For some years now, the UK water supply utilities have been putting considerable resources into detecting and repairing leaks in the drinking water supply pipework. North West Water, who sponsored this development, spends around £3m per year on leak repair operations.

Locating a leak is not easy, and all too often an excavation carried out at the site of a suspected leak will miss it, and another hole has to be dug. Besides being expensive, digging unnecessary holes in roads creates a nuisance for the community.

The interferometer was developed in order to cut out these unnecessary excavations. The interferometer pinpoints the position of a leak before digging begins.

The interferometer works by transmitting a low power radio-frequency (UHF) signal directly into the ground. The signal is reflected off the water issuing from the leak and returns to the antenna, where it is amplified and passed to a sensitive interferometer system. This effectively ignores any signal whose character does not change with time. If there is a leak, the signal reflected from it varies rapidly; the interferometer recognises the signal, which is then amplified, processed, and displayed to show its strength and character.

The system will only detect a leak that is directly below the antenna. Thus by walking the system along a water main, the precise position of a leak can be found.

The photograph below shows the prototype system mounted on a trolley. The RF system, interferometer, the signal processing system and display are housed in the grey box mounted within the red frame. Below it are two aluminium housings containing the transmit and receive antennae (the two can be mounted in a single housing for compactness with a slight loss in sensitivity).