Explain Rebound Hammer - Underwater non-destructive testing?

The underwater rebound hammer system is a surface hardness tester which can be employed to acquire a general condition assessment of concrete. The system contains an underwater rebound hammer, an umbilical cable, and a topside data acquisition unit that is abbreviated as DAU. The rebound hammer is mounted in a waterproof housing which contains an electrical pick up to sense the position of the rebound mechanism. The umbilical cable connects the underwater rebound hammer to the DAU that contains the signal conditioning electronics and data acquisition system.