Define Radiography - Underwater Inspection of Bridge?

Radiographic non-destructive examination of components is based on the phenomenon that materials absorb radiation energy as it passes through them. The amount of energy absorbed depends on several factors, the main ones being the density of the material and the path length in that material. As a rough guide, greater the density of the material and longer the path length, the greater the radiation absorption. In this method, x-rays and γ-rays are used to detect those features that have an appreciable thickness in a direction parallel to the radiation beam. Discontinuities, which have measurable thickness in all directions, can be detected as long as they are not too small in relation to the section thickness. Radiography is more effective when the flaws are not planar.

The radioactive isotope emitting x-rays or γ-rays is secured to the work site and the films in a light proof and watertight cassette is secured on the opposite side. The source is so close to one surface that it does not register on the film, and only the far wall is recorded. Multiple exposures are required in order to achieve all round coverage. Divers are employed to place the source in its secured and safe condition, as well as the film cassette. After securing the source and the cassette at desired location, diver is withdrawn to a safe location. Source is exposed for the correct time by remote control. On completion of the exposure, the source is secured into its safe condition by remote control. Once this has been verified by remote control video camera, the diver is sent to move the source to the next exposure position. This procedure is used for each exposure until total coverage has been obtained.