On the imaging of biogenic and anthropogenic surface films on the sea by radar sensors

Abstract

Radar signatures of sea surface films of different origin are investigated, which have been acquired by airborne and spaceborne multi-frequency/multi-polarisation microwave sensors during the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR), missions in 1994, as well as by the ERS SAR in 1996–98. During SIR-C/XSAR, controlled surface film experiments were performed in the German Bight by deploying various quasi-biogenic substances and mineral oil on the sea surface, in order to study the radar signatures caused by surface films of different visco-elastic properties. In general, our results show that multi-frequency capabilities, rather than multi-polarisation capabilities, are needed for a radar-based system for the discrimination of marine surface films. We show that, under high wind conditions (> 10 m s⁻¹), a discrimination between the different kinds of surface films is very difficult, whereas at low to moderate wind speeds (≤ 5 m s⁻¹) a discrimination seems to be possible. This finding is supported analytically by means of a new model for the wave number-dependent radar contrast at high wind speeds (> 10 m s⁻¹) and statistically through the analyses of more than 700 ERS SAR images. In addition, results of laboratory experiments are presented that were carried out in the wind-wave tank of the University of Hamburg. At certain wind-speed ranges a different damping of bound and freely propagating surface waves by monomolecular surface films is observed, which may explain the high radar contrasts measured by the microwave sensors.