RRS Sir David Attenborough
A new polar research ship for Britain
Mapping the seabed
Echo sounding has been used in shipboard navigation for decades to determine water depth. The ship emits out a single underwater acoustic pulse, which travels through the water and is reflected by the seabed. By measuring how long it takes for the pulse to return to the ship, mariners can calculate how deep the water under the ship is.
In order to find out more about the shape and topography of the seafloor – known as bathymetry – an array of such acoustic pulses is needed. A multi-beam echo sounder sends out hundreds of acoustic pulses at different angles, creating a swath of information on water depth as the ship passes. From these swathes of data, a map of the seafloor can be extrapolated. Often, the ship will sail back and forth in parallel lines to map a larger area. For the highly accurate calculations required for scientific purposes, the exact speed of sound in the waters under the ship can be measured using a sound velocity probe.
Scientists can use the resulting seabed maps for a variety of purposes, from mapping the habitats of different animal groups to calculating the flow of ocean currents and studying how glaciers moved over the ocean floor in the past.
The RRS Sir David Attenborough will be equipped with the following echo-sounding equipment for biological and geophysical studies:
- EM 122 Deep water multibeam system
- EM 712 Medium depth multibeam system
- TOPAS PS 18 Sub bottom profiler
- EA 640 Scientific echo sounder
- EK 80 Biological multi-frequency echo sounder
- ME 70 Biological multibeam echo sounder
- MS 70 Biological multibeam echo sounder
- SH 94 Omni direction sonar
- SX 94 Forward looking sonar (Omni directional)
- K-Sync Synchronisation unit
The multibeam mapping systems will be ice-protected to ensure their reliability and accuracy in polar regions. The systems will be integrated to provide the maximum benefits of integrated datasets to researchers.
The bathymetric maps produced by the multi-beam echo sounding have a variety of scientific applications.
Marine geologists use the maps to find the best sites for taking sediment cores, and to study how glaciers once moved over the coastal areas off Antarctica and the Arctic. Several BAS scientists have used swath bathymetry – both on ships and on AUVs – to explore the depths of the Southern Ocean.
Oceanographers who study the flow of ocean currents use bathymetry maps to understand the exact shape of the ocean basins that they are modelling. This allows them to calculate the flow of currents more accurately, particularly in shallower waters. In the Amundsen Sea Embayment, BAS researchers studying the shape of the seafloor found evidence of the long-term melt history of the Pine Island Glacier. This data helped them interpret the current rapid retreat and melting of the glacier, which is currently one of the biggest contributors to Antarctic ice mass loss.
Marine biologists can use bathymetry data to map wildlife habitats. Scientists at BAS were involved with a study that discovered the first ever deep-sea hydrothermal vents found in the Southern Ocean. These vents, where temperatures can reach more than 300°C, are an environment devoid of light, but rich in chemicals that help sustain an ecosystem unlike any others on Earth. This information is an important tool used to define fishing areas in places like South Georgia in order to avoid overfishing.
Finally, much of the Southern Ocean is very poorly surveyed and ships frequently travel through previously uncharted waters. As a result, there is often an element of discovery in swath bathymetry measurements, which can even help in producing new nautical charts.
Multibeam echo sounders on board a ship can usually map an area about four times as wide as the water depth. For example, if the ship is in 1,000m deep water then it could map a 4km wide swath of the seafloor. However, this data cannot always capture the finer topographic details of the seabed.
As robotic and submarine technologies become more refined, researchers are increasingly using remotely operated vehicles (ROVs) and automatic underwater vehicles (AUVs) to carry multibeam echo sounders far below the surface. From here, they can map a narrower swath of the seafloor but at a much higher resolution. They can also access areas that ships cannot reach, such as the underside of floating ice shelves and the front of glaciers, where large icebergs may break off at any time. Using these technologies, scientists can build a more comprehensive, accurate picture of the seafloor.