A Novel Spatio-Temporal Frequency-Domain Imaging Technique for Two-Layer Materials Using Ultrasonic Arrays

Ultrasonic imaging of multi-layer materials with parallel interfaces is a challenging problem in non-destructive testing. In multi-layer materials, since the sound velocity and the propagation path change when sound travels from one layer into another, calculating the sound travel time is complicated. In this paper, we develop a frequency-domain imaging algorithm for estimating the scattering coefficients of all the points inside the second layer of a two-layer (liquid-solid) medium in order to obtain an image of the region of interest. To do so, we first introduce our data model for the array received signals by modeling the interfaces between the layers of a two-layer medium as a spatially distributed source. Then, we introduce a mapping relationship between the two-dimensional image of the region of interest and the three-dimensional Fourier transform of the received signals. This proposed algorithm has relatively lower computational complexity and it can be used for online imaging. Computer simulations as well as experimental data show the accuracy of the proposed algorithm.