Boundary enhanced coupling processes for rotated horizontal solid aluminum cylinders: Helical rays, synthetic aperture sonar images, and coupling conditions

Abstract

Experiments with solid aluminum cylinders placed near a flat free surface provide insight into scattering processes relevant to other flat reflecting boundaries [J. R. La Follett, K. L. Williams, and P. L. Marston, J. Acoust. Soc. Am. 130, 669–672 (2011); J. R. La Follett, Ph.D. thesis, WSU (2010)]. This presentation concerns the coupling to surface guided leaky Rayleigh waves that have been shown to contribute significantly to backscattering by solid metallic cylinders [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 106, 1673–1689 (1999)]. The emphasis here is on horizontal cylinders rotated about a vertical axis away from broadside viewed at grazing incidence. The range of rotation angles for which helical rays can contribute is limited in the free field by the cylinder’s length [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 112, 528–536 (2002)]. Some examples of surface enhanced backscattering may be summarized as follows. In agreement with geometrical considerations, the angular range for coupling to helical rays may be significantly extended when a short cylinder is adjacent to a flat surface. In addition, the presence of a flat surface splits synthetic aperture sonar (SAS) image features from various guided wave mechanisms on rotated cylinders. [Work supported by ONR.]