Experiments were conducted on the underground pipeline at the EPA's UST Test Apparatus in which three acoustic sensors separated by a maximum distance of 38 m (125 ft) were used to monitor signals produced by 11.4-, 5.7-, and 3.8-L/h (3.0-, 1.5-, and 1.0-gal/h) leaks in the wall of a 5-cm-diameter pressurized petroleum pipeline. The range of line pressures and hole diameters used in the experiments were 70 to 140 kPa (10 to 20 psi), and 0.4 to 0.7 mm (0.015 to 0.030 in.), respectively. Application of a leak location algorithm based upon the technique of coherence function analysis resulted in mean differences of approximately 10 cm between predicted and actual leak locations. Standard deviations of the location estimates were approximately 30 cm.
Spectra computed from leak-on and leak-off time series indicate that the majority of acoustic energy received in the far field of the leak is concentrated in a frequency band from 1 to 4 kHz. The strength of the signal within this band was found to be proportional to the leak flow rate and line pressure. Energy propagation from leak to sensor was observed via three types of wave motion: longitudinal waves in the product, and longitudinal and transverse waves in the steel. The similarity between the measured wave speed and the nominal speed of sound in gasoline suggests that longitudinal waves in the product dominate the spectrum of received acoustic energy. The effects of multiple-mode wave propagation and the reflection of acoustic signals within the pipeline were observed as non-random fluctuations in the measured phase difference between sensor pairs.