- One of the last great challenges in mapping the Earth is mapping the seafloor.
- Acoustics from modern sonar equipment provide the most accurate data.
- The Seabed2030 project aims to map all the oceans by 2030.
Earth’s Last Frontier in Mapping
The seafloor is the last frontier in mapping the Earth. While many believe that the seafloor has been mapped, thanks to laser altimeter measurements that can be viewed on platforms such as Google Earth, the reality is these data are of low resolution, often not better than at about 1-2 degrees resolution.
Why is Mapping the Ocean Floor Important?
Understanding the depth and shape of the seafloor, called bathymetry, is not only a mapping challenge but it is important if we are to better understand are oceans. This includes understanding ocean circulation, which affects climate, tsunamis, environmental change, underwater geo-hazards, resources, and many other processes affecting the environment, safety, and commerce.
In a MapScaping podcast episode with Samuel Greenaway, who is from the National Oceanic and Atmospheric Administration (NOAA), it becomes evident that mapping the seafloor will be a major challenge this decade. However, it is an effort that is well worth it.
Sonars are Critical in Mapping the Seafloor
Mapping the seafloor has been occurring since the early 19th century; however, obtaining accurate data has been a challenge until the invention of the sonar. More recent sonars (short fo SOund NAvigation and Ranging) provide far more accurate data, particularly when multibeam echosounder sonars are used.
The Seabed2030 project is a project attempting to map the seafloor by 2030.[1] Until now, however, only about 20% of the seafloor has been mapped using modern bathymetry methods. In part, the project to map the seafloor will benefit from crowdsourced data obtained from various ocean-going vessels.
However, NOAA is also leading the effort and vessels with sonar equipment are being used to map regions not often travelled by vessels. These vessels are equipped with the latest multibeam sonars that provide hydrographic surveying results that can then build detailed maps with about 0.5 meter resolution.[2]
The mapping efforts are attempting to use different frequencies, from around 12 kHz to closer to 200 kHz, often used in shallower waters. While generally deeper sea levels are easier to map, as sound waves travel and allow a wider region to be surveyed as a ship passes by, shallow areas present challenges, given that multiple passes need to cover less area and interference observed from other sea life and vessels can disrupt data.
One area of potential concern is how the sound waves affect sea life. This is not well know. Lower frequencies are certainly audible, including to humans; however, some sea life also uses higher frequencies and the effects of disruption to sea life need further study.[3]
In addition to the acoustic data, other information is used to create detailed 3D maps.
Using GPS data and sonar data, which is calibrated relative to a datum, information on the latitude, longitude, elevation, role, pitch, and yaw are all obtained, comparable to what a moving object would use to navigate on land.
Speed is also an output from the measurements; global models can be used to estimate salinity and temperature variation, which affect mapping results. As the water surface is dynamic, elevation is dynamic and measured relatively. Mean sea level data are used to produce the estimated and average elevation.
Seafloor Mapping Data Interference
There are a number of things that can interfere with data. Sound is affected by salinity, for instance, and sound refraction needs to be calibrated from the acoustic data. Modern echosounder systems are sensitive and can pickup not only the seabed but also sea life and other data, which have to be filtered out of the data. Acoustic data can also be calibrated to determine substances of the seafloor such as sand, gravel, silt, and clay.
Ground truthing from the seafloor is also conducted, where sediment samples are taken, so that seafloor data can be calibrated for composition. Imagery is also now used using dropped deep cameras or even autonomous submarines to certify readings, including depth and seafloor composition.[4]
In the coming years, autonomous systems will be more important for gathering data, including surface and underwater systems. In fact, autonomous submarines are, in many ways, easier to operate, as deep, subsurface movement is an environment that is more stable and currents are weaker.
Advances in wind-power vehicles, such as sail drones, are also likely to be another form of vessels used to gather data from the sea surface. Crowdsourcing data will also continue to be important[5], although most non-scientific vessels travel limited sea routes, limiting their contribution. (Related: Using Stingrays to Map the Ocean Floor)
With seafloor mapping gaining importance for commerce, government, and scientists, the hope is by 2030 we can develop maps at or near 0.5 meter resolution for all the oceans. This might be a lofty goal but it could be achievable if sufficient resources and assistance from different private and government entities are made possible this decade.
References
[1] For more on the Seabed2030 project, see: https://www.gebco.net/about_us/seabed2030_project/.
[2] For more on NOAA’s seafloor mapping efforts, see: https://oceanexplorer.noaa.gov/explorations/lewis_clark01/background/seafloormapping/seafloormapping.html.
[3] For more on the acoustics and equipment used for seafloor mapping, see: Janowski, Ł., Tęgowski, J., Nowak, J., 2018. Seafloor mapping based on multibeam echosounder bathymetry and backscatter data using Object-Based Image Analysis: a case study from the Rewal site, the Southern Baltic. Oceanological and Hydrobiological Studies 47, 248–259. https://doi.org/10.1515/ohs-2018-0024.
[4] For more on data collected from seafloor mapping, see: Eleftherakis, D., Berger, L., Le Bouffant, N., Pacault, A., Augustin, J.-M., Lurton, X., 2018. Backscatter calibration of high-frequency multibeam echosounder using a reference single-beam system, on natural seafloor. Mar Geophys Res 39, 55–73. https://doi.org/10.1007/s11001-018-9348-5.
[5] For more on crowdsourcing bathymetry, see: https://iho.int/en/crowdsourced-bathymetry.
