Developing a new index to detect optically shallow waters from multispectral satellite images supported by radiative transfer modeling

Behnaz Arabi, Cyrus Samimi, Wouter Verhoef

O 2.3 in Zooming out: Evolution, biomes, global trends

12.10.2023, 15:00-15:15, H 36

Coastal waters often encompass extensive regions of Optically Shallow Waters (OSWs), where observations of remote sensing reflectance (Rrs [sr-1]) are influenced by the sea-bottom. If left unaddressed, the bottom reflectance effect can compromise the accurate retrieval of Water Constituent Concentrations (WCCs) from hyperspectral and multispectral remote sensing data. To delve into this phenomenon further, we modified the existing semi-infinite 2SeaColour Radiative Transfer (RT) model, creating a finite water layer model bounded by a diffusely reflecting surface at the sea-bottom. Our simulations with this new Water - Sea Bottom (WSB) model revealed that a Near-Infrared (NIR) spectral bands ratio, specifically bands 750nm and 900nm, remains nearly insensitive to WCCs and increases with shallower waters. Consequently, this ratio can serve as a robust index for detecting OSWs.

This study introduces the newly established Near-Infrared Bottom Effect Index (NIBEI), which we applied to a series of satellite observations over the Wadden Sea during high and low tidal phases. We processed images from the MEdium Resolution Imaging Spectrometer (MERIS) and the Ocean and Land Colour Instrument (OLCI) to retrieve WCCs of the study area. Our findings demonstrate that the sea-bottom effect in OSWs adversely affects atmospheric correction and retrievals. However, the application of NIBEI to flag OSWs substantially improves the reliability and consistency of WCCs maps.

Notably, applying the proposed NIBEI to satellite images necessitates only Top Of Atmosphere (TOA) radiances at 750nm and 900nm, eliminating the need for atmospheric correction and ancillary local input data (e.g., bathymetry map, bottom type, empirical coefficients, in-situ measurements). This makes NIBEI readily applicable for detecting OSWs across various ocean color remote sensors in different shallow coastal regions.



Keywords: Remote sensing, Water quality monitoring, OLCI, Sentinel-3, MERIS, Optically shallow waters, Tidal waters, The Wadden Sea, Sea-bottom effect, Radiative Transfer Modeling, MODTRAN, 2SeaColour

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