Ultrasonic multi-frequency time-reversal-based imaging of extended targets

In this paper, two multi-frequency time-reversal-based imaging methods, time reversal with multiple signal classification (TR-MUSIC) and its phase-coherent form (PC-MUSIC), are explored for application to the nondestructive testing (NDT) imaging of extended target machined in solids. The size of which is on the order of the ultrasonic wavelength or larger. The principle for multi-frequency TR-based imaging method is presented, and the performance is estimated in terms of point spread function (PSF). Both TR-MUSIC and PC-MUSIC are tested with experimental ultrasonic array data acquired using the full matrix capture (FMC) process. Two experiments in metal solids with different kind of defect, side-drilled hole (SDH) as well as electrical discharge machining (EDM) lines that can be considered as extended targets, were implemented. Here, the EDM lines have different positional relationships with array. For SDH, it is shown that both TR-MUSIC and PC-MUSIC can locate the position, but fail to distinguish the shape. For EDM lines, TR-MUSIC can only detect the defect, but PC-MUSIC can assess the length based on the imaging results under some dimension of signal subspace. In addition, the effect of dimension of signal subspace on the imaging methods is also investigated, and it is shown that TR-MUSIC is sensitive to the dimension when compared with PC-MUSIC.