Elastic Waves in Lake Ice Produced by Impact of Falling Weights

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Experimental studies of the propagation of elastic waves in lake ice were made at Lake Margrethe, Michigan, and Garrison Dam, North Dakota. These studies were designed to investigate, in particular, flexural waves in the ice-water system produced by hammer drops, cylinder drops, and sledge hammer impacts on lake ice and nearby land. A method similar to the standard seismic-refraction techniques was employed for the wave studies. Flexural wave dispersion curves obtained from records of mechanical impulses on ice are, in general, consistent with theoretical curves based on the “layer” theory of Press and Ewing (1951) for the observed ice thickness and elastic parameters. Three groups of points on the observed dispersion curves obtained from Lake Margrethe data are flexural waves with frequencies from 2 to 8 cps, 13 to 25 cps, and 40 to 140 cps. The first and third groups exhibit dispersion consistent with the combined thickness of a two-layer floating ice sheet. The second group follows dispersion for a wave path in the upper thin layer only. Hammer drops on land generate flexural waves in the layer of frozen ground in the frequency range 15 to 40 cps. These waves are transmitted across the lake-shore boundary and follow a wave path in the upper thin layer of lake ice. A continuous train of flexural waves in the frequency range 4 to 140 cps was observed on records of cylinder drops taken at Garrison Dam, North Dakota. (Parts of this work were supported by Tri-service Contract No. DA-36-039-sc-52654.)