LEAKAGE IDENTIFICATION IN A DUCT USING DUCT ACOUSTICS AND WAVELET TRANSFORM

This paper presents a new method for detecting the leak location in a buried gas duct. Gas leakage is often caused by the mechanical impact forces of construction equipment. In general, acoustical technology or non-acoustical technology has been used for such detection. In this work, acoustical technology is employed because the non-acoustical has some limitations depending on the technical theory of the individual detection equipment. The correlation method, which uses the arrival time delay of acoustic waves measured at two sensors attached on the buried duct, has been developed and used for a long time as the acoustical technology. Acoustic waves caused by the impact force are transferred to a far distance through the medium inside of the duct. This wave is very complex since it includes an acoustic wave and solid wave. In general, the solid wave is attenuated faster than the acoustic wave in a buried duct because it is damped out by the soil covering the duct. The acoustic wave is propagated to the far distance with direct and reflective waves on the wall. The directive wave is non-dispersive and the reflected waves are dispersive due to the cavity modes of the duct. The typical correlation method has used both dispersive and non-dispersive waves to estimate the arrival time delay. Thus this method induces a degree of error leak detection. In this paper, time-frequency method is developed to classify the wave speed of direct waves (non-dispersive waves) and the reflective waves (dispersive waves), respectively. It uses also the only direct wave to estimate the arrival time delay. It can determine the leak location with more accuracy than the correlation method.