Comparison of bathymetry estimated from multibeam eco-sounder and optical data

Accurate bathymetry estimation is crucial for various marine and coastal applications, including resource management, navigation, and environmental protection. This study presents a comparison, on sandy bottom, of bathymetry estimated using optical satellite imagery versus bathymetric data acquired through a multibeam instrument. The study area was the Gulf of Sciacca (on the southern coast of Sicily island, Italy, Mediterranean basin). High-resolution PlanetScope SuperDove satellite images were preprocessed to obtain reflectance just below the water surface. Then, a simplified version of the original radiative transfer equation from Jain and Miller (1977) was used to estimate water depth. The model was calibrated and then validated using high-res multibeam data through a k-fold cross-validation. Multibeam data were acquired using a Reason 8125 multibeam Eco-sounder while PDS2000 software provided the functionalities for survey planning, data processing and editing. Multibeam data reveal in the western sector the presence of structural highs, shoals, rocky substrate and Posidonia oceanica meadows extending even beyond ⁓30 m, while, the easternmost sector from Verdura delta up to Capo Bianco shows the presence of sedimentary structures, prairies of Posidonia oceanica and Cymodocea nodosa, and rocky substrate of different nature. Bathymetry was estimated from optical data from the shoreline down to ⁓26 m water depth; while approaching this depth, the reflectance assumed an asymptotic value. The results show a strong correlation between the depths estimated from satellite images and those obtained from multibeam data, with a root mean square error lower than 11% with a confidence interval of 95%. However, some discrepancies were observed in areas in mixed pixels (with submerged vegetation, complex seabeds) or reflectance surface effects (glint, waves due for instance to moving boat wakes).