Bathymetry is the measurement of water depths. Traditional scientific approaches for measuring water depths includes the collection of in-situ measurements and the underlying cost of field campaigns pose serious difficulties in acquiring such information on a global scale, routinely.
Although detailed bathymetric maps can be obtained, only limited areas can be actually surveyed and often no more than once. Thus, apart from the reduced coverage, the dynamics of the changing seafloor, especially along coastal regions where major part of human activities take place, are not well documented. The reasons why space-borne remote-sensing techniques play an essential role in retrieving near-shore bathymetry are threefold. Firstly, space-borne imagery makes it possible to access remote areas with wide coverage at high spatial resolution. Secondly, as space-borne imagery are acquired on a regular basis, a historical data archive from various sensors is available for most areas, which in turns enables scientists to access information from the past. Thirdly, the cost of the data is relatively affordable compared to airborne or ground-based campaigns. While several Earth Observation missions exist, the Copernicus Sentinel-2, operated by the European Space Agency (ESA), is a game changer.
The Sentinel-2 mission comprises twin polar-orbiting satellites in the same orbit, phased at 180° (https://sentinel.esa.int). The satellite constellation carries optical instrument payload (MSI, Multi Spectral Imager) that sample 13 spectral bands. The major advantages of Sentinel-2 mission is its systematic monitoring over wide orbital swath width of 290 km, coupled with high revisit time (5 days at the equator and 2-3 days at mid-latitudes) and high spatial resolution (four multispectral bands at 10m). In this context, an innovative and robust method to retrieve maps of the coastal bathymetry from quasi simultaneous Sentinel-2 imagery is being developed.
Our methodology allows to measure directly both swell celerities (c) and swell wavelengths (λ) from a satellite optical dataset, where c and λ are jointly used to retrieve the depth of the coastal seafloor, robustly. Swell celerity describes the phase velocities of superimposed ocean waves, each of which has a particular wavelength (i. e. the distance between two successive crests). As swell velocity and wavelength are controlled by bathymetry, quantifying these parameters from space allows for robust retrieval of water depths independently of water turbidity, which usually hampers our capacity for robust measurements or limits the maximum reachable water depths. Furthermore, it is the Sentinel-2 mission characteristics of unpreceded spatial coverage, short temporal repeatability and systematic acquisitions at sufficient resolution, which pave the way for advanced scientific developments. The combination of the above mentioned specifications, an innovative scalable approach and the availability of systematically acquired and archived satellite data, would allow for routine bathymetric mapping of shallow-depth coastal areas at global scale. The method preferably applies to the zone between the coast and an area of depth less than or equal to half the wavelength of the waves (typically up to a hundred meters deep), with the exception of the wave breaking zone. Thus, it extends considerably our previous capability for monitoring coastal regions’ dynamics.
de Michele, Raucoules D., Idier D., Smai F., Foumelis M.. Shallow bathymetry from Sentinel 2, by joint estimation of swell celerity-wavelengths. Remote Sensing of Environment, in review, 2020.
Poupardin A., Idier D., de Michele M., Raucoules D., 2016, Water depth inversion from a single SPOT-5 dataset, IEEE trans. In Geosci and Rem. Sensing, vol. 54, 2329-2342.
Poupardin, A., Idier, D., de Michele, M., Raucoules, D., "Water Depth Inversion From Satellite Dataset", proceedings of the IEEE Geoscience and Remote Sensing Symposium, 13-18 July 2014, Quebec City, Canada, DOI 10.1109/IGARSS.2014.6946924.