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Synthetic Aperture Sonar Survey

In August 2010, the new Office of National Marine Sanctuaries (ONMS) East Coast research vessel SRVx (Sanctuary Research Vessel experimental) traveled to the Stellwagen Bank National Marine Sanctuary from Virginia. The vessel served as a platform for a seven-day synthetic aperture sonar (SAS) survey of the sanctuary’s northwest corner. The survey, conducted with Applied Signal Technology, used the PROSAS Surveyor sonar system to map over 169 square kilometers of seafloor on the approaches to Gloucester, Salem, and Marblehead. Lying just off some of America’s oldest ports, this area was thought to have a greater potential for Colonial-Era shipwrecks. In addition to locating archaeological resources, the project sought to assess derelict fishing gear concentrations and characterize seafloor habitats.

Deploying PROSAS
Technicians from Applied Signal Technology deploying the PROSAS Surveyor towfish (courtesy of NOAA/SBNMS and Applied Signal Technology)

SAS creates acoustic images similar to conventional side scan sonar, but the seafloor images are much higher resolution over larger areas. The advanced signal processing allowed scientists to image features as small as 3 centimeters in real time. Each survey line transit mapped a swath of seafloor 300 meters wide.

SRVx
The ONMS research vessel SRVx proved to be well-suited to extended remote sensing survey operations (courtesy of Evelyn Ganson, NOAA/SBNMS)

Over 10 sonar targets with archaeological resource characteristics were located during the SAS survey as well as over a hundred derelict lobster pots. The SAS technology was also used to re-image known shipwrecks such as the steamship Portland and coal schooners Frank A. Palmer and Louise B. Crary to gather a more detailed overall view of the sites. NOAA’s Office of Ocean Exploration and Research and the ONMS Maritime Heritage Program provided project funding.

Portland
Synthetic aperture sonar image of the steamship Portland (courtesy of NOAA/SBNMS and Applied Signal Technology)

Frank A. Palmer and Louise B. Crary
Synthetic aperture sonar image of the collided coal schooners Frank A. Palmer and Louise B. Crary (courtesy of NOAA/SBNMS and Applied Signal Technology)

Derelict lobster pots
Synthetic aperture sonar image of a group of derelict lobster pots (courtesy of NOAA/SBNMS and Applied Signal Technology)


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