By Becky Sanderson, Newcastle University
To know how much ice we have in the world, we need to know the bed of the glacier and the thickness of the ice. But how can we peer down through hundreds of metres of ice to know the ice thickness? Using radar, we see water flowing or ponding under the ice, measure the thickness of the ice, and even link layers of ice across large distances.
Introduction to Radio Echo Sounding
“Radioglaciology” is the study of glaciers, ice sheets and ice shelves using “ice penetrating radar”. Radar stands for “Radio Detection and Ranging“. Ice-penetrating radar is a powerful tool that allows scientists to look down kilometres through the solid ice and visualise the glacier bed: the rock, water and sediment that lies underneath the ice. The bed beneath the ice is crucially important to understand how, and why, the overlying ice behaves as it does.
Using radar to measure things is also referred to as radio-echo sounding (RES), because we transmit a radio wave and listen for the echo reflected from layers within and beneath the ice. It works really well in glacier ice, meaning that the glacier bed can be mapped at depths of up to 4 km.
Radio Echo Sounding is the most widely deployed instrument to measure an ice sheet’s thickness, investigate the properties of the glacier bed, and unravel the layers of the ice (its internal stratigraphy1,2,3). Bed topography, the subglacial networks of water and lakes (subglacial hydrological networks) and ice dynamics are vital physical process components of ice sheet models and can all be imaged and investigated through Radio Echo Sounding.
The information collated from multiple radar campaigns over many years and by different international teams allows us to get a 3-dimensional picture of the ice and bed beneath across large ice sheets including Antarctica and Greenland, as well as over large ice caps of the Canadian and Eurasian High Arctic. Radio Echo Sounding is also used in planetary science to explore the subsurface of the polar ice caps on Mars and comets4.
Why is ice-penetrating radar so important and what can we see?
Ice sheets are important considerations in projections of sea level rise and often contain valuable records of climate history. To make projections of sea level rise we use ice sheet computer models; however, the most accurate way to make these projections is to use ‘real world’ physical inputs such as ice thickness, bed topography, historic climate data and mass balance. Radio Echo Sounding is one of the best ways to measure these inputs.
Bed topography and ice thickness
Ice-penetrating radar is one of the primary methods used to map the bed topography and ice thickness. Most of the Earth’s land surface has been mapped in great detail and we have an extensive understanding of mountain heights, valley depths and coastlines. Antarctic’s bed topography is a notable exception to this.
Collecting highly detailed radar data organised in a grid format allows us to create maps of underlying terrain and explore the complex network of subglacial valleys and mountains that are not visible from the surface.
Radio Echo Sounding has revealed the Gamburtsev Mountains (a mountain range similar size to the Alps) under 2-3 km of ice in East Antarctica, and a system of canyons that may span 1100 km or more in Princess Elizabeth Land.
Similarly collecting more measurements near the grounding line (where ice on the land meets the ocean), is vital to improve our understanding of the ice sheet stability for today and the future in order to make accurate predictions of sea-level rise. RES provides direct measurements of the grounding line and ice thickness around the coast which provides information to understand how much ice is leaving the Antarctic coast.
This is the immediate goal of international groups such as the Scientific Committee on Antarctic Research (SCAR) RINGS Action Group.
The first evidence of subglacial lakes in Antarctica was detected in the late 1960s and came from unusually strong, sharp, continuous and smooth basal reflections imaged by RES. Since then, we have detected 773 subglacial lakes in total globally including 675 from Antarctica, 64 from Greenland, 2 beneath the Devon Ice Cap, 6 beneath Iceland’s ice caps and 26 from valley glaciers6.
Detection and characterisation of subglacial water has improved with innovations in RES and significantly advanced our understanding of subglacial drainage systems.
Recent research has focused on subglacial water flow away from subglacial lakes and significantly advances our understanding of ice stream dynamics, mass balance and the supply of water to the oceans, impacting ocean current circulation and nutrient productivity8.
While Radio Echo Sounding was first developed to investigate ice thickness and bed topography, we can gain an understanding of ice sheet past behaviour through the internal layers observed in radargrams. Analysis of internal layers allows us to observe changes in ice temperature, ice fabric and density, accumulation rates and ice flow9, specifically where the ice currently flows and if this has changed over time.
For example, analysis of internal layers across the ice sheet in Greenland has provided valuable insights into ice sheet dynamics and this history of the ice sheet11. A similar project is being undertaken in Antarctica to achieve a continent-wide dataset for traced and dated internal layers (SCAR AntArchitecture Action Group).
Correlation with ice cores
Increasingly, work is being done to connect internal layers across ice sheets to the ice properties measure directly from a drilled ice core. Investigating internal layers in this way is analogous to studying tree rings, but over much longer time scales. Currently, ice cores in East Antarctica provide a climate record for the last 800,000 years10.
Once you have made the correlation between continuous internal layers and the ice cores, you can trace the layers away from the core and across a large area of the ice sheet, unveiling the age-depth relationship of the ice.
How does Radio Echo Sounding work?
The Radio Echo Sounding technique was first developed in the 1960s to better understand ice volume and shape in regions where ice covers the land, but after quickly discovering its usefulness for other cryospheric scientific projects, radar systems were integrated into long-range aircrafts to image the bed1.
More recent airborne campaigns have utilised short-range aircrafts for specific studies such as locating optimal sites for ice cores, constraining the dimensions of subglacial lakes, or developing our understanding of ice sheet mass balance and flow through resolving internal layers1.
The technique works by emitting electromagnetic waves into the ice and recording the reflections that return from internal layers, the topography beneath the ice and any other features such as liquid water or debris. The signals travel through the ice and are reflected back by changes in the properties of the ice. Radio Echo Sounding measurements can vary in several respects including the platform type (e.g. airborne or ground based), platform speed, central frequency (~1 to > 500 MHz) and by system design (e.g. pulsed, chirp pulse, and continuous waveform).
Collected radar data typically undergoes signal processing depending on the requirements for the study (e.g. to investigate the bed or internal layers). These processing techniques remove clutter and improve the image quality.
There are several factors that can affect the quality of ice-penetrating radar data, including the frequency and power of the radar signal, the ice temperature and density, and the presence of liquid water or debris within the ice. To get accurate and reliable results, it is important to carefully design and calibrate the radar system, this will mean you can get all the information you want out of the data that is being collected.
FAIR access to Geophysical Data
Until recently, many geophysical datasets were not openly available and restricted for further use of the data. A recent drive to standardise and release large datasets has been a major initiative for scientists under the FAIR (findable, accessible, interoperable, and re-usable) data principle12. See the British Antarctic Survey’s Polar Airborne Geophysics Data Portal (https://www.bas.ac.uk/project/nagdp/) and the Center for Remote Sensing and Integrated Systems data products (https://data.cresis.ku.edu/) for access to data across Antarctica and Greenland.
About the author
As a geophysicist I use ice penetrating radar to understand the flow dynamics of ice streams and characterise the internal structure of englacial layer in Antarctica. My previous research has been on identifying and characterising new subglacial lakes across East Antarctica.
For my PhD I am particularly interested in the internal layers of large ice streams including Lambert Glacier and Academy Glacier and how these may have changed over time.
I am passionate about outreach and have organised family events to educate the wider public about climate and ice sheet changes. I enjoy going into schools to talk to children about Antarctica and how to become a scientist.
Further reading and links
- Geophysical Surveys in the Gamburtsev Mountains
- Explore the radar data available via the BAS portal.
- NASA blog about the Operation IceBridge airborne mission.
- How we have used Ice Penetrating Radar to reveal the age of Greenland internal layers.
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