Ultrasonic guided waves will propagate long distances in thin-walled materials such as plate or piping.  They are useful for locating and characterizing defects located at substantial distances from the transducer access site.

HD Laboratories has used guided waves for imaging erosion/corrosion:

Guided WaveTomogram
Image of 2-inch diameter flat-bottom holes in a 1/2 inch steel plate using Lamb wave tomography.
Point-Contact Transducer
25% thru-wall
50% thru-wall

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. 

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