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Practical info

Analysis of acoustic data for reef indicators by Timo Gaida - overview

Acoustic sensors, such as the multibeam echosounder (MBES) and side-scan sonar (SSS), allow to map large areas of the seabed with a high resolution (~decimeters) in a relatively short time period (Figure 1). It provides a remote opportunity to quantify sediment distributions, to discriminate between habitats or identify natural features (e.g., boulders, biogenic reefs) and manmade structures (e.g., pipelines, shipwrecks).

While the SSS is more suitable to detect distinct features on seabed, the MBES allows to produce objectively and automatically large-scale and detailed sediment or habitat maps of the seabed. Ideally, an investigation of the seabed comprises the use of both sensors. If budget, study area or time limitations only allow the use of a single sensor, it needs to be evaluated based on the project purpose and in a preliminary investigation, which sensor should be used.



Advantages and disadvantages of MBES and SSS

SSS

  • Higher resolution
  • Suitable for detection of elevated features (obstacles, boulders, vertical reef growth)

MBES

  • Provides in addition to backscatter (~acoustic reflectivity) bathymetric measurements
  • Precise georeferencing (centimeters to decimeters (MBES); rather meters for SSS)
  • Suitable for automatic and objective sediment or habitat map production (improved processing capabilities)

Ground truthing, via bed sampling or video footage, and a priory geological information support the verification of the acoustic results. Depending on the desired end-product they are even necessary for an appropriate interpretation of the acoustics. Another crucial aspect is the acoustic sensitivity of the seabed. For example, mud generates a very different acoustic pattern than gravel, whereas a mud containing a high amount of shell fragments might result in a similar acoustic response. This example also demonstrates the importance of ground truthing or a-priory knowledge of the study area in order to avoid misleading interpretations of the acoustics.

The graphical chart below illustrates the individual steps on how an acoustic mapping campaign should be planned and executed.

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