Naval Surface Warfare Ctr., White Oak Detachment, R14, Silver Spring, MD 20903-5640
Natl. Defense Res. Establishment, Sundbyberg, Sweden
An earlier study [J. Acoust. Soc. Am. 93, 2412 (A) (1993)] of echoes from submerged elastic shells in the combined time-frequency (t-f) domain is continued. Wigner and pseudo-Wigner distributions as well as time-windowed Fourier transforms are used [cf. L. Cohen, Proc. IEEE 77, 941 (1989)]. These distributions extract from the echoes the time evolution of the target resonances that may be present in the frequency band of operation. Selecting suitable carrier frequencies for the incident pulses and appropriate spectral windows, it is possible to produce (t-f) displays linking resonance features to specific target characteristics. It becomes clear that this type of display is helpful in the processing of the echoes, and in the identification of the scatterer's material properties and geometrical shape. Broader/narrower time windows control the amount of detail in the resulting (t-f) plots, which can be adjusted at will. This is illustrated with examples exhibiting results in a colorized three-dimensional format as beautiful as practical and informative. These examples include the use of various realistic incident pulses, targets, and viewing windows that highlight the target-ID capability of the approach.