Abstract:

The radiation characteristics of individual acoustic events from the central Arctic SIMI experiment data were studied, in the frequency range 10--350 Hz, with the purpose to identify the event generating ice mechanisms. Four event subpopulations and consequently four ice mechanisms were distinguished. The first subpopulation consisted of events attributed to floe unloading, a postfracture process, which was consistently detected in the ambient noise data following intervals of high ice fracture activity. The event radiation patterns were independent of azimuth and were modeled by a stationary vertical dipole. The second subpopulation consisted of events attributed to tensile fracture, the radiation pattern of which was modeled by a weighted superposition of longitudinal octopoles, modified by a Doppler factor to account for source motion. The last two subpopulations consisted of events attributed to shear fracture. A lateral octopole and a weighted combination of lateral and longitudinal octopoles, both modified by a Doppler factor, were used to describe the radiation characteristics of events in these subpopulations, respectively. The adequacy of the latter model indicated that events in the fourth subpopulation had been induced by shear fractures, which propagated through the formation of arrays of tensile cracks in their tips and edges.