Because of the lack of success in using normal incident reflected acoustic longitudinal waves for sea ice thickness measurements and the time consuming process entailed by refraction acoustic ice thickness measurements, a method of ice thickness measurement was tried using normal incident reflected acoustic shear waves or "S" waves. This technique was used for sea ice measurements off Pt Barrow, Alaska during the summer of 1971.
This paper evaluates the feasibility of this ice thickness measurement technique and presents examples of oscilloscope trace results. Equipmentation problems created by the harsh arctic environment are discussed and suitable recommendations are made. The average and mean errors of ice thickness measurement based on the assumption of an average acoustic shear wave velocity for several different locations during the same season are determined.
The propagation of shear waves or "S" waves in ice was studied as early as 1934 by Maurice Ewing, A. P. Crary, and A. M. Thorne at Lehigh University (1). Their work concerned itself with "longitudinal" and "torsional" sound vibrations in long thin rods in a laboratory environment. They obtained velocities of 3163 meters/second and 1913 meters/second respectively, for ice between -5°C and -15°C.
The study of elastic waves in floating ice sheets was seriously studied in the early '50's by Ewing (2) and Crary (3). They conducted their work in the Beaufort Sea near Barter Island, Alaska. The technique they utilized was a seismic-refraction technique. They noted needlelike crystals on the bottom surface of the ice up to 20 cm. long and came to the conclusion that it would be unlikely "any experimental technique assuming this surface to be a plane boundary would prove valid for wavelengths of the order of 20 cm.
The results of their work is summarized in Table I.
During the International Geophysical year, a year-round study of elastic wave propagation was made by Hunkins (4) from Drifting Ice Station Alpha in the central Arctic Ocean. Hunkins found a definite change in velocity over the time frame of the investigations. He also observed the change in velocity with the change in temperature. Horizontally polarized shear waves were generated by a special source and velocity as low as 1349 m/sec were measured in August and as high as 1890 m/sec in June. Hunkins states, "These waves cannot leak into the water and so would be expected to propagate well if excited, but they are not observed on all records. The irregularity of the ice thickness may tend to scatter the waves and this effect probably combined with the effect of insufficient excitation can account for the scarcity of observations." Table II is a summary of the results of Hunkins' work.
In 1963 Cook (5) reported his results in using horizontally polarized shear waves to measure ice thickness using conventional seismic techniques (13).