Introduction
Temperature measurements on several Spitsbergen glaciers indicate that they are of “sub-polar" thermal regime, with accumulation-zone ice at or near the pressure-melting point and ablation-area ice somewhat colder (e.g. Reference SverdrupSverdrup, 1935; Reference SchyttSchytt, 1964; Reference BaranowskiBaranowski, 1975). “Sub-polar" and “temperate" glacier ice is both less homogeneous and more lossy to electromagnetic energy than colder Antarctic and Greenland ice masses. At these higher temperatures both scattering and absorption of radio waves increase relative to colder ice, making radio echo-sounding of glacier thickness problematic (Reference Smith and EvansSmith and Evans, 1972; Reference GoodmanGoodman, 1975; Reference Watts and EnglandWatts and England, 1976).
A primary aim of radio echo-sounding has always been to record an unambiguous bed echo (Reference Smith and EvansSmith and Evans, 1972). However, analysis of airborne echo-sounding data collected in Spitsbergen during 1980 showed bottom returns from certain glaciers at depths 2–3 times those reported in previous echo-sounding studies (Macheret, 1980; Reference Macheret and ZhuravlevMacheret and Zhuravlev, 1982). This paper reports the results of airborne radio echo-sounding in Spitsbergen at a frequency of 60 MHz, and suggests that certain echoes assumed by previous workers to be from the ice-bed interface are in fact returns from an internal reflecting horizon. Reasons for the failure of Soviet 440 and 620 MHz systems to penetrate to the true bed of a number of Spitsbergen glaciers are discussed, along with possible explanations of observed internal reflecting layers.
Radio Echo-Sounding Equipment and Methods
Radio echo-sounding of Spitsbergen glaciers during April and May 1980 used a Scott Polar Research Institute (SPRI) Mk IV sounder mounted in a Bell 206 helicopter (Reference Drewry, Drewry, Liestøl, Neal, Orheim and WoldDrewry and others, 1980). The sounder operated at a centre frequency of 60 MHz with a pulse length of 350 ns, and had a system performance of 140 dB (excluding antenna gain). This was somewhat below its previous specifications in Antarctica due to the use of 20 dB attenuation, which reduced problems of receiver saturation at the low altitudes flown. A list of system parameters for the SPRI Mk IV equipment is given in Table I, which also includes information on the 440 arid 620 MHz systems used for Soviet radio echo-sounding in Spitsbergen (Reference Kotlyakov, Kotlyakov, Macheret and GromykoKotlyakov and others, 1982). Navigation was by visual sightings on known points. Along-track fixing on larger valley glaciers was about 1 km in error at worst, and across-track deviations from the glacier centre-line were normally less than 0.5 km.
Table I. Parameters of SPRI and Soviet Radio Echo-Sounding Equipment Used in Svalbard
During 1980, radio echo-sounding results were recorded as range or “Z” profiles (Fig.1). No quantitative echo-strength measurements were made. Helicopter terrain clearance, ice surface, internal reflector, and bed echoes were digitized, and ice thickness was calculated using a velocity of radio waves in ice of 168 mμ s−1. The ice surface elevations used in constructing glacier profiles were obtained from existing Norsk Polarinstitutt 1:100 000 scale maps of Svalbard. The Spitsbergen glaciers discussed in this paper are located in Figure 2. More detailed discussion of echo-sounding equipment, navigation, and reduction of data collected in 1980 may be found in Reference Dowdeswell, Dowdeswell, Drewry, Liestøl and OrheimDowdeswell and others (in press).
Fig. 1. Radio echo-sounding mnge or “z" profile of Negribroen from SPRI Mk IV 60 MHz equipment.
Fig. 2. Map of Spitsbergen showing the location of glaciers and ice cape mentioned in the text. Airborne radio echo-sounding flight lines are indicated. 1-Raudfjordbreen, 2-Lilliehøøkbreen, 3-Monacobreen, 4-Isachsenfonna, 5-Holtedahlfonna, 6-Austre Brøfggerbreen, 7-Midre Lovenbreen, 8-Utlersbreen, 9-Korgsvegen, 10-Eidembreen, 11-Borebveen, 12-Wahlenberybreen, 13-Mittag-Lefflerbneen, 14-Lomonosovfonna, 15-Tunabreen, 16-Negrlbreen, 17-Austre Grrfnfjordbreen, 18-Fridtjovbreen, 19-Finsterwalderbreen, 20-Penckbreen, 21-Vestre Torellbreen, 22-Werenskioldbreen, 23-Hansbreen.
Results of Airborne Radio Echo-Sounding
Identifiable bed echoes were recorded for only 50% of the 740 track kilometres flown during 1980, illustrating the general difficulty of sounding ice at or near its melting point. However, in this paper discussion concentrates on those Spitsbergen glaciers where discrepancies have been observed between our own and Soviet ice thickness measurements. Results from some 31 further glaciers in Spitsbergen will be reported elsewhere (Reference Dowdeswell, Dowdeswell, Drewry, Liestøl and OrheimDowdeswell and others (in press)).
A range or “Z” profile from Negribreen is shown in Figure 1. Ice-surface and bed echoes are visible, together with an internal reflection varying between depths of 80 and 190 m beneath the surface. Bottom echoes are distinguished from layer echoes by position (i.e. the deepest reflector), and because they are usually stronger than internal echoes (e.g. Fig.1). Digitized and calibrated profiles from seven Spitsbergen glaciers are shown in Figures 3, 4, and 5. These range in size from valley glaciers such as Finsterwalderbreen (12 km long) to relatively large ice-cap outlet glaciers, for example Negribreen (35 km long). All glaciers except Austre Grdøfjordbreen exhibit an internal reflecting horizon. Maximum recorded ice thicknesses for these seven glaciers are 430 m on Negribreen and 400 m for Kongsvegen.
Fig. 3. Reduced radio echo-sounding record of Negribreen, showing SPRI Mk IV 60 MHz surface and bottom echoes (solid lines) and an internal reflector (dashed line). The results of Soviet echo-sounding are also included. Soviet ice-thickness data were measured from continuous or intermittent profiles at 1 km intervals. The point A represents the crossing point between long- and cross-profiles of Negribreen.
Fig. 4. Reduced radio echo-sounding records for three glaciers where independent depth surveys have taken place. Gravity surveys are available for Finster-walderbreen (Reference Husebye, Husebye, Sørnes and WilhelmsenHusebye and others, 1965) and Kongsvegen (Reference OelsnerOelsner, 1966). Bore holes have been drilled on the divide between Austre Grønfjordbreen and Fridtjovbreen (Reference Zagorodnov and ZotikovZagorodnov and Zotikov, 1981). Austre Grønfjordbreen is to the left of the ice divide. Soviet radio echo-sounding results are also shown.
Fig. 5. Comparison of SPRI Mk IV and Soviet radio echo-sounding data for Hansbreen and Penckbreen.
Data obtained using other geophysical methods of ice thickness measurement are included in Figure 4, and Soviet radio echo-sounding results are presented in Figures 3, 4, and 5.
Comparison with Existing Ice Thickness Data
Gravity and bore-hole measurements
Gravity surveys (Reference Husebye, Husebye, Sørnes and WilhelmsenHusebye and others, [1965];Reference OelsnerOelsner, 1966), yielding ice depths averaged over an area equivalent to ice thickness, are compared with glacier thicknesses measured from radio echo returns (Fig.4). For both Finsterwalderbreen and Kongsvegen, ice depths derived from gravity surveys and from 60 MHz radio echo-sounding rarely deviate by more than 25 m or 10% of glacier thickness. Reference DrewryDrewry (1975) has noted that in certain areas of the Antarctic differences between ice thicknesses derived from gravity and radio echo-sounding could be as much as 15–20%.
Thermal drilling on the ice divide between Austre Grønfjordbreen and Fridtjovbreen reached the glacier bed at 211 m (Reference Zagorodnov and ZotikovZagorodnov and Zotikov, 1981), whereas echo-sounding with the SPRI Mk IV system in the vicinity of the drill hole recorded bed returns at 205 m (Fig.4). A second bore hole did not reach the glacier bed.
The limited, but independent, information available from other geophysical methods of ice thickness measurement therefore agrees with the results of 60 MHz echo-sounding to within 10% or better on Finsterwalderhreen, Kongsvegen, and Fridtjovbreen (Fig.4).
Evidence from Soviet radio echo-sounding
Soviet radio echo-sounding in Spitsbergen began in 1974, and ice thickness data for a relatively large number of glaciers have since been published (Reference MacheretMacheret, 1976, 1981; Reference Macheret and ZhuravlevMacheret and Zhuravlev, 1980, Reference Macheret and Zhuravlev1982;Reference Macheret, Macheret, Zhuravlev and GromykoMacheret and others, 1980), Bed and layer echoes recorded by Soviet radio echo-sounding systems (Table I) are plotted in Figures 3, 4, and 5. All these glaciers, except Austre Grønfjordbreen and Fridtjovbreen, show significant discrepancies in ice thickness relative to the data collected in 1980. In every case the Soviet work underestimates glacier depth (Figs 3, 4, and 5). For example, Penckbreen is a maximum of 140 m thick according to Soviet data, but up to 240 m deep from our evidence (Fig.5). Further, ice thickness measurements by Reference MacheretMacheret (1981) are only 30–60% of those from independent gravity surveys of Finster-walderbreen and Kongsvegen.
Figures 3, 4, and 5 also show the correspondence between Soviet echoes assumed to represent the ice-bed interface and the internal reflecting horizons recorded by the SPRI 60 MHz system. More than 60% of Soviet bed echoes coincide closely with these internal echoes. However, on Fridtjovbreen and the lower part of Negribreen an internal echo is explicitly identified as such by Reference Macheret and ZhuravlevMacheret and Zhuravlev (1982). It is almost exclusively in areas where a layer echo is recognized in Soviet work that SPRI Mk IV and Soviet systems record bed echoes at similar depths (Figs 3 and 4).
Further, several relatively large ice masses sounded by Macheret and co-workers, but not investigated during 1980, may also be subject to the mis-identification of bottom echoes, for two reasons. First, these glaciers appear thin (90–200 m deep) relative to their length (15–30 km). Second, internal echoes are not reported from them. Preliminary results of SPRI Mk IV 60 MHz radio echo-sounding during April and May 1983 confirm that the thickness of several more Spitsbergen glaciers has been underestimated considerably in Soviet work. These glaciers include Borebreen, Eidembreen, Lilliehøøkbreen, Monacobreen, Raudfjordbreen, Tunabreen, Uversbreen, and Wahlen-bergbreen (Fig.2).
The presence of apparent depressions or rapid changes in bedrock elevation on certain glaciers was also inferred from Soviet radio echo-sounding records (Reference MacheretMacheret, 1981). According to Macheret’s data for Negribreen, bed elevation rises by over 200 m between 14 and 15 km up-glacier (Fig.3). However, the Soviet bed echo between 15 and 25 km from the glacier terminus closely follows an internal echo from the SPRI Mk IV system (Fig.3). It is suggested that the real bottom echo disappears from the Soviet record at approximately 14 km from the glacier terminus, and from there up-glacier an internal echo at a higher elevation was misinterpreted as the bed. Further, such rapid changes in bed elevation are improbable on glaciological grounds, since the glacier surface profiles do not reflect this presumed change. Similar problems may account for apparently rapid changes in bed elevation in the Kronebreen-Holtedahlfonna region and on Mittag-Lefflerbreen (Reference MacheretMacheret, 1981).
Finally, it should be noted that close agreement (± 15%) exists between Soviet data and results from the 1980 season for several glaciers. Comparative profiles for Austre Grønfjordbreen and Fridtjovbreen are shown in Figure 4. Further examples are Austre Brøggerbreen, Midre Lovenbreen, Vestre Torellbreen and Werenskioldbreen.
Radio Echo-Sounding of Spitsbergen Sub-Polar Glaciers at 60, 440, and 620 MHz
The deepest ice sounded successfully by 620 MHz equipment was 540 m (Reference MacheretMacheret, 1981), whereas the 440 MHz sounder had a maximum penetration of between only 150 and 250 m (Reference MacheretMacheret, 1981; Reference Macheret and ZhuravlevMacheret and Zhuravlev, 1982). The contrast in penetration between the two Soviet systems is due to the higher system performance and antenna gain of the 620 MHz equipment (Table I). The SPRI Mk IV sounder recorded a maximum ice thickness of 530 m on Holtedahlfonna during 1980.
The use of Soviet 440 MHz equipment to sound Svalbard glaciers more than 150–250 m thick could clearly lead to the misinterpretation of layer echoes as basal, since the true bed would be too deep to be recorded. This may explain the discrepancies between our own and Soviet sounding of Finsterwalderbreen and Kongsvegen (Fig.4), which were both sounded at 440 MHz during 1974 (Reference MacheretMacheret, 1976).
The reasons for discrepancies between the results of 60 and 620 MHz sounding are less obvious because the two systems penetrate to approximately similar maximum depths. However, although the Soviet equipment has a higher system performance and antenna gain than the SPRI Mk IV (Table I) a layer echo, rather than the true bed, was generally recorded during 620 MHz sounding of Negribreen, Hansbreen, and Penckbreen (Figs 3 and 5).
Reference Smith and EvansSmith and Evans (1972) showed that absorption and scattering by surface melt water, soaked firn, ice layers and ice lenses increases with radio frequency. Their three-layer model was used to calculate signal attenuation resulting from such inhomogeneities for a glacier 250 m thick at −2°C. For example, the presence of a surface water layer 5 mm thick results in two-way signal attenuation of about 1.5 and 15 dB at 60 and 620 MHz respectively. Reference Davis, Davis, Halliday and MillerDavis and others (1973), using 440 MHz equipment, observed a 10 dB weakening of returned signals during daytime sounding of an east Greenland glacier in the melt season. Model results also showed that attenuation by soaked firn and ice layers increased with radio frequency (Reference Smith and EvansSmith and Evans, 1972). Reference Liestøl, Liestøl, Repp and WoldLiestføl and others (1980) have described supra-glacial lakes on several Spitsbergen glaciers during summer, and on Lomonosovfonna a soaked firn layer up to 2 m thick has been noted (Reference Kotlyakov, Kotlyakov, Macheret, Gordiyenko and ZhuravlevKotlyakov and others, 1980). Ice layers and lenses have also been observed during snow stratigraphic studies in Svalbard. For example, Reference AhlmannAhlmann (1935) reported more than ten discrete ice layers of between a few millimetres and 0.5 m in thickness in several 5 m snow-pits on Isachsenfonna.
The 1980 echo-sounding with SPRI Mk IV 60 MHz radar therefore had two advantages over the Soviet 620 MHz system. It was conducted at a lower radio frequency, and flying took place before the start of the melt season, reducing the effects of absorption and scattering by inhomogeneities within and on the surface of Spitsbergen glaciers. Soviet sounding generally took place during the melt season.
Problems in Radio Echo-Sounding of Glacier Accumulation Areas
Both SPRI Mk IV and Soviet echo-sounding equipment recorded bed echo returns only intermittently. In particular, Reference MacheretMacheret (1981) reported the disappearance of 440 and 620 MHz signals in glacier accumulation areas and near ice divides on thicker glaciers. Bed echoes were only seldom recorded in similar areas during sounding at 60 MHz. For example, bottom echoes were noted for less than 20% of a 109 km fight over Isachsenfonna and Holtedahlfonna. Bottom returns also disappear in the accumulation areas of Negribreen and Kongsvegen (Figs 1, 3 and 4).
Measured 10 m temperatures on a number of Spitsbergen glaciers suggest that firn and ice in accumulation areas are often close to the pressure-melting point (e.g. Reference SverdrupSverdrup, 1935; Reference BaranowskiBaranowski, 1975). The high temperatures, along with liquid water and inhomogeneities associated with melting and refreezing, may partly account for the lack of success of both our own and Soviet equipment in sounding the accumulation zones of thicker glaciers.
Reference Smith and EvansSmith and Evans (1972) also showed that bottom returns can be obscured by the diffuse return from a multitude of scatterers. The masking of bottom-echo returns may be occurring preferentially in glacier accumulation areas if ice lenses and water inclusions (Reference Watts and EnglandWatts and England, 1976) are more numerous in firn and ice at its melting point.
Scattering may also result from heavy surface crevassing, a phenomenon well known from Antarctic radio echo-sounding. Many Spitsbergen glaciers, especially those that surge, have large crevassed areas which may spread considerable distances up-glacier.
Internal Reflecting Horizons in Spitsbergen Glaciers
Internal reflections from depths of 70–180 m were reported by Reference Macheret and ZhuravlevMacheret and Zhuravlev (1980). Reflecting horizons were recorded at between 70 and 190 m by SPRI Mk IV equipment, with about 70% of observations falling between 90 and 110 m depth (Figs 1, 3, 4, and 5). These layers are different from multiples of the ice surface echo. Only single isolated layer echoes were observed on any glacier, although it is not known whether a reflection corresponds to a single discontinuity in the ice or is integrated over the pulse length or wavelength of the radar (Reference HarrisonHarrison, 1973). The multiple layer echoes observed in the Antarctic and Greenland ice sheets have not been reported from radio echo-sounding studies of Spitsbergen glaciers. Further, comparison between spring and summer sounding by our own and Soviet systems indicates that these isolated layers are relatively constant in location, in contrast to the rapidly changing pattern of intraglacial reflectors interpreted as expressions of internal hydrological changes by Reference GoodmanGoodman (1973).
Internal reflectors could result from the presence of impurities (such as moraine or ash layers and chemical precipitates), fluctuations in ice density, bubble content, geometry and crystal axis orientation, the presence of brine, or changes in water content and temperature with depth.
Reflection coefficients cannot be used to estimate the possible causes of layer echoes because quantitative echo-strength measurements were not made in 1980. However, from visual inspection, bottom-echo returns were usually stronger than layer signals (Fig.1). This, along with the lack of an obvious debris source in many areas, indicates that the internal reflector is probably not a moraine layer. Ice cores from Fridtjovbreen (Fig.4) did not reveal any moraine layers, but transparent and impure ice was noted at depths of 50–85 m, 102–106 m, and 145–149 m (Reference Zagorodnov and ZotikovZagorodnov and Zotikov, 1981). Reference Macheret and ZhuravlevMacheret and Zhuravlev (1982) also reported preliminary ice-core data suggesting that ice with a water content of 1–2% was present at depths greater than 115 m, which coincided with the height of the internal reflecting horizon in this area of the glacier. The layer echo might therefore be associated with ice at the pressure-melting point. However, more ice-core drilling, together with data on radio echo signal strengths and reflection coefficient calculations, is needed to provide additional evidence concerning the interpretation of these layer echoes.
Conclusions
The results of radio echo-sounding of certain Spitsbergen glaciers at 60 MHz show that previous investigations using 440 and 620 MHz radar systems (Reference MacheretMacheret, 1981; Reference Macheret and ZhuravlevMacheret and Zhuravlev, 1982) have underestimated ice thickness by two to three times. An unambiguous bed echo, with significant lateral continuity, has been identified consistently from 60 MHz records at greater depths. This conclusion is confirmed by: (1) the agreement (± 10%) between 60 MHz-derived ice thicknesses and independent gravity surveys on two glaciers, in contrast to Soviet ice depths which are only 30–60% of gravity thicknesses; (2) the strong correlation between echoes interpreted as bottom returns by Macheret and co-workers and echoes from an internal reflector in our own study; (3) the unusual glacier morphology required by Soviet results (i.e. very thin glaciers relative to length, and sudden bed depressions unrelated to ice surface topography). Where a layer echo was explicitly recognized as such by Macheret and co-workers there is good agreement with our ice thickness estimates. Preliminary analysis of 60 MHz sounding conducted during the spring of 1983 shows that Soviet results underestimate the thickness of several more Spitsbergen glaciers. Such errors imply that Reference Macheret and ZhuravlevMacheret and Zhuravlev’s (1982) calculation of the ice and water resources of Svalbard is inaccurate.
The lack of penetration by Soviet echo-sounding equipment has probably resulted from the higher frequency of their radar systems, given the relatively high temperatures and surface and internal inhomogeneity of Spitsbergen glaciers. Both absorption and scattering increase with radio frequency, and the use of 440 and 620 MHz equipment, together with the presence of melt water during summer sounding, may explain the lack of penetration relative to investigations using 60 MHz radar. This finding implies that lower-frequency (V.H.F.) equipment has certain advantages over U.H.F. radars in sounding relatively thick ice at or near its melting point, although resolution remains poorer at lower frequencies.
Interpretation of the internal layers reported in this study and Soviet work remains equivocal. Further ice-core drilling, along with detailed echo strength measurements, should lead to a better understanding of the internal structure of Spitsbergen glaciers.
Acknowledgements
This research is part of a continuing cooperative glaciological research programme in Svalbard involving the Scott Polar Research Institute, Cambridge, and Norsk Polarinstitutt, Oslo. We wish to thank Store Norsk Spitsbergen Kullkompani for assistance in Longyearbyen and with field operations. Dr C.S. Neal prepared the radar equipment and took part in field work. SPRI funding was provided by United Kingdom Natural Environment Research Council (NERC) Grant GR3/4463 to Dr D.J. Drewry. J.A. Dowdeswell acknowledges support from a NERC Studentship.
