The utility of Sentinel 2 for bathymetry in turbid waters: &#160;a case study in the Gulf d&#8217;Arguin, Mauritania
Authors: Abileah Ron 
Published: March 2021

&lt;p&gt;The Golfe d&amp;#8217;Arguin (NW Mauritania) is a 15,000 km&lt;sup&gt;2&lt;/sup&gt; shelf, 150 km in N-S, and 150 km offshore.&amp;#160; The Golfe is subdivided into inner and outer shelves.&amp;#160; The shallow (&lt;10m) inner shelf, also known as Banc d&amp;#8217;Arguin, is part of a UNESCO World Heritage Site and renowned for a richly diverse marine ecosystem.&amp;#160; &amp;#160;A steep escarpment separates the deeper (20-200 m) outer shelf.&amp;#160; &amp;#160;&amp;#160;The bottom morphology of both zones is a complex system of shoals, canyons, and channels.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; The water is too turbid for traditional optical remote sensing bathymetry.&amp;#160; The alternative is bathymetry derived from ocean wave celerity as waves shoal over the shelf.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; The celerity method has been practiced for several decades, but only with small sensor footprints (typically 10-100 km&lt;sup&gt;2&lt;/sup&gt;).&amp;#160; &amp;#160;&amp;#160;The Sentinel 2 satellites (now two, with plans to expand to four) have a 290 km push-broom image swath, capturing 2100 km&lt;sup&gt;2&lt;/sup&gt; per second.&amp;#160; The entire Earth is imaged every 4-5 days.&amp;#160; The imagery is free, very accessible, and easy to process.&amp;#160; &amp;#160;The Sentinel 2 image archive is five years and growing daily.&amp;#160; Mapping large regions such as the Golfe d&amp;#8217;Arguin is made possible.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; This presentation describes the Sentinel 2 bathymetry workflow.&amp;#160; &amp;#160;The Golfe d&amp;#8217;Arguin is an excellent case study.&amp;#160; Long swell waves from mid-Atlantic storms are frequent.&amp;#160; Long waves are best for sensing the bottom depth.&amp;#160; (Conversely, the method does not work in locations such as the Persian Gulf shielded from oceanic wave systems.)&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; For the test case, the horizontal resolution is 200m, covers depths 0 to 35 m, and accuracy is 5% of depth. &amp;#160;&amp;#160;Comparisons are made with the General Bathymetric Chart of the Oceans (GEBCO), EMODnet DTM, and various literature sources.&amp;#160;&amp;#160;&amp;#160; GEBCO compares very poorly.&amp;#160; EMODnet is better but with much lower spatial resolution and does not capture the morphological detail seen in Sentinel 2 derived bathymetry.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; There is very limited ground truth bathymetry, e.g., Multibeam, in this area for rigorous validation.&amp;#160; &amp;#160;Other validation methods are presented.&amp;#160; One is Sentinel 2 bathymetry of the Perth Australia area,&amp;#160; with wave conditions similar to Mauritania, and a LiDAR survey for ground truth.&amp;#160; &amp;#160;Another is repeatability with images from different dates.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; Below links preview Golfe d&amp;#8217;Arguin bathymetry with Sentinel 2 and EMODnet for comparison.&amp;#160; GoogleEarth can be used to view.&lt;br&gt;&lt;br&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; http://jOmegak.com/vEGU/28QBH.Sentinel2.kmz&lt;br&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; http://jOmegak.com/vEGU/28QBH.EMOD.kmz&lt;/p&gt;