Chemistry Reference
In-Depth Information
lengths (Bragg scattering, see Valenzuela 1978), thus, wave damping by
oceanic surface films causes a reduction of the measured radar backscatter.
For this reason, dark patches visible on synthetic aperture radar (SAR) im-
ages of the ocean surface can often be related to oil spills or natural slicks
floating on the sea surface.
The layer thickness of a mineral oil spill is much larger than that of a
monomolecular sea slick, which results in different visco-elastic properties
and, therefore, in a different damping of the ocean surface waves (Alpers
and Hühnerfuss 1988). Results of laboratory experiments showed that Ma-
rangoni damping is the dominant mechanism for the damping of short si-
nusoidal gravity waves by monomolecular slicks (Hühnerfuss 1986). Fol-
lowing Marangoni damping theory, monomolecular surface films exhibit a
resonance-like damping behaviour (at wave numbers of about 100 radm
-1
),
whereas this distinct damping maximum is missing for (thick) mineral oil
spills.
Since the application of a single-frequency, fixed incidence radar system
(like the C-band SAR aboard the ERS satellites) can provide information
on the damping of ocean surface waves at only one single Bragg wave-
length, multi-frequency radar techniques have already been used to obtain
more extended information on the damping behaviour of the detected sur-
face films (e.g., Gade et al. 1998c). However, during recent field experi-
ments with artificial monomolecular surface films, a distinct damping-
maximum at intermediate wave numbers was not measured (Gade et al.
1998c). Thus, additional terms describing the energy fluxes on the water
surface, like the energy input by the wind, wave breaking, and non-linear
wave-wave interaction, have to be taken into account for the development
of theoretical models that can explain the measured damping of small
ocean surface waves by different surface films.
During the two Shuttle Imaging Radar C-/X-Band Synthetic Aperture
Radar (SIR-C/X-SAR) missions in April and October, 1994, surface films
of different visco-elastic properties were deployed in the German Bight of
the North Sea, in order to simulate different biogenic and anthropogenic
ocean surface films. The aim of these experiments was to investigate
whether active microwave techniques are capable of yielding information,
on the one hand, on the damping behaviour of different surface films and,
on the other hand, on the damping behaviour of one substance under dif-
ferent environmental conditions. Here we briefly present the results of
these experiments obtained by a 5-frequency/multi-polarisation scat-
terometer, and by the 3-frequency/multi-polarisation SIR-C/X-SAR. They
are complemented by statistical analyses of more than 700 ERS SAR im-
ages acquired over different parts of European coastal waters and by re-
sults of laboratory studies performed in a wind-wave tank. For a more de-
