Chemistry Reference
In-Depth Information
wavelength, 9.5 s period and 2.8 m waveheight was prevalent. At the time
of the Almaz-1 overpass the wind speed had fallen down to 2-3 m s
-1
. This
can explain the higher damping ratio in the Almaz-1 case.
Fig. 3.
Almaz-1 SAR image of August 23
rd
, 1991, at 10:35 UTC showing the first
oil spill released on August 21
st
(incidence angle 27.4q, wind speed 7 m s
-1
, spill
age 40 hrs) (
left
). Variations of the radar backscatter along the scan line are shown
on the Almaz-1 SAR image by white thin arrow (
right
), flight/look directions by
thin black arrows, and wind direction by double black arrow (image courtesy NPO
Mashinostroenia)
This spill was observed from RV
Svanaug Elise
on August 22
nd
at 10:30
as a blue shine and at 21:39 it was reported that the spill had been totally
broken up and no emulsion was formed (Bern et al. 1992b). On August
23
rd
at 10:35 this spill was imaged again by the Almaz-1 SAR with lower
damping ratio (Figure 3). The area covered by the spill almost did not
change with age. As inferred from Table 1, the wave conditions were al-
most the same, too. The reason of the lower damping ratio compared to
that of August 22
nd
could be the higher wind speed (7 m s
-1
).
Another pair of images was acquired on Aug 25
th
, 1991 almost simulta-
neously at 10:44 (Almaz-1) and 10:48 (ERS-1) (Figure 4). Both images
show the footprints of both spills. In that case, on the contrary, in the Al-
maz-1 image the damping ration is about 0, while in the ERS-1 image it is
9.5 dB for the older and 15 dB for the younger spill. Judging from the
weather data, a weak wind (2-3 m s
-1
) was observed. Such a wind is close
to optimum for SAR image formation at vertical polarisation (Bern et al
.,
1992a). The very low damping ratio inferred from the Almaz-1 image can
be apparently explained by the high incidence angle ~53ยบ which is close to
the critical value for receiving useful backscattering from the sea surface.
