Hans C. Strifors
Natl. Defense Res. Establishment, S-17290 Stockholm, Sweden
Guillermo C. Gaunaurd
Naval Surface Warfare Ctr., Silver Spring, MD 20903-5640
Backscattered echoes are studied from submerged elastic targets in the frequency and time-frequency domains when the targets are insonified by short, broadband sound pulses. The combined time-frequency approach seems to give the most information since it can show the time evolution of the resonance features that identify each target. The targets are either an air-filled spherical shell or various solid brass or steel spheres. The incident waveform is generated by weighting a sinusoidal signal with a Blackman time window of a few cycles width. Experimentally obtained data are processed in the time-frequency domain using a pseudo-Wigner distribution (PWD). The associated time window is Gaussian, and its width is adjusted to suppress the interference of cross-terms in the PWD, yet retaining the desired property of time-frequency concentrating the extracted features. Each PWD is displayed in a 3D surface plot using progressively lighter color shades (pseudocolors) determined by the height of the surface (or the magnitude of the PWD), and that surface plot is also projected onto the time-frequency plane to produce an informative 2D intensity image plot. For each target, the time development of significant resonance features extracted from returned echoes is examined and favorably compared with the corresponding theoretically predicted PWD signatures.