Figure 2
Panels (a–d) are the reflected terahertz pulses from various locations on the medallion, the positions that correspond to each measurement are marked on panel (e) It is easy to see that the area where points (a–c) lay presents an internal air gap that generates an echo 35 ps to 45 ps from the surface, which corresponds to 2.3 mm to 3.4 mm from the surface.
The measurement at point (d) shows that the stone is solid and has no internal structure at that position, a dashed line approximately delimits the damaged area. (f) Photograph of the measurement process at position (c), the measurement point is indicated. (Photographs taken by K. Krügener).
