Especially efficient in outside environments, radar detection accurately finds the leak point and provides information about the size of the leak. It uses radar waves that can travel trough most materials.
Parts of the wave that hit a material return back and are received by a device that, by calculating their return time and strength, can determine the material that the wave hit.
This method is especially effective for inside problems, where access to the pipes is easy. The procedure is simple: a certain gas, usually industrial hydrogen, is introduced into a pipe, and then the track of the pipe is carefully inspected with a device sensitive to that particular gas.
Because the gas is pumped at high pressure, if a leak exists, the detector will quickly identify it by detecting the leaking gas. This method can also be applied to outside pipes, but may require digging to get to the pipe, and the detection accuracy depends on the depth the pipe is installed; the deeper the pipe, the less accurate the results will be.
Geophones are similar to stethoscopes used by doctors, and their working principle is the same. These devices catch and amplify sound, and a leak can easily be identified by listening to the ground.
Every leak, big or small, makes a sound, and finding the sound means finding the leak. The irony is that larger leaks may be quite hard to detect using this method because, through a large leak, water can pour out quite easily without much noise.
A small leak in a high pressure pipe usually makes a lot of noise, which, in some cases, can even be heard with the free ear.