# A Comprehensive Review of Acoustic Methods for Locating Underground Pipelines
## 3 Acoustic Methods for Locating Buried Pipelines
### Traditional Acoustic Transmission Technique  
The traditional acoustic transmission technique has its origins in water leak detection [69]. The leak noise propagates along the pipe and through the soil to the ground surface where it can be picked up by a listening rod [15], and then the leakage point can be located. A similar principle is employed in the acoustic transmission technique to determine the location of buried utilities, as shown in Figure 5a. In this case, acoustic waves are generated into the pipeline or the in-pipe product. In addition,  
Figure 4: Overview of acoustic locating methods.  
Figure 3: Block diagram showing the principle of acoustic pipe localization methods.  
they travel through the soil medium until they reach the surface, and thereby they can be detected using sensors such as geophones. The vibration will be strongest directly over the pipe because of the shortest travel distance and less signal attenuation. Therefore, the location of the buried facility can be determined by tracing the strongest sound level intensity. A prototype acoustic transmission system was assembled and evaluated by conducting field tests at four sites [11]. The excitation frequency ranged from 10 Hz to 300 Hz as it is known that the attenuation of soil to sound waves is low in this frequency range. It was found that the frequency range from 50 Hz to 150 Hz was most effective depending on the soil type. A test result is displayed in Figure 5b as an example, where the peaks of the sound level correspond to the location of the pipe.  
There are three methods to generate the acoustic wave into buried structures, including active sonic, passive sonic (relying on the ability of the pipe's product to escape, like water leakage) and resonant sonic [10]. Active sonic method involves inducing a sound onto or into a pipe, which can be realized by attaching a source at an exposed point (like a hydrant) or by inserting a noise source into a pipe. Resonant sonic method relies on the in-pipe medium being a non-compressional fluid (water in most cases), where detectable motion of the pipe can be created by interfacing the fluid surface and generating pressure waves. Most of the commercially available acoustic locators are based on active sonic and resonant sonic methods, see examples of Fuji NPL100 [81], Radiodetection RD500 [82] and  
Figure 5: Traditional acoustic transmission method: (**a**) principle of the method, modified from [9]; (**b**) response versus location for conduit (directly excited) [11].  
SEWERIN System Combiphon [83]. The detailed information of these locators will be demonstrated in Section 4.  
Studies show that the traditional acoustic transmission method proves useful in locating underground utilities and can be an effective supplement to currently existing techniques for locating both metallic and non-metallic buried facilities [11]. However, the requirement of access to the utility systems and its inability of depth-estimation limits the acoustic transmission method as a standard trace technique instead of an effective search technique. In addition, the detection results are susceptible to interference from noise and distortion from surrounding medium properties [10].