Together with radiography, it is classified as a volume method. It means that even defect inside material can be detected. A little bit tricky application, as the probe can be applied on the object accessible surface only.
A typical application is a task of material integrity approval, e.g. manufactured semi-products (sheet, forging or casting) or welds. Internal reflections can be applied in advance. A lot of techniques have been developed, such as pulse-echo, transmission-through, Time-Of-Flight-Diffraction. We need “to see” a defect in specific direction range, however its distance (depth under surface) may be very long (a success of 12 meters in concrete was published). On the other hand, separated layers of CFRP composite can be recognised - it means resolution of about 0.1 mm. A specific application of a pulse-echo technique is a measurement of thickness; both manufactured (tolerances of the sheet) and operated (corrosion on back-wall or thickness decrease due to tube diameter creep).