Spectral signature of highly turbid waters: Application with SPOT data to quantify suspended particulate matter concentrations

An experimental method for determining water composition from “ocean colour” satellite data, in visible and near-infrared (NIR) wavelengths, is applied to highly turbid waters. Numerous spectroradiometric measurements are carried out in the Gironde estuary, for suspended particulate matter (SPM) concentrations ranging between 35 and more than 2000 mg l−1. Empirical relationships are established between remote-sensing reflectance (Rrs) in SPOT-HRV bands and SPM concentration through these numerous in situ measurements. We observed that remote-sensing reflectance increases with SPM concentration and that the SPOT bands saturate at the highest turbidities. The best correlations are obtained for the NIR band XS3 (790–890 nm) and for the reflectance ratios: Rrs(XS3)/Rrs(XS1) and Rrs(XS3)/Rrs(XS2). The XS1 and XS2 visible bands are only used to determine SPM concentrations in the lower part of the estuary (where the SPM concentrations are lower). As a result, SPM concentrations within the surface waters in the estuary are estimated up to 2000 mg l−1 with an accuracy better than ±35%. The algorithm is finally applied to a SPOT scene. Satellite data are corrected for atmospheric effects using a radiative transfer code and in situ reflectance measurements; as a result, the horizontal distribution of SPM is retrieved. Moreover, the high spatial resolution HRV-SPOT sensor shows detailed sedimentary flows, especially in the visible XS1 and XS2 spectral bands.