FLAW SIGNATURE ESTIMATION IN ULTRASONIC NONDESTRUCTIVE EVALUATION USING THE WIENER FILTER WITH LIMITED PRIOR INFORMATION

Flaw signals measured in ultrasonic testing include the effects of the measurement system and are corrupted by noise. The measurement system response is both bandlimited and frequency dependent within the bandwidth, resulting in measured signals which are blurred and distorted estimates of actual flaw signatures. The Wiener filter can be used to estimate the flaw's scattering amplitude by removing the effect of the measurement system in the presence of noise. The optimal form of the Wiener filter requires estimates of the parameters which describe the noise and scattering amplitude distributions, respectively. We present an approach for the optimal implementation of an alternative form of the Wiener filter which requires only estimates of the noise distribution parameters. The first part of the paper addresses the theoretical error for scattering amplitude estimation assuming various levels of available prior information. In the second part of the paper, three estimation techniques, one a maximum-likelihood based method and the other two residual sum of squares methods, are formulated and tested. Results are presented which demonstrate that any of the three approaches could be used to optimally implement the alternative form of the Wiener filter with limited prior information.