ABSTRACT
Low resolution visual polarimetric photographic imagery of the Galveston Bay oil spill from a tanker accident on July 28, 1990, was obtained and analyzed. The low resolution imagery (30-100 m) was obtained concurrently with high resolution imagery (1m) and is representative of what would be seen by a polarimetric satellite. Orthogonal red-greenblue (RGB) polarimetric images obtained with color photography were digitized by Kodalux onto a CD ROM. These polarimetric images were then used to calculate the percent polarization. The positive and negative percent polarized radiation scattered by each of the sea surface waves is seen individually in high resolution imagery. (Percent polarization is defined as positive when the dominant radiation is perpendicular to the plane of incidence and negative when it is parallel.) The analysis of low resolution polarimetry is approached in a different manner than that of high resolution; in high resolution polarimetry, individual waves are imaged, and the positively polarized sky and scattered solar radiation is seen as well as the negatively polarized internally scattered radiation from the back surfaces of the waves. In low resolution imagery, all of the polarized radiation is averaged, and this would be seen by one of the proposed satellite polarimeters. By using the statistically averaged percent polarization in density slices, the areal extent of the oil coating is readily characterized, and the relationship between the oil thickness and the percent polarization is readily determined. Percent polarization has the advantage over photometry and image enhancement in that absolute calibration is retained by virtue of the fact that percent polarization is the ratio of reflected to scattered radiative components and variations in the incident illumination cancel out. Absolute calibration is essential for any quantitative evaluation of oil spill effects just as it is necessary for any legal or enforcement action as well as the evaluation of ecological effects.
