An Evaluation of Waveband Pairs for Water Column Correction Using Band Ratio Methods for Seabed Mapping in the Seychelles

The differential attenuation of visible light (wavelength 400–700 nm) as it travels through the water column confounds the interpretation of remotely sensed imagery acquired over the sea floor. This can be addressed using depth-invariant processing techniques that ratio the radiance values of two wavebands. An evaluation of the performance of different waveband pairs for creating depth-invariant indices of the sea floor is presented. Twenty-eight different band pairs extracted from multispectral Compact Airborne Spectrographic Imager (CASI) data are assessed for the creation of depth-invariant indices over the reef profile of Alphonse Atoll in the Seychelles (water depth 0–30 m). Findings indicate that, for optimal performance, bands selected for depth-invariant processing must be at least 90 nm apart to achieve an optimal ratio of attenuation coefficients within the water column.  This optimal ratio must also lie at a central point of the visible spectrum at which longer wavelength bands are not fully attenuated, while shorter wavelength bands exhibit some attenuation over a depth range that coincides with the features of interest in the water column.

Introduction

The use of remotely sensed imagery to map benthic marine habitats offers a cost effective tool for scaling up field survey effort. The radiance levels over water are low compared with those observed over land and the proportion of useful information contained by the signal reaching the sensor is consequently lower for oceanographic remote sensing applications. The major contribution of unwanted noise is the atmosphere, but variations below the water surface in depth, water quality and benthic substrate also influence at-sensor spectral radiance (Holden and LeDrew 2002). These confounding influences on light as it travels through the water column typically arise because of absorption and scattering. Absorption refers to the conversion of electromagnetic energy into other forms, such as heat or chemical energy, whereas scattering is the deflection of electromagnetic radiation as a result of its interaction with the particles (Curran 1985). The attenuation coefficient incorporates both these components to describe the rate at which energy is lost and attenuation length, the inverse of the attenuation coefficient, is the distance over which attenuation is reduced by a factor
of e (Rees 1999)…