Ocean color satellite data reveals sediment plumes from the SS Sansip and SS Samvurn wrecks near the coast of the North Sea. Image: Jesse Allen/NASA Earth Observatory
By Colleen Otte
When spotting submerged shipwrecks proved difficult, researchers oddly looked to the sky.
Shipwrecks can threaten the Great Lakes environment if remaining onboard fuel leaks or they harbor invasive species that like to stick to them.
A recent study published by the Journal of Archaeological Science suggests that satellite imagery can be used to locate these potentially hazardous wrecks that may otherwise go unnoticed.
Shipwreck searchers rely on sonar in deep waters and airborne laser systems in clear waters. But neither method is as effective for near shore areas with cloudy, shallow waters, according to the study, titled “Detection of shipwrecks in ocean colour satellite imagery.”
The study unveiled a new option: shipwrecks generate telltale plumes of sediment that can extend for nearly 2.5 miles. That is long enough to be identified in satellite imagery.
Maritime archaeologist Phil Hartmeyer dives above the wreck of the Albany in Thunder Bay National Marine Sanctuary.
“The ability to detect submerged shipwrecks from satellite remote sensors is of benefit to archaeological scientists and resource managers interested in locating wrecks and investigating processes driving their evolution,” the study said.
Although the plume identification method has not yet been applied in the Great Lakes and other smaller water bodies, satellites have been used to spot the wrecks themselves, according to experts at the Thunder Bay National Marine Sanctuary in Alpena, Michigan.
“I’ve looked on Google Earth to find shallow shipwrecks, or more likely to confirm the location of some shipwrecks,” said Stephanie Gandulla, media and outreach coordinator for the sanctuary. “But at this stage it isn’t really a tried and true method to go out and discover them. It’s another tool in our toolbox to investigate and look into shipwrecks.”
A diver investigates the wheel from the schooner FT Barney. Image: Joe Hoyt/NOAA, Thunder Bay National Marine Sanctuary
The method could prove useful for the sanctuary’s goal of protecting shipwrecks, Gandulla said. The shipwreck preserve is a cultural resource not only for the community to appreciate. It draws tourism for the community’s economic development.
Additionally, “fish and other lake wildlife like to use them,” she said. “They like the structure of the shipwrecks, and it almost provides an artificial reef for fish and other critters to call home.”
An environmental downside is that invasive species will also call them home, said Sean Ley, development officer for the Great Lakes Shipwreck Historical Society.
In all the Great Lakes except Superior, zebra and quagga mussels affix themselves to shipwrecks. That makes them look like reefs, although reefs’ marine life only occurs in salt water, Ley said. Lake Superior is so cold that those invasive species can’t inhabit it.
“That means the wrecks in Lake Superior are pretty much pristine,” he said.
They are also largely undisturbed by people, as there are strict laws preventing divers from tampering with them, Ley said. That makes the shipwrecks and the artifacts that sink with them valuable to the study of marine histories.
The bow of the two-masted schooner EB Allen. It sunk in 1871 and now lies within the Thunder Bay National Marine Sanctuary. Image: Tane Casserley/NOAA, Thunder Bay National Marine Sanctuary
“If anyone is wondering just how things were done in the maritime world 100 to 200 years ago, the answer lies on the bottom,” he said. “And now we have the technology that can go down and explore these wrecks.”
The Great Lakes Shipwreck Historical Society estimates there are about 6,000 known wrecks in the Great Lakes, and that some 30,000 lives were lost in those wrecks, Ley said. Most of them are in Lake Michigan and Lake Huron, because the vessels were used primarily near population centers when shipping became popular before roads were built.
“In Lake Superior there are about 600,” Ley said. “Our focus is eastern Lake Superior near Whitefish Point, where our museum is, because that’s the greatest concentration of wrecks in Lake Superior. About one third of all the wrecks are right around Whitefish Point.”
Locating these vessels is important as they are navigational hazards, said Laura Rocchio, a member of NASA’s Landsat communication and public outreach team. The Landsat program consistently archives images of Earth so that scientists may assess changes in its landscape.
Also, pinpointing the wrecks allows for the monitoring and rectifying of pollution from corroded heavy metals and leaking fuel.
“There’s a lot of expense involved in that,” Rocchio said. “But at least they know where it is, and then it’s a decision if any cleanup could be done.”
“Any shipwreck can present a hazard if it has anything aboard that doesn’t dissolve or is oily,” Ley said. “The greatest risk we’re always afraid of is diesel fuel leaking out of a shipwreck.”
The sediment plume identification method could be applied to anywhere the water is shallow enough for sediments to reach the surface during tidal swings, Rocchio said. The sediments need to be suspended near the surface to be revealed in the satellite imagery.
“There also needs to be enough sediment in the water,” she said. The plume study was done in the North Sea where rivers bring in lots of sediments.
In the study, the researchers knew where the shipwrecks were to begin with, and their idea was to determine if any sediment plumes extended from those points, Rocchio said. For the ships that had significant portions above the seafloor, they could trace plumes to the submerged vessels.
“The next big step for them is to go anywhere where they don’t know where a ship is, and look at these sediment plumes and see what they find underneath,” Rocchio said.
Initially, the researchers were studying what impact the offshore wind turbines in the English Channel would have on the surrounding area, and they discovered sediment plumes originating from the structures, Rocchio said.
They started realizing that even just small things were making plumes they could detect, she said. “That’s where they got the idea for the study, and they started with where they knew the ships were. Of the four ships they were studying, there were only two they could find, and those are the ones that had pieces of their structure still above the seafloor. The ones that were more buried wouldn’t create the scour pits to makes the signals that were obvious enough.”
The study used Landsat 8 satellite data, which is freely available to the public via the U.S. Geological Survey.