NDTnet 1998 Aug, Vol.3 No.8

Full-Text - Abstract

The main goal of ultrasonic grain noise suppression in material flaw detection is to improve the perceptual possibilities of the operator to observe defect echoes. The suppression is perceptually ideal when a received signal, or image, which contains echoes buried in noise is filtered to yield nonzero values only at the positions of the defect echoes. In statistical terms, this means the amplitude distribution in the filtered signal (image) is much more concentrated around zero than in the raw data. A more concentrated amplitude distribution generally means smaller entropy. Thus, from an operator perception point of view, interesting results should be obtained if the raw data can be filtered to yield low entropy amplitude distributions. However, one should note that the entropy can be minimized by means of a (pathological) filter which always outputs zero or another constant value. Thus, appropriate restrictions must be imposed on the filter construction process.

Filters that output noncoherent detector statistics have, in our recent work, shown to be very powerful for grain noise suppression in ultrasonics. However, such filters require the operator to carefully specify a transient prototype, modelling the defect echoes which should be detected. Here a new approach is presented, based on the above ideas about perception, which eliminates the need for the operator to manually specify a defect prototype.

In this new approach, the prototype is restricted to be a member of a two-parameter signal family, and the parameter values are optimized to minimize the entropy of the amplitude distribution in the filtered signal (image). As the entropy function is rather computationally intensive to evaluate, alternative approaches for measuring the concentration of the amplitude distribution are discussed and more computationally efficient measures are proposed.

Experimental results on real ultrasonic B-scan data, acquired from samples with coarse material structure, are presented to demonstrate the power of the novel approach.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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