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58 Technical Support Information for Active and Passive Acoustic Locating Technology Abstract Injection of an acoustic signal into the fluid medium within a pipe has long been pursued as a means of locating buried pipes. The technique was first applied to cast iron mains and later to plastic piping. Both of these materials have the attri- bute that there may be little or no electrical continuity along the main. Even in cases where a tracer wire is installed along- side a plastic main, the wire can become compromised by corrosion or other issues. In these instances, standard electro- magnetic tracing methods cannot be used, because the main cannot support the tracer signal current. Another technology that is used to locate poorly conduc- tive or nonconductive piping is ground-penetrating radar (GPR). GPR technology works well under some circum- stances but not all. Soils that are wet or mineral laden attenu- ate the radar signal in both directionsâafter launch from the antenna and again on the return trip after reflecting from a target. The other issue is the detectable target size: the target must be large enough to reflect detectable signal energy, and the wavelength of the radar signal must be comparable with the size of the target. Even in benign soils, small targets may produce little or no GPR signal. Active acoustic technologies address some of the limita- tions of GPR and other reflecting technologies. Having the pipe radiate a detectable signal, acoustic or electrical, decreases the signal attenuation in that there is no longer the âround- trip lossâ inherent in reflected signal techniques. The radiated signal can be introduced intentionally to the utility, or it may be intrinsic, as in the case of AC power conduits. A pipe-radiated signal also removes the restriction applicable to reflective sys- tems that the pipe diameter be comparable to at least one wavelength of the signal. The concept of locating plastic pipe using an injected sound signal and an array of surface sensors has been proved to work. Reducing the equipment required to something field-ready has been more problematic. In the 1990s, the data acquisition and processing equipment was bulky and expensive. Current technologies are being applied to drastically reduce the equip- ment size and cost. The same acoustic technology can be used to passively detect buried facilities that radiate an acoustic signature. Examples of this are the 60-Hz vibrations emitted by buried electric power lines and the flow noise emitted by water or steam lines. GTI has extensive experience in the acoustics of utility systems as a result of developing several generations of equip- ment for acoustic pinpointing of gas leaks. Much of the signal processing technique being applied to the SHRP 2 R01C project was developed to differentiate a gas leak signal from the background. Technology Synopsis and Key Performance Indicators Title: Acoustic Location Using an Active Signal Targets: All pipe materials. Depth range: At least 20 ft; greater in some soil types. Depth accuracy: The expected accuracy for pipe parallel to the surface of the ground is ±10% of the pipe depth. For example: 20 ± 2 ft. Location accuracy: The expected accuracy is ±1 ft or ±10% of the pipe depth, whichever is greater. Application: An acoustic driver or speaker is placed in con- tact with the fluid within the pipe to introduce the signal. In the case of potable water, a water hammer can be used to generate the signal. An array of acoustic sensors is placed on the surface of the ground to take readings. Basic principle: The active acoustic pipe locator injects a known acoustic signal into the fluid (gas or liquid) inside a pipe using an acoustic driver or loudspeaker for air or natural gas or a water hammer generated at a hydrant. The acoustic wave propagates through the medium in the pipe, not in the pipe wall. As the signal travels in the fluid, a portion of the signal couples into the pipe wall. Vibrations from the pipe A P P e n d I x e
59 wall propagate to the surface of the ground, where they are detected. Those signals are used to detect the position of the pipe and to estimate its depth. Limitations: The overall depth range is limited by the power of the acoustic signal introduced into the fluid. The sound source must be in direct contact with the gas or liquid in the pipe. For a gas main, a service can be removed and the sound injected there. Additional notes: Because the acoustic signal only travels from the pipe to the ground surface, it suffers less attenuation than reflection-based techniques. The active signal is injected into a specific pipe, positively identifying it. The signal origi- nates in the pipe instead of reflecting from its surface, relax- ing the requirement that the wavelength be scaled to the pipe diameter. Therefore, the active method can achieve extra depth by using lower frequencies, which suffer less attenua- tion. The technique lends itself to locating sewers and other unpressurized facilities because the acoustic driver can be placed or hung in a manhole. Technology Synopsis and Key Performance Indicators Title: Acoustic Location Using a Passive Signal Targets: All pipe materials. Depth range: The depth range depends on the strength of the passive signal. Depth accuracy: To be determined. Location accuracy: The expected location accuracy is ±1 ft or ±10% of the pipe depth, whichever is greater. This assumes that a reasonable passive signal is present. Application: An array of acoustic sensors is placed on the surface of the ground to take readings. The facility being located radiates a characteristic sound signature in normal operation, such as flow noise or 60-cycle hum. Basic principle: The active acoustic equipment can also be used in a passive mode for situations where a passive signal is generated by the normal operation of the pipe. Examples of passive noises include flow noise generated by natural gas or water flowing through the pipe and acoustic hum generated in three-phase electrical cables. In passive mode the active signal generator is not used. Limitations: The overall depth range is limited by the power of the acoustic signal generated by the facility as a by-product of its normal operation. Additional notes: Because the acoustic signal only travels from the pipe to the ground surface, it suffers less attenuation than reflection-based techniques. The signal originates in the pipe instead of reflecting from its surface, relaxing the require- ment that the wavelength be scaled to the pipe diameter.
