Radio Echo Sounding in western Dronning Maud Land, 1971

S. Afr. Journal of Antarctic Research, No.3 1973

Radio Echo Sounding in western Dronning Maud Land, 1971

Introduction

Radioglaciological work has during recent years formed part of the South African Antarctic Earth Science Research studies in western D:ronning Maud Land. The purpose of this paper is to present data obtained during 1971- the year of initiation of the radiogladological studies.

A Scott Polar Research Institute Mark II Echo Sounder was used to record profiles of subglacial topography over a distance of 630 km. The route is shown in Figs. 1 and 2. The echo sounder was housed in the Geophysical Caboose which served as a mobile laboratory for the \Vintering-over party.

The majority of the work was done in the Ahl- mannryggen and along the boundaries of this mountain range (Schaefer, 1972). Position determina~

tion \Vas particularly easy in this area since it has been mapped and consequently fixes relative to the maps, and accurate to within a few hundred metres, could be obtained by making resections with an ordinary prismatic compass.

The echo sounding data are more meaningful if the depths are referred to a sea~level datum, and barometric altitude determinations were thus made every few kilometres along the entire traverse routes, using the single~basc method of Laplace.

The altitudes obtained are estimated to be accurate to \vithin 5 per cent.

From the echo sounding data, navigational and surface altitude data profiles have been drawn, shO\ving surface and subglacial topography along the various traYcrse lines. As more and more \Vork is done in the area it will eventually be possible to draw detailed contour maps of surface and sub~

glacial features from such profiles. This should prove to be of great value in structural geological studies.

Instrumentation

The SPRI Mk II Echo Sounder is a 35 11Hz pulsed transmitter-receiver system \Vith a sensitivity of 155 dB. Folded dipole antennas were employed nnd were mounted 2 metres from the caboose and 2 metres above the snow surface. A detailed de~

scription of the instrumentation and associated equipment has been given in an earlier paper

(Sc/;aifer~ 1972).

45

T.G. Schaefer*

Radio Echo Sounding Engineer, Twelfth South African National Antarctic Expedition

Measurements and Results

The profiles of surface and subglacial topography (Fig. 3) have been dra\Vn with the vertical scale magnified twice. Greater vertical magnification creates a grossly unreal impression of subsurface topography, and at smaller magnification measure~

ment of ordinates becomes less accurate.

In drawing the profiles, surface altitudes and ice depths have only been determined at discrete points and these values have then been plotted.

Landforms between these specific points have mere- ly been sketched in after visual inspection of echo sounder photographic records. However, it is maintained that sufficient points have been plotted in order to avoid misrepresentation of subglacial topography.

The three plates included before the profiles are examples of prints made from the 35 mm echo sounder recording films. It is noteworthy that virtually no echoes due to internal layering in the ice were observed - in fact this proved to be the case for all of the traverse legs. Further, when studying Plates 1, 2 a.nd 3, it must be borne in mind that the surface echoes have been suppressed.

Plate 1 shows a portion of the Fimbulisen.

Plate 2 shows the topography over the portion of the seismic line traversed by Robin (1958) be~

tween Pyramiden and Fasettfjellet. A detailed com~

parison of seismic and echo sounding depths has been given previously (Schaifer, 1972), and will not be repeated here. It should be mentioned that, although no bedrock ecl1o is visible on Plate 2 at the position corresponding to seismic sounding No. 3 (because the echo was so faint that it was not possible to photograph it clearly), it was never- theless visible amid the noise on the monitor os- cilloscope display. It was from this display that depths were accurately measured when bedrock echoes were just too faint to record. This proce~

dure was often used when echoes were very weak.

Clutter on these plates occurs in areas where echoes are weak and receiver sensitivity is at a maximum.

Plate 3 illustrates the phenomenon of multiple echoes frequently originating in areas of :rugged

*Present Address: Information and Research Services, CSIR, Pretoria.

46

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S. Afr. Tydskrif vir Antarkti!!se Navorsing. No. 31973

subglacial features. The actual topography pro- ducing the multiple echoes probably differs con- siderably in form from that shown on the Plate and corresponding profile (KP). It is possible to work back from the echo sounding record and to deduce what landform gives rise to the patterns of echoes recorded. However, the procedure is a lengthy and complicated one and wilt not be entered into here. (I-larri.ro!l (1970) discusses the computational details for deducing the form of the bedrock from echoes.)

The profiles are considered to be an efficient means of presenting data since both surface alti- tudes and levels of bedrock can be shown referred to sea-level, and this information is of particular use in structural geological studies. The echo sounder recording :film on its own gives only an ice depth relative to snow surface of unknown altitude.

The traverse lines WX and XY are shown by broken lines in Fig. 2, since data obtained during the 1972 :field season were used to draw these.

Although this echo sounding was done by Van Z)•l (1973), the5e profiles have been included here because they fall in the nrea under discussion.

The profiles show dearly all data obtained (Fig.

3) and do not require much further comment. The echoes recorded over the icc-shelf, from Sanae to Draaipunt, were strong and cleat and the subsurface was generally flat and uninteresting. The ice-shelf is grounded over Eskimo Ice Rise and the boundary between grounded and floating shelf-ice can be located by distortions in the subsurface (profile CD).

Profiles GH, HI and IJ show some interesting graben-shaped valleys. These valleys are found on the eastern boundary of the Schyttbrecn and could well be normal, steep-walled, fjord-type valleys re- sulting from glacial erosion, but they could also indicate control by faulting. Similar control could also be indicated by the valleys which drop sh.1.rply to below sea-level over relatively short distances along the western and southern boundaries of the Ahlmannryggen (see profiles PQ, RS, STand TW).

Further to the west and south of the Ahlmann- ryggen boundaries the level of the bedrock changes only very gradually.

For determining ice depths, a propagation velo- city of 169 mfp._s was employed. Considering such factors as the rise time of the echo sounder, the width of trace on the photographic record and the accuracy to which depths can be measured on the trace, it is estimated that the ice depth hgu:res given are correct to within

5%.

Fig. 1 (left). Route tral!erscd across Fimbu!imz.

______.c

Traverse leg corrcspo11ding to Profile BC ill Fig. 3.

Fig. 2 (right). Route tra~ersed betwem the soTJthem holmdary of tbe Fimbuli;eu and tbe nortbem boundm:y of the Borgmassivet.

Posiff(Jll of seismic ice depth determination. The llfmJber rorrupond; to ringui number 011 Plates.

S. Afr. Journal of A ntarctiC Research N . . 0.31973 47

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Plate 1. Recording obtained 011 tf,e Fimblllheu. The direction of frapef is from M.7. (jar left) !OIIfliwards to Drnaip11nt (rigbt) (if. profile EF).

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S. Afr. Journal of Antarctic Research, No. 31973

Ice depth iu metres (seismic determination).

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Portion of traNrse over zvhich 110 echo /Pas received.

Results abtaziJed during field JJJork tiz 1972.

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52 S. Afr. Tydskrif vir Antarktiese Navorsing ,No.3 1973

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Acknowledgements

The author wishes to thank the Department of Transport for facilities provided in Antarctica.

The author is also grateful to .:\Tr Brian van Zyl, who made data from his field-work in 1972 avail- able for inclusion in this paper.

The Editor acknmplcdges the assistance of 1·ejerees in evaluating this

paper.

References

Harrison, CH Reconstruction of subglacial relief from radio echo sounding records. Ceopf?pics~

Robin, G de Q Seismic shooting and related in- vestigations. Norm.-Br.-Swed. Antarct. EXped., Scientific Reslfits, V. Norsk Polarinstitutt, Oslo, 1958.

Schaefer, TG Radio echo sounding in western Dronning Maud Land, 1971 - a preview. So!lth Africa!l jolfmal of AHtarctic Research, no. 2, 53-56, 1972.

Van Zyl, RB Radio echo sounding in "\vestern Dronning Maud Land, 1972. South African ]o11mal of Antarctic Research, no. 3, 53-59, 1973.