Satellite Imagery Enhances Coastal Hydrography
For hydrographic and bathymetric professionals, it isn’t just the naturally occurring seafloor topography that matters — underwater archeological sites like shipwrecks are important to trace as well. From a safety perspective, they can pose navigational hazards; from an environmental standpoint, they can contribute to pollution; and from an archaeological aspect, they may hold great historical significance. In nearshore locations, where water depth generally decreases, submerged obstacles such as shipwrecks are especially dangerous to sailors because vessels come closer to the seafloor and are more likely to strike them.
Unfortunately, these sites are especially difficult to locate when they are close to shore. Traditional mapping methods include ship-based acoustic techniques and airborne LiDAR. The trouble with sound-based methods is that they call for very close survey line spacing in shallow water, making the process expensive and time consuming. The challenge with bathymetric LiDAR is that its effectiveness depends on clarity — something the turbid water near shore does not offer.
The good news is that a group of four researchers recently published a study in the Journal of Archaeological Science detailing a new approach that utilizes Landsat 8 satellite data to detect shipwrecks in coastal areas. “This is the first time this method was developed,” says Rory Quinn, a marine geoscientist who teaches spatial mapping, remote sensing and researches maritime archaeology at Ulster University in Northern Ireland.
“Quinn co-authored Detection of Shipwrecks in Ocean Colour Satellite Imagery” with Matthias Baeye and Michael Fettweis, from the Royal Belgian Institute of Natural Sciences; and Samuel Deleu from Flemish Hydrography, Agency for Maritime and Coastal Services.
Their study, focused on four known shipwreck sites near the Belgium port of Zeepbugge. Fettweis and Baeye have worked together in the past on geophysical and geological research projects in the area that have benefitted from satellite imagery. In the midst of looking at the satellite imagery for another study, Fettweis says they started to notice unusual plume-like features. They had used the imagery to look at sediment dynamics before and this time Baeye thought to delve deeper. “He had the nice idea to see if we could link this with shipwrecks. So then we started to look at it and try to explain it,” Fettweis says.
They reached out to Quinn because of his experience with shipwreck erosion and Deleu for his ability to provide them with detailed multi-beam survey images of the four known wreck sites, previously conducted by the Flemish government.
With the help of Landsat 8 images, they were able to map sediment plumes that extend from the shipwrecks. In the images, Fettweis says the plumes show up as a different color than the water surrounding them. He explains them as visible particles that are transported by the current and leave a signal along their flow path. “In these kinds of images we see more particles or a higher concentration of particles of mud. They look a little bit like a plume. So they start very tiny and they get wider further away from the spot.”
The team of researchers found that the exposed parts of the shipwrecks created scour pits that filled with sediment during periods of relatively still currents. During the rougher flood and ebb tides, sediment was re-suspended from the scour pits. Once the sediment reached water’s surface, it created the plumes that show up in satellite imagery.
The results suggest that marine mappers could use the method in reverse, starting with satellite imagery and ending with a newly discovered shipwreck. The approach would require following the plumes upstream to their point of origin.
- The Landsat series of satellites was first launched in 1972.
- The series has produced the longest, continuous record of Earth’s land surface as seen from space. Landsat is a joint effort of USGS and NASA.
- NASA develops the satellites, launches them and validates performance.
- USGS develops the associated ground systems; operates the satellites; and manages data reception, archiving and distribution.
- Since Landsat data was made available free of charge, more than 22 million Landsat scenes have been downloaded through the USGS-EROS website.
The team has only started from known shipwreck sites — not from satellite imagery — and while Fettweis says an apparent plume in an image isn’t guaranteed to lead mappers to shipwrecks, it is an approach worth employing. “It is not for sure reliable, but I would say when you see these kinds of features, go out and look first at these spots instead of scanning a large area of maybe 100-by-100 kilometers and maybe you will have a much faster result. … So I think it could be a way of better planning your hydrographical surveys.”
Quinn agrees that Landsat imagery can make a mapper’s job much easier. He says it simplifies the process on many levels in that it is freely available, the resolution increases with each new sensor, it offers global coverage and it re-occupies the same position regularly.
There are also a lot of efforts to investigate how the depths of shipwrecks vary with time, Fettweis says. In this case, he can see satellite images being used to see what the status of a shipwreck is at any given moment. “When it generates a plume the chance is quite high that it is not submerged in the seafloor. If the intensity of the plume changes, then maybe the shipwreck is also changing. And instead of going out to make multi-beam images, which takes a lot of time — especially ship time and process time — this could also be another way of monitoring the shipwrecks.”
Aside from shipwrecks, he says they have also discovered plumes linked to offshore windfarms and navigational buoys, and that they could be generated from something as natural and unexciting as a large rock on the ocean floor. Whatever the sediment plumes stem from, Fettweis says they are worth looking into because they could be linked to uncharted obstacles or treasures.
What makes this use of satellite imagery particularly handy — its value to marine mappers covering nearshore environments — is also a key weakness. Because satellites only see the surface of the ocean, this approach would not necessarily produce results in locations where the water is more than around 20 meters deep, Fettweis says. It takes strong current and shallow water to suspend sediment plumes to the surface, so in the majority of the ocean, where these characteristics don’t exist, plumes would not show up in satellite imagery like they do closer to the coast.
In addition, Fettweis says the process is not typically as simple as involving the examination of just one satellite image. This particular study required 21 Landsat 8 images and tidal images. He says the need for different pictures stems from the fact that coastal areas come with strong tidal currents and the plumes are not visible during all phases of the tidal cycle. “There are times when you don’t see a plume because the currents are not strong enough, so you have to look at different images at different parts of the tidal cycle in order to really see it.”
Another challenge can be attributed to the location of satellites, so far removed from Earth’s surface. Fettweis says it is not necessarily easy to capture cloud-free imagery and doing so is important because with clouds blocking the ocean’s surface below, it is impossible to gather the information this process calls for.
As for locating more immediate, less historical objects beneath the surface, Fettweis says this method likely would not be useful in the case of a missing plane or cruise ship that has very recently gone off the radar. “I don’t think for emergencies; no. It needs to be in a shallow area, say close to the near shore where you already have a lot of sediments in suspension like in most coastal areas. And when you have an object on the seafloor it takes some time, I think, because it needs to develop some erosion around the object before you can see the plume. So it is not something that will develop that quickly.”
Strength of Satellites
Limitations aside, Quinn says satellite imagery is an extremely significant tool for the mapping community. “Satellite data is revolutionizing the way we map the world and the way we understand the natural and anthropogenic processes acting on Earth.”
While Landsat 8 offers a level of resolution Fettweis is very pleased with, he points out that there are a number of other satellites out there to pull geospatial data from and that image quality will only get with time.
Timothy Newman, U.S. Geological Survey (USGS) program coordinator for land remote sensing, echoes that future optimism. He provides oversight and guidance to the program that operates the Landsat satellites and says the administration has made a 25-year commitment to the program. “Work has begun on the next mission, Landsat 9, with launch scheduled for late 2020. Plans for the next generation of Landsat are also underway, with a series of studies leading to a decision on the Landsat 10 and beyond architecture in 2018.”
He says this is the first his department has heard of such an application for Landsat imagery and that he is excited about it. “We love hearing about new ways Landsat data is used to monitor resources and solve problems around the world. This is testament to the superior quality of the Landsat 8 primary imaging instrument.”
Back in 2008, Newman says USGS made the switch from selling Landsat scenes to providing them free of charge over the Internet. They hoped to spur more use of the imagery and its applications. “The move resulted in an explosion of use across many societal benefit areas. Nearly 1,000 times more Landsat data is distributed today than in 2008,” he says. Those interested in obtaining Landsat imagery can access it via the USGS Earth Explorer website.