Remote sensing of seabed types in the Australian South East Fishery; development and application of normal incident acoustic techniques and associated 'ground truthing'

Calibrated acoustic backscattering measurements using 12, 38 and 120 kHz were collected over depths of 30-230 m, together with benthic epi- and in-fauna, sediments, photographs and video data. Each acoustic ping was envelope detected and digitized by echo sounder to include both the first and second echoes, and specifically designed software removed signal biases. A reference set of distinct habitat types at different depths was established, and a simple classification of the seabed combined both biological and geological attributes. Four seabed types were identified as having broad biological and geological significance; the simple acoustic indices could discriminate three of these at a single frequency. This demonstrates that the acoustic indices are not directly related to specific seabed properties but to a combination of seabed hardness and roughness attributes at a particular sampling frequency. The acoustic-derived maps have greater detail of seabed structure than previously described by sediment surveys and fishers' interpretation. The collection of calibrated digital acoustic data at multiple frequencies and the creation of reference seabed sites will ensure that new shape- and energy-based feature extraction methods on the ping-based data can begin to unravel the complexities of the seabed. The methods described can be transferred to higher-resolution swath-mapping acoustic-sampling devices such as digital side-scan sonars and mull-beam echo sounders.