Sea Technology

SEP 2012

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Use of AUV for Deepwater Shipwreck Search AUV with Side Scan Sonar and High-Resolution Camera Maps Shipwrecks at 1,000 to 1,500 Meters Depth By Garry Kozak Owner GK Consulting Derry, New Hampshire D eepwater searches have traditionally involved deep-towed side scan sonar systems requiring very long tow cables and large winches. This requires large survey sup- port vessels to handle the equipment, resulting in the vessel charter becoming the ma- jor cost in a deep-search project. Towing in deepwater was challenging and frustrating because of the lack of positioning control of the towfish when on a tow cable lagging 3 to 4 miles behind a ship. When a target of interest was located, the time-consuming process of making high- frequency, short-range classification passes with the side scan sonar was more luck than skill. Deep-towed sonars experienced other drawbacks, like susceptibility to heave that causes towfish motion and distorts the sonar im- ages, tow cable failure and poor terrain following. Turnaround times to the next survey line could take hours, lowering search productivity and raising project costs. Today, AUVs are a mature technology that routinely com- pletes search missions with none of these drawbacks. A deep- water project for shipwreck search and classification at 1,000 to 1,500 meters depth with a large search area presented eco- nomic and mission strategy challenges. Looking at the trade- offs between a deep-tow side scan sonar and an AUV solution, it became clear that an AUV offered many advantages over a towed system. Though the initial cost outlay for an AUV is high, the savings made by not requiring a large support vessel and crew made an AUV an economical solution, with better data quality and payload flexibility. Survey Payloads A Bluefin Robotics Corp. (Quincy, Massachusetts) 12-inch 1,500-meter-depth-rated AUV, the Bluefin- 12D, equipped with two swappable payload packages was selected for the project. The first payload was an EdgeTech (West Wareham, Massachusetts) 2200-M side scan sonar operating at 100 and 400 kilohertz. (Top) Preparing to launch the Bluefin AUV. (Middle) The test target, placed at 1,400 meters depth for AUV position accuracy trials. (Bottom) Absolute target position error averaged less than 6 meters. 10 st / SEPTEMBER 2012

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