Research on improving time-domain resolution of pulse-echo methods by compensation filtering

In ultrasonic pulse-echo NDT of structures, the transducers usually operate in its harmonic frequencies in order to maximize the amplitude response. However, there exist some unwanted effects in such mode. For instance, the generated/detected signals will have long ringing, which means the time-domain resolution will be reduced. Presented here is a digital filtering method to compensate the unwanted frequency response characteristics of transducers. The first step is to establish a discrete transfer function model of transducer system based on time-domain system identification algorithms. Then the established model can be realized by a digital compensation filter to reduce the ringing effects of the measured signals. Experimental verification of the proposed method is carried out. After calibrating and modeling for transducers by water-immersion test, a compensation model is established and used to the thickness measurement of a concrete specimen. The compensation results show that the ring of transducer can be greatly reduced and the reflection from bottom of the specimen can be extracted from the original overlapped signals. To further validate the effect of the proposed method, its application in B-scan imaging of a concrete element with embedded anomaly is also given.