Acoustic instruments

RRS Sir David Attenborough is equipped with the following echo-sounding equipment for biological and geophysical studies in both the deep ocean and in shallower waters:

*These units do not have ice protection and therefore can’t be deployed while operating in ice.

Where possible, these systems are integrated to provide the maximum benefits of integrated datasets to researchers.

 

Mapping the seabed

Echo sounding has been used to determine water depth in ship navigation for decades. The ship emits 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. Scientists can use these same acoustic pulses to find out more about the shape and topography of the seafloor –  known as bathymetry.

Multibeam echo sounders sends out hundreds of acoustic pulses at different angles, creating a swath of information on water depth which can be used to create a map of the seafloor. 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 1km deep water then it could map a 4km wide swath of the seafloor. Often, the ship will sail back and forth in parallel lines to map a larger area.

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.

The AUV 'seabed' being deployed from RRS James Clark Ross
The AUV ‘Seabed’ being deployed from RRS James Clark Ross

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.

Using bathymetry maps

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.

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 means they can 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 – currently one of the biggest contributors to Antarctic ice mass loss.

AUTOSUB 3 autonomous underwater vehicle being retrieved after observational missions beneath the floating tongue of Pine Island Glacier.

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 devoid of light but rich in chemicals that help sustain an ecosystem unlike any other on Earth.  This information is an important tool used to define fishing areas in places like South Georgia in order to avoid overfishing.

Marine biologists can also use acoustic techniques can also be used to locate schools of fish, whales and other species living at different depths underwater. Acoustic signals sent into the water by the transducer are reflected by individuals or groups of animals, allowing for biomass estimation. The multi-frequency echosounders aboard RRS Sir David Attenborough will allow for the identification of different species.

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 bathymetry example 1 - Bathymetry around the South Sandwich Islands
An example of a seafloor map created using swath bathymetry off the South Sandwich Islands.

 

Sophie Fielding

Zooplankton Ecologist

Ecosystems team