Ultrasonic imaging of pitting using multilayer synthetic aperture focusing

Synthetic aperture focusing is known to increase both the lateral resolution and signal-to-noise level of pulse-echo ultrasonic images. In this paper, the use of synthetic aperture imaging for detection and sizing of pitting in a plate is considered. It is assumed that a water layer is present between the transducer and the plate, and that the pitting is on the back side of the plate, seen from the transducer. To account for the different wave velocities of the water and plate layers, one of the recently proposed multilayer synthetic aperture algorithms is used. The algorithm can be adapted to both 2D (line scan) and 3D (surface scan) data, and these two variations are tested on a number of artificial pittings in an aluminum plate. Comparison of raw and focused images shows that the algorithm significantly improves the lateral resolution, thus enabling an accurate estimation of the pitting shape.