J. Robert Fricke
Gladys L. Unger
Department Ocean Eng., MIT, 77 Massachusetts Ave., Cambridge, MA 02139
Analytic and numerical modeling calculations predict that fluid and elastic ice floes produce distinctly different scattered fields at low frequency (10--100 Hz): dipolar and quadrupolar, respectively. This led to the hypothesis that first-year ridges, which are loose aggregations of ice blocks, behave like fluid structures, while multi-year ridges, which are frozen solid, behave like elastic structures. Ultrasonic laboratory experiments were conducted at a scale of 1000:1 to test this hypothesis by actually measuring the difference in the scattered field from these two types of structures. Polypropylene with (rho)=0.9 kg/m[sup 3], C[sub p]=2650 m/s, and C[sub s]=1300 m/s was chosen as a surrogate for ice. Floes were modeled two ways using the polypropylene; a half-cylinder modeled the elastic floe, and an aggregation of chips modeled the fluid floe. The scattered field for each case is estimated by subtracting the field measured for a free-surface experiment from the field measured for the scatterer experiment. Experimental results show that loose aggregations of chips produce a dipolar scattered field and that the elastic half-cylinder produces a quadrupolar scattered field. These results support the notion that first-year and multi-year ridges in the Arctic produce distinctly different scattered fields and that scatter from first-year ridges is the dominant scattering loss mechanism in long-range Arctic propagation. [Work supported by ONR and Doherty Foundation.]