Norsk geogr. Tidsskr. Vol. 54, 182–185. Oslo. ISSN 0029-1951
RESEARCH NOTES AND REVIEWS – NOTISARTIKLER OG ANMELDELSER
Continuous measurements of solifluction using carrier-phase differential GPS IVAR BERTHLING, TROND EIKEN & JOHAN LUDVIG SOLLID
Berthling, I., Eiken, T. & Sollid, J. L. 2000. Continuous measurements of solifluction using carrier-phase differential GPS. Norsk Geografisk Tidsskrift–Norwegian Journal of Geography Vol. 54, 182–185. Oslo. ISSN 0029-1951. In this research note, we test the potential of differential carrier-phase GPS measurements (DGPS) for continuous recording of the slow mass wasting occurring in solifluction. DGPS is shown to be capable of yielding records of high temporal resolution and accuracy. Keywords: gelifluction, GPS, solifluction, thaw consolidation Ivar Berthling, Trond Eiken, Johan Ludvig Sollid, Department of Physical Geography, University of Oslo, P.O. Box 1042, Blindern, N-0316 Oslo, Norway. E-mail:
[email protected]
Introduction Solifluction has been given attention by researchers for more than a century. Numerous studies exist that demonstrate down-slope displacements of several centimetres per year on solifluction lobes, terraces and sheets (e.g. Washburn 1967, Benedict 1970, Smith 1987). Recently, laboratory experiments (e.g. Higashi & Corte 1971, Harris et al. 1993, Harris et al. 1995) have provided more detailed information on the processes involved in these displacements. The laboratory offers perfect control of physical parameters and experimental boundary conditions, so both displacements and their controlling factors can be monitored continuously. In the field situation, this is much more challenging, and special problems are involved in measuring the small displacements involved in solifluction continuously and in three dimensions. Since Everett (1966) first introduced the use of displacement transducers for studies of solifluction, some attempts have been made, using sensor technology, to measure displacements continuously relative to some reference frame (Lewkowicz 1992, Matsuoka 1994). The main disadvantage with this method is the need for an absolutely stable frame. In some situations this may be hard to accomplish as a result of the soil creep itself, the possibility of frost heaving and thaw consolidation of the frame, and the effects of snow pressure on top of the frame. A fundamentally different approach is to employ surveying methods for continuous displacement measurements. The use of differential carrier-phase GPS (DGPS) offers the potential for gaining high accuracy in point positioning. This allows the three-dimensional movement of a target point to be monitored through time. In this research note, we present the results from a pilot project testing the applicability of continuous DGPS measurements for solifluction studies.
The study site A variety of soil creep features (ploughing boulders, solifluction lobes, terraces and sheets) is present on the slopes of Jomfrunut mountain at Finse (UTM 185198). One large solifluction lobe situated close to a telecommunication hut (where 220V power is available for battery recharging, computer storage, etc.) was chosen for this study (Fig. 1). The solifluction lobe is developed beneath a semi-perennial snow bank at an altitude between 1350 and 1370 m a.s.l. The lobe is c. 50 m long, 20 m wide and has a frontal riser of
c. 1 m. The surface slope of the lobe is 188. As is commonly found on solifluction lobes, displacements are largest in the upper central part of the lobe, in this instance immediately beneath the snow-bank. A thick snow cover develops early due to wind drift, and the seasonal frozen layer is therefore only a few decimetres deep.
Methodology The GPS system determines positions by measuring distances between one or more receiver antennas and satellites of known position. For surveying purposes, the use of differential carrier-phase measurements enables accuracies at a sub-centimetre level. The principle of differential measurements is to let the GPS receiver at the target point track the same satellites (four or more) as a GPS receiver on a benchmark of known position in the vicinity. Post-processing of the recorded data, or a communication link between the receivers with near real-time processing in the target receiver, provide the target position. Carrier-phase measurements are only possible with specially designed receivers, several times more expensive than the common code-based receivers for navigation use. For our study, two Ashtech Z-X II dual frequency receivers were used. The target points on the solifluction lobe were bolts with internal threads which were drilled into stones on the lobe surface. One of two benchmarks established on bedrock in front of the lobe was used for the base (known position) receiver. The benchmarks are concrete pillars with a screw cemented onto the top to enable the attachment of surveying instruments directly onto the benchmark, thereby eliminating centring errors. For the continuous measurements, one target point in the high velocity zone beneath the snow bank was chosen. Recording was started when this target was still snow covered, and some snow had to be removed to install the antenna. The antenna was attached to the target point with the aid of an ordinary tribrach mounted on a screw that fitted the internal threads of the target. The GPS receiver was placed in a box next to the antenna along with batteries. The system was set to log at 10-second sampling intervals. Because of storage limitations, data were downloaded once a day. Batteries also had to be replaced daily. The thaw depth was measured by probing with an iron rod around the boulder. The GPS measurements were processed using the software Win PRISM version 2.1 (Ashtech). Two different approaches were tried. First, the measurement series for some random days were split into approximately 2-
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Fig. 2. The displacement of the target in the horizontal plane on some sample days, when calculated for 2-hour measurement periods.
period is 4.5 cm in the horizontal plane. The three-dimensional displacement of this series is presented in Fig. 4, and shows that consolidation is a dominant part of total displacement (vertical displacement was 12 cm during the measurement period). The development of the thaw line and the vertical displacement of the target are plotted in Fig. 5.
Discussion Methodology Fig. 1. Key map showing the location of Finse in the central mountains of southern Norway, and a picture of the investigated solifluction lobe. White arrow points to target point position. The frames beneath the arrow are part of an ongoing field experiment.
hour intervals, and mean point position through this time period was calculated. As a second approach, the measurements were split into approximately 12-hour intervals, and mean point position for this interval was calculated.
The GPS method used in this study is capable of resolving solifluction displacement at a high temporal resolution. The method may also be applied to slow mass-wasting features such as rock glaciers or continuous landslides. Although the RMS error was below 1 mm, some apparently random movements back up-slope, and even heaving, were recorded. Some of these movements may be ‘real’, due to tilting of the target boulder during consolidation. However, the influence of signal multipath and possibly also GPS signal distortions due to the telecommunications equipment at the site are also probable sources of errors. Centring errors on the target point due to removing and replacing the antenna cannot be eliminated as another source of error.
Results The measurements started on 19.06.98. The target boulder was embedded in snow and no significant melting of the soil from the surface had started. After three days, the snow bank had withdrawn to the up-slope side of the target boulder, and thaw consolidation began. Frozen ground was encountered with the iron rod until 08.07.97, showing a maximum depth to frozen ground during thaw of 35 cm. The measurement programme was stopped 13.07.98, so that, in total, displacements over a period of 25 days were recorded. The displacement of the target point for the 2-hour intervals is presented in Fig. 2. During one day, positions in x, y and z varied as much as 1.1 cm (x, y) and 1.9 cm (z) without a marked trend, resulting in much apparent back and forth movement. This variability is much larger than that expected from the estimated co-ordinate standard deviations, which are generally less than 2 mm. The reason for this variability is unknown. For the 12-hour intervals, the estimated standard deviations of the resulting co-ordinates were usually below 1 mm in all directions. The change in position of the target points in the horizontal plane is shown in Fig. 3. There are still some back and forth displacements, but the down-slope trend of displacement is nevertheless fairly obvious. Total down-slope displacement during the measurement
Fig. 3. The displacement of the target in the horizontal plane calculated for approximately 12-hour measurement periods.
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Fig. 5. Thaw consolidation and thaw depth development during the measurement period.
Fig. 4. The three-dimensional displacement of the target during the whole measurement period based on the 12-hour measurement series.
Although our system had to be maintained on a daily basis, due to data storage and battery limitations, it is technically possible to set up a corresponding system that would require much less supervision. Solifluction could then be monitored semi-continuously during the whole melting season. Nevertheless, some limitations to the system are obvious. Firstly, the winter period is impossible to monitor. Although the target position may be measured before and during frost heave in autumn, and then again immediately before snow melt, displacements may take place as a response to thawing from underneath the frozen layer in environments where frost penetration is seriously limited by an early developing, thick snow cover. Secondly, a stable positioning of the GPS antenna requires the use of surface stones of a certain size as target points. Even in the case where there are no signs of ‘ploughing’ by these stones, with regard to timing and magnitude of frost heave and thaw consolidation they may have a behaviour that is slightly different from that of the soil elsewhere. Thirdly, only surface displacements can be monitored. The measured variability within days (Fig. 2) and in part between days (Fig. 3) is not fully explained by the estimated errors in point positioning. Even taking into consideration that the estimated standard deviation in GPS processing tends to be too optimistic, the variability cannot be explained.
Implications for the processes occurring in solifluction The displacements occurring in solifluction are generally regarded as being a
result of a combination of frost creep and gelifluction (e.g. Washburn 1967). Frost creep is the displacement caused by ground heave normal to slope and subsequent ground settlement along the vertical, while gelifluction is the slow plastic or viscous straining of a thawing soil. Such straining is due to high pore-water pressures and seepage forces (although not high enough to cause rapid failure) developed during thaw consolidation (Harris et al. 1995). As our measurements do not cover the frost heave period, the frost creep component is not recorded and the detected displacement is therefore entirely due to gelifluction. Gelifluction relies on the development of segregation ice (ice lenses) in the ground during ground freezing. Judging from the total consolidation, the frost heave of the target point in our investigation was c. 10 cm in the preceding winter. Considering the shallow ground freezing at this site, the ice content of the frozen ground must have been high. Low freezing rates, in this case caused by low temperature gradients due to the snow cover, facilitate the time necessary for water migration towards the freezing fringe and thus greater frost heave (e.g. Harris 1993). Further, at this site, excavations in early spring have shown that the water table is situated at the interface between unfrozen and frozen ground, so that availability of water is probably not a restriction on frost heave. During thaw consolidation, the frozen substrate is more or less impermeable to the water being released from thawing of the ice lenses from above. In soils with limited hydraulic conductivity, more water is likely to be released during melting than can be drained away from the soil, leading to a rise in pore-water pressure and a pore-water pressure gradient directed towards the surface (e.g. Harris et al. 1995). The continuous displacements recorded here are similar to the twodimensional displacement vectors measured in laboratory studies (e.g. Harris et al. 1997), and they demonstrate the close association between thaw consolidation and down-slope movement. However, there is a significant difference at the end of ground thawing. Harris & Davies (2000) demonstrated that, in the laboratory, thawing of the deeper part of the soil leads to consolidation only, resulting in a retrograde movement in the horizontal plane similar to that demonstrated in the field by Washburn (1967). In our study, both down-slope movement and soil consolidation was recorded even after the soil had thawed completely. The soil matrix between ice lenses consolidates during the freezing process (Morgenstern & Nixon 1971), and is prone to swelling during thaw (Harris & Davies 2000). Further consolidation of the soil matrix following thaw is therefore not likely, so that the recorded consolidation must be caused by a continued dissipation of excess pore water. These results imply that gelifluction in some cases may proceed for some time after the ground has thawed.
Conclusions This study has shown that use of differential carrier-phase GPS opens the possibility for obtaining continuous records of solifluction movement, or, more generally, of slow mass movements. The results presented here demonstrate the close association between thaw consolidation and gelifluction during spring thaw, and point to the possibility for gelifluction also after the ground has thawed.
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Acknowledgements. – Maintenance of the GPS equipment and downloading of data in the field were performed by Knut Nordahl and Arnleif Bjørstad, former students at the Department of Physical Geography, University of Oslo. The GPS receivers were kindly made available to us by the Norwegian Polar Institute. NORKRING provided access to their telecommunication hut at the site. The authors extend their gratitude to these persons and institutions. Manuscript submitted 15 June 2000; accepted 24 July 2000
References Benedict, J. B. 1970. Downslope soil movement in a Colorado Alpine region: rates, processes and climatic significance. Arctic and Alpine Research 2, 165–226. Everett, K. B. 1966. Slope movement and related phenomena. Wilimovsky, N. J. & Wolfe, J. N. (eds) Environment of the Cape Thompson Region, Alaska, 175–220. U.S. Atomic Energy Commission, Division of Technical Information. Harris, C. 1993. The role of climate and soil properties in periglacial solifluction: evidence from laboratory simulation experiments. Frenzel, B., Matthews, J. A. & Gla¨ser, B. (eds) Solifluction and Climatic Variation in the Holocene. Special Issue: ESF Project European Palaeoclimate and Man 6. Gustav Fischer Verlag, Stuttgart. Harris, C. & Davies, M. C. R. 2000. Gelifluction: Observations from large-
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scale laboratory simulations. Arctic, Antarctic and Alpine Research 32, 202–207. Harris, C., Gallop, M. & Coutard, J.-P. 1993. Physical modelling of gelifluction and frost creep – some results of a large-scale laboratory experiment. Earth Surface Processes and Landforms 18, 383–398. Harris, C., Davies, M. C. R. & Coutard, J.-P. 1995. Laboratory simulation of periglacial solifluction: significance of pore-water pressure, moisture contents and undrained shear strength during thawing. Permafrost and Periglacial Processes 6, 293–312. Harris, C., Davies, M. C. R. & Coutard, J.-P. 1997. Rates and processes of periglacial solifluction: an experimental approach. Earth Surface Processes and Landforms 22, 849–868. Higashi, A. & Corte, A. E. 1971. Solifluction: a model experiment. Science 171, 480–482. Lewkowicz, A. G. 1992. A solifluction meter for permafrost sites. Permafrost and Periglacial Processes 3, 11–18. Matsuoka, N. 1994. Continuous recording of frost heave and creep on a Japanese Alpine slope. Arctic and Alpine Research 26, 245–254. Morgenstern, N. R. & Nixon, J. F. 1971. One-dimensional consolidation of thawing soils. Canadian Geotechnical Journal 8, 558–565. Smith, D. J. 1987. Solifluction in the southern Canadian Rockies. The Canadian Geographer 31, 309–318. Washburn, A. L. 1967. Instrumental observations of mass-wasting in the Mesters Vig District, Northeast Greenland. Meddelser om Grønland 176, 1–303.
