ACOUSTICAL RADIATION FROM THERMALLY STRESSED SEA ICE

Observations of the sound generated by cracking ice in the Canadian Arctic during a cooling period provide an opportunity for study the response of different types of ice to thermal stress. Synthetic aperture radar (SAR) images and air photographs of ice around the experimental site allow mapping of cracking events back to their points of origin, thus providing a correlation between ice type and acoustical activity. It is shown that there are only a few cracking events in the first-year ice during periods of air temperature variation, whereas multiyear ice produces a large number and is responsible for most of the ambient sound recorded. Analysis of several individual events including both natural and artificial sources reveals that failure processes cause acoustic emissions with an angularly dispersive radiation pattern in which higher frequencies tend to be radiated downward and lower frequencies radiated toward the horizontal. This pattern can be explained in terms of two types of eigenmodes used in plate vibration theory. The theory is extended to the case of a shallow surface crack representative of thermal stress effects. Comparisons are made between theoretical predictions and ice cracking events, including artificial sound sources, and the results interpreted in terms of sea-ice properties.