The utility of Sentinel 2 for bathymetry in turbid waters: a case study in the Gulf d’Arguin, Mauritania

The Golfe d’Arguin (NW Mauritania) is a 15,000 km² shelf, 150 km in N-S, and 150 km offshore.  The Golfe is subdivided into inner and outer shelves.  The shallow (<10m) inner shelf, also known as Banc d’Arguin, is part of a UNESCO World Heritage Site and renowned for a richly diverse marine ecosystem.   A steep escarpment separates the deeper (20-200 m) outer shelf.    The bottom morphology of both zones is a complex system of shoals, canyons, and channels. 

              The water is too turbid for traditional optical remote sensing bathymetry.  The alternative is bathymetry derived from ocean wave celerity as waves shoal over the shelf.

              The celerity method has been practiced for several decades, but only with small sensor footprints (typically 10-100 km²).    The Sentinel 2 satellites (now two, with plans to expand to four) have a 290 km push-broom image swath, capturing 2100 km² per second.  The entire Earth is imaged every 4-5 days.  The imagery is free, very accessible, and easy to process.   The Sentinel 2 image archive is five years and growing daily.  Mapping large regions such as the Golfe d’Arguin is made possible.

              This presentation describes the Sentinel 2 bathymetry workflow.   The Golfe d’Arguin is an excellent case study.  Long swell waves from mid-Atlantic storms are frequent.  Long waves are best for sensing the bottom depth.  (Conversely, the method does not work in locations such as the Persian Gulf shielded from oceanic wave systems.)

              For the test case, the horizontal resolution is 200m, covers depths 0 to 35 m, and accuracy is 5% of depth.   Comparisons are made with the General Bathymetric Chart of the Oceans (GEBCO), EMODnet DTM, and various literature sources.    GEBCO compares very poorly.  EMODnet is better but with much lower spatial resolution and does not capture the morphological detail seen in Sentinel 2 derived bathymetry.

              There is very limited ground truth bathymetry, e.g., Multibeam, in this area for rigorous validation.   Other validation methods are presented.  One is Sentinel 2 bathymetry of the Perth Australia area,  with wave conditions similar to Mauritania, and a LiDAR survey for ground truth.   Another is repeatability with images from different dates.

              Below links preview Golfe d’Arguin bathymetry with Sentinel 2 and EMODnet for comparison.  GoogleEarth can be used to view.

                                       http://jOmegak.com/vEGU/28QBH.Sentinel2.kmz
                                       http://jOmegak.com/vEGU/28QBH.EMOD.kmz