Acoustic scattering enhancements for partially exposed cylinders in sand and at a free surface caused by Franz waves and other processes

Abstract

Creeping waves on solid cylinders having slightly subsonic phase velocities and large radiation damping are described as Franz waves because of association with complex poles investigated by Franz. For free-field high frequency broadside backscattering in water, the associated echoes are weak due to radiation damping. It was recently demonstrated, however, that for partially exposed solid metal cylinders at a free surface viewed at grazing incidence, the Franz wave echo can be large relative to the specular echo when the grazing angle is sufficiently small [G. C. Eastland and P. L. Marston, J. Acoust. Soc. Am. 135, 2489–2492 (2014)]. The Fresnel zone associated with the specular echo is occluded making it weak while the Franz wave is partially reflected at the interface behind the cylinder. This hypothesis is also supported by calculating the exact backscattering by half-exposed infinitely long rigid cylinders viewed over a range of grazing angles. Additional experiments concern the high frequency backscattering by cylinders partially buried in sand viewed at small grazing angles. From the time evolution of the associated backscattering by short tone bursts, situations have been identified for which partially reflected Franz wave contributions become significant. Franz waves may contribute to sonar clutter from rocks. [Work supported by ONR.]