft (remote sensing) can be used to map and monitor changes to indicators of coastal ecosystem health. This gives excellent information on sensors that measure reflected sunlight (passive systems) and can be found at: The broad-scale electro-optical sensors are categorised and grouped in to the different types of system and parameters measured as follows:

  1. Aerial photography
  2. Hydrographic LADS/ LIDAR
  3. Airborne hyperspectral Imaging
  4. Satellite multiSpectral

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Fine-scale physical sampling

Fine-scale physical sampling

Underwatrer video, underwater still pictures and physical samples of sediments can provide detailed descriptions of sediment quality and life within the seafloor. The biological community can be determined along with a variety of sediment characteristics including concentration of pollutants, sediment texture, and organic content. Since marine organisms are adapted to live in particular habitats, knowledge of these sediment characteristics can provide insight into the physiology and ecology of the organisms sampled.

The fine-scale sensing and physical sampling techniques are categorised and grouped in to the different types of system and parameters measured as follows:

A. Geophysical

  1. Grab Sampling
  2. Gravity drop and piston coring
  3. Sediment vibrocoring

B. Underwater electro-optical sensors

  1. Sediment Profile Imaging
  2. Underwater photography
  3. Underwater video imaging
  4. Laser Line Imaging
  5. Underwater fish measurements

C. Position Fixing

  1. Differential GPS and subsea tracking

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Note: Non-acoustic Techniques for Habitat Classification

The Toolkit has as its focus an emphasis on the use of acoustics in seabed assessment. Most work in seabed assessment to date has made use of underwater photography, point geological sampling and benthic trawls to inform and evaluate acoustic data. Commonly, such non-acoustic techniques, which are relatively slow and labour intensive are combined with the comparatively rapid data acquisition capability, albeit yielding surrogate measures, provided by acoustic instrumentation. Thus acoustic techniques might be favoured in water depths or conditions where, for example aerial photography, or satellite data products are not suitable. It would also be of value to allow for some overlap between areas covered by acoustic and other techniques for purposes of comparison and evaluation. A recent overview of the roles of video techniques in fisheries, including benthic examples is provided by Harvey and Cappo (2000).

Aerial photography, under optimum conditions of wind/seastate, water clarity and sun position appears to allow e.g. macroalgae and seagrass communities to be distinguished in water of up to approximately ten metres depth (Dr. G. Kendrick is an Associate Professor Lecturer at the School of Plant Biology in the University of Western Australia, Perth, Australia). Similarly, detailed spectral analysis, from airborne or satellite sensors may allow classification of substrate type in suitable conditions. It is understood that DSTO Sydney has had some involvement with satellite based multispectral and hyperspectral techniques, in conjunction with Ball AIMS and CSIRO Land and Water (Held et al., 2000).

A variety of other sensor techniques are under development for seabed assessment. One such technique uses comparatively low frequency electromagnetic waves to estimate bathymetry and to provide some information on sediment properties (Vrbanich et al., 2001a, 2001b). This technique appears to be directed more to geophysical exploration requirements than investigation of benthos. The Laser Airborne Depth Sounder (LADS) system, (see e.g. Sinclair et al., 1999) which has shown significant successes in providing rapid bathymetry information in shallow water, does not seem to be configured to provide benthos information. Airborne lidar has similarly been employed for depth determination, and although providing some evidence of reef structures (West and Lilleycrop, 1999) does not appear to have been greatly employed for benthic classification; however this is certain to change with the introduction of the new generation LIDAR systems of companies such as Optech, with their SHOALS system.

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