SUMMARY 

 Mechanical Properties 



In comparing the results of 1954 and 1955 in tables 1 and 2, a difference of 300 

 to 400 meters/sec exists in the longitudinal plate-wave velocity. In Oliver, Crary, 

 and Cotell's work,* variations in the plate velocity are of the order of 500 

 meters/sec. These variations are significant and can be attributed only to differ- 

 ences in the internal structure of the sea-ice sheet. In comparing the longitudinal 

 plate-wave velocity obtained by seismic methods (table 2) with the longitudinal 

 plate-wave velocity obtained by excitation of a long rod (table 4), a difference 



TABLE 4. Resonant rod measurements of sea ice for 1955. 



Rod Length 

 (meters) 



Resonant Frequency 



(cps) 



Rod Velocity 

 (meters/sec) 



Longitudinal Plate- 

 Wave Velocity 

 (assuming a = 0.33) 

 (meters /sec) 



Young's Modulus 



E 

 X 1010 dynes/cm2 



0.791 



1930 



3050 ± 10 



3240 ± 10 



8.4 



0.812 



1850 



3000 ± 10 



3180 ± 10 



8.1 



0.651 



2330 



3040 ± 10 



3220 ± 10 



8.3 



0.644 



2370 



3040 ± 10 



3220 ± 10 



8.3 



0.666 



2200 



2930 ± 10 



3110 ±10 



7.7 



0.595 



2520 



3000 ± 10 



3180 ±10 



8.1 



0.970 



1475 



2830 ± 10 



3000 ± 10 



7.2 



Temperature Range: — 18°C to — 15°C 



in the plate velocity of the order of 700 to 800 meters/sec is obtained. These re- 

 sults are contrary to the results of Ewing, Crary, and Thorne, Jr.^ on lake ice, in 

 which they found no significant difference in the plate-wave velocity when deter- 

 mined either by use of thin rods or by the seismic method. Poisson's ratio, which 

 is not critical in this relation, is assumed to be 0.33. The rods of ice were obtained 

 with the long axis normal to the ice sheet. The rods were stored for several days 

 at a temperature between — 18°C and — 15°C. The temperature gradient in the 

 ice sheet was — 8°C at the top of the sheet to — 2°C at the bottom of the sheet, 

 as is shown in figures 8, 9, and 10. The difference in temperature of the order of 

 10°C between the ice sheet and the rod might indicate that the temperature of 

 the sea ice has more than a second-order effect on the elastic constants, whereas 

 temperature appears to have only a second-order eflFect with fresh- water ice.'^ 

 Another reason for the discrepancy between the longitudinal plate-wave velocity 

 of the ice sheet and the longitudinal plate-wave velocity as calculated from the 

 rod velocity might be due to attenuation and dispersion. In this case, the faster 

 high-frequency waves would be attenuated first* in the ice sheet as they emanate 

 from the shot point; hence, at various distances from the shot point, a change in 

 the velocity would be detected. However, with the short length of the ice rods, 

 the high-frequency waves should be detected. Malmgren^^ reports sea-ice densities 



19 



