Spatial Resolution of Acoustic Microscopy in the Visualization of Interfaces inside a Solid

The volumetric visualization of materials and inner boundaries of a joint is one of the fields in which long-focus high-frequency beams are used. Visualization is carried out using beams that passed through the immersion–sample interface and changed their geometry due to differences in the refraction of rays being incident at different angles on the interface. The beam structure is substantially distorted upon refraction and transforms into a caustic. The problem of spatial resolution in the formation of acoustic images of the microstructure in the material bulk is relevant. This paper presents a theoretical analysis of the spatial resolution with consideration for refractive aberrations in the formation of images of interfaces that are located at a considerable depth in the sample bulk. The results of the experimental visualization of the interface between jointed materials are presented. It is shown that the theoretical estimate is in good agreement with the experimental value of the resolution for beams of longitudinal waves converging in the sample volume.