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Earth Surface Processes and LandformsEarth Surf. Process. Landforms 30, 1583–1591 (2005)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1238

Technical Communication

Radio-echo layering in West Antarctica:a spreadsheet datasetMartin J. Siegert1*, Magdeline Pokar1, Julian A. Dowdeswell2 and Toby Benham2

1 Centre for Polar Observation and Modelling, Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol,BS8 1SS, UK2 Centre for Polar Observation and Modelling, Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge,CB2 1ER, UK

AbstractOf the various information recovered from radio-echo sounding (RES) of polar ice sheets,internal layering is currently under-utilized by glaciologists, due in part to a lack of avail-able data. Here, RES layering of the West Antarctic Ice Sheet, from the 1970s RES survey ofapproximately 70 per cent of this ice mass, is made available in a series of spreadsheets.Three types of internal layers are evident in the dataset. The first is continuous layers thathave a stratigraphic appearance and can often be traced easily for hundreds of kilometres.The second is buckled layering, which also resembles stratigraphy and can sometimes betraced over tens of kilometres (although layer identification can often be difficult). Theroughness of these layers is often greater than the bed at the same wavelength. The third ishighly distorted or absent layering, which is not possible to trace laterally. Despite debateconcerning the origin of RES layers, they are thought by most glaciologists to representisochronous surfaces. The pattern of internal layering is potentially of importance toglaciologists for three reasons. (1) The position of undeformed layers below the ice surface isa function of accumulation rate, ice flow and basal melting conditions. Numerical modelling(including new ‘data assimilation’ techniques) could be used to discriminate between theseprocesses, so revealing important information about the ice sheet and its environment.(2) Buckled layers are deformed by ice flow process, and so their occurrence can be relatedto the flow dynamics of the ice sheet. (3) Very buckled layers are often associated withice stream flow, which allows their location to mark the positions of past and present fast-flowing ice. Copyright © 2005 John Wiley & Sons, Ltd.

Keywords: radio-echo sounding; internal layers; West Antarctica

*Correspondence to:M. J. Siegert, Bristol GlaciologyCentre, School of GeographicalSciences, University of Bristol,Bristol, BS8 1SS.E-mail: m.j.siegert@bristol.ac.uk

Received: 11 March 2004Revised: 24 January 2005Accepted: 17 February 2005

Introduction

Airborne radio-echo sounding (RES) of polar ice sheets has been taking place for over 35 years, taking advantage ofthe transparency of ice to electromagnetic energy at megahertz frequencies (e.g. Polar Record, 1968). Although thetechnique is used principally as a measure of ice thickness, it was also apparent to pioneers of the technique that RESwas capable of measuring englacial layers that could be followed for considerable distances across the ice sheet (e.g.Robin et al., 1969). There have been numerous more recent investigations of internal layers, and glaciological studiesinvolving their use. While early investigations concentrated on identifying the causes of internal layers (e.g. Paren andRobin, 1975; Millar, 1981a,b), later studies employed internal layers to reveal information about ice-sheet flowprocesses (Robin and Millar, 1982; Siegert et al., 2003a), ice surface accumulation rates (e.g. Vaughan et al., 1999;Fahnestock et al., 2001a) and subglacial melt rates (e.g. Bell et al., 2002, Fahnestock et al., 2001b).

Radio waves are reflected from surfaces where there is a contrast in the complex dielectric properties of the ice(Paren and Robin, 1975). Three major causes of dielectric contrasts within ice have been identified. The first is a

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change in dielectric permittivity due to density fluctuations (Paren and Robin, 1975). This is thought to be importantin the upper few hundred metres of the ice sheet, below which the ice density does not change. The second cause isfrom changes in electrical conductivity due to variations in the acidity of ice (Paren, 1981; Millar, 1981a). In particu-lar, internal reflections that occur deeper than about 700 m or so have often been attributed to this process. The originof the acidity is most probably from volcanic aerosol products contained within the ancient surface snow and, hence,reflections from such surfaces have been regarded as isochronous (Millar, 1981a,b). The third mechanism is frompermittivity changes below the upper few hundred metres (Fujita et al., 1999). The permittivity variations at depthwithin an ice sheet have been attributed to the development of preferred crystal fabrics that may form under enhancedenglacial stresses (Fujita et al., 1999; Siegert and Fujita, 2001).

It should also be noted that distinct reflections have been identified from several other types of location within theAntarctic Ice Sheet. Bell et al. (2002) interpret reflections from a few hundred metres above the surface of LakeVostok, the huge subglacial lake in East Antarctica, to originate from the boundary between meteoric ice and accretedice derived from frozen lake water. Similarly, internal reflectors within the ice above the base of the Filchner-RonneIce Shelf have been ascribed to the freeze-on of saline water at the ice-shelf base (e.g. Bombosch and Jenkins, 1995;Joughin and Padman, 2003).

In reality, RES layers are formed from the constructive interference of reflections off a number of layers. Theapparent thickness of an RES layer is dependent on the pulse length of the radio wave used. For example, the RESdata presented here were collected using a radio-wave pulse length of 250 ns, which represents about 40 m in ice at avelocity of 168 m µs−1.

Although there have been a number of airborne campaigns over the ice sheets of Antarctica, the most extensivesurvey (by area) was undertaken in the 1970s by a consortium of the Scott Polar Research Institute (SPRI, Universityof Cambridge, UK), the US National Science Foundation (NSF) and the Technical University of Denmark (TUD).Over the course of four field seasons (1971/72, 1974/75, 1977/78 and 1978/79), approximately 400 000 km of trackwas flown across both the West and East Antarctic ice sheets. These data have been used in a number of applicationssince their acquisition: for example, in making up the bulk of the information used to construct both ice-thickness andsubglacial-bed maps of Antarctica (Drewry, 1983; Lythe et al., 2001), and in an inventory of subglacial lakes (Siegertet al., 1996). The internal layers, however, have yet to be fully utilized for glaciological study. One of the reasons forthis is that they have, until now, not been reduced from the original analogue records and made available for use indigital form.

In this paper, RES layers extracted from the SPRI-NSF-TUD survey of the West Antarctic Ice Sheet (Figure 1) arepresented. Information regarding the distribution of internal layers in the ice sheet, the RES layer quality, and theenglacial positions of internal layers are provided. The layers from each analogue RES transect are digitized to anExcel spreadsheet, which can be downloaded from the Wiley Interscience website (www.interscience.wiley.com). Theaims of this work are to establish the englacial structure of West Antarctica, as recorded by RES, and to make theresults available to the glaciological community.

SPRI RES Data

Specification of RES dataThe RES data presented in this paper were collected using a 60 MHz system with a 250 ns pulse length. Data wererecorded onto 35 mm photographic film in two formats. The first is the ‘Z-scope’ mode, in which single RES pulsesare stacked next to each other in a time-dependent manner. These data represent pseudo-cross-sections of the ice sheetin a manner similar to a seismic section (Figure 2). It is from this format that internal layering can be observed, tracedand extracted. The second data format is the ‘A-scope’ mode, which records ice-sheet reflections from a single pulseof electromagnetic energy as a graph of two-wave travel time (or distance) against reflected power. This form of RESdata can be used to identify the power reflected off an individual surface, from which information about the reflectingboundary can be obtained. A-scope information has been used previously to help determine englacial characteristics(e.g. Millar, 1981a,b). However, A-scope data are not included in this dataset of internal layers, as they do not on theirown provide information on layer positions with depth and cannot therefore be used to build englacial structures. Inaddition, A-scope records were collected over only a small proportion of the total flight tracks.

About 70 per cent of the West Antarctic Ice Sheet was covered during the SPRI-NSF-TUD survey on thirteenRES flights (Figure 1). In each case, the identifier given to each line in the respective season the data were acquiredis used here. Line identifiers beginning with the number ‘1’ were collected in the 1974/75 season. Ice-thickness andice-surface elevation data from these flights were read to datafiles every 20 seconds. On each occasion a tick-mark

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Figure 1. RES lines acquired by the SPRI-NSF-TUD airborne survey of West Antarctica during four field seasons in the 1970s.Although fourteen RES lines are shown, line 036 is only available at a frequency of 300 MHz (and not 60 MHz like the others).Internal layers are not provided for this particular line.

was placed on the analogue Z-scope record (Figure 2a), and the position at that moment was also recorded. Linesbeginning with ‘00’ or ‘01’ were recorded in the 1977/78 season. Lines prefixed with ‘03’ or ‘04’ were from the1978/79 season. In the latter two seasons navigation and ice-sheet data were recorded much more frequently than in1974/75. However, tick marks were provided on the top of each Z-scope to denote every 50th navigational controlpoint (Figure 2b).

Classification of RES layersRES layering is classified into three types, following Drewry (1983), as follows.

(1) Good (laterally continuous) internal layers; continuous and traceable within the ice sheet for at least 10 km, andsometimes hundreds of kilometres (Figure 2a). Internal layers appear smooth and virtually parallel to each otherover short distances.

(2) Buckled (moderate) internal layers. These layers are often traceable for tens of kilometres, but they are not asclear as ‘good’ layers. The Z-scope shows a generally stratified structure, but not all individual layers may betraced (Figure 2b). Layers often undulate at the same (or greater) wavelength as the bed and with greateramplitude (Figure 2b). Such layers may form from ‘good layers’ under enhanced flow of ice, such as ice streamtributaries (Siegert et al., 2003a).

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Figure 2. Z-scope RES data and the classification of RES layers. (a) Continuous layering from line 148 collected in the 1974/75season. (b) Buckled layering from line 005 taken during the 1977/78 season. (c) Absent layering from line 138 acquired in the 1974/75 season.

(3) Poor (absent) internal layers. Internal reflections may be present, but there is no stratigraphic form to theirappearance (Figure 2c). Internal layers are often absent within ice streams, but this may have a number of reasons.It could be because the flow of ice has buckled the layers beyond recognition as stratigraphy. Lack of layers couldalso be due to the presence of surface crevasses, which cause enhanced surface scattering of energy (and obscurethe signal from beneath), and to relatively high aircraft flying altitudes, which reduced the power of the trans-mitted wave in ice through spreading of the cone of energy.

Distribution of RES layer typeDrewry (1983) provided a map of internal layer types in West Antarctica. The systematic survey of internal layersundertaken in our study allows Drewry’s map to be revised. Internal layer type is marked onto each RES transect. Thelayer-classified transects are then projected onto a map, providing a revised distribution of layer types in WestAntarctica (Figure 3). A number of differences exist between the distribution of layer types shown in Figure 3, and

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Figure 3. Distribution of internal layer types within the West Antarctic Ice Sheet. Black lines represent continuous layers, whitelines buckled layers, and thin black lines highly buckled or absent layers. See Figure 2 for examples of each type of layering. Part ofthis map (i.e. lines 137, 138, 139, 144, 147, 148) is adapted from a previous investigation of RES layering around the upstreamregion of Ice Stream D (Siegert et al., 2003a).

Drewry’s (1983) map. For example, very few layers have been detected in the SPRI-NSF-TUD data across the SipleCoast ice stream regions, while Drewry (1983) assigns this region entirely to ‘buckled (moderate)’ layers. Further,over the centre of the ice sheet, right across the main ice divide, excellent internal layers have been measured withinthe ‘buckled (moderate)’ zone of Drewry (1983). In fact far more ‘good’ internal layers are classified across WestAntarctica in Figure 3 than in Drewry (1983) in general.

RES Layering

Picking internal layersThere is no reliable automated method by which analogue RES data can be processed to reveal internal layers.Consequently, internal layers have to be ‘picked’ and digitized by hand to extract the positions of internal layers to aspreadsheet. The method used is as follows. First, photographic copies of the raw 35 mm film records were scanned,and each scan was reformatted to form a single electronic copy of an RES transect (i.e. a ‘jpeg’ or ‘tif’ file). Theelectronic file was loaded into an image analysis package (ERDAS Imagine™ was used in this study), in whichinternal layers were traced. Not every internal layer was picked, but the broad full distribution of available layerswas captured during the tracing procedure. The traced layers, in addition to the ice surface and the bed, were thendigitized at each available navigation control point. The resulting dataset was then standardized with respect to the icesurface, such that the data relate to depths below the surface of the ice sheet. Figure 4 provides an example of an800 km long section of raw RES data (part of line 005 from the 1977/78 season) and the digitized plot that wasextracted from it.

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Figure 4. Comparison of an 800 km section of raw Z-scope RES data and the digitized data used to build the spreadsheets ofinternal layers. (a) The 60 MHz Z-scope data from line 005, acquired during the 1977/78 field season (Siegert and Payne, 2004).(b) The digitized version of line 005. Modified with permission of the American Geophysical Union.

The accuracy of internal layer picking requires discussion. The results presented must be viewed in the context ofthree potential limitations. The first is because internal RES layers originate from a number of closely spaced physicalhorizons. It is, therefore, possible that individual layers might appear to migrate up and down the ice column asthe effect of radio-wave interference from a multitude of reflection boundaries changes. Millar (1981b) expectedthe errors associated with this issue to be small, however, compared with the pulse length of the radio wave. Thesecond potential source of error is where the layer quality is not good. Tracing ‘good’ layers is a straightforwardoperation. However, where layers buckle and are less obvious, the ability to trace them becomes difficult. In suchcases, short (c. <5 km) data gaps are filled by matching the apparent englacial stratigraphy on either side. Furthermore,on occasions, internal layers are observed to merge or split. In these circumstances, only one internal layer is tracedand retained in the dataset. The third problem involves operator error in the tracing process. Independent tracing of thesame RES line reveals that this issue is not important over short (<50 km) distances, but can be serious when tracingbuckled (less good) layering over greater distances. In our study, only ‘good’ internal layers (that can be extractedwith little ambiguity) are traced over distances in excess of 50 km.

Dataset of RES layeringThe dataset of RES layering in West Antarctica is arranged into a series of individual spreadsheets relating to theoriginal flightlines. In each spreadsheet, the latitude and longitude are provided along with the depth (below the icesurface, which is set at zero) of individual internal layers and the bed. Each continuously traceable internal layer isgiven a discrete column in the spreadsheet. Individual layers are, thus, easy to extract from the spreadsheets. Columnheadings allow each spreadsheet to be ‘self-contained’. The user is therefore able to download and understand theRES layering from any single flightline.

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Tracing Layers about the Centre of the Ice Sheet

One of the RES flights undertaken in the 1974/75 season (flight 148) deserves special attention in the dataset, as itcomprises 18 short transects across the Byrd Station Strain Network, between the ice divide in central West Antarcticaand Byrd Station, where a 2164-m deep ice core was extracted in 1968 (Whillans, 1976).

Eight internal layers were traced continuously across all 18 transects within line 148, and the short sections of radarline that connect them. The resolution of the radar stratigraphy is particularly good in this part of central WestAntarctica. In fact, the layers here are among the best quality recorded in the whole SPRI survey. Tracing layers alongline 148 was, consequently, a relatively easy operation. Internal layers were traced continuously and without change totheir elevation across the intersection of orthogonal transects in 29 cases along line 148. These data can be used tobuild a three-dimensional image of the internal structure of this part of West Antarctica. An example of how the datacan be used for this purpose is provided in Figure 5. The internal layers can be assigned ages at their intersection withthe Byrd ice core location by correlating layer depth with the ice core’s depth–age function (at 80°S, 120°W; fordetails see Blunier and Brook, 2001).

Uses of Internal RES Layering

The RES layering presented here has a number of potential uses as a glaciological dataset. For example, internalice-sheet layering has been used on a number of occasions to construct past accumulation rates, and their spatialdistribution, using a relatively simple ice compression model (e.g. Fahnestock et al., 2001a). This is particularlyrelevant close to the centre of the ice sheet where vertical compression accounts for the majority of ice flow. Awayfrom the ice divide accumulation rates can still be determined, but a more sophisticated flow model is required (as iceflow processes also control the position of internal layers) (e.g. Siegert et al., 2003b). Where the accumulation andflow rates of ice are known, the RES layering can also be used to determine the rates of basal melting (Fahnestocket al., 2001b).

In some cases, internal layers are disrupted and buckled. Siegert et al. (2003a) showed the coincidence of buckledlayering with enhanced ice flow. The positions of buckled layers may, therefore, reflect ice flow processes (past andpresent) in West Antarctica. As more information on ice-surface velocities is gained from satellite radar interferometry,this use of internal layering may provide important insights into the englacial flow history of the ice sheet.

Another important potential use of this dataset, and of RES layering in general, is in data assimilation techniques inice-sheet modelling. Data assimilation involves the use of ‘data’ as an integral part of the modelling process, ratherthan as an input or a boundary condition. Such modelling is commonly used in the study of geophysical flows andatmospheric circulation, but has yet to be employed in glaciology. The potential of RES layering to understand the fullthree-dimensional flow of the ice sheet at a continental scale makes this technique potentially powerful for ice-sheetmodellers.

Finally, the dataset developed in this investigation should not be considered exhaustive of all the informationregarding englacial layers in West Antarctica. There have been several regional airborne RES investigations inWest Antarctica undertaken by the US scientists, and a number of ground-based traverses by the InternationalTrans Antarctic Scientific Expedition (ITASE). Internal layers have been extracted from these more recently acquireddata, and used to infer information on past accumulation rates (e.g. Morse et al., 2002; Welch and Jacobel, 2003). Asthese new data become widely available, they could be added to the information currently included in this RES layerinventory.

Summary

RES has been used successfully to measure ice-sheet thickness and subglacial conditions. Such information is appliedcommonly to build subglacial topographies, which are an essential boundary condition to numerical ice-sheet models.RES also reveals isochronous internal ice sheet layering. Despite the glaciological insights that are possible from thestudy of internal RES layers, their use has been limited because of the lack of availability of data. Analogue RES datafrom the most extensive (by area) survey of West Antarctica has been digitized and, using image analysis software, theinternal layers were extracted. In total, thirteen flightlines of data were processed, revealing internal layer patternsacross much of the West Antarctic Ice Sheet. Information for each flightline is available on an individual spreadsheet,which can be downloaded from the Wiley Interscience website (www.interscience.wiley.com). The resulting internallayer dataset can be used for a number of glaciological purposes, including the construction of past and present ice

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Figure 5. Ice-sheet structures along SPRI flight 148 measured from internal layering. (a) Thickness between the ice surface andthe shallowest traceable internal layer. (b) Between internal layers 2 and 3. (c) Internal layers 4 and 5. (d) Internal layers 6 and 7.(e) Separation between internal layer 8 and the ice base, which is shown in all diagrams.

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accumulation rates, and ice flow patterns. The internal layers can also be used in the future for next-generation icesheet models using ‘data assimilation’ techniques.

AcknowledgementsFunding for this project was provided by the UK Natural Environment Research Council’s Centre for Polar Observation andModelling (CPOM).

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