Models Comparison for the scattering of an acoustic wave on immersed targets

Ultrasonic telemetry techniques consist in locating various immersed structures (for instance, components in the main vessel of fast breeder reactors). The interactions beam-targets give rise to different scattering phenomena: tip diffraction of boundaries and edges of the different parts, specular reflection, and corner effect. In order to conceive and design such imaging techniques, simulation tool needs to account for these effects. Classical methods have been studied for such problems. The diffraction coefficients based on the Geometrical Theory of Diffraction (GTD) fail in the transition regions adjacent to shadow and reflection boundaries. The uniform diffraction theory provides continuous solutions in these regions, but with more sophisticated formulation. Another simple approximation based on the integral equation, widely used for scattering problems, is the so-called Kirchhoff approximation. The Kirchhoff approximation has good performance in the specular reflection zone but fails at predicting amplitude of diffracted waves by edges. A refinement of the Kirchhoff approximation which is based on the Physical Theory of Diffraction (PTD) and combines GTD and Kirchhoff edge diffraction coefficients has been studied. This refined Kirchhoff approximation provides a simple formulation and correct results for all scattered directions, which will be illustrated in the case of a rigid halfplane or wedge.