Measuring Depth Of Antarctic Ice

Press, Volume CIII, Issue 30592, 7 November 1964, Page 19

Measuring Depth Of Antarctic Ice

(By

DR. S. EVANS,

of the Scott Polar Research Institute,

Cambridge, England.)

Four research teams are mapping the “profile” of the Antarctic regions, trying to plot out the real shape of the ground under thousands of feet of permanent ice-cap. One of the teams is from the Scott Polar Institute at Cambridge, England. Scientists on this team have helped to develop a technique and the appropriate machinery for ‘“echo sounding” the depth of the ice to get a clear picture of the ground beneath. Given the right machine the problem of sounding the ice, which can be up to three miles deep, is a straightforward task because of the uniform character of the icefield through which the machine sends its echoes. Picture of Bottom With an exact picture of the bottom of the ice—that is

to say, the surface on which the ice rests—much more will be known about the way the ice forms and flows. Radio waves at ordinary broadcast frequencies penetrate ice with very little absorbtion. Radar systems use much higher frequencies (corresponding to wavelengths of a few centimetres) both to permit the radiation to be confined within a narrow beam which sweeps the field of view, and to achieve high range resolution—about a metre in short range systems. However, centimetre waves are rather strongly absorbed in ice, but in the intervening (V.H.F.) band useful penetration of ice and acceptable range resolution can be achieved. The V.H.F. radio echo sounding apparatus developed at the Scott Polar Research Institute in Cambridge has now been used in several field trails. Mark I was used by the British Antarctic Survey on the Brunt Ice Shelf at Halley Bay in the Weddell Sea, where strong echoes were obtained from the bottom of the ice shelf in small isolated areas. The shelf is about 600 feet thick and is usually supposed to be formed by ice from the inland plateau, squeezed outwards by the accumulation of snow on the continent, and floating out to sea, in this case for a distance of about 30 miles, but in some cases for hundreds of miles. The reason for a lack of echoes over a great part of the area is unknown, as is the nature of the bottom surface. The Mark II appartus has been used in 1963 and, more extensively, in 1964 in northwest Greenland. Continuous Profile

For glaciologists the obvious advantage of the new ; radio-echo technique is that it provides a continuous profile of the bottom features. This will be an immeasurable help towards our understanding of the factors governing the surface form and flow of ice sheet, since our knowledge has been limited hitherto by the difficulty of forming an impression of the bottom relief from the spot soundings (usually 10 to 100 miles apart) obtained by the seismic technique. It is a problem capable of fairly exact solution because of the uniform nature of the material and its simple mechanical properties, a situation rare in geography or geophysics. The deepest ice in the Antarctic is nearly three miles thick: the greatest thickness which the radio apparatus will penetrate is unI known, but it is known that • it depends very much on the temperature. The colder the ice, the less the absorbtion, and in Greenland a maximum depth of more than a mile was penetrated. The average temperature at the top surface of the ice in Antarctica may be as low as minus 50 degrees Centigrade in some inland situations, the temperature near the bottom is expected to be appreciably higher, but there are no direct measurements I The presence, or absence, of

radio echoes will give some clues to this and will also give some information about the nature of the bottom surface, its roughness, and whether it is wet or dry. In addition to the bottom echoes which are recognisable by their continuity on the record as the vehicle ■ moves along the trail, there is also a large number of over- ■ lapping echoes from discon-

tinuities in the snow structure near the top surface. The discontinuities arise from the seasonal variation in snow accumulation: they are especially strong if there is melting in the summer so that an ice stratum is formed. Depth of 1640 ft However, echoes have been obained from the body of the snow at depths of 1640 feet, much greater than can be explained in this way. We suppose they are due to prolonged warm periods, not simply summer periods but climatic fluctuations persisting for a century or so, many centuries ago. This promises to be a new and interesting field of study.