Arctic ice cap velocity variations revealed using ERS SAR interferometry Beverley Victoria Unwin A thesis submitted to the University of London in fulfilment of the requirements for the degree of PhD Mullard Space Science Laboratory Department of Space and Climate Physics University College London 1998 ProQuest Number: 10013365 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10013365 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract This thesis will examine the velocity structure of Austfonna, a large ice cap in the Svalbard archipelago. The remoteness of its location had previously hindered detailed observation by traditional methods, but indirect evidence suggested that it had the potential to be dynamically interesting. A recently developed remote sensing technique, SAR interferometry (inSAR), has allowed us to obtain the most detailed map of Austfonna's topography to date, plus unprecedented synoptic measurements of its velocity field. A four year time series of data acquired by the European Remote Sensing satellites ERS-1 and ERS-2 has been used to delineate active and inactive areas of the ice cap, which suggest that past ideas about Austfonna's thermal structure may need to be re-examined. It has also revealed large temporal velocity variations in one of its major drainage basins. These are difficult to classify because intermittent sampling has prevented us from determining their temporal wavelength, and also because globally the database of observed glacier velocity variations is so sparse that the range of possible variable flow scenarios is unknown. The work here demonstrates the huge potential for inSAR in helping to resolve such issues, and in providing an invaluable resource for scientists monitoring the stability of the world's ice fields. Acknowledgements I would like to thank my supervisor Professor Duncan Wingham for his guidance, particularly over the last few months during the writing of this thesis. In addition I'd like to acknowledge the benefits of some useful discussions with Anne-Marie Nuttall and Professor Julian Dowdeswell at Aberystwyth University's Centre for Glaciology. Julian was also instrumental in helping me to obtain external data-sets with which to validate my results. Ian Joughin, at JPL, probably saved me many months' work by allowing me to use his phase-unwrapping software, and I am very grateful for this. I would also like to thank colleagues at MSSL, past and present, who collectively have been a rich source of advice, support and information. I'd particularly like to acknowledge the excellent computer support from which I have benefited throughout my time here. On a personal note, I thank my family, particularly my parents, and my friends, for their support and encouragement. I'm very grateful to my cousin Carol, and her partner Andrew, who welcomed me into their lovely, comfortable home 18 months ago, and are remarkably good humoured about the fact that I'm still there. Finally, I give very special thanks to my boyfriend John whose support has been invaluable and whose talent for making me laugh has taken the edge off some difficult days. Contents List of figures List of tables List of acronyms List of symbols Chapter 1 : Introduction 12 LI Background of study 12 1.2 The glaciology of Austfonna 14 1.3 Questions addressed by this thesis 19 1.4 The evolution and glaciological application of InSAR 20 1.5 Structure of thesis 22 Chapter 2 : Principles of the interferometric technique 23 2.1 Basic geometry for topographic estimation 23 2.2 Determination of range-difference 24 2.3 Interferometric imaging of a moving target 25 2.3.1 Isolating the topographic component of the range-difference 27 2.3.2 Isolating the velocity component of the range-difference 31 2.4 Determination of three dimensional target velocity 32 Chapter 3 : Methodology 35 3.1 Data origin 35 3.2 Registration and formation of single-pair interferograms 36 3.3 Determination of ice cap topography 42 3.4 Determination of line-of-sight velocity component v^. 45 3.5 Determination of the three-dimensional velocity vector 47 3.5.1 Determination of v by combining ascending and descending data 47 3.5.2 Determination of v by assuming a flow direction 49 Chapter 4 : Error Model 50 4.1 Phase error 50 4.1.1 Phase errors due to changes in the target's scattering properties 50 4.1.2 Phase errors due to travel-time anomdies 53 4.2 Errors in the computation of target height 55 4.2.1 Relationship between height error and phase error 55 4.2.2 Errors introduced during the processing procedure 56 4.2.3 Height errors due to errors in knowledge of the imaging geometry 57 4.2.4 The effect of spatially variable surface penetration 61 4.3 Errors in the computation of line-of-sight velocity 63 4.3.1 Sensitivity to phase error 63 4.3.2 Sensitivity to height and baseline error 63 4.4 Errors in the computation of the 3D velocity vector 67 4.4.1 Validity of surface parallel flow assumption 67 4.4.2 Problems with the "method b" velocity determination 67 Chapter 5 ; Verification of results 69 5.1 Interferometrically derived DEMs of Austfonna 69 5.1.1 Comparison of the 1992 and 1994 results 69 5.1.2 Comparison of the interferometric DEMs with an external data set 71 5.1.3 The accuracy of the 1994 DEM 72 5.2 Line-of-sight velocity field 73 5.2.1 Accuracy and repeatability of ascending line-of-sight velocity results 73 5.2.2 Accuracy of the descending line-of-sight velocity results 81 5.2.3 Temporà line-of-sight velocity variations 81 5.3 Ice velocity vector v 83 5.3.1 Results obtained by combining ascending and descending data 83 5.3.2 Results obtained by assuming a flow direction 87 Chapter 6 : Discussion 91 6 .1 Summary of results 91 6.2 Discussion of Austfonna's velocity structure 94 6.3 Discussion of the observed velocity variations 99 6.4 Some comments on Austfonna's mass balance 102 Chapter 7 : Conclusions 104 References 110 Appendix 1 : Justification of use of a spherical coordinate svstem 116 Appendix 2 : Baseline constraint using tie-points 119 List of figures 1.1 The Svalbard archipelago 15 1.2 Austfonna's major drainage basins 16 1.3 Ice surface elevations on Austfonna 17 1.4 Austfonna's subglacial bedrock elevation 18 1.5 Austfonna's ice thickness 18 1.6 The pattern of driving stresses on Austfonna 19 2.1 Interferometric imaging geometry for a stationary target 23 2.2 Interferometric imaging geometry for a moving target 26 2.3 Baseline components 27 2.4 Geometry for topographic phase isolation 28 2.5 Geometry of the differential system 30 2.6 The measured velocity component 32 2.7 Components of ice surface velocity 33 2.8 Relationship between two look geometries 33 3.1 ERS SAR imaging geometry 35 3.2 ERS SAR frames used in this research 36 3.3 Amplitude component of Austfonna SAR image 37 3.4 Wrapped, flattened phase-difference image 40 3.5 Correlation images for each interferometric pair 41 3.6 Topographic phase-difference images 44 3.7 Phase-difference images showing only motion effects 47 3.8 Descending SAR imagery 48 4.1 Relationship between phase error and correlation 51 4.2 Phase scatter error for image pair acquired 13th/16th February 1992 52 4.3 Overlapping ascending/descending correlation images 53 4.4 Height error due to phase scatter error 56 4.5 Predicted height error per centimetre of baseline error 58 4.6 Height error that was obtained from experiment 3 61 4.7 Line-of-sight velocity error that is due to phase scatter error 64 4.8 Error in line-of-sight velocity that arises due to topographic error 65 4.9 Predicted line-of-sight velocity error per centimetre of baseline error 67 4.10 Percentage error introduced by "method b" velocity determination 68 5.1 Height difference between the 1992 and 1994 interferometric DEMs 70 5.2 Correlation related height errors across the CD profile 70 5.3 Details of NP-SPRI radio echo sounding survey 71 5.4 Difference between interferometric DEMs and RES heights 72 5.5 Line-of-sight velocity field for each image pair 74 5 .6 Line-of-sight velocity along the KL profile 7 5 5.7 Difference between each line-of-sight velocity result and a reference 7 6 5 .8 Profiles across the velocity-difference results of Figure 5.7 77 5.9 Velocity difference produced using a deliberately mal-registered DEM 78 5.10 The effect of using the 1992 DEM in computing the velocity results 7 9 5.11 Descending line-of-sight velocity 80 5.12 Phase-difference images for three prominent drainage basins 82 5.13 Line-of-sight velocity profiles for three prominent drainage basins 83 5.14 Velocity field obtained by combining ascending/descending data 84 5.15 Flow azimuth of "method a" velocity field 85 5.16 Position of surveyed velocity profiles 86 5.17 Comparison between interferometric and surveyed velocities 87 5.18 Flow direction taken as direction of maximum slope 88 5.19 Velocity field obtained by assuming a flow direction 89 5.20 Velocity conversion factor 89 5.21 Difference between velocity results derived using different methods 90 5.22 Difference between velocity results