Non-recursive synthetic aperture imaging for multilayered media with irregular boundary

The synthetic aperture focusing technique (SAFT) has been widely used for detection and sizing of material discontinuities (flaws) in ultrasonic nondestructive testing (NDT) because of its significant improvements in lateral resolution. But the efforts of researching on SAFT for irregularly multilayered objects that are inhomogeneous in both the vertical and lateral directions are relatively insufficient. In this paper, a new non-recursive synthetic aperture imaging algorithm, called modified direct Fourier migration (M-DFM), is proposed to deal with such irregularly multilayered situations. The derivation of the algorithm was inspired by reformulating Stolt f-k migration as a nonstationary filter, which could be extended to generally multilayered cases by the root-mean-square velocity. The experiment shows that the proposed algorithm is well suited for restoring images of regularly multilayered objects with almost the same accuracy as phase shift migration (PSM) which has recently succeeded in planar layered structures. And the performance of the proposed algorithm is also evaluated using immersion test data from a steel block with a convex interface and a side drilled hole, in this way investigating the method's capability of improving the lateral resolution of flaws detection in irregularly multilayered media. Furthermore, the proposed algorithm provides a non-recursive means to recover the images by performing a single 2-D inverse Fourier transform on the filtered spectrums rather than by recursive wave field extrapolation needed for PSM.