Sea ice noise-generating processes

An ambient noise model is presented for sound generated in compact ice by ridging, microcracking, and mixed layer shearing. The model describes sound intensity as the sum of these different processes and the sum of intensities propagated from all source locations. The source intensity changes as a function of time for each frequency and can depend on material parameters that affect behavior, such as ice thickness. The processes are identified by their driving forces. The large-scale stress state is related to the local stress in each ice floe, which, when small, can cause local cracking but does not contribute to large-scale deformations. If the stress level is large enough to cause ridging, rafting, and leading, then large-scale deformations occur. Independently of these large-scale deformations, the ice can move as a rigid body, which shears the mixed layer. Each of these processes is assumed to have its own spectral signature, and each is driven by a different environmental variable. Local cracking is driven by the stress. Ridging is driven by the stress and deformation. Mixed layer shearing is driven by the ice velocity relative to the current. Each driving force is converted to an energy dissipation to serve as a source level to the model. These variables provide the key to successfully modeling the different processes and to isolating them from one another and from other processes. Ambient noise measurements taken during the October 1988 CEAREX Drift Experiment were successfully simulated by the model. Day-to-day temporal variations in the 31.5-Hz noise intensity were reproduced by the mixed layer shearing, pressure ridging, and shear ridging models. Microcracking was apparently not active during this time period.