# A Comprehensive Review of Acoustic Methods for Locating Underground Pipelines
## 3 Acoustic Methods for Locating Buried Pipelines
### Seismic Wave Methods
#### 3.4.2 Shear Wave Method  
Shear waves in underground imaging has been proposed were used to detect relics, based on the stacking of reflected scattered waves. However, the resolution of the image is not sufficient [96]. Accordingly, studies have been conducted to improve imaging of underground objects, by using a high-pass filter and peak position correction of received waves [104], adopting the envelope display method [105] and the deconvolution method [106]. As reported in [103], experimental work was carried out to validate the feasibility of the proposed method.  
Figure 12: (a) Experimental set-up for compressional wave method; (b) specially designed platform used to couple the shaker to ground [101], Copyright AIP Publishing, 2011.  
Figure 13: Cross-sectional image produced from compressional wave method. The white bold line shows the location of the target [101], Copyright AIP Publishing, 2011.  
A method proposed by [PERSON] et al. [103] was then modified to the compressional wave method as we introduced previously. However, the imaging results have poor resolution, as shown in Figure 13. To overcome this problem, a shear wave was suggested to be employed to illuminate the object, as it has lower speed than compressional wave which implies a smaller wavelength for a given frequency [101]. Consequently, a shear wave ground surface vibration technique was proposed and presented in [102] for the detection of buried pipes. The experimental set-up is displayed in Figure 14a. Another advantage of using shear wave is that time separation between the Rayleigh wave and reflected shear wave can be maximised, thus preventing the smaller reflected signal from being obscured by the dominant surface wave [99]. The rationale of this technique is similar to the compressional wave method while a shear wave is the predominant wave type among the main seismic waves. This was achieved by attaching the shaker to a specially designed rake (see Figure 14b) to preferentially excite horizontally polarised shear waves. The excitation signal was a swept sine from 10 Hz to 400 Hz.  
In addition to the modification of hardware, the signal processing algorithms have also been improved. A modified form of the cross correlation, which includes the excitation signal as an additional reference, was employed to mitigate the contaminants from clutter. The smoothed coherence transform was used to pre-whiten the signals to improve the resolution of the image. Figure 15 shows the cross-sectional staking images revealing the location of a buried plastic pipe. The experimental results have demonstrated that the shear wave method is successful in locating both plastic and cast iron pipe despite the low resolution of images.  
Figure 14: (**a**) Experimental set-up for shear wave method [102] Copyright Elsevier, 2014; (**b**) specially designed platform used to couple the shaker to ground [100].  
Figure 15: Cross-sectional image produced from shear wave method using two different wave speeds. The black bold line shows the location of the target [102], Copyright Elsevier, 2014.