While AusSeabed aims to publish data to the level of adherence based on the requirements stated in the AusSeabed multibeam guidelines (version 1), we will also publish interim products (version 0) that are currently available, but have not yet been standardised (version 1). Users should be aware that V1 products will always supersede V0 products.<br>

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The detailed workflow is presented in Lebrec, 2023 (Lebrec, U., Paumard, V., De Reau, C., O’Leary, M.J., and Lang S.C., 2023, Seismic-derived bathymetry data asset, Australia. [in, preparation]) and the Individual processing steps performed on each survey are reported in the supplementary spreadsheet (see attached zip file). It can be summarized as follow:<br>

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1. Data Collection. All publicly available 3D seismic surveys within the area of interest, including PSTM SGY and P190 files were sourced from Nopims and QA/QC.<br>

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2. SGY processing. The first reflector was extracted using PaleoScantm software from each SGY file. The resulting surfaces are in time domain.<br>

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3. P190 Filtering. Navigation files were filtered to extract depth values at the vessel echo-sounder location. If unavailable, the vessel reference point was selected instead. Depth values within P190 files are in metres, but typically obtained using constant velocities. As such, whenever velocities are specified, depth values were converted back to time for further calibration.<br>

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4. Time\depth conversion. All depth values are converted from time to depth using site-specific synthetic velocity profiles derived from WOA2018 salinity and temperature databases using Doris software. <br>

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5. Gridding. Individual surveys were gridded using inverse distance weight algorithm, and exported as 32-bit floating point Geotif. The bin size is defined as the square root of the average area occupied by a point.<br>

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6. Datum Calibration. All grids and point clouds were converted to WGS84 and reduced to EGM2008. To do so, datasets were reduced to LAT using AHO depth sounding as calibration points before being converted to EGM2008 using AusCoastVDT software. It should be noted that LAT and EGM2008 are regarded as equivalent for areas beyond AusCoastVDT coverage, that are typically in water depth of more than 400m (Fig. 2).<br>

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Notes about the spatial and vertical accuracy of the surveys.<br>

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• The vertical accuracy of the reflection-derived bathymetry decreases rapidly in water depths of less than 150 m. Morphologies become vertically distorted and the absolute depths unreliable. In such water depths, values from the navigation-derived bathymetry should be preferred.<br>

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• Navigation-derived bathymetry data transects are often spaced by 500 to 1,000 m, corresponding to the vessel path. Resulting rasters therefore have cell size varying between 50 and 100 m to limit gridding artefacts. When using navigation-derived bathymetry grids, refer to the xyz files for precise measurements.<br>

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• The vertical accuracy could not be precisely determined for each individual survey due to the lack of comparative datasets: MBES data ( http://dx.doi.org/10.26186/5c63832e3ed8e) are sparse and only intersect a few 3D seismic surveys. ENC depth soundings were already used as part of the calibration process and can therefore not be used as an independent dataset. As a result, the vertical accuracy was evaluated using a merged of all MBES datasets available in the region against a merge of the seismic-derived bathymetry grids (Fig. 2). The resulting average vertical accuracy is of 11.51 m (mean average error), which qualifies for the hydrographic accuracy of 1 m +2%d. This value can however vary significantly between surveys. It should be noted that it is often possible to observe a constant vertical offset between MBES and seismic-derived bathymetry grids (Fig. 2) The vertical accuracy of the seabed features relative height is therefore expected to be much higher.<br>

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• ENC depth soundings often show inconsistent values. For example, the mean average difference between ENC depth soundings and MBES depth values is of 10.92 m (Fig. 3). ENC depth soundings are however the only dataset available throughout the area of interest to be used for vertical calibration. This uncertainty was factored in by integrating neighbourhood surveys as well as MBES values whenever possible. <br>

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• 3D Seismic surveys are initially acquired and processed for Oil and Gas exploration to image targets located thousands of meters below the seabed. It is therefore common to observed geometrical distortions at the seabed (e.g., vertical offsets between adjacent surveys are not constant). Such artefacts could not be easily corrected due to the lack of reference surfaces (i.e., ENC datapoints are not dense enough to perform such analysis and Nav datapoints are often not tide corrected). This effect is the main contributor to the vertical uncertainty of the datasets and, as a result, vertical offsets of up to tens of meters can be locally observed between adjacent surveys.<br>