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Mapping Techniques: Acoustics

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Summary Bathymetry Surveys Side-scan Sonar

Sub-bottom Profiling Acoustic Seafloor Classification Systems (ASCS)

Acoustic Seafloor Classification Systems

The RoxAnn Ground Master unit. Courtesy: U.S. Environmental Protection Agency

Acoustic seafloor classification systems (ASCS) process the acoustic return signals from standard single-beam echosounders, and can be used to make qualitative estimates of the seabed composition. They gather information about bottom type, bottom sediments, and aquatic plants. Different seabed bottom types can be discriminated by extracting data on bottom roughness (that is, irregularities in topography) and hardness (that is, type of substrate – rock, sand, mud, and so forth). There are several commercially available acoustic seafloor classification systems, such as the RoxAnn^TM and QTC View^TM systems.

Acoustic seafloor habitat characterization requires field verification. This can be done through either physical sampling of the bottom using sediment cores or grabs or through visual observations by divers or underwater cameras. All types of substrate encountered must be verified to interpret the data accurately and link the acoustic signatures to the seabed classification scheme. Extensive fine-scale sampling may be required, especially where the seafloor is complex. Additionally, these systems require initial calibration in each unique study location in order to interpret the signal returns and classify benthic cover types.

Post processing of an acoustic data series showing depth changes and signal strength relative to seabed classes (indicated by different colors). Courtesy: Maine Department of Marine Resources

Advantages and Limitations

There are some advantages to using an acoustic seafloor classification system. These systems are commonly available at relatively low cost. They are portable, so they can be deployed on boats of opportunity, and they have minimal power requirements. They are also capable of collecting data quickly.

These systems are highly dependent on field calibration, so the data they produce are very difficult to interpret without physical sampling, video, or imagery collection. Additionally, their narrow swath width makes continuous coverage of the seafloor difficult, and their acoustic "footprint" is relatively small and dependent on depth.

Uses

ASCS are used in a variety of benthic habitat assessment projects. Delaware Coastal Programs are characterizing benthic habitats in inland bays using a seabed classification system. Through analysis of the acoustic echoes, the system can distinguish between sand, mud, shell, clams, rocks, and algae found on the seafloor. Detailed maps of the bottom substrate and bathymetry will help to manage the benthic resources in the bays.

How can ASCS data be used for habitat mapping?

Maine Department of Marine Resources is using ASCS to help characterize and map juvenile fish habitats, and the Cooperative Oxford Laboratory's Mapping and Analysis Project (COLMAP) is charting benthic habitats in the Chesapeake Bay. They are evaluating the use of ASCS to determine the quality of oyster habitats.

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