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a b s t r a c tThis paper is concerned with time delay estimation for the detection of leaks in buried plastic water pipes using the cross-correlation of leak noise signals. In some circumstances the bandwidth of over which the signal analysis can be conducted is severely restricted because of resonances in the pipe system, which manifest themselves as peaks in the modulus of the power spectral and cross spectral densities, and deviations from straight-line behaviour in the phase of the cross spectral density. The result can be a crosscorrelation function in which it is difficult to estimate the time delay accurately. This paper describes a procedure in which the shape of the cross-correlation function can be significantly improved, resulting in an unambiguous and clear estimate of the time delay. The frequency response function(s) of the resonator(s) responsible for the resonance effects are first determined and then the data is processed using the model(s) of the resonators to remove these effects. This enables more signal processing to be conducted, potentially over a much wider bandwidth, further improving the shape of the cross-correlation function. The process is illustrated in this paper using hydrophone measured data at a leak detection facility. The current limitation in the process is that it is carried out manually, which could potentially restrict its application in practical acoustic correlators. The challenge now is to develop an algorithm to carry out the procedure automatically.

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Abstract:The axisymmetric (n=0) fluid-borne (s=1) wave has been exploited with varying 10 degrees of success in practical surveys for determining the location of buried pipes. 11Difficulties are sometimes encountered in interpreting ground surface vibration data, whilst 12 attempting to locate the pipes, due to the occurrence of abrupt changes in the phase response 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 model, theoretical predictions of ground surface displacements are compared with 1 experimental data from a dedicated MDPE pipe rig.

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