The use of satellite imagery for monitoring coastal environment in Taiwan

Abstract

The authors use satellite remote sensing imagery data to monitor the near-shore environment in the coastal area. In particular, the use of ERS 1/2 SAR data to detect oil slicks near shore is described. Because the presence of oil slicks on the sea surface increases the surface tension of sea water, the surface wave motion is significantly depressed or eliminated. This effect will relatively lower the sea surface roughness, and accordingly, will decrease the radar backscattered energy. These damping effects are now well understood and it is such effects that enable the oil slicks to be discernible from the radar image. In this study, the authors are concerned with the digital techniques that effectively delineates the oil slicks pattern from SAR image. Directly using SAR images to detect the oil slicks, however, is not fully straightforward because of complex process of SAR imaging mechanism and the existing of inherent multiplicative speckle noise. The technique the authors develop is based on the fact that the oil slicks on the image gray value surface is a concave area. In order to correctly identify these concave areas and suppress the effect of speckle noise, an image pyramid with multiresolution layers is generated sequentially from the original image. Then a top-down approach, which applies both first and second order derivative operators, Difference of Gaussian (DoG) and Laplace of Gaussian (LoG), to the image pyramid, is used to detect oil patches. Two ERS-1 SAR images acquired over Taiwan water area on August 5, 1994 and September 5, 1994 were used for testing.