Frequency-Wavenumber Migration of Ultrasonic Data

In ultrasonic nondestructive evaluation (NDE), the depth of the image is usually calculated by multiplying the traveling time of the echoes with the velocity of the medium. If the flaw is not a horizontal plane, the flaw images may be distorted and mislocated. Although the lateral resolution and sizing accuracy of the flaw image can be improved using the digital signal processing methods, e.g., the utilization of the blocking filter or deconvolution filter, these methods do a little favor about the distortion and mislocation problems. The migration, an image-processing method used widely in reflection seismology, is introduced to process the ultrasonic data in this study. Since the signals are coherent and the noises are random, the flaw image can be transformed from its apparent position to the true position using the migration method and the resolution of the image may be improved. Not only the real positions of the oblique cracks can be found upon applying the frequency-wavenumber (F-K) migration to process ultrasonic data, but, in addition, the dimensions of the flaws can be estimated more accurately. Results presented in this study show that the migration method can be applied successfully to the ultrasonic data processing and can improve the quality of ultrasonic image in both size and location of the flaws.