A New Model for Array Spatial Signature for Two-Layer Imaging With Applications to Nondestructive Testing Using Ultrasonic Arrays

Imaging multilayer materials is a common challenge in seismology, medical diagnosis, and nondestructive testing. One of the applications of multilayer imaging is ultrasonic immersion test where the material under test and the transducer array are immersed in water. The main imaging challenge in immersion test (or in imaging any multilayer medium) is that since the sound wave propagates with different speeds in different layers of a multilayer medium, such a medium cannot be assumed homogenous. As a result, calculating the sound travel time for the received signal due to backscattering from such a nonhomogenous medium is not as straightforward as in the case of homogenous materials. In this paper, we propose a new model for the array spatial signature which can be used in frequency-domain algorithms that are used for imaging a two-layer medium when an array of transducers is utilized. To do so, we model the interface between the two layers as a spatially distributed source which consists of infinite number of point sources. Then, we use this model to develop a new array spatial signature for any point inside the second layer of a two-layer medium. This new array spatial signature can be used for multilayer ultrasonic imaging in frequency-domain imaging techniques including the conventional beamforming technique, the MUSIC method, and the Capon algorithm. Numerical simulations as well as experimental data are used to examine the accuracy of the proposed model.