Modeling of Subsurface Scattering from Ice Sheets for Pol-InSAR Applications

Remote sensing is a fundamental tool to measure the dynamics of ice sheets and provides valuable information for ice sheet projections under a changing climate. There is, however, the potential to further reduce the uncertainties in these projections by developing innovative remote sensing methods. One of these remote sensing techniques, the polarimetric synthetic aperture radar interferometry (Pol-InSAR), is known since decades to have the potential to assess the geophysical properties below the surface of ice sheets, because of the penetration of microwave signals into dry snow, firn, and ice. Despite this, only very few studies have addressed this topic and the development of robust Pol-InSAR applications is at an early stage. Two potential Pol-InSAR applications are identified as the motivation for this thesis. First, the estimation and compensation of the penetration bias in digital elevation models derived with SAR interferometry. This bias can lead to errors of several meters or even tens of meters in surface elevation measurements. Second, the estimation of geophysical properties of the subsurface of glaciers and ice sheets using Pol-InSAR techniques. There is indeed potential to derive information about melt-refreeze processes within the firn, which are related to density and affect the mass balance. Such Pol-InSAR applications can be a valuable information source with the potential for monthly ice sheet wide coverage and high spatial resolution provided by the next generation of SAR satellites. However, the required models to link the Pol-InSAR measurements to the subsurface properties are not yet established. The aim of this thesis is to improve the modeling of the vertical backscattering distribution in the subsurface of ice sheets and its effect on polarimetric interferometric SAR measurements at different frequencies. In order to achieve this, polarimetric interferometric multi-baseline SAR data at different frequencies and from two different test sites on the Greenland ice sheet are investigated. This thesis contributes with three concepts to a better understanding and to a more accurate modeling of the vertical backscattering distribution in the subsurface of ice sheets. First, the integration of scattering from distinct subsurface layers. These are formed by refrozen melt water in the upper percolation zone and cause an interesting coherence undulation pattern, which cannot be explained with previously existing models. This represents a first link between Pol-InSAR data and geophysical subsurface properties. The second step is the improved modeling of the general vertical backscattering distribution of the subsurface volume. The advantages of more flexible volume models are demonstrated, but interestingly, the simple modification of a previously existing model with a vertical shift parameter lead to the best agreement between model and data. The third contribution is the model based compensation of the penetration bias, which is experimentally validated. At the investigated test sites, it becomes evident that the model based estimates of the surface elevations are more accurate than the interferometric phase center locations, which are conventionally used to derive surface elevations of ice sheets. This thesis therefore improves the state of the art of subsurface scattering modeling for Pol-InSAR applications, demonstrates the model-based penetration bias compensation, and makes a further research step towards the retrieval of geophysical subsurface information with Pol-InSAR

Feasibility of GNSS-R ice sheet altimetry in Greenland using TDS-1

Radar altimetry provides valuable measurements to characterize the state and the evolution of the ice sheet cover of Antartica and Greenland. Global Navigation Satellite System Reflectometry (GNSS-R) has the potential to complement the dedicated radar altimeters, increasing the temporal and spatial resolution of the measurements. Here we perform a study of the Greenland ice sheet using data obtained by the GNSS-R instrument aboard the British TechDemoSat-1 (TDS-1) satellite mission. TDS-1 was primarily designed to provide sea state information such as sea surface roughness or wind, but not altimetric products. The data have been analyzed with altimetric methodologies, already tested in aircraft based experiments, to extract signal delay observables to be used to infer properties of the Greenland ice sheet cover. The penetration depth of the GNSS signals into ice has also been considered. The large scale topographic signal obtained is consistent with the one obtained with ICEsat GLAS sensor, with differences likely to be related to L-band signal penetration into the ice and the along-track variations in structure and morphology of the firn and ice volumes The main conclusion derived from this work is that GNSS-R also provides potentially valuable measurements of the ice sheet cover. Thus, this methodology has the potential to complement our understanding of the ice firn and its evolution.Peer ReviewedPostprint (published version

A new high-resolution elevation model of Greenland derived from TanDEM-X

In this paper we present for the first time the new digital elevation model (DEM) for Greenland produced by the TanDEM-X (TerraSAR add-on for digital elevation measurement) mission. The new, full coverage DEM of Greenland has a resolution of 0.4 arc seconds corresponding to 12 m. It is composed of more than 7.000 interferometric synthetic aperture radar (InSAR) DEM scenes. X- Band SAR penetrates the snow and ice pack by several meters depending on the structures within the snow, the acquisition parameters, and the dielectricity constant of the medium. Hence, the resulting SAR measurements do not represent the surface but the elevation of the mean phase center of the backscattered signal. Special adaptations on the nominal TanDEM-X DEM generation are conducted to maintain these characteristics and not to raise or even deform the DEM to surface reference data. For the block adjustment, only on the outer coastal regions ICESat (Ice, Cloud, and land Elevation Satellite) elevations as ground control points (GCPs) are used where mostly rock and surface scattering predominates. Comparisons with ICESat data and snow facies are performed. In the inner ice and snow pack, the final X-Band InSAR DEM of Greenland lies up to 10 m below the ICESat measurements. At the outer coastal regions it corresponds well with the GCPs. The resulting DEM is outstanding due to its resolution, accuracy and full coverage. It provides a high resolution dataset as basis for research on climate change in the arctic

Radarinterferometrische Untersuchungen mit ERS-1/2 auf der Antarktischen Halbinsel

Synthetic Aperture Radar Interferometry (InSAR) ermöglicht effiziente Untersuchungen von polaren Regionen, mit denen ein Beitrag zur Bestimmung von Massenbilanzen geleistet werden kann. Dieser Parameter hat entscheidenden Einfluss auf den globalen Meeresspiegel. Die Antarktische Halbinsel ist ein Raum, der besonders empfindlich auf Änderungen der klimatischen Bedingungen reagiert und für den von einem positiven Beitrag zum Meeresspiegelanstieg ausgegangen wird. Zur Quantifizierung dessen, sind präzise Kenntnisse des Verhaltens von Eisströmen, Gletschern und Schelfeisen erforderlich. Die eingesetzte Methodik kann dafür glaziologische Schlüsselparameter wie Fließgeschwindigkeiten und Höheninformationen bestimmen. Diese werden in der vorliegenden Arbeit für zwei Untersuchungsgebiete – King George Island, mit einer auf Klimaänderungen empfindlich reagierenden temperierten Eiskappe und Wilkins Schelfeis, das mit am nördlichsten verbleibende Schelfeis auf der Westseite der Antarktischen Halbinsel, das bereits teilweise zerfällt – abgeleitet. Aufgrund der sich schnell ändernden Oberflächenbedingungen in diesen Regionen, können nur Aufnahmen in zeitlich sehr kurzen Abständen verwendet werden. Daher kommen ausschließlich Radardaten der European Remote Sensing Satellites (ERS) aus bestimmten Missions Phasen, die dies erfüllen, zum Einsatz. Dadurch ermöglicht sich eine Bestimmung von Ergebnissen, die ausschließlich aus Mitte der 1990’er Jahre stammen. Nachdem in dieser Arbeit die Grundlagen der differentiellen Radarinterferometrie erläutert werden, wird die komplette Prozessierungskette der Verarbeitung der Radardaten vorgelegt. Zudem wird auf die Besonderheiten, die sich speziell bei diesen Untersuchungen ergeben, eingegangen. Die im Folgenden genannten, wesentlichen Ergebnisse, erlauben Aussagen mit hoher Genauigkeit, hoher räumlicher Auflösung und gleichzeitig flächenhafter Abdeckung für die Untersuchungsgebiete. Für King George Island wird ein Geschwindigkeitsfeld bestimmt, das die gesamte Insel abdeckt und Geschwindigkeiten von bis zu 120 m/a im Bereich von Auslassgletschern aufweist. Es wird zusätzlich für ein Teilgebiet der Eiskappe ein Höhenmodell mit einer vertikalen Genauigkeit von ± 18 m erstellt. Für das Untersuchungsgebiet Wilkins Schelfeis wird zum ersten Mal ein Geschwindigkeitsfeld, das einen Teil des Schelfeises abdeckt, bestimmt. Dies weist eine Genauigkeit von ± 10 m/a auf und enthüllt differenzierte Fließstrukturen in räumlich hoher Auflösung von 50 m. Es werden erstmalig Geschwindigkeiten für einen Teil der Zuflussgletscher zum Schelfeis mit gleicher Genauigkeit hergeleitet. Die Geschwindigkeiten erreichen dabei Werte von bis zu 265 m/a. Für das Schelfeis werden größtenteils Werte zwischen 35 m/a – 105 m/a gemessen. Für einen Teil der Zuflussgletscher auf dem Gebiet von Alexander Island wird ein Höhenmodell mit 50 m Auflösung und einer Genauigkeit von ± 18 m erstellt. Dies bedeutet eine enorme Verbesserung zu den bisher für diesen Raum verfügbaren Höhenmodellen, sowohl was die vertikale Genauigkeit als auch die räumliche Auflösung betrifft. Zusätzlich werden Untersuchungen zum Einsatz von Tidenmodellen zu InSAR Zwecken angestellt und die Schwimmgrenze des Schelfeises in einem Teilgebiet neu bestimmt. Die aus der Arbeit gewonnenen Ergebnisse sind zum Teil bereits in geophysikalische Modelle (King George Island) eingeflossen bzw. werden in solchen zum Einsatz kommen (Wilkins Schelfeis). Des Weiteren stellen sie eine Basis für Vergleiche mit Daten aus anderen Zeitepochen, wenn verfügbar, zur möglichen Bestimmung von Änderungen im Fließverhalten dar. Damit würde eine Aussage über Reaktionen auf Klimaänderungen und einen potentiellen Beitrag zum Meeresspiegelanstieg möglich

Wenchuan Earthquake Deformation 3D Modelling based on ALOS/PALSAR Data

A devastating earthquake of magnitude Mw 7.9 occurred in Wenchuan area of Sichuan Province, China on 12th May 2008 and caused great casualties and economic damage. This study is aiming to investigate the faulting geometry and motion of the major seismic faults in Longmenshan fault thrust belt that caused this earthquake, based on the surface rupture displacement data measured using differential interferometric synthetic aperture radar (DInSAR) and SAR amplitude pixel-offset techniques. The cross-event Japanese ALOS PALSAR data have been used for this study. First, the methodology for recovering the missing data in the decoherence zone of the DInSAR line-of-sight (LOS) surface motion maps was developed. In the area along the seismic fault zone, the coherence between pre- and post-event SAR images is completely lost because of the earthquake induced violent and chaotic destruction on the land surface and as the result, no surface displacement can be measured using the DInSAR technique. An Adaptive Local Kriging (ALK) technique has then been developed to retrieve the interferometric fringe patterns in the decoherence zone. The novel ALK operating in a multi-step approach enables to retrieve and interpolate the values with high fidelity to the original dataset. Thus a map of continuous radar LOS displacement was generated. Then, the horizontal displacement motion maps in ground range and azimuth direction were derived from cross-event SAR amplitude image pairs using advanced sub-pixel offset technique, Phase Correlation based Image Analysis System (PCIAS). Though the ground range pixel-offset is proportional to the LOS displacement, the azimuth pixel-offset data provide extra information of the coseismic motion. Thus the horizontal displacement vector field can be obtained in order to constrain the faulting motions in key areas. Finally, with the constraints of the ALK refined DInSAR data and the horizontal displacement data together with the published seismic focal mechanism solutions, seismic reflection profiles and field observations, forward modelling was proceeded using the Poly3D software to decide the most likely faulting geometry based on the optimal matching between the simulated and the measured surface displacement. In the much disputed Beichuan – Pengguan area, the best fit is achieved only when the Pengguan fault is set as the main fault that intercept the Yingxiu-Beichuan fault at a depth of about 13 kilometres. This geometric relationship between the two faults and the distribution of slip is compatible with them being two adjacent splay faults on a propagating thrust

Arctic sea ice trafficability: new strategies for a changing icescape

Thesis (Ph.D.) University of Alaska Fairbanks, 2017Sea ice is an important part of the Arctic social-environmental system, in part because it provides a platform for human transportation and for marine flora and fauna that use the ice as a habitat. Sea ice loss projected for coming decades is expected to change ice conditions throughout the Arctic, but little is known about the nature and extent of anticipated changes and in particular potential implications for over-ice travel and ice use as a platform. This question has been addressed here through an extensive effort to link sea ice use and key geophysical properties of sea ice, drawing upon extensive field surveys around on-ice operations and local and Indigenous knowledge for the widely different ice uses and ice regimes of Utqiaġvik, Kotzebue, and Nome, Alaska. A set of nine parameters that constrain landfast sea ice use has been derived, including spatial extent, stability, and timing and persistence of landfast ice. This work lays the foundation for a framework to assess and monitor key ice-parameters relevant in the context of ice-use feasibility, safety, and efficiency, drawing on different remote-sensing techniques. The framework outlines the steps necessary to further evaluate relevant parameters in the context of user objectives and key stakeholder needs for a given ice regime and ice use scenario. I have utilized this framework in case studies for three different ice regimes, where I find uses to be constrained by ice thickness, roughness, and fracture potential and develop assessment strategies with accuracy at the relevant spatial scales. In response to the widely reported importance of high-confidence ice thickness measurements, I have developed a new strategy to estimate appropriate thickness compensation factors. Compensation factors have the potential to reduce risk of misrepresenting areas of thin ice when using point-based in-situ assessment methods along a particular route. This approach was tested on an ice road near Kotzebue, Alaska, where substantial thickness variability results in the need to raise thickness thresholds by 50%. If sea ice is thick enough for safe travel, then the efficiency of travel is relevant and is influenced by the roughness of the ice surface. Here, I develop a technique to derive trafficability measures from ice roughness using polarimetric and interferometric synthetic aperture radar (SAR). Validated using Structure-from-Motion analysis of imagery obtained from an unmanned aerial system near Utqiaġvik, Alaska, I demonstrate the ability of these SAR techniques to map both topography and roughness with potential to guide trail construction efforts towards more trafficable ice. Even when the ice is sufficiently thick to ensure safe travel, potential for fracturing can be a serious hazard through the ability of cracks to compromise load-bearing capacity. Therefore, I have created a state-of-the-art technique using interferometric SAR to assess ice stability with capability of assessing internal ice stress and potential for failure. In an analysis of ice deformation and potential hazards for the Northstar Island ice road near Prudhoe Bay on Alaska's North Slope I have identified a zone of high relative fracture intensity potential that conformed with road inspections and hazard assessments by the operator. Through this work I have investigated the intersection between ice use and geophysics, demonstrating that quantitative evaluation of a given region in the ice use assessment framework developed here can aid in tactical routing of ice trails and roads as well as help inform long-term strategic decision-making regarding the future of Arctic operations on or near sea ice

Temporal fluctuations in the motion of Arctic ice masses from satellite radar interferometry

This thesis considers the use of Interferometric Synthetic Aperture Radar (InSAR) for surveying temporal fluctuations in the velocity of glaciers in the Arctic region. The aim of this thesis is to gain a broader understanding of the manner in which the flow of both land- and marine-terminating glaciers varies over time, and to asses the ability of InSAR to resolve flow changes over timescales which provide useful information about the physical processes that control them. InSAR makes use of the electromagnetic phase difference between successive SAR images to produce interference patterns (interferograms) which contain information on the topography and motion of the Earth's surface in the direction of the radar line-of-sight. We apply established InSAR techniques (Goldstein et al., 1993) to (i) the 925 km2 LangjÖkull Ice Cap (LIC) in Iceland, which terminates on land (ii) the 8 500 km2 Flade Isblink Icecap (FIIC) in Northeast Greenland which has both land- and marine-terminating glaciers and (iii) to a 7 000 km2 land-terminating sector of the Western Greenland Ice Sheet (GrIS). It is found that these three regions exhibit velocity variations over contrasting timescales. At the LIC, we use an existing ice surface elevation model and dual-look SAR data acquired by the European Remote Sensing (ERS) satellite to estimate ice velocity (Joughin et al., 1998) during late-February in 1994. A comparison with direct velocity measurements determined by global positioning system (GPS) sensors during the summer of 2001 shows agreement (r2 = 0.86), suggesting that the LIC exhibits moderate seasonal and inter-annual variations in ice flow. At the FIIC, we difference pairs of interferograms (Kwok and Fahnestock, 1996) formed using ERS SAR data acquired between 15th August 1995 and 3rd February 1996 to estimate ice velocity on four separate days. We observe that the flow of 5 of the 8 outlet glaciers varies in latesummer compared with winter, although flow speeds vary by up to 20 % over a 10 day period in August 1995. At the GrIS, we use InSAR (Joughin et al., 1996) and ERS SAR data to reveal a detailed pattern of seasonal velocity variations, with ice speeds in latesummer up to three times greater than wintertime rates. We show that the degree of seasonal speedup is spatially variable and correlated with modeled runoff, suggesting that seasonal velocity changes are controlled by the routing of water melted at the ice sheet surface. The overall conclusion of this work is that the technique of InSAR can provide useful information on fluctuations in ice speed across a range of timescales. Although some ice masses exhibit little or no temporal flow variability, others show marked inter-annual, seasonal and even daily variations in speed. We observe variations in seasonality in ice flow over distances of ~ 10 km and over time periods of ~10 days during late-summer. With the aid of ancillary meteorological data, we are able to establish that rates of flow in western Greenland are strongly moderated by the degree of surface melting, which varies seasonally and secularly. Although the sampling of our data is insufficiently frequent and spans too brief a period for us to derive a general relationship between climate and seasonality of flow, we show that production of meltwater at the ice surface and its delivery to the ice bed play an important role in the modulation of horizontal flow speeds. We suggest that a similarly detailed investigation of other ice masses is required to reduce the uncertainty in predictions of the future Arctic land-ice contribution to sea level in a warming world

Untersuchungen zu gezeitenbedingten Höhenänderungen des subglazialen Lake Vostok, Antarktika

Lake Vostok, der größte der über 70 subglazialen Seen in der Antarktis, ist derzeit einer der Forschungsschwerpunkte der geowissenschaftlichen Polarforschung. Der See erstreckt sich unter einer 4 000 m dicken Eisschicht auf über 250 km Länge mit einer Wassertiefe von bis zu 1 000 m. Ziel der hier vorliegenden Arbeit ist die Untersuchung des Einflusses der Gezeiten auf den Wasserstand des Sees, die eine bisher nicht betrachtete Komponente in der Zirkulation im See darstellen. Auf Grund seiner Ausdehnung ist das Gezeitenpotential an verschiedenen Punkten auf dem See nicht gleich, sondern weist differentielle Unterschiede auf. Unter der Annahme, dass sich die Seeoberfläche entlang einer Äquipotentialfläche ausrichtet, ergeben sich Gleichgewichtsgezeiten des Sees mit Amplituden von bis zu 4,6 mm für die größte ganztägige Tide K1 und 1,8 mm für die größte halbtägige Tide M2. Differenzen des Luftdruckes zwischen Nord- und Südteil des Sees rufen zusätzlich einen differentiellen inversen Barometer-Effekt hervor. Der inverse Barometer-Effekt besitzt im wesentlichen die spektralen Eigenschaften eines roten Rauschens. Die Variationen erreichen bis zu +/- 20 mm. Zum messtechnischen Nachweis derartiger Höhenänderungen an der Eisoberfläche über dem See wurden drei unterschiedliche Verfahren herangezogen. Differentielle GPS-Messungen zwischen einem Punkt auf aufliegendem Eis und einem zweiten in der südlichen Seemitte bestätigen die Modellvorstellungen und zeigen sowohl mit der Luftdruckdifferenz korrelierte Höhenänderungen als auch Höhenänderungen mit ganz- und halbtägigen Perioden. Die SAR-Interferometrie als flächenhaft arbeitende Methode zur Bestimmung von Höhenänderungen lässt den räumlichen Verlauf der Deformation erkennen. Dabei zeigt sich, dass sich die Aufsetzzone auf dem etwa 50 km breiten See bis in die Seemitte ersteckt. Erdgezeitenregistrierungen, die im Jahr 1969 in der Station Vostok durchgeführt wurden, zeigen zwar Auffälligkeiten wie etwa einen stark erhöhten Luftdruckregressionskoeffzienten und einen Phasenvorlauf der K1-Tide, diese können jedoch nicht eindeutig als Resultat von Höhenänderungen der Seeoberfläche identifiziert werden. Auf Grund der Lage der Station Vostok nahe dem Ufer des Sees ist die Deformation dort schon stark gedämpft. Die zu erwartenden Effekte liegen daher unterhalb der Auflösung der damaligen Messungen. Damit sind die theoretischen Grundvorstellungen über die Reaktion des subglazialen Sees auf Gezeiten- und Luftdruckanregungen herausgearbeitet, sowie diese Effekte mit zwei unabhängigen und komplementären Messverfahren nachgewiesen.Lake Vostok, the largest of more than 70 subglacial lakes in the Antarctic, is one of the prominent topics of recent geoscientific polar research. The lake extends beneath the 4,000 m thick ice sheet to a length of more than 250 km with a water depth of up to 1,000 m. This thesis aims to investigate the influence of tides on the lake level which has not been considered so far in the discussion of water circulation within the lake. Due to the extent of the lake the tidal potential at different positions on its surface is not equal but exhibits a differential effect. Under the assumption of the lake level to be parallel to an equipotential surface the equilibrium tides of the lake yield amplitudes of up to 4.6 mm for the largest diurnal tidal constituent K1 and 1.8 mm for the largest semi-diurnal wave M2. In addition, differences in air pressure between the northern and the southern part of the lake result in a differential inverse barometric effect. This effect shows red noise characteristics with variations of up to +/- 20 mm. Three different types of measurements were used to verify corresponding height changes of the ice surface above the lake. Differential GPS measurements between one station on grounded ice and one in the southern centre of the lake confirm the concept and show height changes correlated to air pressure differences as well as changes with diurnal and semi-diurnal periods. SAR interferometry as a spatial method to determine height changes reveals the areal extent of the deformation with a grounding zone extending to the centre of the about 50 km wide lake. Gravimetric earth tide data recorded at Vostok Station in 1969 show pecularities such as an increased regression with air pressure and a phase lead of the K1 tide. However, these effects cannot be explicitly attributed to height changes of the lake surface. Due to the position of the station near the edge of the lake the effect is highly attenuated and below the noise level of these measurements. This work introduces the concept of the response of the subglacial lake to the tidal potential and to air pressure forcings and presents evidence for the effect by two different techniques proving the validity of the model