Ultrasonic measurements of defects in metals using cepstral processing

Abstract

We consider the problem of measuring the size of a spherical inclusion in a host metal. Ultrasound is directed at the test piece, and the echo returns are detected. Among other components, the received signal contains two echoes of the incident pulse, whose time separation is a measure of the diameter of the inclusion, or defect. A modified version of cepstral processing is used to determine this time separation. Simulated data are generated using a certain scattering model, and processed with good results. Sample test pieces were also constructed, and data collected and processed. The resulting measured defect diameters agreed with the true diameters.1.

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“…These benefits have of course been discussed intensively in the past already, in particular in the medical ultrasound community. Postprocessing of digitally sampled ultrasonic data was already discussed in the 70s regarding spatial filtering [4], deconvolution via inverse filtering in time [5], and Cepstral processing [6], respectively. Deconvolution techniques became increasingly elaborate with the growing availability of digital signal processing power [7][8][9].…”

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“…The cepstrum --a relatively new method --has been demonstrated on sonar, radar and image processing applications for separating two or more overlapped responses [6]. The basis for the cepstrum lies in the fact that when two signal echoes are combined then the composite signal can be modeled as a convolution of primary signal with a pair of impulses: one at the origin and another at a time delay corresponding to echo arrival time.…”