Synthetic aperture focusing technique for correction of poorly-focused ultrasonic pressure tube inspection data

Defect size measurement of ultrasonic pressure tube inspection within the CANDU (CANada Deuterium Uranium) reactor has challenges when the transducer is poorly-focused with respect to the tube surface. Environmental effects, such as diametrical creep and sagging over time can lead to displacement of the tube surface from the optimal inspection position. In order to improve the accuracy of defect sizing, a Synthetic Aperture Focusing Technique (SAFT) method based on single element focused transducer inspection is proposed. In SAFT processing, the transducer focal point operates as a virtual source to transmit ultrasound with a corresponding beam angle relating to the transducer aperture and focal length. To correct for the image distortion caused by poor focusing in the region beyond the transducer focal point, every pixel value of the B-scan image is calculated using the delay-and-sum of echo signals collected at multiple positions on the synthetic aperture, while the synthetic aperture length is determined by the beam angle of the virtual source. Application of the SAFT approach significantly enhances image resolution through which more accurate feature size measurements can be determined, with the accuracy of a -6dB measured surface defect width improved by 50% compared to the original B-scan image when the surface is located more than 1.5mm beyond the focal point.