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Title: Measurements of ultrasonic wave velocities in ice cores from Greenland and Antarctica
Authors: Michigan State University. Dept. of Geology.
Bennett, Hugh F. (Hugh Frederick), 1931-
Keywords: Acoustic birefringence
Anisotropic material
Crystal acoustics
Ice cores
Ice sheet
Ice cap
Isotropic media
Seismic field data
Shear wave birefringence
Ultrasonic laboratory measurements
Ultrasonic wave velocity measurements
Wave propagation in crystal aggregates
Publisher: Cold Regions Research and Engineering Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Research report (Cold Regions Research and Engineering Laboratory (U.S.)) ; 237.
Description: Research Report
Abstract: Detailed ultrasonic velocity measurements were made on snow and ice cores from Greenland and Antarctica in order to study velocity anisotropy and its relationship to the petrofabric analysis of these cores. In addition, ultrasonic velocities were measured in the near-surface snow layers at Byrd Station and South Pole Station, Antarctica, to provide a detailed velocity profile in the region of the ice sheet where the velocity is greatly influenced by the snow structure. The experimental arrangement, including the design of equipment, measurement errors, techniques, and problems encountered in the study, is discussed. The theory of wave propagation in a general anisotropic medium is reviewed and a detailed presentation of this theory, concerning transversely isotropic media, is given. A method is developed for calculating a theoretical velocity model from the petrofabric analysis of the ice cores, thus providing a means of testing the theory with field and laboratory observations. Based on a comparison of the field and laboratory observations with the theoretical predictions, these conclusions were made: 1) the surface snow layers act as a high-frequency-cut filter on sonic wave propagation; 2) there is good agreement between the theoretical velocity models based on the petrofabric ice core analysis and the observed seismic and ultrasonic shear-wave velocity observations, but poorer agreement with the compressional wave velocities; 3) there is good agreement between the theoretical ray path calculations and the observed data in the near-surface anisotropic snow layers; 4) acoustic birefringence is demonstrated in the single ice crystal and observed in the ice sheets of Greenland and Antarctica; and 5) the ice sheets of Greenland and Antarctica display varying degrees of anisotropy. Three possible zones of anisotropy in the ice sheets include a) a near-surface layer with marked transverse structural isotropy, b) an intermediate layer with slight transverse isotropy due to weak vertical crystal orientations, and c) deep layers displaying high degrees of anisotropy due to strongly oriented petrofabrics. In zone (a) the anisotropy is due to the snow structure, in zone (b) the crystals may orient preferentially in response to a nonhydrostatic stress condition, and in zone (c) the strong crystal orientations may be caused by high shear stresses at the base of the ice cap.
Appears in Collections:CRREL Research Report

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