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Feature Article

Passive Acoustic Detection of an ROV In Ports and Shallow-Water Environments
Experiments Measure an ROV's Acoustic Signature To Improve Monitoring Systems for Detecting Underwater Threats

By Meng Cai
Ph.D. Student
University of California
Berkeley, California

As terrorism threats become more technologically advanced, port and harbor security continues to be of increasing interest to government and defense agencies. Acoustic signal analysis can be used in exploration and detection applications, but existing commercial and military systems for active sonar system detection are still limited by many factors. For example, these systems require costly transmitters and their constant transmission may be detected by enemy systems. Therefore, they are not a viable defense against low-cost, readily available underwater robotic platforms that might be used for delivering dangerous explosives to port facilities.

Past technologies and research projects have focused on underwater threat detection that employs passive acoustic methods to detect divers, small surface vessels and ships in ports and harbors. Research centers, such as the U.S. Naval Research Laboratory, the Maritime Operations Division and the NATO Undersea Research Centre, have focused on marine surveillance systems and have provided support to find and track marine mammals and the next generation of harbor threats.

A comparison of different acoustic tests when the ROV was running in (a) the laboratory tank, (b) the outdoor tank and (c) the field test in a shallow-water port.

The author has developed a passive acoustic method as a complement to active sonar systems for protecting ports and harbors from potential threats involving underwater robotic platforms. Through a series of experiments, the author used this method to test the passive acoustic detection of a small commercial off-the-shelf ROV in shallow waters, ports and harbors, and deep-ocean environments by characterizing the ROV's acoustic signature. These tests also assessed the feasibilities and limits of passive acoustic methods and estimated the spatial bounds for detecting ROVs in port environments.

Tests for Characterizing the ROV's Acoustic Signature
Three experiments were conducted in an effort to characterize the acoustic signature of an ROV with commercially available technologies. The experiments were conducted in a laboratory test tank, an outdoor tank and finally in a shallow-water coastal environment with a Sea'Botix (San Diego, California) LBV 150-4 ROV. The acoustic collections were carried out with an Aquarian Audio Products' (Anacortes, Washington) H1a hydrophone with a sensitivity of -190 decibels referenced to volts per micropascal and a range of 1 hertz to 100 kilohertz.

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Meng Cai is a Ph.D. student in mechanical engineering at the University of California, Berkeley. He received his master's in science from the University of Hawai'i at Manoa. His research on the passive acoustic detection of ROVs was done for his master's thesis in Hawaii. His research interests are fluid hydrodynamics, renewable energy, naval architecture and underwater acoustic detection and navigation.

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