Ultrasound tomography based on the coefficient inverse problem as a way to combat structural noise

The paper proposes to use the ultrasound tomography method based on the solution of the inverse coefficient problem to reduce the level of structural noise. Mathematical models used in ultrasonic tomogra-phy well describe such physical effects as refraction, diffraction and redispersion effects. It is logical to ex-pect that reconstruction of the internal structure of metallic samples using ultrasound tomography will be more efficient compared to digital antenna focusing (DAF) techniques. Due to the nonlinearity of the inverse problem of ultrasound tomography, an iterative MultiStage method is used to ensure convergence to the glob-al minima of the non-convexity functional. The paper presents the results of numerical experiments to restore the image of the internal structure of the welded joint, which may contain lateral cylindrical holes and crack models. The region of the welded metal is represented in the form of sections constructed according to the principle of Voronoi diagrams. In each section the velocity is constant and its value is randomly distributed. In the model adopted in the paper, the structural noise is formed due to multiple scattering at the boundaries of sections with different sound velocity. It was assumed that the antenna array is located on the outer surface of the control object of known thickness. The results obtained show that the tomographic method allows us to determine the shape and speed of sound in low-contrast reflectors, for which the CFA method is ineffective.

Inverse coefficient problem, ultrasonic tomography, ultrasonic nondestructive testing (NDT), structural noise, antenna array, digital antenna focusing (DAF) method