Observations of the sound generated by ice cracking events in the Amundsen Gulf provide an opportunity for studying the mechanisms of the generation and propagation of acoustic signals beneath the Arctic ice cover. Contributions from waves traveling in the ice (longitudinal waves) are identified and shown to be small compared to the direct acoustic wave. Both high-frequency (∼5 kHz) and base-band (200–300 Hz) components occur and are interpreted in terms of a moving source model modulated by the ‘‘slip-stick’’ process considered in earthquake mechanics. It is shown that the high- and low-frequency components can be related to the vertical and horizontal length scales of the crack.
Observations of the sound generated by cracking ice in the Canadian Arctic during a cooling period provide an opportunity for studying 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.
Detailed avoidance reactions of adult migrating salmon to a mobile survey vessel were successfully observed with side-looking dual-frequency identification sonar (DIDSON) in the lower Fraser River (British Columbia, Canada). Both adult sockeye (Oncorhynchus nerka) and pink salmon (Oncorhynchus gorbuscha) returning to the river were found to avoid the approaching vessel by initiating lateral movements away from the vessel, making the fish unlikely to be insonified by the downward-looking transducers towed by the vessel. The vessel was found to have an estimated mean interference range of 4 m from its propeller. Analyses of the data concluded that once the vessel and fish were separated by more than 7 m, the vessel no longer affected the normal migration behaviour of the fish.Résumé : Un sonar latéral d'identification à double fréquence (DIDSON) nous a permis d'observer avec succès les réac-tions détaillées d'évitement des saumons adultes en migration face à un navire mobile d'inventaire dans le cours inférieur du Fraser (Colombie-Britannique, Canada). Tant les saumons rouges (Oncorhynchus nerka) que les saumons roses (Oncorhynchus gorbuscha) qui retournent dans la rivière évitent le navire qui s'approche en entreprenant des déplacements latér-aux pour se distancer du navire, ce qui fait que les poissons risquent peu d'être repérés par les capteurs sonar orientés vers le bas et tirés par le navire. La portée moyenne de l'interférence du navire est estimée à 4 m depuis l'hélice. Les analyses de ces données mènent à la conclusion que lorsque le navire et les poissons sont séparés par une distance de plus de 7 m, le navire n'affecte plus le comportement normal de migration des poissons.[Traduit par la Rédaction]
When landfast ice breaks up and starts to move under the influence of wind and current, it produces a distinctive acoustical signature that can include nearby pure tones. Observations of this signal obtained in Amundsen Gulf are interpreted in terms of the initial fracturing mechanism followed by the rubbing of adjacent ice floes. The ice rubbing sound excites SH waves in the ice that evidently dominate the response, resulting in the pure tone that is observed. The frequency of this signal is determined by ice thickness and the shear wave speed.
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