title: Noise reduction in ultrasonic NDT using undecimated wavelet transforms.
abstract: Translation-invariant wavelet processing is applied to grain noise reduction in ultrasonic non-destructive testing of materials. In particular, the undecimated wavelet transform (UWT), which is essentially a discrete wavelet transform (DWT) that avoids decimation, is used. Two different UWT processors have been specifically developed for that purpose, based on two UWT implementation schemes: the "à trous" algorithm and the cycle-spinning scheme. The performance of these two UWT processors is compared with that of a classical DWT processor, by using synthetic grain noise registers and experimental pulse-echo NDT traces. The synthetic ultrasonic traces have been generated by an own-developed frequency-domain model that includes frequency dependence in both material attenuation and scattering. The experimental ultrasonic traces have been obtained by inspecting a piece of carbon-fiber reinforced plastic composite in which we have mechanized artificial flaws. Decomposition level-dependent thresholds, which are suitable for correlated noise, are specifically determined in all cases. Soft thresholding, Daubechies db6 mother wavelet and the three well-known threshold selection rules, Universal, Minimax and SURE, are applied to the different decomposition levels. The performance of the different de-noising procedures for single echo detection has been comparatively evaluated in terms of signal-to-noise ratio enhancement.
authors.affiliation: Dpto. Señales, Sistemas & Tecnologías Ultrasónicas, Instituto de Acústica (CSIC), Serrano 144, 28006 Madrid, Spain.
authors.forename: E
authors.initials: E
authors.lastname: Pardo
authors.forename: J L
authors.initials: JL
authors.lastname: San Emeterio
authors.forename: M A
authors.initials: MA
authors.lastname: Rodriguez
authors.forename: A
authors.initials: A
authors.lastname: Ramos