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Comparisons are made between bottom categorisations obtained by the RoxAnn and QTC-View (version 4) acoustic bottom classification systems in the Great Barrier Reef lagoon, Australia. RoxAnn classification is based on energy calculations for first and second echosounder returns, while QTC version 4 calculates first echo shape parameters. QTC provides automatic classifications and confidence estimates, whereas RoxAnn relies on arbitrary manual calibration. QTC bottom classes generally had consistent grain size and texture properties, and followed grainsize trends, but RoxAnn classes were difficult to define. QTC classes mapped to well-defined parallelograms in RoxAnn space, indicating with other considerations that the concept of “RoxAnn squares” for RoxAnn calibration is not optimal. RoxAnn performance was strongly dependent on ship speed, but QTC was not. The RoxAnn ship speed dependence was a major limitation, which appears to have primarily affected the low-energy second echo. Both systems experienced problems over rough terrain which could sometimes be monitored by simple depth correlations. The acoustic systems provide useful information, but are not suitable for full stand alone usage. Bottoms should also be classified with conventional techniques, after which acoustic data can be used to fill in the gaps, but with ambiguities; or to indicate areas needing further sampling. Despite the shortcomings of RoxAnn, there are indications it could be used as a remote sensor for bottom porosity and related parameters. |
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