A Hybrid Bio-Optical Transformation for Satellite Bathymetry Modeling Using Sentinel-2 Imagery

The article presents a new hybrid bio-optical transformation (HBT) method for the rapid modelling of bathymetry in coastal areas. The proposed approach exploits free-of-charge multispectral images and their processing by applying limited manpower and resources. The testbed area is a strait between two Greek Islands in the Aegean Sea with many small islets and complex seabed relief. The HBT methodology implements semi-analytical and empirical steps to model sea-water inherent optical properties (IOPs) and apparent optical properties (AOPs) observed by the Sentinel-2A multispectral satellite. The relationships of the calculated IOPs and AOPs are investigated and utilized to classify the study area into sub-regions with similar water optical characteristics, where no environmental observations have previously been collected. The bathymetry model is configured using very few field data (training depths) chosen from existing official nautical charts. The assessment of the HBT indicates the potential for obtaining satellite derived bathymetry with a satisfactory accuracy for depths down to 30 m.

4SM Method Tested in the Gulf of California Suggests Field Data are Not Needed to Derive Satellite Bathymetry

Satellite derived bathymetry methods over coastal areas were born to deliver basic and useful information like bathymetry. However, the process is not straightforward, the main limitation being the need of field data. The Self-calibrated Spectral Supervised Shallow-water Modeler (4SM) method was tested to obtain coastal bathymetry without the use of any field data. Using LANDSAT-8 multispectral images from 2013 to 2016, a bathymetric time series was produced. Groundtruthed depths and an alternative method, Stumpf’s Band Ratio Algorithm, were used to verify the results. Retrieved (4SM) vs groundtruthed depths scored an average r2 (0.90), and a low error (RMSE = 1.47 m). Also 4SM showed, over the whole time series, the same average accuracy of the control method (40%). Advantages, limitations and operability under complex atmosphere and water column conditions, and high and low-albedo bottom processing capabilities of 4SM are discussed. In conclusion, the findings suggest that 4SM is equally accurate as the commonly used Stumpf’s method, the only difference being the independence of 4SM to previous field data, and the potential to deliver bottom spectral characteristics for further modelling. 4SM represents a significative advance in coastal remote sensing potential to obtain bathymetry and optical properties of the marine bottom.

Exploring the Utility of Bathymetry Maps Derived With Multispectral Satellite Observations in the Field of Underwater Archaeology

Bathymetry maps derived with satellite-based multispectral sensors have been used extensively for environmental and engineering coastal studies and monitoring. However, so far this technique has not been widely exploited in other coastal applications, such as underwater archaeology. Submerged settlements and shipwrecks are often located in water depths where the application of multispectral satellite data is feasible. This could lead to more efficient field work practices thus enabling more optimal allocations of costs and labour during archaeological excavations. This study explores the contribution of processed satellite bathymetry maps to the recording of two archaeological coastal sites: a submerged prehistoric settlement in Greece and a shipwreck of a modern cargo vessel in Italy. The results indicate that even though the accuracy of satellite derived bathymetry is high, the level of detail (spatial resolution) is not sufficient to fully replace field-based measurements. However, the use of satellite data complements the existing techniques and can help to place the archaeological sites within a broader spatial context as well as to efficiently monitor the deterioration of a site due to natural causes or human activity, which inevitably leads to risk management. When the study of larger objects is involved (for example First World War shipwrecks) the potential of using satellite data in underwater archaeological studies becomes more promising.