274
A.J. SEPHTON AND K.C. PARTINGTON
year ice floes as these are the most distinctive floes in the image pair). By comparison
with Fig. 7 it can be seen that the algorithm has accurately tracked the motion of each
of the five floes, without generating any spurious matches, despite variations in region
boundary caused by the segmentation process.
In comparison, for ERS-1 data of primarily marginal ice recorded in the Gulf of St.
Lawrence in March 1992, the region matching algorithm does not perform well. This is
mainly due to the higher concentration of ice (above 8hoths) in which the floe boundaries are not so distinct, and the much smaller average floe size (below 1 km in dimension) and hence less distinctive floe shape. Where groups of ice floes are moving together' then increased matching accuracy might be obtained by using contextual information (relations to and attributes of neighboring regions such as the direction and extent
of adjacency, degree of "surround ness;' and region area) to assist the matching of individual floes.
12.4
Iceberg Detection
Drifting icebergs can be found many kilometers outside the ice edge in areas such as
Baffin Bay, the Labrador Coast, the Grand Banks, and the North Atlantic shipping lanes.
Because they are extremely hard (consisting of nearly pure water crystals), and can be
of enormous size and mass (drafts of 300 - 400 m are not uncommon, whilst an iceberg of 13 m diameter will typically weigh some 8000 tons), they represent a major hazard to oil rigs and shipping.
The spatial resolution and relatively steep (23 0 ) incidence angle of ERS-1 means that
less than 100% detection rates are achievable, though useful information can potentially still be derived for planning and as an adjunct to marine radar data. It is also
demonstrated here that although the icebergs can be small compared to the imaging
spatial resolution, their effect on the local environment, and hence their signature size,
can be much larger. An example is shown in Fig. 9 where an iceberg of approximately
100 m diameter is shown in a full-resolution ERS-1 SAR subscene. The large signature
size is a result of the iceberg shadow, which in this case extends to approximately 500
m.
There are three characteristics of an iceberg which can be used in its identification:
- The iceberg itself (generally bright)
- The iceberg "shadow" (dark)
- The iceberg wake (linear, if visible)
The iceberg itself is generally brighter than surrounding open water and ice, as a result
of strong internal scattering from air bubbles and, in some cases, double-bounce scattering from the sides of the iceberg and surrounding water. The "shadow" is a dark
region adjacent to the iceberg which may be caused by fresh water damping of capillary waves or locally lower wind speeds (in the lee of the iceberg). A wake may be present, depending on wind conditions, iceberg size, and currents. Certainly in areas of sea
ice, the wake can show up as strongly scattering linear regions of brash ice (Thomas
and Roth 1996). In open water, a wake is rarely visible to the eye. The detection characteristics of icebergs are described in more detail by Willis et al. (1996).
A.J. SEPHTON AND K.C. PARTINGTON
year ice floes as these are the most distinctive floes in the image pair). By comparison
with Fig. 7 it can be seen that the algorithm has accurately tracked the motion of each
of the five floes, without generating any spurious matches, despite variations in region
boundary caused by the segmentation process.
In comparison, for ERS-1 data of primarily marginal ice recorded in the Gulf of St.
Lawrence in March 1992, the region matching algorithm does not perform well. This is
mainly due to the higher concentration of ice (above 8hoths) in which the floe boundaries are not so distinct, and the much smaller average floe size (below 1 km in dimension) and hence less distinctive floe shape. Where groups of ice floes are moving together' then increased matching accuracy might be obtained by using contextual information (relations to and attributes of neighboring regions such as the direction and extent
of adjacency, degree of "surround ness;' and region area) to assist the matching of individual floes.
12.4
Iceberg Detection
Drifting icebergs can be found many kilometers outside the ice edge in areas such as
Baffin Bay, the Labrador Coast, the Grand Banks, and the North Atlantic shipping lanes.
Because they are extremely hard (consisting of nearly pure water crystals), and can be
of enormous size and mass (drafts of 300 - 400 m are not uncommon, whilst an iceberg of 13 m diameter will typically weigh some 8000 tons), they represent a major hazard to oil rigs and shipping.
The spatial resolution and relatively steep (23 0 ) incidence angle of ERS-1 means that
less than 100% detection rates are achievable, though useful information can potentially still be derived for planning and as an adjunct to marine radar data. It is also
demonstrated here that although the icebergs can be small compared to the imaging
spatial resolution, their effect on the local environment, and hence their signature size,
can be much larger. An example is shown in Fig. 9 where an iceberg of approximately
100 m diameter is shown in a full-resolution ERS-1 SAR subscene. The large signature
size is a result of the iceberg shadow, which in this case extends to approximately 500
m.
There are three characteristics of an iceberg which can be used in its identification:
- The iceberg itself (generally bright)
- The iceberg "shadow" (dark)
- The iceberg wake (linear, if visible)
The iceberg itself is generally brighter than surrounding open water and ice, as a result
of strong internal scattering from air bubbles and, in some cases, double-bounce scattering from the sides of the iceberg and surrounding water. The "shadow" is a dark
region adjacent to the iceberg which may be caused by fresh water damping of capillary waves or locally lower wind speeds (in the lee of the iceberg). A wake may be present, depending on wind conditions, iceberg size, and currents. Certainly in areas of sea
ice, the wake can show up as strongly scattering linear regions of brash ice (Thomas
and Roth 1996). In open water, a wake is rarely visible to the eye. The detection characteristics of icebergs are described in more detail by Willis et al. (1996).