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be interpreted. Furthermore, the model can explain the similarities between
the results obtained from quasi-biogenic (TOLG) and anthropogenic
(IFO180) surface films under high wind conditions.
3 Results obtained from analyses of SIR-C/X-SAR data
Various SIR-C/X-SAR images showing biogenic and anthropogenic sur-
face films at different parts of the world's oceans were analysed, in order to
investigate whether or not SIR-C/X-SAR is capable of discriminating be-
tween biogenic and anthropogenic oceanic surface films. Using SAR im-
ages of the surface film experiments in the German Bight of the North Sea,
again, we found that the measured damping ratios strongly depend on the
wind speed, which is in accordance with the results obtained by
HELISCAT (see Figure 1). The results obtained from SAR images of vari-
ous biogenic and anthropogenic surface films are shown in Figures 3 and
4, respectively. The classification between biogenic and anthropogenic
surface films was made based on the shape and the size of the observed
surface films. E.g., the SAR images shown in Figure 3 show typical signa-
tures of biogenic surface films encountered in large sea areas during ongo-
ing algal blooms: the surface-active material accumulates on the water sur-
face and the long, narrow, dark streaks follow the surface currents. In con-
trary, signatures caused by spilled mineral oil look different: the dark elon-
gated patch in Figure 4 is very likely caused by an oil spill, because of its
irregular shape and the fact, that its contrast is independent of the different
wind speed on either sides of the varying wind speed in that very area. In
general, however, no sampling of any film material was done for our stud-
ies.
It is obvious that biogenic surface slicks cause a strong damping at L-
band (Figure 3), whereas anthropogenic oil spills cause low L-band damp-
ing ratios (Figure 4). Considering the large scatter of the data, the observed
damping ratios at C- and X-band are similar, which, particularly at C-band,
is due to an insufficient signal-to-noise ratio of the SIR-C/X-SAR system.
The presented results, however, show that multi-frequency SAR im-
agery yields more information about the damping characteristics of oce-
anic surface films than single-frequency SAR imagery. This is, in turn,
needed for a better discrimination between different kinds of surface films,
particularly under low to moderate wind conditions. Moreover, our results
show that L-band data is crucial for a successful discrimination of different
surface films. Although OLA represents natural slicks, its damping ratios
look similar to those of mineral oil spills. A possible reason for this is the
limited spatial extent of these films. In contrast, natural slicks usually
