EXPERIMENTAL CHARACTERIZATION OF ELASTIC-WAVES IN A FLOATING ICE-SHEET

Frequency-wave-number analysis is applied to geophone data to study phase velocities of ice waves as a function of frequency. Previous measurements of phase velocities were based primarily on initial arrival time of individual waves which were valid only for the frequency components associated with the initial arrival. By processing the geophone array data coherently, this frequency-wave-number analysis measures the phase velocity as a function of frequency for a wide frequency band. This processing is applied to geophone data collected on a line array of three-axis geophones; the data were generated by striking sledge hammers against a steel pipe frozen into the ice during an Arctic experiment in 1988. As expected, the longitudinal and horizontal shear waves travel with constant phase velocities below 100 Hz. Applying this analysis to vertical-axis geophone data, we find two families of dispersion curves for the flexural waves. Only one dispersion curve is expected for the flexural wave for a uniform sheet of ice. The origin of the additional (flexural) wave is not established. Time-frequency analysis was applied to individual time series to estimate the group velocities of the flexural wave and use the line array data to estimate ice attenuations. Frequency-wave-number analysis is used to identify ice waves in a complex signal in which the various ice waves are no longer separated by travel time. This is demonstrated by the analysis of a noise event. The origin of this noise event based on the analysis of the hammer blow data is discussed.