# A Comprehensive Review of Acoustic Methods for Locating Underground Pipelines
## 3 Acoustic Methods for Locating Buried Pipelines
### Pipe Excitation Method--Improved Acoustic Transmission Technique  
As stated earlier, the traditional acoustic transmission method employs rather crude ways of excitation and only a single receiver is used to collect the resultant vibration signals on the ground surface. Moreover, only magnitude information of the gleaned signals is processed for interpretation of the detection results. Consequently, most acoustic locating systems for commercial use suffer from poor performance. To overcome these drawbacks, exploiting the phase of the ground vibration data has been proposed [84]. Previous studies show that in a buried fluid-filled pipe, the fluid-dominated wave can be well-coupled with surrounding soil and radiate effectively towards the ground surface [63; 64; 85], which means this mode is the ideal wave type for detection at ground surface. Therefore, a dedicated pipe rig was designed and several excitation methods were tested to find out the most effective means to generate the fluid-dominated wave [86]. Based on the findings mentioned above, pipe excitation method was proposed and elaborated in [69], providing a robust and promising technique in buried water pipe localization, especially for plastic pipes. The pipe excitation method shares similar principles with the traditional acoustic transmission method. When compared with each other, there are three main improvements employed in the pipe excitation method. Firstly, the acoustic source is specially designed and installed, with an inertial shaker mounted on a plastic end-plate bolted to the flanged end to the pipe as it come up to the ground surface, as shown in Figure 6a,b. In this way, the fluid-dominated wave can be excited to most extent. The shaker was excited with a stepped sine input from 10 Hz to 400 Hz. Secondly, the ground vibration is measured using an array of I/O SM-24 geophones over a rectangular measurement grid depicted in Figure 6c, which enables the multiple vibrations to be measured simultaneously. Most importantly, the phase information of ground vibration is employed to map the run of the buried pipes, which is proved to be more robust than magnitude estimates in presence of noise and different types of ground surfaces [69]. The pipe excitation technique has been shown to be successful for locating both plastic and metal water pipes, laid under grass and under tarmac [12]. Figure 7 shows detection results at a single frequency of 62 Hz for the same MDPE water pipe, with the unwrapped phase clearly revealing the entire run of the pipe. Magnitude information can supplement this and may be particularly useful in identifying discontinuities where reflections can occur, for example a bend in the pipe, a change in pipe material or dimensions, even a leak. When unwrapping the phase in the frequency domain at a single location above the pipe, abrupt phase changes occur, which may not be properly accounted for. To rectify this problem, an analytical model was established to predict the ground surface displacements due to fluid-dominated wave motion in a buried water pipe [70; 87]. This work extended the study conducted by [PERSON] and [PERSON] [12], where the ground surface displacements accompanying the propagation of acoustic waves in a buried gas-filled pipe was initially investigated. The theoretical model in [70] explained the reason for abrupt phase changes by demonstrating the radiated wave interference that occurs in some soil conditions, and underpins the pipe excitation method for locating buried pipes. A recent application of the vibro-acoustic was to locate the tree root from ground surface vibration measurements [88, 89], where, fortuitously, the run of a plastic pipe adjacent to the tree root was plainly visible in the contour plots at low frequencies. This finding implies new possibilities for non-touching  
Figure 6: Experimental set-up of pipe excitation method: (**a**) pipe rig layout; (**b**) shaker mounted on plastic end-plate; (**c**) grid of measurement points. [69], Copyright Elsevier, 2011.