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: ivar.berthling@geografi.uio.no 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 experi- ments (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 measure- ments 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 telecommu- nication 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 measure- ments 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- Norsk geogr. Tidsskr. Vol. 54, 182–185. Oslo. ISSN 0029-1951 RESEARCH NOTES AND REVIEWS – NOTISARTIKLER OG ANMELDELSER