AUV-based bed roughness mapping over a tropical reef
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Identifying fixed bed roughness scales of hydrodynamic relevance to waves and currents is challenging around coral reefs due to their highly inhomogeneous bathymetry. In order to characterize the spatial variability in reef roughness, a quantitative analysis of high-resolution sidescan sonar backscatter is performed for the identification of distinct substrates around a tropical reef and is related to echo sounder-based roughness measurements. Data were collected in the vicinity of the Kilo Nalu Observatory on the south shore of Oahu using sidescan sonar and a narrow beam echo sounder incorporated in a REMUS-100 (Remote Environmental Monitoring UnitS) autonomous underwater vehicle (AUV). With basic statistics and principal component analysis of variables derived from the backscatter data, it is possible to discriminate between areas of rough reef, bare reef, and rippled sand. Echo sounder-derived spectral analysis did not reveal dominant length scales. However, by combining the seabed classification obtained from sidescan measurements with echo sounder data, spectral root mean square (RMS) height values of approximately 3.3 cm and 7.3 cm are assigned to the bare reef and rough reef areas, respectively, for roughness with wavelengths between 0.2 and 6 m.
KeywordsSeabed roughness Sidescan sonar Substrate classification Autonomous underwater vehicle
The authors thank Judith Wells, Jonathan Fram, and Kumar Rajagopalan for their helpful suggestions during our weekly meetings. We are indebted to Roy Wilkens and Roger Davis for providing the sidescan preprocessing software and for their assistance in its use. We are also thankful to Jennifer Patterson, Chris Colgrove, Kimball Millikan, and Brian McLaughlin for their support during field operations and to Alyssa Glass for her help with the visual verification of sidescan data. This work was carried out with funding from the Office of Naval Research Coastal Geosciences Program (Grants N00014-07-1-1182 and N00014-10-1-0414).
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