Analytical simulation of low frequency wave radiation from a buried water pipe

Abstract

Vibro-acoustic techniques have been developed with the aim of determining the location of buried pipes. However, the characteristics of the recorded conical wave radiation from the pipe wall to the surface, at very low frequencies, have not been investigated in depth. By applying any type of excitation, at low frequency, to a typical water pipe, only the axisymmetric waves termed n=0, s=1,2 are dominant and most of the energy is carried by these waves which are well coupled to the pipe wall. This excitation can be applied, from a location where there is an access point to the buried pipe from the surface, e.g. a fire hydrant. Exciting the pipe using this technique generates two types of wave in the surrounding medium; a spherical and a conical wave. A distortion of the spherical wave at the surface, above the pipe axis, indicates the location of the pipe. In this paper, the speed of the seismic wave within the surrounding medium and the acoustic waves in the pipe are estimated by looking at the phase data obtained from Cross Spectrum Density function (CSD), between the input voltage to the shaker and the measured ground vibration at the surface. Using phase measurements are preferable to magnitude, since they are more resistant to noise effects and provide more reliable data, even for low quality measured data. An analytical study was performed based on the results obtained from the experimental study. Furthermore, a new way of unwrapping phase data was proposed so as to simplify the process of pipe location using this technique.