HIGH-FREQUENCY FORWARD SCATTERING FROM THE ARCTIC CANOPY - EXPERIMENT AND HIGH-FREQUENCY MODELING

A high-frequency forward scattering experiment carried out under multiyear ice is outlined. The experiment employed an omnidirectional transmitter emitting short tone bursts centered at either 25 or 45 kHz. The receiver was a four quadrant array, and the quadrants were recorded separately. Each quadrant had a broadbeam pattern, greater than 40 (70) deg at the 3-dB down points in the horizontal (vertical) direction at 45 kHz. A large number of transmitter/receiver geometries were obtained by varying both the transmitter depth and horizontal range. Horizontal transmitter/receiver separations ranged from 400 to 2800 m. Model/experiment comparisons are shown for horizontal ranges less than 500 m, since these ranges can be most readily compared with our high-frequency scattering models developed to date, which ignore refraction in the water column. Both individual realizations and ensemble experimental results are shown and compared with models. Specular point theory [D. E. Funk and K. L. Williams, J. Acoust. Sec. Am. 91, 2606 (1992)] is used as a starting point, and then catastrophe theory [P. L. Marston, in Physical Acoustics, Vol. XXI, edited by R. N. Thurston and A. D. Pierce (Academic, 1992); K. L. Williams et al., J. Acoust. Sec. Am. 94, 1890(A) (1993)] is used where inclusion of diffracted effects is essential. The models capture many of the attributes of the data. Much of the difference between models and experiment can be traced to limited knowledge of the ice canopy topography. Although the experiment was carried out along paths where the one-dimensional profile of the under-ice surface had been previously determined with upward looking sonar, and these profiles were used in the models, out of plane scattering could not be included. That would require full two-dimensional knowledge of the under-ice surface.