Using quasi-holographic backpropagation to understand high-frequency 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 backpropagation (QHBP) limited to supersonic wave numbers can reveal the location in space and time of the virtual sources of scattered sound [Baik, Dudley, and Marston, 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, the use of QHBP is emphasized to determine information about the backscattered signal, including evidence of axial focusing associated with glory scattering by axisymmetric objects. The location and diameter of virtual ring sources of the glory scattering are revealed. In a bistatic underwater experiment, illuminating an elastic object with a short pulse gives raw data in the form of a space-time plot. From this raw data image, it is shown how backpropagating data is used to obtain information about such features for elastic objects varying in size, shape, and density.