Wrecks: HMS Erebus

In 1845, two ships led by Sir John Franklin, the expedition's captain, left Kent bound for the Arctic, tasked with being the first to navigate the Northwest Passage, a hoped-for trade route between Europe and Asia through the Arctic Ocean. Franklin's two ships were some of the first polar vessels to be equipped with steam engines—repurposed railway engines—leaving port with 12 days coal aboard, for example, as well as state-of-the-art Massey double action bilge pumps.The ships never returned to England.

It is thought the two vessels, HMS Erebus and HMS Terror, were abandoned when they became icebound, leaving the crew to begin a trek on foot across Canada in the hope of finding supplies, or human settlements along the way. The crewmen never made it to safety, and subsequent investigations of remains, found over 100 years later, found traces of starvation, lead poisoning, scurvy, pneumonia, and cannibalism among the party.
The history of the Erebus and Terror has been built up piecemeal since the ships were lost, using testimony from local Inuit, the objects left behind by the crew on their desperate journey, and even notes written by the acting captains following the death of Sir John Franklin, the expedition's captain.
The Inuit reported seeing one of the ships go down off the coast of King William island in around 1850, and they would be the last humans to lay eyes on the vessels for the century and a half that followed. (Anyone interested in a hair-raising fictional and spooky account of the trip might read The Terror by Dan Simmons.)

Recovery efforts have been narrowed to two areas, one in the Victoria Strait, another in Queen Maud Gulf, prompted by testimony from local Inuits who reported going aboard the vessel after its desertion by Franklin's men.

The Parks Canada returned every year, surveying the two areas for traces of the lost vessels. With ice making the areas inaccessible for much of the year, the archaeologists had only a handful of weeks at a time to hunt for the missing ships.

In 2011, the searchers drafted new technology to aid the search: aircraft equipped with lidar symmetry, which could scan the shore areas to a depth of around 20 meters. While the lidar systems weren't expected to be able to pick up signs of a wreck, they could help the team put together better maps of the region, which is still largely uncharted even today.

With better maps, the team could use side-scan sonar and multibeam echosounding, which can build up a picture of the seafloor, without risk of damage to the environment or to their equipment. After six monotonous years of scanning in September last year, an image loomed out of the sonar data.

A shipwreck. It proved to be the Erebus.

A robot, the Saab SeaEye falconer remotely operated vehicle (ROV), went first. The, later, scuba divers. While the ice closed over the site and eventually put an end to explorations, the team were able to return to the Erebus in April 2012, carrying a new piece of equipment that would allow them to access the site even in winter. Defence Research and Development Canada, the military's technology arm, lent the archaeologists a tool that uses a jet of hot water to cut through ice. Using DRDC's 'hot water knife', a two meter section of the ice was removed, allowing the divers to slip beneath the ice and onto the wreck site.

The Parks Canada team face the difficulty of navigating within the ship itself, mapping the location of the objects within it and any subtle associations with them. The team used stereophotogrammetry for that. Harris said, "It's an extremely important tool for us now. Essentially it uses a whole bunch of still photos, and software is able to determine the three dimensional relationship between subsequent exposures and produces a three dimensional model or a point cloud of what the camera saw, so in just a couple of hours you can aquire a whole bunch of data and produce three dimensional images of the entire wreck site."
The expedition is also experimenting with laser scanning, in partnership with Canadian firm 2G Robotics which makes underwater scanners normally used for detecting damage on oil pipelines. The company developed a longer range scanner for the Franklin expedition, which can map up to a five meter range with millimeter resolution, used to image the outside of the wreck. The expedition also used a smaller machine, with a range of between 50cm and 20cm, for investigating the interior, allowing the team to record the position of small objects, like plates, where they lay within the ship.

The company developed a longer range scanner for the Franklin expedition, which can map up to a five meter range with millimeter resolution, used to image the outside of the wreck. A smaller machine, with a range of between 50cm and 20cm, is used for investigating the interior.
 A 7.5-meter autonomous underwater vehicle, the Arctic Explorer, can stay underwater for 72 hours, and was packed with all sort of tech: inertial guidance systems and doppler velocity logs to plot the position and speed of the vehicle, as well as an interferometric synthetic aperture sonar (InSAS) system that can record a far wider swathe of radar (630 meters) than the towed side-scan sonar system the survey boat normally uses.
Said Harris, "It can resolve a target the size of your thumb anywhere in that sonar record, because it's using almost like synthetic aperture radar—it's using multiple radars and its synthesising that into one coherent very, very accurate image."