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Feature Article

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

Feature Author
Garry Kozak
GK Consulting
Derry, New Hampshire

Preparing to launch the Bluefin AUV.
Deepwater searches have traditionally involved deep-towed side scan sonar systems requiring very long tow cables and large winches. This requires large survey support vessels to handle the equipment, resulting in the vessel charter becoming the major 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 images, 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 completes search missions with none of these drawbacks. A deepwater project for shipwreck search and classification at 1,000 to 1,500 meters depth with a large search area presented economic 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. The system was selected because its high frequency did not degrade with depth. The search phase used the 100-kilohertz frequency on a 500-meter range (1,000-meter swath) to acquire targets, then the 400-kilohertz frequency classified the target using a 75-meter range (150-meter swath). The second payload was a high-resolution digital camera with LED strobe built by the Woods Hole Oceanographic Institution (WHOI), which was used to photograph mosaic targets that had been classified by the high-frequency side scan to be of interest.

AUV Positioning Accuracy
Vehicle positioning accuracy was a major mission concern since the high-frequency sonar runs required short ranges to classify targets of interest. The positioning control to run a photomosaic mission required a line spacing of 3 meters.

The AUV's positioning capability was tested using a target designed to test both the sonar target position accuracy and the photomosaic line control. The target was placed on the seafloor at 1,500 meters depth. An acoustic beacon was attached to the target so that a precise position could be calculated as a benchmark to evaluate the target position error generated from the sonar and photomosaic.

The test results exceeded expectations by confirming that the AUV positioning of the target from the sonar had an error of less than 6 meters, with the average about 5 meters, and that the AUV could maintain a 3-meter line spacing to photograph the target to produce a quality photomosaic. To continue this article please click here.

Garry Kozak of GK Consulting specializes in shipwreck location and has been involved in undersea search for the past 40 years. He is a recognized expert in undersea search operations, providing global consulting and training expertise to navies and companies with a critical underwater search need. He now works with EdgeTech.

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