Prestack migration imaging of hardened layer depth based on equivalent offset

Accurately measuring surface-hardened layer depth is essential for evaluating workpiece quality. This paper proposes a novel method based on the full matrix capture (FMC) mode of ultrasonic phased arrays. By dividing equivalent offsets, multi-channel signal energy is remapped into common scattering point (CSP) gathers. A velocity-scan-based method is introduced to invert the hardened layer’s wave velocity using the hyperbolic characteristics of CSP time-distance curves. Combined with Kirchhoff integral migration, this enables high-resolution spatial imaging of the layer’s boundary. Simulations demonstrate high agreement with preset velocities and a relative depth error of only 0.4%. Experiments confirm the expected wave velocity increase in the quenched layer. Validation against metallographic analysis reveals a low average depth error of 2.8%. Compared to traditional common midpoint (CMP) and B-scan imaging, this method acquires richer interface details, shows higher sensitivity to inclination variations, and delivers superior resolution and reliability, offering a robust tool for non-destructive quality assessment.