Greg Duckworth Ted Farrell Kevin LePage Cola Atkinson
Bolt Beranek and Newman, Inc., 70 Fawcett St., Cambridge, MA 02138
Recent advances in modeling coherent loss in the Arctic have made it possible to attempt a constrained inversion of monostatic Arctic reverberation for the ice scattering matrix. An analysis is presented of low-frequency active reverberation data collected in the Central Arctic in 1992 where such an inversion yields meaningful results and compare them to theoretical predictions. Historical and measured ice roughness statistics collected in tandem with the acoustic portion of the experiment were used to obtain elastic perturbation theory predictions of scattering loss and backscatter. These losses were integrated into the Kraken normal mode model to yield highly accurate predictions of the coherent signal structure to very long ranges in the Arctic waveguide, as verified by TL measurements. Based on these predictions, a significant portion of the time/frequency structure of the backscattered return received on a 256-element hydrophone array was modeled. To construct a well-conditioned inverse problem, propagation in 10-Hz bands was divided into three distinct groups; surface duct, mid depth and RSR. Modeling of TL and travel time for each band/group made it possible to invert the energy in the backscattered return for the average scattering cross section of the ice cover at the three discrete group grazing angles. Comparison of the derived scattering strengths with the elastic perturbation theory predictions yielded good agreement. [Work supported by ONR.]