Air‐coupled flexural waves in floating ice

Experimental studies of the propagation of elastic waves on floating ice sheets were made on the ice of Lake Superior and Lake Cayuga. Elastic waves were produced by small explosive charges detonated at various depths in the water, within the ice, and in the air. Seismic detectors consisting of a spread of geophones, a microphone, and a hydrophone recorded the resultant wave motion at varying distances. Shots in the water produced the normal sequence of dispersive flexural waves. For shots in the air the dispersive flexural waves were absent and a train of constant frequency waves was observed, beginning gradually at the approximate time t = r/2v a and culminating with the arrival of the air wave at the time t = r/2v a (r is the range and v a is the speed of sound in air). These waves were interpreted as air‐coupled flexural waves, The frequency of the air‐induced waves is that of flexural waves whose phase velocity equals the speed of sound in air. The generated wave train precedes the air disturbance since the group velocity of flexural waves exceeds the phase velocity, all in accordance with classical theory. The frequency of the air‐coupled vibration is simply related to the ice thickness. The interpretation was supported by subsequent tests which consisted of shooting on shore and recording on the ice, shooting and recording on ice sheets of varying thickness, and recording air‐coupled flexural vibrations with microphones.