|
|
Internal layers in the Antarctic Ice Sheet: electronic repository including three-dimensional visualisation |
|
|
Summary This website provides access to internal layers digitised from airborne radio-echo sounding (RES) surveys conducted across the Antarctic Ice Sheet by a consortium of the U.K. Scott Polar Research Institute, U.S. National Science Foundation and the Technical University of Denmark between 1974 and 1979. This RES survey programme remains to this day the most areally extensive ever conducted across the Antarctic Ice Sheet, but because the internal layers at the time of surveying were only recorded onto film, the internal layering information has remained largely unused by modern glaciological methods. In 2004 the original film records were scanned into a digital database, and from 2004-2006 ‘digitised’ internal layers were retrieved from the new electronic records. This website presents the retrieved internal layers, and also provides 3-D views of the internal layers across the Antarctic Ice Sheet. Compilation of the database was sponsored by the NERC Centre for Polar Observation and Modelling, and this website is hosted by the NERC British Antarctic Survey. >> Skip the introduction and go straight to the data |
 RES antennae installed on wings of U.S.C110 Hercules © Charles Swithinbank |
|
Introduction If all of the ice contained in the Antarctic Ice Sheet were to melt, global sea level would be raised by 65 m. Observations of global warming have therefore precipitated the requirement to model whether the Antarctic Ice Sheet is responding to this warming, and if so how quickly; and calibration and tuning of these models requires data pertaining to past accumulation rates and rates and directions of ice flow. Internal layers, spatially coherent englacial surfaces that are detected by radio-echo sounding (RES), and which can often be traced for several hundreds of kilometres across the ice sheet, are often taken as isochrones, and therefore provide critical information on past accumulation rates and variations in ice flow that can be used to drive ice-sheet models. However, the increasing complexity of models, conducted in one-, two- and three-dimensions, is rapidly outpacing the availability of the internal layering datasets that are required to calibrate them. In many regions of Antarctica, the only information that is available on internal layers was collected using analogue means, and methods to digitise these datasets have only recently become available. Consequently, internal layers are currently under-utilised by glaciologists. Moreover, modelling efforts would be significantly aided by the ability to view internal layers in a three-dimensional context. This website provides a repository of internal layering data retrieved from RES surveys conducted across the Antarctic Ice Sheet by the SPRI-NSF-TUD consortium between 1974 and 1979. The database contains digitised internal layers from the original analogue records, as well as 3-dimensional visualisations of the internal layers overlaid over Antarctic subglacial topography (BEDMAP). Compilation of the database was sponsored by the NERC Centre for Polar Observation and Modelling, and this website is hosted by the NERC British Antarctic Survey. |
|
Genesis and detection of internal layers Internal layers are detected by radio-echo sounding (RES) and represent echoes off any boundary where there is a contrast in the dielectric properties of the ice, resulting, variously, from contrasts in ice density, electrical conductivity, and/or ice crystal fabrics. Near the surface, density variations, exemplified by contrasts between firn and occasional ice lenses, probably account for most internal reflections. Below 1 km of ice, however, where compression negates density contrasts, most internal layers probably manifest variations in conductivity, resulting from fluctuations in the acidity of volcanically-derived aerosols incorporated into the ice when it was first laid down during discrete snow deposition events. In very deep ice (> 3km), subject to very large englacial stresses, changes in ice permittivity, related to the development of anisotropic or preferred crystal fabric orientations, may also be discernible as discrete internal layers. Regardless of their origins, most internal layers picked out on RES radargrams are thought to represent constructive interference formed by radiowave reflections off several parallel and closely-spaced layers; thus the apparent thickness of individual layers observed in RES data is a function not only of its physical origins, but also the radiowave pulse lengths and frequencies used. |
 Example of scanned Z-scope from SPRI-NSF-TUD database, showing clear internal layering through ~3 km depth of ice. |
|
Collection, reduction and representation of Antarctic internal layers The data presented in this website were collected between 1974 and 1979 by a consortium of the Scott Polar Research Institute (SPRI, University of Cambridge, UK), the US National Science Foundation (NSF) and the Technical University of Denmark (TUD). The RES surveys conducted by this SPRI-NSF-TUD consortium took in approximately 400 000 km of flight track across both the West and East Antarctic Ice Sheets. Covering ~70% of the ice sheet overall, this remains the most areally extensive single programme of RES surveys ever conducted across Antarctica. The surveys were driven principally by a desire to measure ice thickness, but also captured numerous internal layers existing through most of the depth profile across large swathes of the ice sheet. |
 Analogue records (radargrams) of internal layers stored at SPRI © Katrina Dean |
 Canisters of film comprising Z-scope radargrams © Katrina Dean |
RES data were collected using a 60 MHz system with a 250 ns pulse length. Navigation proceeded through the use of an inertial aircraft system, and although not as accurate as that which can be achieved using modern GPS, navigational accuracy was typically attained to within 1 km, an acceptable resolution for most ice-sheet modelling applications. Because the survey programme preceded the ‘digital age,’ data recording was analogue only. Data were recorded onto 35 mm photographic film as ‘Z-scope’ ‘radargrams,’ in which single RES pulses are stacked next to one another in a time-dependent manner, and navigational tick-marks added to these records enabled tying-in of the records with geographical waypoints. These data represent pseudo-cross-sections of the ice sheet from which internal layers can be viewed, traced and extracted. Nevertheless, prior to 2004, the analogue records pertaining to internal layers remained largely unused in glaciological applications: this was primarily because the data were not in digital form thus it was difficult and time-consuming to extract information relevant to modelling. |
| In 2004, as part of a NERC Centre for Polar Observation and Modelling (CPOM) initiative led by Professor Martin Siegert, then based in the Bristol Glaciology Centre, photographic copies of the raw 35 mm film records were scanned, and each scan was reformatted to form a single electronic copy of a RES transect (i.e. a ‘jpeg’ or ‘tiff’ file). Each transect was loaded into an image analysis package (i.e. ERDAS ImagineTM) in which internal layers were traced. The traced layers, in addition to the ice surface and bed, were then digitised at each navigational control point. The resulting dataset was then standardised with respect to the ice surface, such that the data relate to depths below the surface of the ice sheet. A CPOM follow-up to this work, conducted between 2005 and 2006 by Dr. Robert Bingham, then also based at the Bristol Glaciology Centre, was concerned with emplacing these data into a 3-D environment. This website provides a repository for both the 2-D traced internal layers and their visualisations within a 3-D environment. |
 Original RES flight logbooks stored at SPRI © Katrina Dean |
|
Data formats We present three forms of the internal layering data:
>> To the data |
| For further information and any queries please contact Dr. Robert Bingham, British Antarctic Survey. |
|
|
|