Near surface sub-seabed boulder detection using a 3D acoustic profiler

In 2021, Kraken Robotics (formerly PanGeo Subsea) was contracted to conduct a Sub-Bottom Imager (SBI) survey to support a harbour deepening campaign. To perform the dredging safely and effectively in the harbour, all subsea boulders and other obstructions must be identified to minimise the risk of impeding the operation or damaging the dredging equipment. The SBI data was acquired to support the bathymetry data within the channel approach and inner harbour in areas where water depth was recorded under 16 m. Since the channel deepening campaign will dredge up to 2 m below the seabed, the bathymetry data alone is insufficient to identify boulders that lie below the seabed, which introduces a danger of potentially excluding a significant number of obstructions. Data interpretations were performed in four sections based on a priority system, and approximately 36,600 discrete anomalies were interpreted. Using GIS techniques, 11 'blocks' of dense boulder accumulations, '100 boulders per ha at 3 m depth of burial (30,000 m3), were analysed.