Catherine Stamoulis- caterina@mit.edu
Ira Dyer, idyer@aol.com
Massachusetts Institute of Technology
Department of Ocean Engineering
77 Massachusetts Ave. Rm 5-212
Cambridge, MA 02139
Popular version of paper 1pUW6
Presented Monday afternoon June, 22, 1998
ICA/ASA '98, Seattle, WA
Just as noise is present in the air of cities and the countryside, noise is in the ocean's waters. In the Arctic ocean noise results mainly from ice activity, fracture and deformation processes in particular. These radiate sound in the water in the form of individual acoustic events. For the past twenty years, researchers have tried to identify the major noise generating ice mechanisms and understand their characteristics. In the past, military interest in the strategic role of the under-ice environment in the Arctic motivated much of the research on ambient noise. In regard to the ice itself, multi-year ice in particular which is of great concern in navigation and off-shore structural engineering, research was and still is driven by the need to understand the response of the ice cover to different environmental forces. The research on the behavior of Arctic ice under different environmental conditions is of significant interest also for climatological purposes. For example, monitoring of Arctic ice thickness variation may provide useful information on atmospheric phenomena, such as global warming. In order to understand the characteristics of ice fractures, noise data collected during an experiment in the central Arctic were analyzed. The detected acoustic events were studied at two levels, namely local motion and large-scale propagation of simultaneous events.
Any fracture process can be considered as a series of local motion offsets and arrests which in the aggregate result in large faults, such as those formed during earthquakes. Local motion of ice fractures can be estimated directly from the detected pressure signals. The three parameters that characterize this motion is the local displacement of the fault, its velocity and the time required to complete a displacement offset. The results of the analysis of acoustic events at this level showed that, in comparison to earthquake faults, ice faults can be considered as micro-tremors, with displacements about four orders of magnitude lower that those of seismic faults. However, their local velocity is only about 50% lower.
Fractures generate different types of elastic waves in the ice. The speed of propagation is often limited by the speed of one of the surface waves. In the absence of accurate speed estimates, this limit value is assumed to be the fracture speed. The data analyzed in this work enabled accurate estimation of propagation speed. The wide range of estimates (200-1100 m/s) indicate that multiple and possibly distinct mechanisms radiate sound in the water. In order to assess the validity of this result, the characteristics of sound radiation of individual events were also analyzed. According to the fracture process, these characteristics are expected to vary. For example, radiation from a tensile fracture, which involves opening in the ice, is expected to be different than that from a shear fracture which involves sliding. Indeed, the results of the analysis showed that not only shear and tensile fracture are plausible event generating mechanisms but also that ice fault formation is more complicated, as is the case in rock fractures in earthquakes. Large faults in ice propagage through the formation of secondary cracks which also radiate sound in the water. Therefore, the radiation characteristics of the main process (usually shearing in the case of large faults) are affected by such fracture details.