Interpretation of radio echo sounding in polar ice sheets

Abstract

Experimental results are presented from a traverse over the ice sheet of north western Greenland in 1964, during which a continuously recorded profile of ice thickness was obtained for the first time. Interpretation of data from this traverse is consistent with results of subsequent work to December 1967. The parameters of the apparatus are presented briefly, while the details of electronic circuits are being published separately. Theoretical problems of radio wave propagation in an ice sheet and, in particular, the factors affecting accuracy are discussed. The uncertainty in depth, over a small area, is ±5 m ±1.5% and this is verified by comparison with the seismic results for a range of depths up to 1.5 km. It is found that the only real uncertainty arises in irregular terrain. The effectiveness of the radio echo technique is dependent on the absorption of radio waves in ice. Temperature, and to a lesser extent the impurity content of ice, appear to be the main variables affecting field performance. Earlier laboratory results on the variation of absorption with temperature for ice cores from northwest Greenland, together with theoretically predicted temperature distributions throughout the ice mass, have provided estimates of the total loss by absorption. These estimates are reasonably consistent with the observed echo strengths over most of the traverse. Consequently, it is predicted that echoes can be obtained over considerable areas of the ice sheets of Greenland and Antarctica, as has been verified by subsequent observations. The reflexion coefficient at the ice/rock interface is of the order of —15 dB. It could rise to 0 dB for an ice/water interface and one area was found in Greenland where it appeared to fall to — 30 dB. Results from this traverse have shown that local surface slopes on the ice sheet are largely controlled by variations of longitudinal stress along the line of flow. Regional slopes over several kilometres vary with the velocity of movement of the ice, but appear to be less dependent on basal ice temperatures than laboratory results would suggest. The velocity of ice movement increases in proportion to the square or cube of the basal shear stress, but the stress itself shows no obvious dependence on basal ice temperature. Partially reflecting layers discovered within the ice mass are discussed mainly in terms of small density variations between adjacent layers of ice. One particularly prominent layer is calculated to be about 1000 years old and its variation of depth with position provides evidence in favour of the steady state model of the ice sheet.