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When an underwater pipeline is damaged because of corrosion, a leakage is taken place. The high-pressure fluid inside the pipeline may leak out from the corroded orifice. This kind of leakage may cause direct loss of resource from the pipeline. Sometimes, it may induce more serious industrial damage and pollution. Hence, it is worthwhile to study the feasibility of designing a sensing system for the leak detection of an underwater pipeline. The characteristic of this acoustic field is a wideband noise field. Because the sound intensity of the leaky signal is very weak, we need an interferometer. Because the location of the leakage is uncertainty, it is not easy to detect this position. Also, because it is an underwater pipeline, the leakage may take place far away from the station on land. Hence, it may not proper to detect the position by some traditional sensors. Hence, a distributed sensing scheme is better for this kind of detection. In this paper, an in-line hybrid Sagnac and Mach-Zehnder interferometer is proposed to detect the location of the leaky point. In our experiment, the sensitivity of the sensing system is 3.3/spl times/10/sup -4/ rad//spl radic/Hz. Its dynamical range is 75 dB.