A sparse array of Lamb wave transducers can directly image pits or corrosion using ultrasonic tomography. The velocity of the guided Lamb waves is tomographically imaged throughout the area of interest. Since Lamb wave velocity is a function of plate thickness, the result is an image that can be calibrated in thickness.
The example shown above used a square array of 32 point-contact transducers. The transducers were located on the border of 12-inch square segment of the plate encompassing a pair of machined flaws. Test frequency was 75 KHz.