A new way of looking at sea ice thickness

The Autonomous Underwater Vehicle (AUV) being lowered off the stern of the ship. The data from the AUV is used to make 3-D maps of the underside of the sea ice.
The Autonomous Underwater Vehicle (AUV) being lowered off the stern of the Aurora Australis during SIPEX-II in 2012. The data from the AUV is used to make 3-D maps of the underside of the sea ice. (Photo Wendy Pyper)
A preliminary 3-D map produced from multibeam sonar data collected by the AUV under an ice floe This under ice view shows the thick, jumbled structure of ice under an ice floe, which makes it very hard to dig holes through to the ocean surface. (Photo: ROV team)A view of the Autonomous Underwater Vehicle under the sea ice.

24th November 2014

Antarctic scientists have used an underwater robot to produce the first detailed three-dimensional maps of Antarctic sea ice, showing it may be thicker than previously thought.

The research, published in Nature Geoscience today, shows that ice floes are much thicker and more variable than previous ship-based ice drilling measurements have shown.

The study was conducted during a United Kingdom-led voyage to the Weddell and Bellingshausen seas in 2010 and the Australian Antarctic Division-led Sea Ice Physics and Ecosystems eXperiment-II (SIPEX-II) in 2012.

Co-lead author of the paper, Dr Guy Williams, from the Antarctic Climate and Ecosystems Cooperative Research Centre, said the ‘autonomous underwater vehicle’ (AUV) technology had proved it could accurately measure and map ice thickness in difficult to access areas.

“Over the two voyages, our teams covered some 500 000 square metres and mapped 10 ice floes, providing the most comprehensive and only high resolution three-dimensional view of sea ice structure to date,” Dr Williams said.

Much like its surface, the underside of sea ice is often covered with peaks, ridges, depressions and rubble fields, formed under high pressure by moving pack ice.

“Our maps revealed heavy ice deformation in all three study regions, producing a mean sea ice thickness well in excess of that typically observed from drilling data,” Dr Williams said.

“The mean thickness ranged from 1.4 to 5.5 metres, with the thickest ice measuring 16 metres.

“This technology will fill the gaps in ice thickness data collected by other methods and improve our understanding of the role of ice deformation in controlling total sea ice volume, and our ability to evaluate sea ice and climate models.”

The AUV used in the study was developed and deployed by the Woods Hole Oceanographic Institution in the United States.

The vehicle was equipped with an upward-looking multibeam sonar that recorded the structure of the under-ice surface by sending out a swath of ‘pings’ and measuring the amount of time it took for the sound to bounce back.

“We operated at a depth of 20 to 30 metres beneath the ice and drove the AUV in a lawnmower pattern across a 400 by 400 metre grid, with overlapping swaths,” Dr Williams said.

“This allowed us to map the ice thickness beneath the ocean surface at a resolution of better than 0.25 metres, which enabled us to discriminate individual ridge keels and rafted ice blocks.”

Scientist have typically measured sea ice thickness using ’point-based’ measurements, by drilling hundreds of holes in a floe and measuring the ice depth. However, drill profiles can only be made in areas accessible by ship, are labour intensive, and tend to avoid the thickest ice. Satellite observations can measure large-scale thickness, but snow cover on the ice can make data interpretation difficult.

Dr Williams said broader scale surveys were now needed to determine the thickness and formation of sea ice throughout the Antarctic pack ice area, particularly in regions and in seasons that have been traditionally inaccessible.

Watch the video below about measuring sea ice thickness during SIPEX-II in 2012.

Related story: First 3D map of under the East Antarctic Sea ice


Mapping East Antarctic sea ice

Video transcript

Dr Guy Williams – Antarctic Climate and Ecosystems Cooperative Research Centre

Sea ice thickness represents one of these sort of holy grail at the moment. It’s something that we have difficulty in measuring with great accuracy and with any sort of great success on large scales. So thickness is important because we want to know how much there is. We’ve got a good idea of the area from the satellites, but the satellites can’t tell us the thickness and without the thickness we won’t know the total volume or the total amount of sea ice.

Dr Clay Kunz – Woods Hole Oceanographic Institution

So this is an Autonomous Underwater Vehicle, or AUV, and what it does is it’s a free swimming underwater robot. So it carries on board all of its power and intelligence and navigation equipment so that it is basically free swimming through the water and doing its own thing, as opposed to be being remotely controlled over a tether.

On this particular trip, since we are looking at the underside of the ice, we want to be pretty close to it. So we are driving around, so far we’ve been generally 20 metres underneath the water actually which is less distance under the ice because of course the ice sticks down into the water quite away.

The AUV has a lot of waypoints that it’s trying to get to as it is driving around underwater and the last waypoint that its set to get to is basically back where it started again, which is in open water off the stern of the ship.

Dr Guy Williams – Antarctic Climate and Ecosystems Cooperative Research Centre

It represents a leap forward in observational capability in terms of how we can measure thickness. The multi-beam sonar that we have on this AUV will provide us with a 3-D view of the underside of the sea ice. That will, together with the surface measurements that we are getting from other platforms, like the helicopter, we’ll have a full 3-D map of the entire sea ice flow.

Dr Jan Lieser – Antarctic Climate and Ecosystems Cooperative Research Centre

We are here in Antarctica to measure the thickness of the snow cover and the sea ice which is separating the atmosphere from the ocean. When we know how the thickness of the sea ice cover is changing over time we can estimate the influence of global changing climate on the overall environment down here, which includes not only the physical environment, in terms of sea ice, atmosphere and ocean, but also the biosphere.

We have this helicopter equipped with a whole heap of instruments which we call our flying toolbox. The flying toolbox consists of an aerial photography which is in this bucket down here, we have a radar, a snow thickness radar, which is mounted beneath the skids back there. We have a laser scanner and pyrometer on the front over here. And the whole thing will be combined together with an INS and GPS so that we know where we are and how we are orientated in a 3-D space. It is all driven with an electronics control unit which is in the centre here. This time around we also have a microwave radiometer from our Japanese colleagues which is installed in the boot there. So we fly about 60 nautical miles in one direction, then turn 120 degrees, fly 60 nautical miles in the next direction and then fly back to the ship.

What I like most about working in Antarctica is that so many people from so many different skills come together, work seamlessly, know what they are doing and we are all working towards one goal of gathering as much data as possible on sea ice environment down here.

[end transcript]