CHAPTER
31
Mapping blue-ice areas and crevasses in
West Antarctica using ASTER images, GPS,
and radar measurements
Andre´s Rivera, Fiona Cawkwell, Anja Wendt, and Rodrigo Zamora
ABSTRACT
Before the satellite era, relatively little was known
about the interior of the West Antarctic Ice Sheet
(WAIS). Of special interest are the rock outcrops
associated with blue-ice areas (BIAs), which have
been exploited for logistical purposes as well as
being the subject of scientific research. The blue
ice consists of relatively snow-free glacier ice that
is undergoing ablation.
One of these BIAs is Patriot Hills (80 18 0 S,
81 22 0 W) where aircraft with conventional landing
gear have been landing for more than 20 years. This
is now the main hub supporting large terrestrial
expeditions conducted by Chilean scientists within
Antarctica. Kinematic GPS has been used to map
BIAs since 1996, with areas delineated on ASTER
images since 2001 using both manual and automated approaches. The GPS method typically
delimits the largest area, and supervised classification of the images by an algorithm demarcates the
smallest area due to thin patchy snow cover overlying blue ice. These areas do not display a unique
spectral response when mostly snow covered, so
that they can only be visually discriminated. This
detailed record of BIA extent shows no significant
areal change with time, but does display interannual
variability, which most likely is connected to prevailing meteorological conditions. BIAs around
other nunataks in the region have been mapped
from ASTER imagery, with the aim of identifying
other landing sites for aircraft, as well as providing
a detailed map for meteorite seekers. ASTER composite images have also been used to map safe
routes for terrestrial traverses through crevasse
zones. High-pass filters enhanced crevasse features,
but visual analysis proved to be the most reliable
method of identifying all crevasses. ASTER images
were superior to microwave imagery for crevasse
detection, as the latter can lack sufficient contrast;
however, only Radarsat imagery provided coverage
of higher latitude regions. Information gleaned
from visible imagery can be combined with that
of field-based radio-echo sounding and groundpenetrating radar profiles through the ice to map
internal layers and bedrock topography with the
objective of enhancing our knowledge of this
remote region.
31.1
INTRODUCTION
Blue-ice areas (BIAs) are a rare feature in Antarctica (occupying between 0.8 and 1.6% of the continent; Winther et al., 2001) and are commonly, but
not exclusively, associated with nunataks. These
rock outcrops represent a barrier to the strong
katabatic winds that flow constantly from the interior of the ice sheet, with the resulting turbulent air
flow responsible for removing surface snow, leaving
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Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
a bare ice face. BIAs have no net annual accumulation, and ablation occurs mainly through sublimation, which can be much higher than over adjacent
snowfields (Bintanja and Reijmer 2001), and
through wind erosion, resulting in a local negative
surface mass balance. BIAs tend to be smooth,
although they may be rippled, which facilitates their
use for aircraft with conventional landing gear
rather than aircraft with skis. Other BIAs are associated with steep slopes, glacial valleys, or the lower
parts of glacial basins, where accelerated katabatic
winds can effectively remove snow leaving a smooth
ice surface. For more information on BIAs see
Bintanja (1999).
Under steady state conditions, ice flows converge
horizontally in the vicinity of BIAs, with upward ice
flows at the margins of nunataks balanced by ice
mass losses mainly due to sublimation at the surface
(Bintanja 1999). These ice flows transport englacial
material to the surface, with unusually large volumes of surface deposits found along the margins
of BIAs adjacent to many nunataks and mountain
ranges. Thus BIAs are also of interest because they
can be a concentrated source of meteorites that
have fallen over a wider region over many millennia, been trapped and transported within the ice,
and covered by snow, before being exhumed on the
surface (Corti et al. 2003). As of 1999, more than
20,000 meteorites had been discovered in Antarctic
BIAs (Bintanja 1999).
Patriot Hills, with a maximum altitude of 1,246 m
asl (USGS 1966), are located at the southeastern tip
of Ellsworth Mountains. They comprise one of
many Antarctic nunataks, which act as obstacles
to katabatic winds (Figs. 31.1 and 31.2). Increased
wind speeds on the leeward side of Patriot Hills
limits the accumulation of snow, and as a consequence has led to the generation of a BIA with an
area of approximately 12 km 2 . This BIA is located
at the southern edge of Horseshoe Valley and has
slopes of less than 1 degree and surface topography
varying in altitude between 1,100 m asl on the western side to 700 m asl on the eastern side. Ice flows
from west to east, with very low velocities at the
BIA, and a maximum velocity of 14 m yr1 at the
center of Horseshoe Valley (Wendt et al. 2009).
No meteorites were found in the BIA of Patriot
Hills during an initial expedition in 1997/1998 (Lee
et al. 1998). However, in 2000 a meteorite was
found in a nearby moraine band (Grossman and
Zipfel 2001). As for other BIAs in the region,
meteorites have been found at Martin Hill and
Pirrit Hill (Lee et al. 1999), and more recently at
Thiel Mountains (Choi et al. 2007). One of the
reasons commonly given to explain the scarcity of
meteorites in these BIAs, especially Patriot Hills, is
the occasional occurrence of warm events, which
melt the ice surface, causing surface materials to
sink into the ice (Lee et al. 1998). One of these warm
events took place in December 1997, when air
temperatures reached 2.5 C (Carrasco et al.
2000), resulting in a pond at the margin of the
BIA (Casassa et al. 2004). Very little is known,
however, about the frequency of such warm events,
or whether there is any longer term trend of change
in the areal extents of BIAs.
Analysis of satellite images has been shown to
be especially useful for mapping remote areas in
Antarctica (Bindschadler 1999), not only to identify
potential meteorite sites (Choi et al. 2007), but also
to monitor fluctuations in the extent of BIAs
(Casassa et al. 2004), to detect possible crevasse
fields near terrestrial traverse routes (Bindschadler
and Vornberger 2003), to determine ice velocities
(Stearns and Hamilton 2006), and to detect the position and variations of grounding and hinge lines
(Rignot 1998). In this work we combine ASTER
satellite images with GPS and radar data collected
on the ground to map BIAs and crevasse fields in
West Antarctica. These studies have proven to be
an important precursor to more detailed analyses
related to the age and origin of Antarctic ice features, especially with respect to glaciers flowing into
ice shelves that may be susceptible to future collapse, as observed farther north in the Antarctic
Peninsula (Rignot et al. 2005). Union Glacier and
ice in Horseshoe Valley, where the Patriot Hills’
BIA is located, flow into the Ronne Ice Shelf; local
grounding lines are only a few tens of kilometers
downstream from the surveyed areas. These glaciers
have subglacial topographies well below present sea
level, so upstream migration of grounding lines in
the future could affect the stability of these glaciers,
most likely inducing an acceleration of flow and
dynamic thinning.
31.2
BLUE-ICE AREAS
31.2.1 Mapping BIA extent in the field
and on imagery
The extent of the Patriot Hills’ BIA was first
surveyed in 1996/1997, using topographic quality
Trimble Geoexplorer II GPS receivers (single frequency). Differential correction procedures were
Blue-ice areas
745
Figure 31.1. Map of Antarctica showing the nunataks studied in this chapter and some of the main stations in the
area. Figure can also be viewed as Online Supplement 31.1.
applied to the GPS data, giving a horizontal precision of between 5 and 10 m. In 2005, 2006, and
2008 geodetic-quality Javad Lexon GD GPS receivers (dual frequency) were used to outline the
BIA. After correction, submeter vertical and horizontal accuracies were obtained. These GPS surveys
attempted to follow the snow/blue-ice interface.
However, in many places intermittent patches of
thin snow covering the surface made the margin
difficult to distinguish. Consequently, a maximum
extent criterion was applied which joined up ice
areas separated by patches of snow, as specified
in the first survey by Casassa et al. (2004). The
southernmost extent of the BIA could not be
mapped in the field because of the proximity of
the ice margin to the lateral moraine; therefore, this
margin was defined from satellite imagery (Fig.
31.3).
BIAs are easily distinguishable on visible imagery
as a result of their color and unique spectral signature. Thus they can be readily delineated either by
manual digitization or supervised classification. By
manually defining the margin, the same maximum
extent criterion as that used in the field was applied
to estimate the greatest extent of blue ice likely
covered by thin snow. Applying this same rule to
a supervised classification procedure, however, is
more complex as the surface expression of blue
746
Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
Figure 31.2. Radarsat mosaic from October 20, 1997 showing the main features discussed in the text and the GPS
tracks of the 2004 and 2007 traverses to the South Pole, the 2006 traverse to subglacial Lake Ellsworth, and the
2008 traverse to Union Glacier. Figure can also be viewed as Online Supplement 31.2.
ice covered by snow is no different than other snow
and ice regions. Nevertheless, an automated
approach is simpler, more objective, and faster than
manual digitizing, and once training sets have been
defined spectral signatures can also be applied to
other BIAs.
Five midsummer ASTER satellite images of
Patriot Hills’ BIA (Table 31.1) were radiometrically
and geometrically corrected using the internal
parameters of each scene. Standard false-color
composite images were created from bands 3N, 2,
1 (RGB), using histogram equalization to improve
the contrasts between the BIA and snow surface
areas. A maximum likelihood classification with
seven classes (two snow classes, three blue-ice
classes, rock, and shadow) was performed on
unstretched (raw) bands, followed by combining
classes, sieving, and filtering using a majority
5 5 kernel to remove isolated pixels (Fig. 31.4).
31.2.2 Interannual fluctuations in the
extent of Patriot Hills’ BIA
Patriot Hills’ BIA experienced relatively little net
change between 1996 and 2008 (Table 31.2 and
Fig. 31.5); its maximum extent measured by GPS
survey in December 1997 was 13.8 km 2 . With only
one exception, GPS surveys indicate the largest
area, although it should be noted that none of the
survey dates coincided exactly with the image
dates—the closest two were November 2005
(image) and January 2006 (GPS). On this occasion
the image-defined area exceeded the GPS-defined
Blue-ice areas
747
Figure 31.3. Outline of the Patriot Hills’ BIA derived from field GPS measurements and manual digitization of
ASTER images (Tables 31.1 and 31.2). ALE ¼ location of the summer base camp of the company Antarctic and
Logistic Expeditions; L ¼ runway for aircraft with conventional landing gear; M ¼ location of the moraine debris
band discussed in the text. The background ASTER composite image (bands 1, 2, and 3N) was acquired on
November 25, 2005. The white arrows indicate the main ice flow directions based on Wendt et al. (2009). Figure
can also be viewed as Online Supplement 31.3.
area by 0.3 km 2 , which could be due to the difference in exposed blue ice in the earlier and later parts
of the season. Comparing the images from November 2002 and January 2003 shows a similar change,
with a decrease in BIA extent over the 2-month
period, suggesting that by January seasonal snow
cover is already masking some of the BIA.
On three occasions the GPS survey recorded
identical areal values (Table 31.2), although as
shown in Fig. 31.3 the actual ice margin did vary,
particularly in the northern and western margins
when comparing satellite image results. The difference in manual and automated BIA outlines can be
seen for three dates in Fig. 31.4, with good overall
agreement despite areas of considerable divergence.
The ice margin area as defined by the supervised
classification approach was consistently the lowest
value, and as noted above this is most likely due to
the difficulty in discriminating snow-covered BIAs
from spectral responses alone. Thus, where human
judgment in the field or image interpretation may
indicate or suggest that there is blue ice under thin
748
Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
Table 31.1. ASTER image details.
Local Granule ID
yyyy-mm-dd
hh:mm:ss
Band
Solar
Solar
azimuth elevation
angle
angle
AST14OTH_00311232006121131_20071009100408_26781
2006-11-23
12:11:31.789 1, 2, 3N
77.5
22.6
AST14OTH_00311252005122939_20071009100408_26778
2005-11-25
12:29:39.823 1, 2, 3N
73.6
23.7
AST14OTH_00301092003111150_20071009095957_25205
2003-01-09
11:11:50.149 1, 2, 3N
100.1
20.6
AST14OTH_00311172002123124_20071009095957_25200
2002-11-17
12:31:24.930 1, 2, 3N
72.5
22.3
AST14OTH_00311242001113329_20071009100438_27079
2001-11-24
11:33:29.390 1, 2, 3N
87.8
21.2
Figure 31.4. Outline of the Patriot Hills’ BIA from manual digitization and supervised classification for selected
dates. Note that the manually derived outline tends to delimit a greater area than the automated approach. The
background ASTER composite image (bands 1, 2, and 3N) was acquired on November 17, 2002. The black arrows
indicate the main ice flow directions. Figure can also be viewed as Online Supplement 31.4.
snow, replicating this automatically is much more
difficult. For the January 2003 image a minimum
criterion approach was adopted in which bare ice
areas interspersed among snow patches allowed
manual delineation of the BIA. This yielded an area
of 11.2 km 2 (Casassa et al. 2004), which is 1 km 2
less than the maximum criterion approach of manual analysis and 0.7 km 2 less than the result given
by automated classification. This suggests that the
automated approach can be trusted to give realistic
map areas of exposed BIA, a result that is confirmed by visual analysis of the imagery.
Blue-ice areas
749
Table 31.2. Extent of blue-ice area of Patriot Hills between 1996 and
2007 as derived by different techniques (sources of information acknowledged).
Date
Area
(km 2 )
Type of data
Source
This study
December 2008
12.8
GPS
November 2006
12.5
ASTER—digitization
12.39
ASTER—classification
January 2006
12.6
GPS
Wendt et al. 2009
November 2005
12.9
ASTER—digitization
This study
12.35
ASTER—classification
January 2005
12.6
GPS
Wendt et al. 2009
January 2003
12.2
ASTER—digitization
This study
11.90
ASTER—classification
12.9
ASTER—digitization
12.41
ASTER—classification
12.0
ASTER—digitization
11.87
ASTER—classification
December 1997
13.8
GPS
December 1996
12.6
GPS
November 2002
November 2001
As shown in Fig. 31.5, there was no long-term
trend in change of the areal extent of the Patriot
Hills’ BIA during the period of mapping, but the
year with the maximum extent (1997) did correlate
well with meteorological conditions and some of
the highest temperatures recorded that summer
(Carrasco et al. 2000). Unfortunately the meteorological record is not long enough to extend this
analysis, but it does suggest that interannual variations are closely related to local meteorological conditions. Similar results were shown by Brown and
Scambos (2005) using a longer time series of
Landsat and MODIS imagery to analyze the extent
of the BIA near Byrd Glacier in East Antarctica.
Midsummer images for this area over 15 years indicated that there can be 10–30% changes in extent
from one year to another with no net long-term
trend, and a more detailed analysis over a period
of three years indicated that the timing of BIA
exposure can vary rapidly and be very dependent
Wendt et al. 2009
on whether the meteorological conditions allow for
removal of the overlying snow cover.
31.2.3 Interannual fluctuation in the
extent of other BIAs
To the west of Patriot Hills there are additional
BIAs, not yet mapped by GPS, in the lee of the
Heritage Range. The same classification procedure
was applied to these images in order to delineate the
BIAs. In general, this approach worked well,
despite the need for a small amount of manual
editing of misidentified regions—most commonly
associated with cloud shadows misclassified as blue
ice. Elsewhere, however, cloud shadows precluded
automatic identification of BIAs; a separate class of
topographic shadow had been defined and, using
only the visible bands of ASTER, it was not possible to automatically distinguish between the two.
Visual analysis of the imagery also revealed that a
750
Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
Figure 31.5. Graph to show the difference in areal extent of the Patriot Hills’ BIA over time as denoted by manual
digitisation and automated classification of ASTER images and GPS measurements.
few very small areas of high-elevation blue ice,
much paler in color than the BIAs of Patriot Hills
were also omitted. Had the same training set as
previously defined been applied these would have
needed to be characterized by an additional blue-ice
class.
Despite these problems, the supervised classification approach is a viable way of rapidly delineating BIAs from high-resolution optical ASTER
imagery, and under clear sky conditions works very
well. Comparing the results from several years
revealed the same degree of interannual variability
as at nearby Patriot Hills.
31.3
CREVASSE DETECTION ON
SATELLITE IMAGERY
Crevasse fields can be detected on ASTER falsecolor composite images (bands 3-2-1, RGB) and
in the Radarsat mosaic (produced during the
Radarsat Antarctic Mapping Project, RAMP).
These crevasses appear as discontinuities on the
snow surface, sometimes as open linear features
but more often covered by snow and only evident
by the shadows cast by snow built up at the edges of
the crack.
The fact that the crevasses are marked by linear
discontinuities on the image allows them to be
detected both visually and by applying filters. A
high-pass filter not only enhances the crevasses,
but also other features on the snow surface such
as nunataks, blue-ice areas, and shadows (Fig.
31.6). Although topographic normalization can
reduce some shadow effects where there is deep
shadow, the reduced spectral information precludes
any distinct features from being identified. Due to
their linearity, applying a directional filter to the
high-pass image further enhances some crevasses
but, because of the different orientations of crevasses across the whole image, this approach is
not valid over large areas. Similarly, applying a
grayscale co-occurrence texture filter can enhance
some features but, due to the varying sizes of the
crevasses, texture feature extraction does not
appear to be universally applicable. Visual analysis
of the high–pass filtered image therefore proved to
be the most reliable method of detecting crevasses
that had a surface expression. It should be noted,
however, that crevasses cannot be detected in the
imagery when there are just a few tens of centimeters of snow cover.
Preliminary mapping of crevasses based on the
ASTER scenes proved to be extremely useful for
planning safe routes for tractor traverses in Antarctica (Fig. 31.7). Radio-echo sounding and groundpenetrating radar systems were used to collect
additional information on subsurface features.
All the planned routes were found to be safe,
except for one crevasse that was identified in the
field approximately 50 m from the tractor (Zamora
et al. 2007). On this occasion the crevasse had not
previously been detected because its position
(87 30 0 S, 82 25 0 W) prevented it from being within
Crevasse detection on satellite imagery 751
Figure 31.6. An area of crevasses near the Heritage Range as shown on an ASTER composite image (bands 1, 2,
and 3N) (left), and enhanced by a high-pass filter (right). Note that the filter also emphasizes edges due to cloud,
topographic shadows, and variability within a BIA. The black arrow indicates the main ice flow direction.
Figure 31.7. ASTER mosaic, based on composite bands 1, 2, and 3N, showing the track to Union Glacier and the
crevasse fields along the way. The black arrows indicate the main ice flow directions at Horseshoe Valley and at
Union Glacier. For more details of the ice flow at Union Glacier see Rivera et al. (2010). Figure can also be viewed as
Online Supplement 31.5.
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Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
range of the ASTER orbit. In theory, the southern
limit for ASTER images is 86 S with pointing of the
instrument (Kargel et al. 2005); no useful images,
however, have been acquired poleward of 85 S.
Furthermore, this crevasse could not be identified
in the Radarsat mosaic because of its lack of contrast with the surrounding surface; a limitation that
favors the use of ASTER images over the Radarsat
mosaic when both are available.
31.4
RADIO-ECHO SOUNDING AND
GROUND-PENETRATING RADAR
MEASUREMENTS
In 1996 and 1997, a profiling impulse radar with a
central frequency of 2.5 MHz was used to survey
Horseshoe Valley, near Patriot Hills. The radioecho sounding (RES) system (Plewes and Hubbard
2001) was towed by snowmobiles and sledges,
carrying the Ohio State University (OSU) transmitter connected to a notebook computer where
ice thickness data were stored. More details of the
RES system can be found in Rivera and Casassa
(2002) and Casassa et al. (2004). In 2008, a 155
MHz pulse compression radar system designed
and built at the Centro de Estudios Cientı́ficos
(CECs) was used to map the subglacial topography
between Patriot Hills and Union Glacier to determine total ice thickness in the region.
In December 2008, a 400 MHz model GSSI
ground-penetrating radar (GPR) was used for crevasse detection along the traverse between Patriot
Hills and Union Glacier. The GPR system was able
to survey the upper 20–40 m of the internal structure of the ice, with discontinuities due to internal
layers and crevasses appearing as hyperbolae on the
radar trace (Zamora et al. 2007). This system was
particularly intensively used in areas previously
detected on ASTER satellite imagery as being
highly crevassed. Normally, the transmitter was
located a few meters behind the convoy while transiting ‘‘safe’’ areas. When the tracks were near
crevasse fields that had been previously detected
on satellite imagery the transmitter was installed
at the tip of an 8 m long arm projecting ahead of
the tractor cabin. This system allowed the radar
trace to be monitored in real time and crevasses
to be detected a few seconds in advance, allowing
the driver to stop the convoy before reaching them.
The GPR survey along the track to Union
Glacier allowed detection of many more crevasses
than were previously mapped with ASTER and
Radarsat imagery. All of these additional crevasses
were covered by at least 2 m of snow and, despite
knowledge of their location, they could not be distinguished on the satellite imagery. Fig. 31.8 shows
a number of crevasses detected on the ground by the
GPR system (purple crosses) along one of the tracks
surveyed in 2008. Two more parallel tracks were
also surveyed (not shown), with a similar number
of crevasses and snow bridges. The upper part of
Fig. 31.8 shows the radargram collected along this
track, with the crevasses located at a depth of 5–10
m, which ensured the snow bridges were thick
enough to sustain the heavy weight of the tractor
and convoy used during the survey. Notably, all
snow-covered crevasses were located close to crevasse fields previously detected on ASTER images
(Fig. 31.8); therefore, allowing a buffer zone around
known crevasses permits routes to be planned from
the imagery that avoid these hidden dangers as well.
Surface features are often linked to subglacial
topography; for example, as ice flows over undulating bedrock the change in slope can lead to surface
crevasses. To further understand the location of
these surface features analysis of the internal ice
structure and bedrock topography via groundbased remote-sensing tools is valuable. Although
the surface elevation of Horseshoe Valley is of
the order of 1,000 m, the subglacial topography is
largely below sea level (1,300 m at the center of
the valley), with the bedrock composed of several
subglacial peaks. The maximum penetration range
of the 2.5 MHz impulse radar system was 1,300 m,
and no returns were obtained from bedrock in the
valley center, presumably due to attenuation of the
radar signal and power loss (Casassa et al. 1998a,
b). However, the 155 MHz pulse compression radar
system was able to penetrate through the total
thickness of ice, which at the center of Horseshoe
Valley was measured to be 2,300 m.
Internal layers have been recognized as a common feature in many Antarctic areas (Bogorodsky
et al. 1985), and at less than 1,000 m depth are
usually associated with changes in ice density, and
at greater depths are more commonly related to
highly acidic material generated by large volcanic
eruptions (Siegert 1999). These layers can be
detected on RES profiles, with additional internal
reflectors caused by solid materials embedded
within the ice. The RES profiles located close to
ALE (a private company, Antarctic Logistic and
Expeditions) base camp, recorded a strong nearsurface englacial reflector layer, located at an average ice thickness of 360 m (Fig. 31.9). This internal
Discussion
753
Figure 31.8. ASTER mosaic with crevasses detected from satellite imagery shown as red lines, and crevasses
detected in the field using the GPR system shown as purple crosses (bottom). The upper part of the figure is a
radargram showing several snow-covered wedge-shaped crevasses (numbers 13 to 18) as recorded by the GPR
system. Figure can also be viewed as Online Supplement 31.6.
layer was almost parallel to the surface of the ice,
but near the edge of southeastern Patriot Hills’s
BIA it becomes steeper, with an upward dip angle
(Fig. 31.9). Unfortunately, the first two microseconds (160 m of ice) of the return signal are
obscured by a direct air pulse, so the surface position of the internal reflector cannot be detected.
However, extrapolating the same dip angle of the
layer to the surface suggests that the internal reflector is related to a sediment/debris band observed on
ASTER imagery on the surface of the BIA (Fig.
31.3). Observations in the field showed that the
band was composed of small particles, large blocks
(1–100 cm), and occasional debris with heterogeneous geological composition, mainly limestone,
calcite, igneous metamorphic rock, sandstone, and
other nonvolcanic rocks. These clasts were generally pointing upward, indicating the upthrusting
movement of the ice at the BIA. Just a few hundred
meters to the north of the BIA the band disappears
under the snow (Fig. 31.3).
The origin of this moraine band at Horseshoe
Valley is not known. To trace the moraine back
to its origin, a more comprehensive ice flow map
of the area is needed, including a more extensive
map of surface velocity and subglacial topography
than those given by Casassa et al. (2004) and Wendt
et al. (2009).
31.5
DISCUSSION
The interior of the WAIS is relatively unknown but
is believed to have scientific value regarding meteorite collection and glaciological studies. The region
has the potential to change dramatically should
754
Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
Figure 31.9. (Left) The BIA and location of the RES profile A–A 0 . M ¼ moraine bands detected in the RES data;
IFD arrows ¼ main ice flow directions. (Upper right) The radargram A–A 0 collected in 1997. M ¼ structure of the
subglacial moraine band. (Lower right) The corrected topographic profile based on the A–A 0 radargram and GPS
data collected in the field. Figure can also be viewed as Online Supplement 31.7.
peripheral regions of the ice sheet experience major
changes. For scientific studies of the interior to be
undertaken safely and accurately, it is important
to have a thorough understanding of the terrain,
to identify potential landing sites for aircraft,
and to plan optimal routes for overland traverses.
Satellite imagery has proved invaluable for these
objectives, and ASTER imagery, perhaps more
than any other sensor, has been of greatest value.
Visual analysis of the imagery provides an initial
indication of the nature of the terrain, but the greatest benefit comes from processing the images to
highlight specific features, whether on the basis of
their spectral signature (e.g., the BIAs) or their
geometry (e.g., crevasses). Moreover, a time series
of imagery can be used to monitor changes in the
area, as shown by variations in the outlines of BIAs
delineated on ASTER imagery since 2001.
The use of satellite data, however, does have
limitations. The ASTER time series is relatively
short, does not have full latitudinal coverage, geographic coverage is limited, and cloud and cast
shadows can mask some surface features. By contrast, the RAMP mosaic does provide full coverage,
but it is a dataset that does not allow for change
detection and the limited contrast in some areas
precludes feature detection. It is therefore essential
to utilize satellite imagery with data collected in the
field to get a full understanding of the region.
As discussed above, RES can be used to detect
subsurface features to a depth of several hundred
meters, some of which can be traced to a surface
manifestation that is also evident in satellite
imagery. Ice dynamics may force a body of material
located at depth towards the surface, but from the
RES profile alone little can be learned about the
Conclusions
composition of this material. Conversely, examination of imagery reveals the surface location of the
material but provides no information on its subglacial extension; indeed, surface deposits such as
those at the Patriot Hills’ BIA have been widely
observed at the surface of BIAs in Antarctica, but
analysis of their full extent to date has been limited.
Despite being able to show small near-surface features, low-frequency RES has its limitations in that
the signal may not be sufficiently powerful to penetrate the full depth of ice to the bedrock; higher
frequency radar systems, however, can record a
profile through several kilometers of ice but without
the same detailed resolution. The return signal from
these deeper profiles can again be examined in conjunction with satellite imagery and GPR profiles to
link variations in the bedrock topography to surface expressions in the form of crevasses.
ASTER imagery combined with GPS, RES, and
GPR data permit a much more detailed interpretation of the form, origin, and development of
features within the Antarctic Ice Sheet than can be
obtained from any one data source alone.
31.6
CONCLUSIONS
This is the first time ASTER images have been used
to map parts of West Antarctica, thereby enhancing
what we know about the region. Prior to this work
the location of many of the nunataks of the area
was only available from USGS maps constructed
in the 1950s, with crevasse locations marked
approximately and little information regarding
surface features.
ASTER imagery was used to map areas of blue
ice in West Antarctica, which are of critical importance for aircraft logistics, meteorite collectors, and
glaciological dynamics. Fluctuations in the outline
of Patriot Hills’ BIA over the period 1996–2008
were mapped from a combination of field GPS
measurements, manual analysis of visible imagery,
and automated analysis of spectral data. There is no
significant long-term trend in the extent of Patriot
Hills’ BIA over this time frame, although there can
be large variations from one year to another and
within a season, as patches of snow cover accumulate and are removed depending on the prevailing
meteorological conditions. This patchy snow cover
makes it difficult to identify the BIA margin, either
on the ground or on imagery. Nevertheless, we
adopted a maximum extent approach enabling us
to link clearly visible areas of blue ice. This can be
755
more readily undertaken by human analysis in the
field or in viewing images; however, a supervised
classification approach allows all areas with a clear
or mixed pixel blue-ice spectral response to be identified. The area mapped in this way is typically
smaller in size than that defined by either of the
visual approaches, but is greater than that mapped
by a minimum extent criterion, suggesting that for
cloud-free and shadow-free areas it is a valid,
simple, and objective method of detecting BIAs
reliably.
The linearity of crevasse fields also makes them
easily detectable on ASTER scenes when a highpass filter is used. However, the filter also identifies
other features, and the varied shape and orientation
of crevasses prevents them from being singled out
by simple filtering or texture enhancement. As safe
route planning through crevasse fields demands the
utmost reliability in identifying all linear features,
human analysis of the filtered image is preferred to
any automated approach. Although a Radarsat
mosaic provides greater latitudinal coverage than
ASTER images, a lack of radar image contrast
makes optical imagery a better option for detecting
crevasses. Despite there being no accidents related
to crevasses in recent years as a result of route
determination based on image analysis, more work
is needed to map crevasses covered by tens of
centimeters to tens of meters of snow, which can
completely obscure their surface expression in
visible and microwave imagery.
ASTER images show many interesting additional
features, such as moraine bands, whose presence
can be further understood by combining visible
imagery with subsurface profiles collected by
radio-echo sounding and ground-penetrating radar.
By matching features in more than one dataset
much can be learned about their spatial extent
and composition. Many internal layers within the
RES trace are isochronous, but where ice dynamics
in the vicinity of nunataks and BIAs force an
upward ice flow, englacial sediments cut across
the other layers at an angle of thrust associated with
the degree of compressive ice flow. This was demonstrated in Patriot Hills’ BIA by linking a band of
surface deposits evident on the ASTER image at the
BIA margin with a RES profile showing the subglacial extension of this moraine to depths of about
360 m some 4 km distant.
As this chapter shows, combining different
remotely sensed products is extremely useful when
it comes to learning more about the ice conditions
of Antarctica and to avoiding crevasse-related acci-
756
Mapping blue-ice areas and crevasses in West Antarctica using ASTER images, GPS, and radar measurements
dents in the field. Field-based GPR profiles have
been used to detect many hidden snow-covered
crevasses and thereby remove the significant risk
they pose to Antarctic expeditions. The challenge
now is to see whether other image-derived parameters, such as measures of surface roughness, can be
used to identify these features and thus ensure
the contribution of satellite imagery to Antarctic
research continues to have both scientific and
safety benefits.
31.7
ACKNOWLEDGMENTS
This research was funded by INACH/CONICYT
under the Ring Project ARTG02-2006. GLIMS
provided the ASTER satellite images. Jorge Quinteros, the late Victor Villanueva, Rubén Carvallo,
Heiner Lange, the late Jens Wendt, and Gino
Casassa collaborated in many ways in this research,
including data collection during field campaigns,
support in the field, and discussion of results. José
Araos collaborated with radar and GPS analysis.
Claudio Bravo and David Farı́as helped with the
figures. ALE provided valuable support to campaigns conducted at Patriot Hills and Union
Glacier since 2004. This research has been supported by the Centro de Estudios Cientı́ficos
(CECs). CECs is funded by the Chilean Government through the Centers of Excellence Base Financing Program of CONICYT. Andrés Rivera thanks
the Guggenheim Foundation. ASTER data courtesy of NASA/GSFC/METI/Japan Space Systems,
the U.S./Japan ASTER Science Team, and the
GLIMS project.
31.8
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