A high-fidelity time-frequency representation for ultrasonic signal analysis

The time-frequency (TF) characteristics of ultrasonic echoes provide valuable information leading to the characterization of materials and localization of defects. The conventional TF analysis methods such as Wigner-Ville distribution (WVD) and short-time Fourier transform (STFT) perform inadequately when applied to ultrasonic signals because they introduce cross terms, offer poor resolution and are sensitive to noise level. In this study, we present a TF representation for ultrasonic echoes based on a matching pursuit (MP) method. MP decomposes a signal into a linear expansion of Gabor functions and maintains energy conservation. MP offers a TF distribution of the signal by enabling the addition of the TF distribution of each composing Gabor function. This TF representation is free of cross terms and adaptive to signal characteristics. The performance of this TF method has been tested using simulated and experimental ultrasonic data, and then compared to conventional techniques such as WVD and STFT. In particular, we present TF results for ultrasonic flaw detection where the microstructure scattering echoes dominate the flaw echoes (SNR is about 0 dB).