Monitoring of water quality by satellite ocean colour data requires high quality atmospheric correction and especially the accurate quantification of the aerosol contribution to the top of atmosphere radiance. Several methods have been proposed for atmospheric correction over turbid waters, including modelling the marine contributions to the NIR signal or switching to longer short-wave infrared (SWIR) wavelengths where the signal even in turbid waters can be assumed zero. Here we present the use of the high quality SWIR bands of the Operational Land Imager (OLI) on Landsat-8, launched in 2013, to extend our existing turbid water atmospheric correction to extremely turbid waters. The atmospheric correction is image based, and no external measurements are required. The aerosol type is estimated using NIR and SWIR bands in clear water pixels, or in all water pixels using the two SWIR bands. The aerosol type is assumed to be constant over a single Landsat-8 tile (170 by 185 km), or allowed to vary spatially when using both SWIR bands. Realistic spatial patterns of marine reflectances are retrieved, uncorrelated with the estimated aerosol reflectance. Taking spatial and temporal variability into account, products from Landsat-8 compare well with those of MODIS Aqua and Terra — also using the SWIR bands for atmospheric correction. The limitations of our previously published method (Vanhellemont & Ruddick, 2014a) are illustrated at higher turbidities, and removed by using the new method. The uncertainty caused by using a single aerosol type per scene is assessed. The advantages of the high spatial resolution L8/OLI data are clear for applications in coastal and estuarine waters. As an example of the advantage of high quality SWIR bands, and a SWIR-based atmospheric correction, an algorithm for detecting black suspended sediments from dredging and dumping operations is demonstrated here. In conclusion, L8/OLI is a powerful new tool for remote sensing of extremely turbid waters, and can be used as a precursor for future ocean colour missions with SWIR bands. |
