In several big Japanese cities, the leakage of the water supply system is greater than 20% of the whole supplied water. So the leakage detection became an urgent problem to those cities. We developed a new method to this problem.
At first we investigated the basic properties of water leakage. We found how the size of a leak is related to the leaking sound intensity which usually gives the clue in detecting a leak. We took the frequency spectrum of the leaking sound. We also found how the sound propagates along the underground water supply pipe. The conventional method consists in detecting the leaking sound by a simple sound detector.
Secondly, we improved the noise characteristics of the leak detector by more than 20dB. The detection of the leaking sound had been heavily disturbed by the internal noise of the amplifier and by the ambient urban noises. We thus improved the utility of the conventional method. But the problem of the inaccurate detection of the leakage location still existed.
Thirdly, our new method of leak detection consists in measuring the propagation time of the leaking sound along the plpe. By the cross-correlation of the two simultaneously recorded random signals of the leaking sound on the pipe we calculate the propagation time. When a leak exists between the two recording positions, the maximum value of the cross-correlation occurs at the time corresponding to the difference in propagation times to the recorders from the leak. Our investigation has revealed that the propagation velocity is constant along the pipe in service. From the velocity and the propagation time difference we get the distance from each observing point to the leak along the pipe under examination. A practical test proved the accuracy and the utility of our method of the leak detection. The accuracy of the detected location was better than 0.5m. The method also greatly improved the signal to noise ratio of the leakage sound detection.

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