The calibration of optical backscatter sensors for suspended sediment of varying darkness levels

Abstract

A series of calibration experiments were run in order to determine the effect of the colour of suspended sediment on the response of optical backscatter sensors (OBS). Munsell notation describes the properties of soil and sediment colour using the three parameters, hue, chroma, and value, which represent, respectively, the spectral position, lightness level, and amount of pure chromatic colour present. The exposure of the OBS to Munsell Soil Chart patches revealed that the OBS is sensitive to variations in Munsell value and not Munsell hue or chroma. Since OBS output decreased with darkness level (and increased with lightness level and Munsell value), it is suggested that the level of blackness of particles acts to absorb the near-infrared (IR) signal of the OBS, modifying its output. A strong correlation (r2=0.984) was found between the observed and predicted OBS outputs of varying concentrations and ratios of black and white suspended sediment. The OBS output was predicted using a linear mixture-model incorporating end-member calibrations of black and white sediment (similar to Green and Boon [Green, M.O., Boon, J.D., III, 1993. The measurement of constituent concentrations in nonhomogeneous sediment suspensions using optical backscatter sensors. Marine Geology, 110, pp. 73–81.]). A strong correlation (r2=0.907) was also observed between luminance factor and IR reflectance for various minerals, suggesting a link between Munsell notation and OBS output. Formulae were developed to predict the IR reflectance of sediment of known Munsell value, concentration, and size. Models predicting sediment transport should consider the effect of the level of blackness of particles on the OBS response.