I have heard of it, and saw a ...

I have heard of it, and saw a case done by helicopter (expensive) using thermal imaging. With thermal imaging, the water will have to be a different temperature than the surroundings. It would also only detect leaks coming to the surface. Leaks going down into sewers or drainage systems would likely be missed. I'd think depth of the pipe would be a factor as well as deep leaks are even harder to detect acoustically than shallow ones from ground level. I'd think it would be a good shotgun approach survey tool, but you would need others such as loggers and acoustic devices to fully survey.