SPECTRAL CORRELATION IN ULTRASONIC PULSE-ECHO SIGNAL-PROCESSING

This paper considers the effects of using spectral correlation in a maximum likelihood estimator (MLE) for backscattered energy corresponding to coherent reflectors embedded in media of microstructure scatterers. The spectral autocorrelation (SAC) function is analyzed for scatterer configurations based on the regularity of the interspacing distance between scatterers. It is shown that increased regularity gives rise to significant spectral correlation between different frequencies, while scatterers that are uniformly distributed throughout a resolution cell result in no significant correlation between spectral components. This implies that when a true uniform distribution for the effective scatterers exists, the power spectral density (PSD) is sufficient to characterize their echoes. However, as the microstructure scatterer distribution becomes more regular, SAC terms between different frequencies become more significant. Experimental results compare the performance of an adaptive MLE using the SAC and PSD characterizations of the grain echoes. The MLE results for 15 A-scans from stainless steel specimens with three different grain sizes indicate an average 6-dB signal-to-noise ratio (SNR) improvement in the coherent scatterer (flat-bottom hole) echo intensities for estimators using the SAC characterization, as opposed to estimators using the PSD characterization.