Ultrasonic array imaging for multi-layer materials in non-destructive testing applications.

Abstract

Ultrasonic imaging for a multi-layer medium is a common challenge in seismology, medical diagnosis, and non-destructive testing. One application for multi-layer imaging is ultrasonic immersion test where the material under test and transducer array are immersed in water. The main imaging challenge in immersion test (or in any multi-layer medium) is that since the sound wave propagates with different speeds in different layers, the homogeneity assumption is not valid. Thereby calculating the sound travel time for the backscattered received signal is complicated. In this dissertation, we propose a new approach to model the array received signals in order to image the material under test. In the first approach, we propose a distributed reflector modeling approach to characterize the interface between water and the solid as well as any crack inside the solid test sample. This approach relies on incoherently distributed reflector modeling. A distributed reflector can be modeled as infinitely many point sources located close to each other. We use distributed reflector modeling in order to estimate the shape of the reflectors. To do so, we present our data model in a two-dimensional coordinate system, and then develop a covariance fitting based approach to parametric estimation of the shape of the interface between the two media and that of a crack inside the test material. Numerical computer simulations show the accuracy of the proposed approach. However the proposed approach is a parametric localization method which needs the repetition of the ultrasonic test. In the second approach we present a data model to use for image reconstruction of a multi-layer medium without need to repeat the ultrasonic experiment. In this approach, we also use the spatially distributed source to model the interfaces between the layers of a multi-layer medium. Then, based on the Huygens principle, we develop a new array spatial signature for all the points inside a multi-layer medium. This new array spatial signature can be used in existing imaging techniques including the conventional beamforming technique, the MUSIC method, and the Capon algorithm in order to image a multi-layer medium. These aforementioned three algorithms are traditionally applied for a homogeneous medium where the sound velocity is constant in the material under test. Numerical simulations as well as experimental data show that the distributed reflector modeling outperforms other approaches such as rooted mean square velocity. In the third approach, to reduce the execution time for the imaging process, we develop a Fourier-based imaging technique to estimate the scattering coefficient of the points inside the second layer of a two-layer medium in order to obtain an image of the region of interest. First, we use an approximation of the proposed data model for the array backscattered signals due to the scattering of the point scatterers inside the second layer of the material under test. Seeking the similarity with the definition of Fourier transform, we propose a Fourier-based imaging algorithm, for imaging the second layer of the material under test. In this proposed algorithm, the execution time is considerably reduced compared to the three aforementioned imaging algorithms. This proposed algorithm can be used in an online imaging process.