CHAPTER 31

Mapping blue-ice areas and crevasses in West using ASTER images, GPS, and radar measurements

Andre´s Rivera, Fiona Cawkwell, Anja Wendt, and Rodrigo Zamora

ABSTRACT from ASTER imagery, with the aim of identifying other landing sites for aircraft, as well as providing Before the satellite era, relatively little was known a detailed map for meteorite seekers. ASTER com- about the interior of the posite images have also been used to map safe (WAIS). Of special interest are the rock outcrops routes for terrestrial traverses through crevasse associated with blue-ice areas (BIAs), which have zones. High-pass filters enhanced crevasse features, been exploited for logistical purposes as well as but visual analysis proved to be the most reliable being the subject of scientific research. The blue method of identifying all crevasses. ASTER images ice consists of relatively snow-free glacier ice that were superior to microwave imagery for crevasse is undergoing ablation. detection, as the latter can lack sufficient contrast; One of these BIAs is Patriot Hills (80180S, however, only Radarsat imagery provided coverage 81220W) where aircraft with conventional landing of higher latitude regions. Information gleaned gear have been landing for more than 20 years. This from visible imagery can be combined with that is now the main hub supporting large terrestrial of field-based radio-echo sounding and ground- expeditions conducted by Chilean scientists within penetrating radar profiles through the ice to map Antarctica. Kinematic GPS has been used to map internal layers and bedrock topography with the BIAs since 1996, with areas delineated on ASTER objective of enhancing our knowledge of this images since 2001 using both manual and auto- remote region. mated approaches. The GPS method typically delimits the largest area, and supervised classifica- tion of the images by an algorithm demarcates the 31.1 INTRODUCTION smallest area due to thin patchy snow cover over- lying blue ice. These areas do not display a unique Blue-ice areas (BIAs) are a rare feature in Antarc- spectral response when mostly snow covered, so tica (occupying between 0.8 and 1.6% of the con- that they can only be visually discriminated. This tinent; Winther et al., 2001) and are commonly, but detailed record of BIA extent shows no significant not exclusively, associated with nunataks. These areal change with time, but does display interannual rock outcrops represent a barrier to the strong variability, which most likely is connected to pre- katabatic winds that flow constantly from the inter- vailing meteorological conditions. BIAs around ior of the ice sheet, with the resulting turbulent air other nunataks in the region have been mapped flow responsible for removing surface snow, leaving 744 Mapping blue-ice areas and crevasses in using ASTER images, GPS, and radar measurements a bare ice face. BIAs have no net annual accumula- Thiel Mountains (Choi et al. 2007). One of the tion, and ablation occurs mainly through sublima- reasons commonly given to explain the scarcity of tion, which can be much higher than over adjacent meteorites in these BIAs, especially Patriot Hills, is snowfields (Bintanja and Reijmer 2001), and the occasional occurrence of warm events, which through wind erosion, resulting in a local negative melt the ice surface, causing surface materials to surface mass balance. BIAs tend to be smooth, sink into the ice (Lee et al. 1998). One of these warm although they may be rippled, which facilitates their events took place in December 1997, when air use for aircraft with conventional landing gear temperatures reached 2.5C (Carrasco et al. rather than aircraft with skis. Other BIAs are asso- 2000), resulting in a pond at the margin of the ciated with steep slopes, glacial valleys, or the lower BIA (Casassa et al. 2004). Very little is known, parts of glacial basins, where accelerated katabatic however, about the frequency of such warm events, winds can effectively remove snow leaving a smooth or whether there is any longer term trend of change ice surface. For more information on BIAs see in the areal extents of BIAs. Bintanja (1999). Analysis of satellite images has been shown to Under steady state conditions, ice flows converge be especially useful for mapping remote areas in horizontally in the vicinity of BIAs, with upward ice Antarctica (Bindschadler 1999), not only to identify flows at the margins of nunataks balanced by ice potential meteorite sites (Choi et al. 2007), but also mass losses mainly due to sublimation at the surface to monitor fluctuations in the extent of BIAs (Bintanja 1999). These ice flows transport englacial (Casassa et al. 2004), to detect possible crevasse material to the surface, with unusually large vol- fields near terrestrial traverse routes (Bindschadler umes of surface deposits found along the margins and Vornberger 2003), to determine ice velocities of BIAs adjacent to many nunataks and mountain (Stearns and Hamilton 2006), and to detect the pos- ranges. Thus BIAs are also of interest because they ition and variations of grounding and hinge lines can be a concentrated source of meteorites that (Rignot 1998). In this work we combine ASTER have fallen over a wider region over many millen- satellite images with GPS and radar data collected nia, been trapped and transported within the ice, on the ground to map BIAs and crevasse fields in and covered by snow, before being exhumed on the West Antarctica. These studies have proven to be surface (Corti et al. 2003). As of 1999, more than an important precursor to more detailed analyses 20,000 meteorites had been discovered in Antarctic related to the age and origin of Antarctic ice feat- BIAs (Bintanja 1999). ures, especially with respect to glaciers flowing into Patriot Hills, with a maximum altitude of 1,246 m ice shelves that may be susceptible to future col- asl (USGS 1966), are located at the southeastern tip lapse, as observed farther north in the Antarctic of Ellsworth Mountains. They comprise one of Peninsula (Rignot et al. 2005). Union Glacier and many Antarctic nunataks, which act as obstacles ice in Horseshoe Valley, where the Patriot Hills’ to katabatic winds (Figs. 31.1 and 31.2). Increased BIA is located, flow into the Ronne Ice Shelf; local wind speeds on the leeward side of Patriot Hills grounding lines are only a few tens of kilometers limits the accumulation of snow, and as a conse- downstream from the surveyed areas. These glaciers quence has led to the generation of a BIA with an have subglacial topographies well below present sea area of approximately 12 km2. This BIA is located level, so upstream migration of grounding lines in at the southern edge of Horseshoe Valley and has the future could affect the stability of these glaciers, slopes of less than 1 degree and surface topography most likely inducing an acceleration of flow and varying in altitude between 1,100 m asl on the west- dynamic thinning. ern 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 31.2 BLUE-ICE AREAS center of Horseshoe Valley (Wendt et al. 2009). No meteorites were found in the BIA of Patriot 31.2.1 Mapping BIA extent in the field Hills during an initial expedition in 1997/1998 (Lee and on imagery et al. 1998). However, in 2000 a meteorite was found in a nearby moraine band (Grossman and The extent of the Patriot Hills’ BIA was first Zipfel 2001). As for other BIAs in the region, surveyed in 1996/1997, using topographic quality meteorites have been found at Martin Hill and Trimble Geoexplorer II GPS receivers (single fre- Pirrit Hill (Lee et al. 1999), and more recently at quency). 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 pre- mapped in the field because of the proximity of cision of between 5 and 10 m. In 2005, 2006, and the ice margin to the lateral moraine; therefore, this 2008 geodetic-quality Javad Lexon GD GPS re- margin was defined from satellite imagery (Fig. ceivers (dual frequency) were used to outline the 31.3). BIA. After correction, submeter vertical and hori- BIAs are easily distinguishable on visible imagery zontal accuracies were obtained. These GPS surveys as a result of their color and unique spectral signa- attempted to follow the snow/blue-ice interface. ture. Thus they can be readily delineated either by However, in many places intermittent patches of manual digitization or supervised classification. By thin snow covering the surface made the margin manually defining the margin, the same maximum difficult to distinguish. Consequently, a maximum extent criterion as that used in the field was applied extent criterion was applied which joined up ice to estimate the greatest extent of blue ice likely areas separated by patches of snow, as specified covered by thin snow. Applying this same rule to in the first survey by Casassa et al. (2004). The a supervised classification procedure, however, is southernmost extent of the BIA could not be 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 , 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 classes, sieving, and filtering using a majority and ice regions. Nevertheless, an automated 5 5 kernel to remove isolated pixels (Fig. 31.4). approach is simpler, more objective, and faster than manual digitizing, and once training sets have been defined spectral signatures can also be applied to 31.2.2 Interannual fluctuations in the other BIAs. extent of Patriot Hills’ BIA Five midsummer ASTER satellite images of Patriot Hills’ BIA (Table 31.1) were radiometrically Patriot Hills’ BIA experienced relatively little net and geometrically corrected using the internal change between 1996 and 2008 (Table 31.2 and parameters of each scene. Standard false-color Fig. 31.5); its maximum extent measured by GPS composite images were created from bands 3N, 2, survey in December 1997 was 13.8 km2. With only 1 (RGB), using histogram equalization to improve one exception, GPS surveys indicate the largest the contrasts between the BIA and snow surface area, although it should be noted that none of the areas. A maximum likelihood classification with survey dates coincided exactly with the image seven classes (two snow classes, three blue-ice dates—the closest two were November 2005 classes, rock, and shadow) was performed on (image) and January 2006 (GPS). On this occasion unstretched (raw) bands, followed by combining 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 km2, which could be due to the differ- when comparing satellite image results. The differ- ence in exposed blue ice in the earlier and later parts ence in manual and automated BIA outlines can be of the season. Comparing the images from Novem- seen for three dates in Fig. 31.4, with good overall ber 2002 and January 2003 shows a similar change, agreement despite areas of considerable divergence. with a decrease in BIA extent over the 2-month The ice margin area as defined by the supervised period, suggesting that by January seasonal snow classification approach was consistently the lowest cover is already masking some of the BIA. value, and as noted above this is most likely due to On three occasions the GPS survey recorded the difficulty in discriminating snow-covered BIAs identical areal values (Table 31.2), although as from spectral responses alone. Thus, where human shown in Fig. 31.3 the actual ice margin did vary, judgment in the field or image interpretation may particularly in the northern and western margins 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 less than the maximum criterion approach of man- difficult. For the January 2003 image a minimum ual analysis and 0.7 km2 less than the result given criterion approach was adopted in which bare ice by automated classification. This suggests that the areas interspersed among snow patches allowed automated approach can be trusted to give realistic manual delineation of the BIA. This yielded an area map areas of exposed BIA, a result that is con- of 11.2 km2 (Casassa et al. 2004), which is 1 km2 firmed 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 acknowl- edged).

Date Area Type of data Source (km2)

December 2008 12.8 GPS This study

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

November 2002 12.9 ASTER—digitization

12.41 ASTER—classification

November 2001 12.0 ASTER—digitization

11.87 ASTER—classification

December 1997 13.8 GPS

December 1996 12.6 GPS Wendt et al. 2009

As shown in Fig. 31.5, there was no long-term on whether the meteorological conditions allow for trend in change of the areal extent of the Patriot removal of the overlying snow cover. Hills’ BIA during the period of mapping, but the year with the maximum extent (1997) did correlate 31.2.3 Interannual fluctuation in the well with meteorological conditions and some of extent of other BIAs the highest temperatures recorded that summer (Carrasco et al. 2000). Unfortunately the meteoro- To the west of Patriot Hills there are additional logical record is not long enough to extend this BIAs, not yet mapped by GPS, in the lee of the analysis, but it does suggest that interannual varia- Heritage Range. The same classification procedure tions are closely related to local meteorological con- was applied to these images in order to delineate the ditions. Similar results were shown by Brown and BIAs. In general, this approach worked well, Scambos (2005) using a longer time series of despite the need for a small amount of manual Landsat and MODIS imagery to analyze the extent editing of misidentified regions—most commonly of the BIA near Byrd Glacier in . associated with cloud shadows misclassified as blue Midsummer images for this area over 15 years indi- ice. Elsewhere, however, cloud shadows precluded cated that there can be 10–30% changes in extent automatic identification of BIAs; a separate class of from one year to another with no net long-term topographic shadow had been defined and, using trend, and a more detailed analysis over a period only the visible bands of ASTER, it was not poss- of three years indicated that the timing of BIA ible to automatically distinguish between the two. exposure can vary rapidly and be very dependent 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, as nunataks, blue-ice areas, and shadows (Fig. much paler in color than the BIAs of Patriot Hills 31.6). Although topographic normalization can were also omitted. Had the same training set as reduce some shadow effects where there is deep previously defined been applied these would have shadow, the reduced spectral information precludes needed to be characterized by an additional blue-ice any distinct features from being identified. Due to class. their linearity, applying a directional filter to the Despite these problems, the supervised classi- high-pass image further enhances some crevasses fication approach is a viable way of rapidly delin- but, because of the different orientations of cre- eating BIAs from high-resolution optical ASTER vasses across the whole image, this approach is imagery, and under clear sky conditions works very not valid over large areas. Similarly, applying a well. Comparing the results from several years grayscale co-occurrence texture filter can enhance revealed the same degree of interannual variability some features but, due to the varying sizes of the as at nearby Patriot Hills. crevasses, texture feature extraction does not appear to be universally applicable. Visual analysis of the high–pass filtered image therefore proved to 31.3 CREVASSE DETECTION ON be the most reliable method of detecting crevasses SATELLITE IMAGERY that had a surface expression. It should be noted, however, that crevasses cannot be detected in the Crevasse fields can be detected on ASTER false- imagery when there are just a few tens of centi- color composite images (bands 3-2-1, RGB) and meters of snow cover. in the Radarsat mosaic (produced during the Preliminary mapping of crevasses based on the Radarsat Antarctic Mapping Project, RAMP). ASTER scenes proved to be extremely useful for These crevasses appear as discontinuities on the planning safe routes for tractor traverses in Antarc- snow surface, sometimes as open linear features tica (Fig. 31.7). Radio-echo sounding and ground- but more often covered by snow and only evident penetrating radar systems were used to collect by the shadows cast by snow built up at the edges of additional information on subsurface features. the crack. All the planned routes were found to be safe, The fact that the crevasses are marked by linear except for one crevasse that was identified in the discontinuities on the image allows them to be field approximately 50 m from the tractor (Zamora detected both visually and by applying filters. A et al. 2007). On this occasion the crevasse had not high-pass filter not only enhances the crevasses, previously been detected because its position but also other features on the snow surface such (87300S, 82250W) 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. 752 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 Radarsat imagery. All of these additional crevasses limit for ASTER images is 86S with pointing of the were covered by at least 2 m of snow and, despite instrument (Kargel et al. 2005); no useful images, knowledge of their location, they could not be dis- however, have been acquired poleward of 85S. tinguished on the satellite imagery. Fig. 31.8 shows Furthermore, this crevasse could not be identified a number of crevasses detected on the ground by the in the Radarsat mosaic because of its lack of con- GPR system (purple crosses) along one of the tracks trast with the surrounding surface; a limitation that surveyed in 2008. Two more parallel tracks were favors the use of ASTER images over the Radarsat also surveyed (not shown), with a similar number mosaic when both are available. 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 31.4 RADIO-ECHO SOUNDING AND m, which ensured the snow bridges were thick GROUND-PENETRATING RADAR enough to sustain the heavy weight of the tractor MEASUREMENTS and convoy used during the survey. Notably, all snow-covered crevasses were located close to cre- In 1996 and 1997, a profiling impulse radar with a vasse fields previously detected on ASTER images central frequency of 2.5 MHz was used to survey (Fig. 31.8); therefore, allowing a buffer zone around Horseshoe Valley, near Patriot Hills. The radio- known crevasses permits routes to be planned from echo sounding (RES) system (Plewes and Hubbard the imagery that avoid these hidden dangers as well. 2001) was towed by snowmobiles and sledges, Surface features are often linked to subglacial carrying the Ohio State University (OSU) trans- topography; for example, as ice flows over undulat- mitter connected to a notebook computer where ing bedrock the change in slope can lead to surface ice thickness data were stored. More details of the crevasses. To further understand the location of RES system can be found in Rivera and Casassa these surface features analysis of the internal ice (2002) and Casassa et al. (2004). In 2008, a 155 structure and bedrock topography via ground- MHz pulse compression radar system designed based remote-sensing tools is valuable. Although and built at the Centro de Estudios Cientı´ficos the surface elevation of Horseshoe Valley is of (CECs) was used to map the subglacial topography the order of 1,000 m, the subglacial topography is between Patriot Hills and Union Glacier to deter- largely below sea level (1,300 m at the center of mine total ice thickness in the region. the valley), with the bedrock composed of several In December 2008, a 400 MHz model GSSI subglacial peaks. The maximum penetration range ground-penetrating radar (GPR) was used for cre- of the 2.5 MHz impulse radar system was 1,300 m, vasse detection along the traverse between Patriot and no returns were obtained from bedrock in the Hills and Union Glacier. The GPR system was able valley center, presumably due to attenuation of the to survey the upper 20–40 m of the internal struc- radar signal and power loss (Casassa et al. 1998a, ture of the ice, with discontinuities due to internal b). However, the 155 MHz pulse compression radar layers and crevasses appearing as hyperbolae on the system was able to penetrate through the total radar trace (Zamora et al. 2007). This system was thickness of ice, which at the center of Horseshoe particularly intensively used in areas previously Valley was measured to be 2,300 m. detected on ASTER satellite imagery as being Internal layers have been recognized as a com- highly crevassed. Normally, the transmitter was mon feature in many Antarctic areas (Bogorodsky located a few meters behind the convoy while tran- et al. 1985), and at less than 1,000 m depth are siting ‘‘safe’’ areas. When the tracks were near usually associated with changes in ice density, and crevasse fields that had been previously detected at greater depths are more commonly related to on satellite imagery the transmitter was installed highly acidic material generated by large volcanic at the tip of an 8 m long arm projecting ahead of eruptions (Siegert 1999). These layers can be the tractor cabin. This system allowed the radar detected on RES profiles, with additional internal trace to be monitored in real time and crevasses reflectors caused by solid materials embedded to be detected a few seconds in advance, allowing within the ice. The RES profiles located close to the driver to stop the convoy before reaching them. ALE (a private company, Antarctic Logistic and The GPR survey along the track to Union Expeditions) base camp, recorded a strong near- Glacier allowed detection of many more crevasses surface englacial reflector layer, located at an aver- than were previously mapped with ASTER and age 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, movement of the ice at the BIA. Just a few hundred but near the edge of southeastern Patriot Hills’s meters to the north of the BIA the band disappears BIA it becomes steeper, with an upward dip angle under the snow (Fig. 31.3). (Fig. 31.9). Unfortunately, the first two micro- The origin of this moraine band at Horseshoe seconds (160 m of ice) of the return signal are Valley is not known. To trace the moraine back obscured by a direct air pulse, so the surface pos- to its origin, a more comprehensive ice flow map ition of the internal reflector cannot be detected. of the area is needed, including a more extensive However, extrapolating the same dip angle of the map of surface velocity and subglacial topography layer to the surface suggests that the internal reflec- than those given by Casassa et al. (2004) and Wendt tor is related to a sediment/debris band observed on et al. (2009). 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 31.5 DISCUSSION (1–100 cm), and occasional debris with hetero- geneous geological composition, mainly limestone, The interior of the WAIS is relatively unknown but calcite, igneous metamorphic rock, sandstone, and is believed to have scientific value regarding meteor- other nonvolcanic rocks. These clasts were gener- ite collection and glaciological studies. The region ally pointing upward, indicating the upthrusting 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–A0.M¼ moraine bands detected in the RES data; IFD arrows ¼ main ice flow directions. (Upper right) The radargram A–A0 collected in 1997. M ¼ structure of the subglacial moraine band. (Lower right) The corrected topographic profile based on the A–A0 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 The use of satellite data, however, does have changes. For scientific studies of the interior to be limitations. The ASTER time series is relatively undertaken safely and accurately, it is important short, does not have full latitudinal coverage, geo- to have a thorough understanding of the terrain, graphic coverage is limited, and cloud and cast to identify potential landing sites for aircraft, shadows can mask some surface features. By con- and to plan optimal routes for overland traverses. trast, the RAMP mosaic does provide full coverage, Satellite imagery has proved invaluable for these but it is a dataset that does not allow for change objectives, and ASTER imagery, perhaps more detection and the limited contrast in some areas than any other sensor, has been of greatest value. precludes feature detection. It is therefore essential Visual analysis of the imagery provides an initial to utilize satellite imagery with data collected in the indication of the nature of the terrain, but the great- field to get a full understanding of the region. est benefit comes from processing the images to As discussed above, RES can be used to detect highlight specific features, whether on the basis of subsurface features to a depth of several hundred their spectral signature (e.g., the BIAs) or their meters, some of which can be traced to a surface geometry (e.g., crevasses). Moreover, a time series manifestation that is also evident in satellite of imagery can be used to monitor changes in the imagery. Ice dynamics may force a body of material area, as shown by variations in the outlines of BIAs located at depth towards the surface, but from the delineated on ASTER imagery since 2001. RES profile alone little can be learned about the Conclusions 755 composition of this material. Conversely, examina- more readily undertaken by human analysis in the tion of imagery reveals the surface location of the field or in viewing images; however, a supervised material but provides no information on its sub- classification approach allows all areas with a clear glacial extension; indeed, surface deposits such as or mixed pixel blue-ice spectral response to be iden- those at the Patriot Hills’ BIA have been widely tified. The area mapped in this way is typically observed at the surface of BIAs in Antarctica, but smaller in size than that defined by either of the analysis of their full extent to date has been limited. visual approaches, but is greater than that mapped Despite being able to show small near-surface feat- by a minimum extent criterion, suggesting that for ures, low-frequency RES has its limitations in that cloud-free and shadow-free areas it is a valid, the signal may not be sufficiently powerful to pene- simple, and objective method of detecting BIAs trate the full depth of ice to the bedrock; higher reliably. frequency radar systems, however, can record a The linearity of crevasse fields also makes them profile through several kilometers of ice but without easily detectable on ASTER scenes when a high- the same detailed resolution. The return signal from pass filter is used. However, the filter also identifies these deeper profiles can again be examined in con- other features, and the varied shape and orientation junction with satellite imagery and GPR profiles to of crevasses prevents them from being singled out link variations in the bedrock topography to sur- by simple filtering or texture enhancement. As safe face expressions in the form of crevasses. route planning through crevasse fields demands the ASTER imagery combined with GPS, RES, and utmost reliability in identifying all linear features, GPR data permit a much more detailed inter- human analysis of the filtered image is preferred to pretation of the form, origin, and development of any automated approach. Although a Radarsat features within the Antarctic Ice Sheet than can be mosaic provides greater latitudinal coverage than obtained from any one data source alone. ASTER images, a lack of radar image contrast makes optical imagery a better option for detecting crevasses. Despite there being no accidents related 31.6 CONCLUSIONS to crevasses in recent years as a result of route determination based on image analysis, more work This is the first time ASTER images have been used is needed to map crevasses covered by tens of to map parts of West Antarctica, thereby enhancing centimeters to tens of meters of snow, which can what we know about the region. Prior to this work completely obscure their surface expression in the location of many of the nunataks of the area visible and microwave imagery. was only available from USGS maps constructed ASTER images show many interesting additional in the 1950s, with crevasse locations marked features, such as moraine bands, whose presence approximately and little information regarding can be further understood by combining visible surface features. imagery with subsurface profiles collected by ASTER imagery was used to map areas of blue radio-echo sounding and ground-penetrating radar. ice in West Antarctica, which are of critical impor- By matching features in more than one dataset tance for aircraft logistics, meteorite collectors, and much can be learned about their spatial extent glaciological dynamics. Fluctuations in the outline and composition. Many internal layers within the of Patriot Hills’ BIA over the period 1996–2008 RES trace are isochronous, but where ice dynamics were mapped from a combination of field GPS in the vicinity of nunataks and BIAs force an measurements, manual analysis of visible imagery, upward ice flow, englacial sediments cut across and automated analysis of spectral data. There is no the other layers at an angle of thrust associated with significant long-term trend in the extent of Patriot the degree of compressive ice flow. This was demon- Hills’ BIA over this time frame, although there can strated in Patriot Hills’ BIA by linking a band of be large variations from one year to another and surface deposits evident on the ASTER image at the within a season, as patches of snow cover accumu- BIA margin with a RES profile showing the sub- late and are removed depending on the prevailing glacial extension of this moraine to depths of about meteorological conditions. This patchy snow cover 360 m some 4 km distant. makes it difficult to identify the BIA margin, either As this chapter shows, combining different on the ground or on imagery. Nevertheless, we remotely sensed products is extremely useful when adopted a maximum extent approach enabling us it comes to learning more about the ice conditions to link clearly visible areas of blue ice. This can be 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 Brown, I.C., and Scambos, T.A. (2005) Satellite monitor- been used to detect many hidden snow-covered ing of blue ice extent near Byrd Glacier, Antarctica. crevasses and thereby remove the significant risk Annals of Glaciology, 39, 223–230. they pose to Antarctic expeditions. The challenge Carrasco, J.F., Casassa, G., and Rivera, A. (2000) A now is to see whether other image-derived param- warm event at Patriot Hills, Antarctica: An ENSO eters, such as measures of surface roughness, can be related phenomenon? In: J.F. Carrasco, G. Casassa, used to identify these features and thus ensure and A. Rivera (Eds.), Sixth International Conference the contribution of satellite imagery to Antarctic on Southern Hemisphere Meteorology and Oceanog- research continues to have both scientific and raphy, April 37, 2000, Santiago, Chile, American Meteorological Society, Boston, MA, pp. 240–241. safety benefits. Casassa, C., Brecher, H., Ca´rdenas, C., and Rivera, A. (1998a) Mass balance of the Antarctic ice sheet at Patriot Hills. Annals of Glaciology, 27, 130–134. 31.7 ACKNOWLEDGMENTS Casassa, G., Rivera, A., Lange, H., Carvallo, R., Brecher, H., Ca´rdenas, C., and Smith, R. (1998b) Radar and This research was funded by INACH/CONICYT GPS studies at Horseshoe Valley, Patriot Hills, Ant- under the Ring Project ARTG02-2006. GLIMS arctica. In: H. Oerter (Ed.), Filchner–Ronne Ice Shelf provided the ASTER satellite images. Jorge Quin- Programme (FRISP), Report No. 12 (1998), Alfred teros, the late Victor Villanueva, Rube´n Carvallo, Wegener Institute for Polar and Marine Research, Heiner Lange, the late Jens Wendt, and Gino Bremerhaven, Germany, pp. 7–18. Casassa collaborated in many ways in this research, Casassa, G., Rivera, A., Acun˜ a, C., Brecher H., and including data collection during field campaigns, Lange, H. (2004) Elevation change and ice flow at support in the field, and discussion of results. Jose´ Horseshoe Valley, Patriot Hills, Antarctica. Annals Araos collaborated with radar and GPS analysis. of Glaciology, 39, 20–28. Claudio Bravo and David Farı´as helped with the Choi, B., Lee, J.I., Ahn, I., Han, J.M., and Kusakabe, M. figures. ALE provided valuable support to cam- (2007) Antarctic meteorites recovered from Thiel paigns conducted at Patriot Hills and Union Mountains, West Antarctica by the First Korea Glacier since 2004. This research has been sup- Expedition for Antarctic Meteorites, 70th Annual ported by the Centro de Estudios Cientı´ficos Meeting of the Meteoritical Society (Abstract d5173). (CECs). CECs is funded by the Chilean Govern- Corti, G., Zeoli, A., and Bonini, M. (2003) Ice-flow ment through the Centers of Excellence Base Finan- dynamics and meteorite collection in Antarctica. Earth cing Program of CONICYT. Andre´s Rivera thanks and Planetary Science Letters, 215, 371–378. the Guggenheim Foundation. ASTER data cour- Grossman, J., and Zipfel, J. (2001) The Meteoritical Bul- tesy of NASA/GSFC/METI/Japan Space Systems, letin, No. 85. Meteoritics and Planetary Science, 36, the U.S./Japan ASTER Science Team, and the A293–A322. GLIMS project. 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