Stagnant ice at the bed of White Glacier, Axel Heiberg Island, N.W.T., Canada

A total of 400 soundings along 15 profiles were obtai ned on White Glacier, Axel Heiberg Island, N.W. T. with a monopulse radar equipment that was rebuilt according to a model of the US Geological Survey . The resulting data allowed maps to be compiled of the ice thickness for the glacier tongue. The radio echo-sounding data and englacial temperature measurements give some indication of the existence of stagnant ice in depressions of the glacier bed in the accumulation zone of White Glacier. INTRODUCTION Since 1959 glaciological and climatological data have been collected on the White Glacier, Axel Heiberg Island, N.W. T. , Canada, by members of the McGill-Axel Heiberg Island Research Expedition of the late Professor F. Muller. In the first years, the main studies focused on mass balance (Miiller 1963[a)), shallow depth ice temperature measurements (Miiller 1963[b)) and the study of ice movement and fluctuations (Iken 1974). On 3 transverse profiles, ice thickness measurements with seismic (Redpath , 1965) and gravimetric (Becker 1963) soundings were made. In the years 1974 to 1981, vertical profiles of the ice temperatures were measured at 32 sites, some reaching close to bedrock (Blatter 1985). A more detailed and accurate picture of the bedrock topography was gained by radio echosounding in May 1984. White Glacier is a valley glacier situated at altitudes ranging from 80 to 1600 m a.s. with a correspondingly wide range of conditions. The mass balance, for example, varies from about -2.5 m water equivalent on the lowest part of the tongue area to about +0.4 m. w.e. in the higher parts. The mean annual temperature of the glacier is around _18 0 C; the annual precipitation is 170 mm , mainly fallin g during the spring and summer season. The melt seaso n starts in June with strong radiation and usually ends in mid-August. However, the year-to-year variations may be rather high with the equilibrium line varying between 400 m and 1500 m a.s. l. and the mean altitude being at around 1100 m a.s. l. The glacier is cold except for a layer which is up to 40 m thick near the bedrock in the lowest tongue area (Blatter 1985). The surface ice velocities are up to 40 m a-I in the central part of the glacier. During strong melt events or periods of rainfall, peak velocities may be up to 5 times faster than the mean annual value. DEPTH SOUNDING For the sounding campaign on the White Glacier, the US Geological Survey Monopulse Ice Radar (Watts and England 1976; Hodge unpublished; Watts and Wright 1981 ) was used. The equipment was partly built with the help of the Laboratory for Hydraulics, Hydrology and Glaciology at the Swiss Federal Institute of Technology, Zurich (Haeberli and others 1983). The operating equipment consists of a transmitter and a receiver placed about SO m apart on the glacier surface, both equipped with identical, resistively loaded, centre-fed dipole antennas. The antenna arms used were 20 m long, which gives the choice of 10 MHz radio frequency. The transmitter works with avalanching transistors, giving a pulse repetition rate of about 10 kHz (Hodge unpublished). The signal from the receiver antenna is fed directly into a battery-powered oscilloscope. The direct signal from the transmitter and the reflected signal from the glacier bottom are displayed on the screen, where the difference of the transmission time of the signals can be read to an accuracy of ai / lOO 1lS . The equipment allowed identification of reflected signals to a depth of 500 m. Soundings of glacier thicknesses of less than SO m proved to be unsatisfactory. This may be caused by the length of the antenna which records a diffuse transmission time due to diffuse path lengths. The sounding profiles were chosen transverse to the valley axis and the antennas were laid out orthogonal to the profile direction on the ice surface. The equipment, including theodolite and laser distomat for surveying the sounding locations, weighed less than 100 kg and could be handled and operated by three people. BEDROCK TOPOGRAPHY The evaluation of the sounding data was perfomed by direct geometrical means. It was assumed that the reflection points lie within the vertical plane of the surface profile . The possible locations for the reflection points, the ellipse with the focal points at the transmitter and at the receiver station could be closely approximated by circles for ice thicknesses of more than 100 m. The envelope of the circles of one profile is then assumed to give an upper limit to the elevation of the glacier bed, as is illustrated for example in Figs 3, 4, and 5. The velocity of the radio waves in ice was chosen at 168 mi llS (Robin 1975 , Glen and Paren 1975). The same value was used for firn and the refraction was neglected (Rasmussen 1986). Since firn layers are usually about 50 m thick, this error is negligible in the order of the reading accuracy. Finally, it is believed that the relative accuracy of the depth sounding is about 5 to 10% (Haeberli and Fisch 1984), owing to the uncertainty of wave velocity, transmission time reading, and the location of the reflection point at the bedrock . The above method proved to work very well in the tongue area of the glacier where the bedrock forms a simple U-shaped valley. Usually, the radar soundings confirmed drilling depths within the limits of accuracy . On the other hand, the results obtained from earlier seismic soundings (Red path 1965) and gravimetric soundings (Becker 1963) now show substantial differences (sometimes by as much as 50%) from the radar sounding thicknesses . The data from 1I transverse sounding profiles have enabled compilation of a map of the glacier bed topography and thus for the glacier thickness in the tongue area (Fig. I ), although smaller features «50 m in the horizontal) of the bedrock cannot be spatially resolved. The soundings performed in the acumulation zone , however, frequently produced multiple echoes. To explain