From lead lines to sonar beams
Today, we can map the seabed in exquisite detail, but it hasn’t always been that way. “There’s a statue from an Egyptian tomb dating back to 2000 BC showing an ocean mapper using a weight at the end of a rope,” says Professor Larry Mayer, Director of the Center for Coastal and Ocean Mapping at the University of New Hampshire. Lead-line mapping, as the technique is known, is the earliest known form of seabed mapping. It involves marking a rope at fixed intervals to indicate distance and then dropping the weighted end into the water. When the end hits the bottom, the mapper looks at the marking at the surface and records the depth. Sounding poles, which involved a pole instead of a rope, worked similarly. “It’s a technique that stayed with us for a long time,” says Mayer. “If we jump 4000 years later to the 1800s, we see [mappers] are doing exactly the same thing. In 1940? The same thing.”
While lead-line mapping continued, the early 1900s saw the introduction of a related technique – wire-drag surveying. The method involved attaching a long wire to two vessels held in place at a predetermined depth with weights (to keep the wire at the desired depth) and buoys (to prevent the wire from sinking). The vessels would move in parallel, dragging the wire between them. The wire would catch around any rocky outcrops or other obstacles and, as the vessels continued forward, would become taught, form a V-shape. This V indicated the location of the obstacle, while the depth of the wire gave some indication as to the obstacle’s depth.
Wire-drag surveys allowed people to map more of the seabed faster than possible with lead-line mapping, but it wasn’t long before a new technology arrived that would truly revolutionise seabed mapping.
An acoustic revolution
Single-beam echosounders use the movement of sound through the water to measure distances. The echosounder transmits a pulse down towards the seabed. When the pulse hits something solid, it is reflected back towards the surface, where the echosounder can hear it. By knowing how long it takes for the pulse to be sent and received and the speed of sound in the water, we can calculate the distance. “There's no question that that leap from lead line to echosounder was a huge step forward,” says Mayer, noting that not only did mapping become much faster after their introduction in the 1920s, but it became more accurate too.
The technology quickly evolved. In the 1960s, side-scan sonar entered into commercial operation. While single-beam echosounders transmit a pulse downwards, side-scan sonar sends a pulse at a downward angle, creating a fan that sweeps the seabed on either side of the vessel. Side-scan sonar provides unprecedented images of the seabed, but depth has to be inferred rather than measured.
In the 1970s, satellite altimetry, which emits a radar pulse from a satellite to the surface, where it is reflected back up, arrived on the scene. On a broad scale, bumps and dips in sea surface height roughly follow bumps and dips on the seabed, so we can infer depth by measuring bumps and dips in sea surface height and comparing them to actual measurements of depth where we have them. “That's been phenomenal, but it's quite coarse. I did a presentation in Monaco where I apologised to the Prince [of Monaco], saying if we look at Monaco at the scale of our predicted depths from satellite altimetry, Monaco doesn't exist.”
Then, there was the commercialisation of multibeam echosounders. “To me, this was the huge revolution in our ability to start looking at the seabed,” says Mayer.
“Instead of one big fat sonar beam, multibeam gives hundreds of very thin laser-like beams of sound across a wide swath. That swath is something like three to five times the water depth, so if you are in 4000 meters of water, you're looking at 20 kilometres swath with hundreds of individual depth measurements.” These measurements sped up seabed mapping and provided greater accuracy and detailed imagery than previously possible. It also improved our ability to understand what type of material the pulses are reflecting off.
“If we can capture the amplitude of the sound reflected back to the echosounder – the backscatter – we can get a hint about what material it is. So, is it hard like rock or soft like mud,” Mayer explains. “At the moment, we can’t tell exactly what the material is just from the backscatter, but there is a lot of research focusing on systems that will automatically tell us if it’s sand, silt, clay, seagrass… achieving this will be the Holy Grail.”
These technological advancements have helped support seabed mapping for many different uses, including mapping the seabed habitats that support rich, vibrant communities of marine life.
Mapping more than the seabed
Benthic habitat mapping involves mapping the seabed and the plants and animals that live there. By understanding what conditions a particular species lives in – the depth, the type of sediment, the temperature, for example – we can design different management actions that help protect them and the services they provide for people, such as food.
“It’s really in the last twenty years that we’ve seen some of the most rapid change [in the technology used for benthic habitat mapping],” says Dr Benjamin Misiuk, Assistant Professor at Memorial University of Newfoundland and Labrador. Alongside Dr Craig Brown (Dalhousie University), Misiuk published a review of the last thirty years of benthic habitat mapping. “It used to be a lot of single-beam, but since the 2000s, multibeam started to pick up. In the last five to ten years, we’ve also seen a huge uptake in satellite and LiDAR methods for shallow-water mapping.”
Like echosounders, LiDAR – Light Detection and Ranging – measures distance by transmitting a laser light signal, which is then reflected off a surface. The time between transmitting and receiving the signal can be used to calculate distance. “Lidar is very high resolution, but they depend on water clarity because the waves of light can only penetrate the water so far,” says Misiuk, noting that dedicated bathymetric LiDAR systems can reach approximately 30 metres depth if conditions are optimal. “Satellite systems have the same limitation. If the water is not clear, you may only be able to get down a couple of metres. The flip side is that for coastal areas, using these systems can be more affordable than using a boat with a multibeam.”
With systems that can automatically detect what type of substrate an echosounder or indeed LiDAR or satellite-based signal is reflecting off still in the works, benthic habitat mappers need to gather detailed information about the seabed and the life that resides there with more manual methods. “You can have great seafloor data, but if you don't have great observations of the subject you are trying to map, it will never come together.”
The exact methods for garnering those observations vary. “In less than ten metres depth, it’s still extremely common for people to wade or snorkel out and record observations with a pencil and paper and take photos. As you move deeper, we see people sitting on boats with cameras or a grab sampler on a tether. Sometimes, we see small trawls or scientific fishing. “After two to three hundred metres, it becomes less common to send down a camera on a tether. Instead, we start to see AUVs [autonomous underwater vehicles] and ROVs [remotely operated underwater vehicles],” Misiuk explains, noting that “they are a more expensive and specialised technology, so they are not accessible to many researchers.”
The future of seabed mapping
Every potential use of seabed mapping – deciding where to put offshore renewables, understanding Earth processes, exploring the unknown, for example, comes with its list of “future wishes.” For benthic habitat mapping, Misiuk would like to see “camera systems that integrate all the components for high-resolution seabed observations – a camera that takes close-up images of the substrate so you can differentiate grain size, a 360-camera so you would get spatial context, a USBL, which is an acoustic positioning system that lets you pinpoint where your observation is located on the seafloor. Then you could scale your observations to the resolution of the other seabed data you’re using, whether that’s satellite data or multibeam.”
More generally, Mayer is looking towards robotics.
“We've done some calculations, and we think it's on the order of five billion US Dollars to map the waters deeper than 200 metres,” Mayer explains. Those costs arise from myriad sources, but a significant proportion comes from the need for crewed vessels and fuel. Moreover, “the ocean is very big, and we have very small ships.”
“So what if we increase that footprint and reduce the costs with uncrewed vessels?”
Mayer suggests several options – a crewed “mothership” with a group of smaller uncrewed vessels that go alongside. “Instead of mapping just one 20-kilometre swath, we can map, say, five 20-kilometre swaths, all at once.” Alternatively, a series of larger uncrewed vessels could potentially do the work without the need for a crewed mothership at all.
Another option could be to harness Argo floats, robotic drifters already collecting information about ocean conditions, such as temperature. “The key issue is power because an echosounder takes much more power than the sensors on an Argo float,” says Mayer. “A company called Seatrec has something they call the infinTE float. It derives power from a material that expands and contracts with the thermal differences the float goes through as it descends and ascends. So, we’ve got two infiniTE float prototypes [with single-beam echosounders] built, and we’ll be testing them in January [2025].”
Similarly, “some colleagues of mine at Lincoln Labs have been looking at ways to use something called sparse arrays,” says Mayer. “The idea is to put many little autonomous vehicles or floats over, say, a 40-kilometre footprint with several of them pinging and all of them listening for the signal. Theoretically, you can get 40 kilometres of swaths with on the order of one meter resolution in 4000 meters depth of water. The jump between theoretical and real is large, but it is worth exploring.”
As for the funding, Mayer would like to see an international ocean mapping program “where many countries contribute, perhaps the wealthier ones more, and then share the data globally. There are a lot of political issues, but the benefits to the global community are overwhelming. Just look at Google Earth and the unbelievable benefits that it has provided. We can do the same with the Ocean.”
This story was written for Seabed 2030
