Locating a Lost Underwater Mooring with an EdgeTech 4125

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On 22 May 2011 a trawl resistant mount with an ADCP was deployed in 20m of water near Deer Island Boston, Massachusetts. Over the next few months, it was expected that the mooring was collecting data as required by the NOAA program. The data were to be used to update NOAA tide current predictions in the region. Unfortunately, during a scheduled retrieval trip on 4 August it became clear that the ocean had tossed another challenge to the awaiting crew. It was suspected that the mooring may have been moved, turned over, or damaged by fishing activity. The crew knew the mooring was close by, but they needed an exact position for the divers. The EdgeTech 4125 ultra high-resolution side scan sonar was called in to assist in the important task.

EdgeTech’s 4125 Side Scan Sonar System is a rugged, portable, easy-to-use system that provides ultra high-resolution underwater imagery. The 4125 utilizes EdgeTech’s Full Spectrum® CHIRP tech- nology, which provides higher resolution imagery at longer ranges than traditional systems. This translates into more accurate results and faster surveys, both critical components for missions such as mooring recovery. The 400/900kHz frequency set provides an excellent combination of long range search capability and high-res- olution images for detection of very small objects like the NOAA trawl resistant mooring in this case. The 4125 can be powered by both AC and DC; in this case, it was running off a standard 12V car battery on the vessel. EdgeTech’s easy-to-use Discover acquisition software is included with the system and has both Target Logger and Coverage Mapper modules. The standard 50m multi-conductor Kevlar tow cable was used to tow the systems behind the boat at approximately 7m off the seafloor.

Using the EdgeTech 4125, operators Rob Morris from EdgeTech and Stephen O’Malley from OceanTechUSA, Inc., were able to locate and precisely position the lost underwater object within 1 hour of searching the suspected area. The mooring had been moved approximately 70m from the deployment location. They were also able to determine that the anti-trawl mount was most likely upside down. As seen in the images here the mooring was visible in the high resolution imagery and was easily identified. The use of the EdgeTech 4125 side scan sonar successfully enabled the location and recover of an important underwater mooring.