A novel fiber optical speckle sensor based on multimode interference for underground pipeline intrusion detecting is investigated both theoretically and experimentally. The sensor utilizes a single mode to multimode to single mode (SMF-MMF-SMF) structure as a sensing unit, which cascade to create a quasi-distributed optical sensor. To pave it near the underground pipeline we can detect any valid intrusion caused by motion, sound or vibration acts on the fiber. Experimental test shows that the detecting distance can be about 30Km and the locating accuracy of intrusion event is ±500m. Compared with Mach-Zehnder interferometer based configuration it has a much higher sensitivity.

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