Elastic features visible on canonical targets with high frequency imaging during the 2014 St. Andrews Bay experiments

Abstract

During the 2014 St. Andrews Bay experiments some canonical metallic targets (a hollow sphere and some circular cylinders) were viewed with a synthetic aperture sonar (SAS) capable of acquiring data using a 110–190 kHz chirped source. The targets rested on mud-covered sand and were typically at a range of 20 m. Fast reversible SAS processing using an extension of line-scan quasi-holography [K. Baik, C. Dudley, and P. L. Marston, J. Acoust. Soc. Am. 130, 3838–3851 (2011)] was used to extract relevant signal content from images. The significance of target elastic responses in extracted signals was evident from the frequency response and/or the time-domain response. For example, the negative group velocity guided wave enhancement of the backscattering by the sphere was clearly visible near 180 kHz. [For a ray model of this type of enhancement see: G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994).] In another example, the timing of a sequence of near broadside echoes from a solid aluminum cylinder was consistent with reflection and internal reverberation of elastic waves. These observations support the value of combining reversible imaging with models interpreted using rays. [Work supported by ONR and SERDP.]