Near-Normal Incidence Scattering from Rough, Finite Surfaces: Kirchhoff Theory and Data Comparison for Arctic Sea Ice
WASHINGTON UNIV SEATTLE APPLIED PHYSICS LAB
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We apply Kirchhoff theory for the target strength of a rough, circular surface whose roughness is characterized by a two-dimensional, isotropic power-law wavenumber spectrum W2K n2K-p2. Three nondimensional parameters are found that govern the target strength Koa, nn2ap2-4, , and PI P2-1, where KO is the acoustic wavenumber, a is the radius of the surface, and p 1 is the spectral exponent of the one-dimensional power-law wavenumber spectrum. First, we discuss the general influence of p 1, and n on the target strength. Following that are calculations of average target strength of the icewater interface of a submerged cylindrical block of ice, which are compared with individual realizations of measured target strengths of ice blocks for frequencies between 20 kHz and 80 kHz. Data and theory show that the smooth surface form function for a finite surface does not describe the observed diffraction pattern. Instead, the lobes of the pattern diminish and the nulls fill in - i.e., the total backscatter becomes more incoherent - as frequency increases or as the large wavenumber components of the roughness spectrum contribute more to the total acoustic return. These comparisons also allowed us to infer the rough surface statistics of the ice surface and the compressional sound speed structure within the skeletal zone of the ice.
- Snow, Ice and Permafrost
- Target Direction, Range and Position Finding