Ninth International Conference on Permafrost ... - IARC Research

Contribution of Terrestrial Laser Scanning for Studying the Creep ofMountain PermafrostFrançois RiffGeographic Institute of Lausanne UniversityChristophe LambielGeographic Institute of Lausanne UniversityThierry OppikoferInstitute of Geomatics and Risk Analysis, University of LausanneIntroductionUp to now, differential GPS and triangulation usingtheodolite were the most used terrestrial techniques formeasuring the creep of permanently frozen sediment bodies.Recently, the studies of Bauer et al. (2003) and Bodin (2007)demonstrated that terrestrial laser scanning (TLS) offersgood opportunities to provide precise measurements onmountain permafrost creep.This abstract presents positive results of TLS measurementscarried out on two permafrost-related creeping landformslocated in the Swiss Alps: the tiny Lac des Vaux rockglacier, located in a wider complex slope, and on the Coldes Gentianes push moraine (cf. Lambiel 2006). First, thetwo principal stages of the methodology (data acquisitionand treatment) will be presented.Data AcquisitionTLS is based on the contactless and reflectorless acquisitionof a XYZ point cloud of the topography using a time-of-flightprinciple. The TLS data acquisition stage is relatively quickand easy. However, two recurrent problems were identifiedusing the Optech ILRIS 3D in alpine periglacial landforms:• The localization of ideal viewpoints is crucial in orderto avoid occlusion and too far distances to the object. At adistance shorter than 100 m and with a point cloud resolutionbetter than 5 cm, we were able to identify every object in aunique way. This allows the observation of the movement ofthe matrix and the bigger blocks. Beyond a distance of about300 m and a resolution higher than 10 cm, this advantage islost, but measurement of mass movement is still possible.• No points can be obtained from snow-coveredsurfaces, since snow does not reflect the TLS signal.mass loss or depletion.In addition, the following operations can be carried out:• Precise volume calculation by creation of parallelcross-sections or use of 3D point clouds.• Creation of movement vectors by point pairidentification. However, using real time kinematics GPS toobtain this kind of information is more practical, faster andmore precise than the TLS, but provides only information onselected monitoring points.• Easy integration of the results in a GIS, since allinformation is georeferenced.ResultsLac des Vaux rock glacierIn the upper part of the rock glacier, a distinct blue area (1)indicates a loss of elevation of about 35 cm (Fig. 1). Directlybelow (2), the light green to yellow colors correspond topositive values, which indicate a general slight increase(5–15 cm) in elevation. Several big blocks even show apositive displacement of about 50 cm. In the lower part(3), the increase in elevation is a bit lower. The GPS dataindicates slightly larger displacements.According to geomorphologic evidence, the successiveloss (1) and gain (2-3) in elevation may be the result ofData TreatmentThe longest stage in the TLS methodology is data treatmentwith the Polyworks software. The raw TLS point cloudsneed to be cleaned from unwanted objects, unified to a singlepoint cloud and finally georeferenced using GPS points.Sequential TLS point clouds, or TLS time-series, enablethe calculation of relative differences, which are related toslope movements (e.g., Oppikofer et al. in review). Positivedifferences (yellow to red colors in Figs. 1, 2) are related toadvances or elevation increases of the creep, while negativedifferences (blue to pink colors in Figs. 1, 2) are signs ofFigure 1. Topographic evolution of the Lac des Vaux rock glacierbetween 19 July and 16 October 2007, represented with a 50 cmscale. Top left are represented the D-GPS movements extrapolatedbetween the same dates.255