G. R. Giellis
T. C. Yang
Naval Res. Lab., Washington, DC 20375
Ice cracking/breaking induces ice vibrations, hence measurements of acoustic waves in ice can be used to probe the forcing function which cracks/breaks the ice. Previous measurements using ice-implanted geophone sensors (array) have revealed the existence of longitudinal (plate), flexural, and horizontally polarized shear waves at low (<100 Hz) frequencies. Applying frequency-wave number analysis to hammer blow data received on a line array of three-axis geophones, the phase velocities of these waves were measured. As expected, the longitudinal and horizontal-shear waves travel with constant phase velocities whereas the phase velocity of the flexural wave is frequency dependent. The frequency wave number analysis also reveals the existence of a new wave, heretofore not noticed, with a different frequency dispersion characteristic compared with the flexural wave. This wave travels with a phase velocity comparable to the flexural wave, consequently identifying it in the time series data is difficult. This wave is hypothesized as the forced wave excited by a point force. Measurements of group velocities and attenuation for each type of wave are also discussed.