Internal pipeline inspection using virtual source synthetic aperture ultrasound imaging

Focused ultrasonic monolithic transducers are often used for internal imaging of pipelines. The transducers yield a high lateral resolution as long as the distance between the pipe wall and the transducer corresponds to the transducer focal length. If the distance to the wall changes, for example due to increasing pipe diameter or de-centering of the inspection robot, the lateral resolution is decreased. We propose using synthetic aperture focusing with the virtual source method to extend the effective range of the focused transducer. It is assumed that the measurements are performed over a cylindrical surface, and we employ the Cylindrical Phase Shift Migration algorithm to perform the synthetic aperture focusing. The imaging algorithm is applied to scans of both pointlike scatterers and a corroded pipe surface, and the results demonstrate that a high lateral resolution can be obtained even when the pipe wall is several focal lengths away from the transducer.