Abstract - By employing a number of complementary space-borne, air-borne,
and surface remote sensing techniques significant advances are being made
in the study of glacial hydrological systems. A case study from Bylot Island
in the Canadian Arctic provides examples of the application of several techniques
for examining spatial and temporal characteristics of the glacial hydrological
system.A time series of Landsat imagery and aerial photographs was employed
to identify areas of recent glacial retreat and icing formation. Synthetic
aperture radar data was used to provide information on the seasonal variability
in surface runoff patterns. This data was found to be especially effective
for detecting water flow within the snowpack during the winter months. Ground
penetrating radar was employed to map the three-dimensional location of
englacial drainage tunnels. The contrasting dielectric properties of ice,
water and air enabled the determination of whether the drainage tunnels
were currently active.
One of the greatest challenges in the study of glacial hydrology has always been acquiring complete spatial and temporal measurements of the hydrological network. As glaciers tend to be in remote locations, direct year-round monitoring is difficult. As well, many remote sensing techniques are not suitable to image winter hydrologic activity (e.g.. icing development) due to the blanket of snow that covers the ground surface and the low light levels experienced at higher latitudes in winter.
The significant contrast in dielectric properties between water and ice enables SAR imaging radar systems to be used to map the presence of liquid water on glaciers, and the variability of the moisture content in the snowpack. Since SAR systems supply their own illumination they are not affected by Arctic winter darkness. However, SAR systems only image the ground surface. To map the subsurface component of the hydrological network a time-domain reflection technique such as ground-penetrating radar (GPR) is required.
Over the last three decades, radio echo devices and time-domain profiling radar systems have been developed for a variety of large scale glaciological applications (e.g. Davis et al., 1973; Bentley et al., 1979). However, until recently these systems tended to be very bulky, and operating them in the rugged terrain around a glacier terminus was difficult to impossible. Ground penetrating radar systems operated from aircraft have provided some subsurface data, but the strong reflection from the air/ground interface dramatically limits the amount of energy entering the ice and thus the depth of penetration (Arcone et al., 1995).
Recently developed GPR systems, designed for general geomorphologic and geotechnical work, are backpack portable and have technical specifications appropriate to glaciological work. However, they have yet to be extensively utilized to study the glacial environment (Moorman, 1998). The purpose of this project was to assess the feasibility of using a combination of traditional optical imagery along with SAR and GPR data to study the entire surface and subsurface hydrological system in a glacial environment.
The southwest portion of Bylot Island in the Canadian Arctic (73°N 80°W) was chosen for this research as it has varied glacial and hydrological conditions (Fig. 1). Glaciers originating in a central ice field of the Byam Martin Mountains flow down deep valleys onto the southern lowlands. Icings form at the terminii of several glaciers on the island due to englacial and subglacial drainage continuing during the cold winter months.
Historical Data: A series of aerial photographs and Landsat images dating back to 1948, was used to identify temporal changes in the glacial extent, and the presence of icings. Due to the limitations of optical imagery, all of the images were collected during the summer months.
It was found that 8 of the 18 larger glaciers on the south side of the island are rapidly retreating. The bare moraine surface exposed as a glacier retreats has a unique spectral signature that was quantified with areal photograph and Landsat TM data (Fig. 2).
Icings vary in size and shape from one year to the next, but generally form in front of the same glaciers each year. On-site observations reveal that most of the icings completely melt each summer. However, a portion of the icing in front of Glacier B26 appears to have persisted year-round since before the first aerial photographs were taken in 1948.
Ice-dammed lakes are generally present up-glacier from icings. These lakes are hypothesized to provide the source of water in the winter to feed the icings. However, drainage patterns within the glaciers and the complete seasonal regime could not be measured using optical imagery.
Surface Hydrology: Winter SAR imagery from the Canada Centre for Remote Sensing CV580, ERS, JERS, and Radarsat sensors, display large unfrozen water features (100 m in scale), where icings form. Springs, on or in front of glaciers, bring water to the surface. The water then flows over the surface in channels, through the snow pack, or as slush flows. The CV580 data has fine enough resolution to detect small flowing and dry supraglacial channels (1-10 m in scale) (Fig. 3). Areas of ponded water appear dark on the SAR images due to the high dielectric constant of water. Areas where the snow was saturated with water or there was slush on the surface the reflectance values were between than of water and dry snow. This data provides information on the location and flow regime of the springs that feed the icings, but not the subsurface flow paths or water source.
Subsurface Hydrology: The relative transparency of ice simplifies the detection of anomalous features within it using GPR. State-of-the-art GPR systems can image through hundreds of metres of ice and detect objects less than 50 cm in diameter.
As Arcone et al. (1995) discuss, by examining the polarity of the reflections, the dielectric constant (k) of the target relative to the ice can be determined. The strong reflection from the ice (k=3) to sediment (k25) interface at the base of the glacier shows a distinctive +-+ polarity. Thus, targets within the ice with the same polarity are expected to have a higher dielectric constant than ice. Most geologic materials including water have a higher dielectric constant than ice, while air has a lower dielectric constant than ice. Thus air filled voids would be expected to generate a reflection with a -+- polarity.
Two anomalies within the glacier, at the 255 m and 369 m positions in the GPR profile shown in Fig. 4, have a -+- polarity and are interpreted as air filled voids. The size of these voids cannot be determined as they are smaller than the resolution of the GPR and act as simple point source reflectors. The location of four other air filled voids are shown in Fig. 5. Extrapolation of the diffraction tails to surrounding GPR profiles using the method described by Moorman (1998) indicated that these voids are not connected. It is likely that they are remnant air cavities from when crevasses closed on the surface. There are several heavily crevassed areas up glacier from the study area.
Several of the reflections from within Stagnation Glacier have the same +-+ polarity as the base of the glacier, indicating that the reflectors have a higher dielectric constant than the ice (e.g., water). By interpolating between the GPR profiles surveyed in the grid on Glacier B28, the route of a water filled tunnel was mapped (Fig. 5).
Using historical aerial photograph and Landsat data, area of icing formation and glacial retreat could be mapped, but current levels of activity could not be measured. SAR imagery enabled the identification of winter hydrologic activity and the study of icing formation processes. GPR profiling of the subsurface provided information on the three dimensional position of the internal glacial hydrological network. It was shown that with the technology now available, the entire hydrological system in the glacial environment can be studied.
Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada. The Polar Continental Shelf Project provided logistical support. The Canada Centre for Remote Sensing provided much of the airborne and satellite data. Chuck Livingston coordinated the CV580 data acquisition and Dr. Paris Vachon arranged for the JERS, ERS, and Radarsat data acquisition. Dr. Fred Michel, Mark Elver, and Lynn Moorman provided field assistance. Ron Pietsch processed some of the Landsat data. All of these contributors to this research are gratefully acknowledged.
Arcone, S.A., D.E. Lawson and A.J. Delaney, 1995. Short-pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of Matanuska Glacier, Alaska, U.S.A. Journal of Glaciology, vol. 41, pp. 68-86.
Bentley, C.R., J.W. Clough, K.C. Jezek and S. Shabtaie, 1979. Ice thickness patterns and the dynamics of the Ross Ice Shelf, Antarctica. Journal of Glaciology, vol. 24, pp. 287-294.
Davis, J.L., J.S. Halliday and K.J. Miller, 1973. Radio echo sounding on a valley glacier in east Greenland. Journal of Glaciology, vol. 12, pp. 87-91.
Moorman, B. J., 1998. The Development and Preservation of Tabular Massive Ground Ice in Permafrost Regions. Ph.D. Thesis, Carleton University, Ottawa, 308 p.
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