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consideration of the shape of the spill, or more accurately, on its displace-
ment from the assumed ship course of a straight line. The displacement
serves to estimate the average sea surface current. It is known that the
shape as well as the drift and state (whether it is broken or not, etc.) of oil
spill are determined by the wind and current history between the oil release
and the time of SAR image acquisition (Espedal et al. 1998, Espedal and
Wahl 1999, Reed et al. 1999). In the cases under study, a wind field in the
spill area changed slightly during radar sensing, as follows from the SAR
image brightness distribution. Moreover the surface analysis maps show
that at the time of acquisition and 6 hours earlier, the sea east of Taiwan
was in a low-gradient pressure field at the anticyclone periphery. Wind
speed to the south and east of Taiwan was roughly 5 ms
-1
and thus the
wave breaking affecting the oil transport was absent. At such conditions
the displacement from the assumed ship course allowed us to relate them
to current patterns, reflecting in particular, the interaction of the Kuroshio
Current with the bottom topography and an island.
The remainder of the paper is organized as follows. First, the features
revealed on two ERS-1 SAR images, covering the sea southeast of Taiwan,
are analysed, together with the bathymetry and the weather maps (section
2). The results of calculations of average surface currents based on the ana-
lysis of the geometry of the spills and also on estimates of wind drift are
given in section 3. The spreading of the spill in the transverse direction and
the dependence of the width of the band on time are considered in section
4. Additional data confirming the interpretation of disturbances in the spill
shape and a discussion of the results of the analysis are presented in sec-
tion 5.
2 Interpretation of the SAR signatures
A route east of Lutao, parallel to Taiwan is usual for ships (tankers) mov-
ing to and from Japan and South Korea. These ships have been regularly
detected on the SAR images acquired by the Chung-Li (Taiwan) ground
station. Some of the ships clean their tanks of oil along this route. As a re-
sult, spills from moving ships were observed on the SAR images.
Two kinds of theses spills have been detected. When the amount of oil
in the cleaning waters was small (within permissible limits), a spill appear-
ed as a narrow dark band, the width of which practically did not change,
and its radar contrast decreased with the increase in distance from the ship
(spill age). It may be assumed thus that only the amount of oil and its re-
duction due to evaporation were the factors inhibiting an increase in the
width of a spill. The spread of oil (widening of a band) occurred as long as
