Satellites reveal undersea landscapes
Conventional methods of mapping the sea floor are slow and expensive. EU-funded researchers used satellite-derived techniques combined with crowd-sourced depth soundings to map large areas quickly and at lower cost.
© EOMAP GmbH & Co KG, 2018
Bathymetry is the practice of mapping the sea floor. Yet even with modern technology, high-quality bathymetric data remains scarce and expensive. As a consequence about half of the worlds coastal waters are not well charted.
By far the most common method uses sonar to build up images of underwater terrain. In shallow water, aircraft-based lidar can measure distances via the reflected pulses from laser light. But these are expensive and are not a solution for people who require data very quickly or for extended areas or for simple purposes such as planning or modelling, says Knut Hartmann, of EOMAP in Germany, a partner in the EU-funded BASE-PLATFORM.
If you wanted to do a proper survey of those areas it would take several decades using, ship-based methods, he explains.
Instead, the EU-funded BASE-PLATFORM used data from Earth observation satellites, notably the EUs Copernicus programme, to make cost-effective bathymetric maps of the marine environment from space.
Measurements from space
The project partners used three techniques to collect information from satellite data, developing models and algorithms to translate the data into depth maps.
In shallow waters, satellites can sense depth by measuring the intensity of sunlight reflected from the sea floor. Resolution is good and this may be the most important of the satellite methods. Shallow waters are the areas where most people have interests, Hartmann says.
Over the deep oceans, the sea level is affected by the gravitational pull of underwater features, such as mountains. By using radar to measure the height of the satellite above the ocean surface the underlying terrain can be inferred. Maps can be rough but are often the only available method for surveying the deepest parts of the oceans.
A more experimental method, applicable to intermediate depths, uses radar to measure the length and speed of waves which can be related to the depth of the water.
Alongside the satellite work, the project invited shipowners to install a simple device to record routine depth readings from the ships echo sounder. The readings are used to validate the data gathered from space but are also valuable in their own right.
With around 10 million ships on the worlds oceans this crowd-sourcing method has great potential. It is not the same quality as data from a modern survey vessel, says Hartmann, but could be used to improve our knowledge of the bathymetry, especially in areas which are very poorly charted.
Bathymetry on the map
The project finished in late 2017 and data is now available commercially through the BASE- PLATFORM portal. Partners are also developing their own products, and free maps are provided for the project demonstration areas of the Balearic Islands, Channel Islands, Wadden Sea and Mauritius.
Customised maps can also be created. One client wanted to run a tsunami forecast model, Hartmann says, and needed depth data from the shore out to the deep ocean. So we made a map with a specific design algorithm to depict depths from shore to deep, including all the different technologies.
Other applications range from planning of offshore wind farms and oil and gas installations to dredging, cable laying and port operations. The Shell oil multinational is reported to have saved USD1 million by using satellite data for surveying waters near Qatar.
The demand for bathymetry data, especially in shallow water, is becoming increasingly significant, Hartmann says. For industry, it all comes back to saving costs and reducing risks.