A Distributed Reflector Localization Approach to Ultrasonic Array Imaging in Non-Destructive Testing Applications

In array-based immersion ultrasonic non-destructive testing (NDT), an ultrasonic array and a solid test sample are immersed in water for the purpose of imaging and flaw detection inside the test sample. In such a test scenario, the upper surface of the test sample has two effects: i) it produces a strong interference signal in the backscattered received signal, and ii) its shape determines the array spatial signature of every point inside the material under test. Hence, in immersion NDT, to achieve a precise localization of a crack inside a test sample, the knowledge of the shape of the upper surface of the test sample is required. In this paper, we propose a distributed reflector modeling approach to characterize the interface between water and a solid test sample as well as any crack inside the solid test sample. This approach relies on the so-called incoherently distributed reflector modeling, where a distributed reflector can be modeled as infinitely many point sources located close to each other. Using such an approach, we present a model for the array data, and then develop a covariance fitting based technique to estimate the parameters of the shape of the interface between the two media and those of the shape of a crack inside the test material. Our numerical experiments show that our proposed approach yields a lower root mean squared error for the parameter estimates, compared to a state-of-the-art method, called root mean squared velocity technique.