3. Imaging and acoustic signature

3.1 Volume and surface modes

These two complementary concepts have emerged as the instrument has developed, to some extent copying the evolution of electron microscopy techniques. The use of modalities with different interactions with the material to be observed offers cross-checking possibilities to facilitate the interpretation of results. C-mode imaging, i.e. imaging planes parallel to the surface, was the first to be developed, but soon the rather qualitative nature of the observations gave way to quantitative analysis. When a highly convergent acoustic beam strikes the surface of an object, mode conversion occurs. As in optics, the most classic modes are reflected and transmitted, but there are many others in acoustics, known as surface and interface modes. The information coming...