Directionality of ice cracking events

Abstract

This paper presents the measured vertical directivity of ice cracking events extracted from a 341 s sample of ambient noise recorded under the polar ice pack. The sample was taken with a vertical linear array of four hydrophones suspended over the Arctic continental shelf in 500 m of water. The hydrophone spacing was 100 m. The samples from each hydrophone were bandpass filtered between 40 and 1250 Hz, and searched for corresponding peaks in the acoustic pressure. The acoustic events are believed to be due predominantly to thermal ice cracking. The angular distribution of the 765 arrivals detected has its maximum at 15 deg below the horizontal, with most of the arrivals occurring between 0 and 25 deg below the horizontal. An explanation for this observation is given in terms of a simple ray-based model of the propagation of ice cracking noise. The asymmetry in the angular distribution is shown to be due mostly to the source directivity favoring bottom reflected and refracted rays arriving at negative angles. A dipole source directivity is shown to produce the best fit to the data.