Using quasi-holographic back propagation to understand scattering from curved elastic objects in water

High-frequency scattering from elastic objects in water can include significant contributions from structural guided waves. Those contributions can appear to be radiated from virtual sources within the structure. Quasi-holographic back propagation limited to supersonic wavenumbers can reveal the location in space and time for the virtual sources of scattered sound [Baik etal., J. Acoust. Soc. Am. 130, 3838–3851 (2011)]. Using this technique, the scatter signals of interest can be isolated, eliminating tank reflections and other anomalies. In the present research, we emphasize the use of quasi-holographic back propagation to determine information about the backscattered signal including evidence of axial focusing associated with glory scatteringby axisymmetric objects. The location and diameter of virtual ring sources of the glory scattering are revealed. In a bistactic underwater experiment illuminating an elastic object with a short pulse gives raw data in the form of a space-time plot. We show how, from this raw data image, back propagation images are used to obtain information about such features for elastic objects varying in size, shape, and density. [Work supported by the Office of Naval Research.]