That indeed works, but is a time consuming manual process, pipes arent always leaking clearly (visually), and that only works when a failure has occured. I built a few fibre based instruments three or four years ago that could detect and gauge changes in the pipe caused by wall thinning (before leaks have occured), small leaks (too small to notice soil changes) and also fouling. Longevity and robustness of the sensors was always an issue though
