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FIRST INTERNATIONAL SYMPOSIUM ON GEOSCIENCE AND REMOTE SENSING. PG. XXXX.XXX. VALDIVIA, CHILE, JUNE 15-16, 2017 1 Inferring Subglacial Topography of the Emerald Icefalls (King George Island, Antarctica) from Ice Surface Terrestrial Laser Scanning Michal Petlicki Abstract—Information about ice thickness and subglacial to- pography is critical for ice dynamics modelling and, consequently, for better understanding of current response of glaciers to climatic forcing. While there is a great progress in measurements of glacial surface, ice thickness measurements have limited coverage and are usually heavily interpolated. Still, there are some areas where typical remote sensing techniques fail because of adverse climatic conditions, mainly persistent cloud cover, and very steep topography. A perfect example of such feature are icefalls dominating landscapes of the maritime Antarctic. In order to close that gap, a simple method based on repeated terrestrial laser scanning of the ice surface is proposed. Based on such measurements, inverse Shallow Ice Approximation ice flow modelling was applied to infer the subglacial topography Fig. 1. Location of the study site: a) Antarctica b) King George Island of the Emerald Icefalls, King George Island. The icefalls were (WGS1984) c) Emerald Icefalls in Ezcurra Inlet, King George Island. surveyed twice within 8 day period, allowing to perform feature Basemap is a Sentinel-2 image from 23 Mar 2017 (WGS1984, UTM zone tracking analysis and, hence, to derive surface ice flow velocity 21S). field. The measured flow velocities are spatially highly variable, suggesting the existence of four separated trunks. The estimated ice depths are low with the mean value of 30.3±8.3 m. Therefore, are also frequently limited by the steep topography typical for as suggested by previous studies, the overall ice flux of Emerald this area. Icefalls is low despite relatively high surface ice flow velocities. Speckle intensity tracking of satellite SAR imagery was Index Terms—Antarctica, cryosphere, glaciology, ice thickness, successfully applied to measure the dynamics of King George LiDAR, Terrestrial Laser Scanning Island [10] and Livingston Island [11] ice caps. However, the spatial window used was too large (1.2 km) to properly mea- I. INTRODUCTION sure ice speed of both smaller and slower glaciers, especially small and steep icefalls terminating in ocean, a set up that is HERE is a large uncertainty concerning the dynamic very common in the region. response of the outlet glaciers in the Antarctic periphery T On the other hand, recently there was a significant progress to the observed climate change [1], [2]. Still, despite large in the development of close remote sensing techniques, such progress in ground penetrating radar mapping of Antarctica, as Structure-from-Motion [12], terrestrial radar interferometry the spatial coverage of ice thickness measurements remains [13] or terrestrial laser scanning (TLS, [14], [15]) that allows very limited. Still the most common method to estimate ice for quick acquisition of very precise glacial digital elevation volume and thickness is area-volume scaling [3], a highly models. Hence, it provides an opportunity to perform ice questionable and limited method [4], [5]. Recently, a large in- thickness estimation based purely on high-resolution digital terest has been on more physically-based ice thickness models elevation model analysis. [6], [7] that allow for more robust estimation of ice thickness The main goal of this work was to preliminary map bedrock than purely statistical models [5]. Such models usually rely on topography under a highly crevassed, inaccessible glacial ter- simple inverse modelling of ice dynamics and hence require rain that is unfavourable for detailed ground penetrating radar information on ice surface topography and velocity. surveys and, consequently, to give an insight into subglacial In the Antarctic Peninsula (AP) region, the ground based conditions and possible future deglaciation pattern. measurements of ice dynamics and geometry are scarce [8], [9] and remote sensing techniques frequently fail to provide II. STUDY SITE comprehensive data because of adverse climatic conditions that e.g. limit the availability of optical imagery. While radar King George Island is located in the region of northern satellite measurements have proven useful in such case they AP and is the largest island of the South Shetland Islands archipelago. Large part of the island is covered by an extensive M. Petlicki is now with the Centro de Estudios Cientificos (CECs) Valdivia, ice cap [16] that is drained to the ocean by a series of icefalls Chile, e-mail: [email protected]. He was with the Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland. and several fast-flowing outlet glaciers [10]. In the second Manuscript received ; revised 2017. half of 20th century its outlet glaciers fronts have witnessed a FIRST INTERNATIONAL SYMPOSIUM ON GEOSCIENCE AND REMOTE SENSING. PG. XXXX.XXX. VALDIVIA, CHILE, JUNE 15-16, 2017 2 to 0.5×0.5m gridded DEMs with CloudCompare software, one for each measurement date. Density of interpolated points per DEM pixel was exported as an additional scalar field (Figure 2). Subsequent processing of the resulting DEMs was made in QQIS software; surface slope was calculated from the DEM using a 100x100m moving window. B. Surface ice velocity Surface ice velocity fields of the Emerald Icefalls were calculated from subsequent DEMs by tracking surface features (mainly crevasses) with ImGRAFT toolbox [26]. In order to improve the performance of feature tracking, point density Fig. 2. TLS surveys of the Emerald Icefalls: acquired point density. Basemap rasters were used. Normalized cross-correlation algorithm is a Sentinel-2 image from 23 Mar 2017, coordinates in WGS1984, UTM zone was used for template matching [26]. The template window 21S. had 101×101 pixels (50×50m) and the search window had 121×121 pixels (60×60m). severe retreat [17]. While most of the interest of the scientific C. Ice thickness modelling community has been on the larger glaciers of the ice cap [10], the retreat of the smaller glacier fronts of Ezcurra Inlet Ice thickness was calculated with inverse SIA modelling revealed new rock outcrops for geological mapping giving [5] using TLS–derived surface slope and ice velocity fields. insight to the climate conditions preceding the development According to Glen’s flow law, ice surface velocity can be of the Antarctic Ice Sheet [18], [19]. Unfortunately, ground calculated as: penetrating radar measurements of the ice thickness were 2A n n+1 Ui = (ρig tan α) H (1) limited to the main icefields of the King George Island ice n +1 cap [20], [21] and the bedrock topography of the smaller where Ui is the surface ice velocity, A and n are constants, glaciers and icefalls remains unknown [22], preventing better ρi is ice density, g - acceleration due to gravity, α - surface understanding of the future glacial changes in this region. slope and H - ice thickness. This equation can be inverted in order to find H when α ETHODS III. M and Ui are known: Ice surface topography of the Emerald Icefalls was repeat- 2 edly surveyed with a long-range Riegl VZ-6000 terrestrial n+1 Ui H = n (2) laser scanner over a period of 8 days during the austral A (ρig tan(α)) summer 2015/16. Subsequently, the ice flow velocity field was Previous ice dynamics modelling studies confirmed that calculated by tracking recognizable features, like crevasses, the ice of King George Island ice cap is temperate below between TLS-derived Digital Elevation Models (DEMs). Next, 400 m a.s.l [27], thus the Arrhenius enhancement factor A theoretical ice thickness was modelled with inverse Shallow is considered constant with the value typical for temperate ice Ice Approximation (SIA) flow modelling using ice flow ve- −15 −3 − (3.2·10 kPa s ). Glen’s factor n is assumed to be equal locities and driving stress derived from measured ice surface to 3. geometry. Finally, bedrock topography was calculated by spatially differencing surface topography and calculated ice D. Subglacial topography thickness. Subglacial topography is calculated by subtracting the mod- elled ice thickness from the surface topography obtained with A. Terrestrial Laser Scanning TLS surveys. Ice surface of the Emerald Icefalls was surveyed on 18 and IV. RESULTS 26 Jan 2016 with a long range Riegl VZ-6000 terrestrial laser scanner located on Pond Hill and overlooking the measured A. Ice surface topography glacier from the south (Figure 1). This instrument is consid- Two high-resolution DEMs of Emerald Icefalls were ac- ered a top-tier for close remote sensing of snow and ice and quired on 18 and 26 Jan 2016. Due to shadows cast by soon after release have found application in studies of glacier undulating ice surface, some higher reversed-slope areas were dynamics [23] and mass balance [14], [22], [24]. Previously, not measured (Figure 2). The acquired point density was much TLS based studies of glaciers suffered from the low effective higher on vertical surface features that were normal to the range of instruments [15], [25], impeding detailed analysis of laser beam, such as frontal parts of seracs, DEM derived glacial processes. surface slope shows high spatial variability with two main Acquired point clouds were co-registered and georeferenced altitudinal zones of steep topography followed by more plain with RiSCAN PRO software using differential GNSS mea- areas (Figure 3). The areas of high surface slope coincide surements of the scanner positions (see [22] for more details). with severe crevassing, indicating high strain rates and flow Next, they were exported to a LAS format and interpolated velocities. FIRST INTERNATIONAL SYMPOSIUM ON GEOSCIENCE AND REMOTE SENSING. PG. XXXX.XXX. VALDIVIA, CHILE, JUNE 15-16, 2017 3 Fig. 3. Surface slope of the Emerald Icefalls. Basemap is a Sentinel-2 image Fig. 5. Modelled ice thickness of the Emerald Icefalls in Jan 2016.

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