NEAR-NORMAL INCIDENCE SCATTERING FROM ROUGH, FINITE SURFACES - KIRCHHOFF THEORY AND DATA COMPARISON FOR ARCTIC SEA-ICE

The Kirchhoff theory is applied for the target strength of a rough, ci rcular surface whose roughness is characterized by a two-dimensional, isotropic power-law wave number spectrum, W2(kappa) = eta2kappa(-p2). The reflection coefficient for ice and three nondimensional parameters are found to govern the target strength. These parameters are zeta=ka ppa0a, eta=eta2a(p2-4) , and p1=P2-1 where kappa0 is the acoustic wave number, a is the radius of the surface, and p1 is the spectral expone nt of the one-dimensional power-law wave-number spectrum from which W2 (kappa) is derived. The general influence of zeta, p1, and eta on the target strength is discussed. Calculations of average target strength of the ice/water interface of a submerged cylindrical block of ice are shown, which are then compared with individual realizations of measur ed target strengths of ice blocks for zeta between 25 and 100, corresp onding to frequencies between 20 and 80 kHz for a=0.29 m. Data and the ory 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 backsca tter becomes more incoherent-as frequency increases or as the large wa ve-number 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 soun d-speed structure within the skeletal zone of the ice.