The Influence of Propagation Focusing on Clutter Statistics

Fluctuations in reverberation (clutter) from scatterers at long range in shallow water can be affected by many mechanisms. Their statistics, in particular the scintillation index (SI), depend not only on the statistics of the scatterers (their physical distribution, shape, orientation, etc.) but on the outward and return propagation paths. This paper investigates the focusing effects due to forward scattering from weak undulations in the seabed. Some existing Fresnel-Kirchhoff theory for monostatic, vertical incidence on a 2-D surface is adapted to demonstrate the behavior of the intensity's SI in bistatic geometry. In addition, using a simulation of bistatic geometry and a 1-D surface the intensity statistics are extended to include autocorrelation function and probability distribution. The behavior of these properties is investigated for various Gaussian-correlated roughnesses with or without an added component of uncorrelated roughness. Bistatic geometries include source and receiver moving with constant separation, and moving source with fixed receiver. Fairly modest vertical undulations can result in significant fluctuation due to reflection focusing with scintillation indices possibly greater than unity.