Chemistry Reference
In-Depth Information
The NOSV gives a better estimate of the detectability of marine oil pol-
lution, independently of the local wind conditions of this particular study.
These results show that higher wind speeds cause lower detectability of oil
pollution, and the maximum (model) wind speed where oil spill detection
in European coastal waters can be inferred. In particular, at wind speeds
below 7 m s
-1
oil spills are well detectable, whereas above 10 m s
-1
wind
speed the definite detection of marine oil pollution seems to be almost im-
possible. At wind speeds between 7 and 10 m s
-1
the detectability of oil
spills is rather low. These results can explain why less oil pollution was
detected in the northern test areas during winter time. E.g., the mean wind
speed in the central North Sea during winter time lies above 10 m s
-1
(Fig-
ure 5), thus making it unlikely that every oil spill in that area was detect-
able by SAR sensors.
5 Results obtained from laboratory measurements
In order to complement the results of the radar backscatter measurements
on the open sea, laboratory measurements of the wave amplitude and slope
and of the radar backscatter at X- and Ka-band were carried out in a wind-
wave tank with mechanically generated gravity waves as well as with
wind-generated waves on a slick-free and a slick-covered water surface. In
this paper, we concentrate on the results of the radar measurements with
wind-generated waves. For a full description of the obtained results the
reader is referred to Gade et al. (1998c).
The measurements were carried out in the wind-wave tank of the Uni-
versity of Hamburg, which is 26 m long and 1 m wide. The mean water
depth is 0.5 m, the wind tunnel height is 1 m, and the effective fetch is
19.5 m. The measurement area was at a fetch of 15.5 m. The scatterome-
ters used for the present investigation are working at 9.8 GHz (X-band)
and 37 GHz (Ka-band) and the measurements were carried out with up-
wind looking radar antennae and at VV-polarisation.
Figure 7 shows the measured radar Doppler shifts, both at X- and Ka-
band, for the entire wind speed range. The solid and dashed lines in both
panels denote theoretical Doppler shifts assuming freely propagating
Bragg waves and Bragg waves bound to the dominant wind waves, respec-
tively. We can conclude that the X-band backscattering from a slick-free
water surface at low wind speeds (up to approximately 4.5 m s
-1
) and long
fetches (15.5 m in our investigation) is determined mainly by bound
waves, whereas the Ka-band backscatter is caused by both bound and
freely propagating Bragg waves. For wind speeds higher than 4.5 m s
-1
the
X-band backscattering in our experiments is caused by both parasitic har-
