The first-ever high-definition sonar images of the Civil War ironclad USS Monitor wreck have been released by the National Oceanic & Atmospheric Administration (NOAA), which manages the marine sanctuary surrounding the revolutionary shipwreck in North Carolina.
Last September space, aeronautics and defence company Northrop Grumman’s Technology for Conservation team and researchers from NOAA and water company Stantec deployed an AUV equipped with the advanced µSAS sonar system to capture new imagery of the historic site.
The initiative was intended to help the public explore, understand and protect the Monitor National Marine Sanctuary, stated NOAA.
“Monitor was a technological marvel of its day, and it continues to serve the nation today through technology, innovation and education,” it said, drawing attention to the advances in seabed scanning since the earliest Monitor images were produced more than half a century ago. The new images have been used as the basis for creating a series of animations and 3D models.
Rotating gun-turret
Designed by Swedish-American engineer John Ericsson, USS Monitor was launched at the start of 1862, built almost entirely of iron and powered by steam rather than sail. Its most distinctive feature was a rotating armoured gun-turret carrying two heavy cannon – an innovation that would transform naval warfare.
Barely two months after entering service for the Union, Monitor fought the Confederate ironclad CSS Virginia at the Battle of Hampton Roads in March 1862.
The engagement ended in stalemate but marked the first clash of ironclad warships, demonstrating how armoured steam vessels could replace traditional timber fleets.
Later that year, on 31 December, while being towed south Monitor sank in a storm off Cape Hatteras, North Carolina, taking 16 sailors to their deaths.
The wreck remained lost for more than a century until it was located beyond 70m deep in 1973. Two years later NOAA designated its resting place as the USA’s first national marine sanctuary.
µSAS (micro-Synthetic Aperture Sonar) is designed for small platforms such as AUVs and claimed to offer far higher imaging resolution (2-3cm) than conventional side-scan sonar by combining echoes from many consecutive pings.
Synthetic aperture processing is said to maintain almost constant resolution across the full scan width, so that targets distant from the AUV can be imaged almost as clearly as nearby ones.
Scanning wide swathes while still detecting small objects speeds up the process because fewer passes are needed over the same area. SAS processing is also said to improve detection of weak or partially buried objects, while a single pass can produce multiple viewing angles of the same target.
With Circular Synthetic Aperture Sonar (CSAS) the AUV circles the wreck and software combines the echoes from all angles to create very high-resolution images.
With the wreck viewed from every direction, acoustic shadows that normally hide details are reduced and the combined data can produce very detailed 2D maps or 3D models of wrecks such as the USS Monitor. The concept is not new but practical deployment on small AUVs with very high-frequency SAS sensors is.
Free permits are available for technical divers to visit USS Monitor, but requests are evaluated based on potential impact on sanctuary resources versus resource protection benefits.
Recovered Monitor artefacts can be seen at the sanctuary’s visitor centre and main repository and conservation facility, the Mariners’ Museum & Park in Newport News, Virginia. The revolving gun-turret, Dahlgren guns and steam engine are conserved by the museum at its Batten Conservation Complex.
Also on Divernet: Divers map historic North Carolina wrecks
