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moto et al. 1993, Masuko et al. 1995, Gade et al. 1998). It was shown that
maximum damping by artificial films is encountered around the Bragg
waves related to C-band (Gade et al
.
1998).
However, radars' possibilities are limited by the respective conditions
on the air-sea interface and depend on oil spill parameters. Different proc-
esses in the ocean-atmosphere system can result in similar SAR image sig-
natures and equal contrasts. In order to discriminate between natural sea
slicks and oil spills additional information is always required. Neverthe-
less, the ability of all-weather monitoring by radar makes it a powerful tool
for ocean remote sensing.
The papers mentioned above did not include data obtained by the S-
band SAR aboard the Russian Almaz-1 satellite. In order fill this gap we
herein comparatively analyse the Almaz-1 and ERS-1 SAR imagery taken
during the Dedicated Oil Spill Experiment-91 (DOSE-91) (Bern et al.
1992a, Ivanov et al. 1998). The S-band SAR aboard the Almaz-1 satellite
was operating at 3.1 GHz (9.6 cm wavelength) at an orbital altitude be-
tween 300 and 400 km during the flight, horizontal polarisation of the sig-
nal (HH) for transmission and reception, resolution 10-15 m and variable
incidence angles (25-60q). More detailed specifications of the Almaz-1
SAR can be found in Wilde et al
.
(1993).
2 Experiment and results
The DOSE-91 experiment was conducted between August 21 and Septem-
ber 2, 1991 in the Norwegian Sea, 100 km off the western coast of Norway
around the Haltenbanken during simultaneous operation of the Almaz-1
and ERS-1 satellites. Twenty tons of artificial oil were spilled on the sea
surface in a point with coordinates 64q 30c N and 07q 30c E (Figure 1) on
August 21
st
(20:40 UTC), August 24
th
(19:05) and August 27
th
(19:00)
.
Only the two first spills were imaged by either Almaz-1 or ERS-1 SAR
and are considered here. The oil consisted of a mixture of 35 % Statfjord
crude, 20 % Gullfaks crude and 45 % DUC crude (Bern et al. 1992a). Such
mixture was selected because it does not form steady emulsions in the sea
water and in this sense can be considered as ecologically safe (for details
see Bern et al. 1992a, and Bern et al. 1992b). The spills existed in the sea
between 3 and 5 days. In the area of the experiment three oceanographic
buoys were installed to measure sea and wind parameters. The observa-
tions of the spill evolutions were also produced from a Norwegian aircraft
equipped with SLAR, IR and UV sensors. Twelve data acquisitions from
the Almaz-1 and 4 from the ERS-1 satellite were taken. However, the
spills were only identified on four Almaz-1 and on two ERS-1 SAR im-
