Radio-Echo Sounding Over Polar Ice Masses Robert G. Bingham* and Martin J. Siegert { Centre for Polar Observation and Modelling, Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS Email: rgbi@bas.ac.uk *Now at: British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET { Now at: Centre for Polar Observation and Modelling, School of GeoSciences, University of Edinburgh, Grant Institute, King’s Buildings, West Mains Road, Edinburgh, EH9 3JW ABSTRACT Radio-echo sounding (RES) constitutes the principal means by which glaciologists investigate the subsurface properties of the polar ice sheets and ice caps. Developed in the 1960s as a method for locating and mapping the subglacial interface beneath extensive regions of ice- covered terrain, thereby to constrain ice volume and morphology, it was quickly discovered that RES supplies numerous additional cryospheric parameters, including strong reflectors derived from subglacial lakes, and isochronous internal reflectors derived from burial of snow deposition events. Soon after its establishment, RES was integrated into long-range aircraft primarily to image the bed across Antarctica and Greenland (1960s/1970s). More recent airborne campaigns (1980s/1990s), while supplementing this coverage and extending to the ice caps of the High Arctic, have utilised only short-range aircraft, and were designed explicitly to support specific scientific studies, such as locating optimal sites for deep ice-coring, constraining the dimensions of subglacial lakes, or resolving internal layers for studies of ice sheet mass balance, form and flow. In parallel with these developments, ground-based (over-snow) RES equipment has also been used to investigate the englacial and subglacial conditions at a number of key locations across the polar ice sheets. This article discusses the many scientific advances which have resulted from these efforts, and offers recommendations for future developments in terms of (i) reanalysis of existing data and (ii) suggestions for future RES campaigns. Introduction Radio-echo sounding (RES) is a technique through which scientists investigate the subsurface properties of polar ice masses. RES, also known as ice-penetrating radar (IPR) or radar sounding, primarily exploits electromagnetic (EM) waves in the HF/VHF (megahertz) bands, to which, it was discovered in the late 1950s, cold ice is largely transparent. Prior to this discovery, glaciological calculations reliant upon such basic quantities as ice thickness and volume were based on a very limited and widely-spaced set of active seismic measurements, which were highly labor-intensive to obtain; and the englacial properties of ice sheets and ice caps remained enigmatic. Today, RES comprises an efficient method for the collection of a wide range of fundamental subglacial and englacial data from polar ice masses, complementing the extensive array of observations on surface elevation and flow that are now obtained using satellite remote sensing methods (Bindschadler, 1998). To constrain these boundary conditions is important, because it is widely recognised that ice sheets and ice caps fluctuate in response to climate change, thereby impacting significantly on sea levels; and our ability to predict future behaviour is fundamentally dependent upon the accurate determina- tion of past and present ice volumes, subglacial morphology, accumulation and melting rates, and rates of ice flow. RES equipment comprises a transmitter that emits EM waves and a receiver that records their reflections (or ‘echoes’) from any surfaces where there is a contrast in dielectric properties. Over ice sheets and ice caps, the most common reflectors constitute the ice surface, the basal interface, and englacial (internal) layers; although additional features such as subglacial lakes, subsurface crevasses and thermal boundaries can also be discerned. RES apparatus can be mounted either on to airborne or ground-based (over-snow) platforms. For comprehensive studies of ice sheets and ice caps over scales of 10 s to 100 s of km, airborne RES constitutes the most logistically feasible and efficient mechanism for 47 JEEG, March 2007, Volume 12, Issue 1, pp. 47–62