Underwater light characterisation for correction of remotely sensed images

Abstract

Objective measurement of habitat change using remote sensing requires processing of the images to enhance the bottom reflectance signal. This process typically uses correction techniques to remove the influence of the water column on bottom reflectance, and to enable the accurate correction of the imagery for varying bathymetry. Such correction measures depend on reliable estimates of water column light attenuation. An investigation into the spatial variation in attenuation in a typical tropical region was undertaken. Measurements of gross spatial variations in downwelling attenuation around San Andres and Old Providence islands in the western Caribbean were made using a PAR sensor. Measurements of specific attenuation were also made for blue, green and red light using filters fitted to the sensor. High spectral resolution attenuation measurements were also made using a spectroradiometer. Results showed a four-fold variation in light attenuation in shallow littoral regions alone. Spectral attenuation measurements suggested that this variation was largely the result of scattering by particulate matter rather than varying concentrations of dissolved yellow substances. These findings suggest that the results of studies where single measurements of 'average' attenuation have been used to depth-correct remotely sensed imagery should be interpreted with a high degree of caution. The paper goes on to show that simple models can be empirically obtained where attenuation can be spatially predicted with confidence, based on the variables of water depth, distance to and size of mangrove beds, and distance to and size of towns. The models obtained showed high statistical significance, with 89% and 78% of the spatial variation in attenuation explained for San Andres and Old Providence, respectively. It is postulated that the use of such approaches for the estimation of attenuation will lead to more accurate depth correction and hence improved interpretation of remotely sensed imagery for littoral regions.