A technique for generating a target image utilizing acoustic transient response signature data is explained and demonstrated for elastic shells in water. The technique combines features of physical optics inverse diffraction and feature-space target identification approaches. Complex harmonic backscattering spectrum in the low resonance range is used as input data for this approach. A periodic ramp in response waveform synthesized from these data is shown to be correlated to the cross-sectional area versus distance along the line-of-sight, or profile function, of the targets measured. Visual images simulating an isometric view of the target are constructed.
An image recons.coction algorithm has been developed based on the Physical Elastodynamics Approximation which leads to a relationship between thebackscattered ramp response to an incident ultrasonic ramp pulse and the area of cross section of the object along the "line of sight". This particular inverse algorithm is well suited for NDE applications since a limited number of "look angles" are enough (usually 3 to 5); the data can be bandwidth limited; and high frequency noise does not affect the algorithm. The imaging algorithm is based on the concept o f an approximate limiting surface. Images generated using both synthetic data and measured data for voids of various shapes embedded in an elastic solid are presented.
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