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See detailTowards operational use of combined Split-Band Interferometry and Multidimensional Small Baseline Subset: application to geohazard monitoring in the Kivu region
Libert, Ludivine ULiege

Doctoral thesis (2018)

The vulnerable region of Kivu is strongly affected by geohazards such as volcanism and landslides. An essential ingredient for modelling the geophysical processes from which these geohazards originate is ... [more ▼]

The vulnerable region of Kivu is strongly affected by geohazards such as volcanism and landslides. An essential ingredient for modelling the geophysical processes from which these geohazards originate is the measure of the Earth’s surface displacements. In this complex region, monitoring these ground deformations can be efficiently performed by using Synthetic Aperture Radar Interferometry (InSAR). Nevertheless, geophysical modelling requires accurate measurements of the displacements over time, which are not always manageable with conventional InSAR due to area-related limitations such as geometric distortions. Consequently, we consider advanced processing techniques such as Split-Band Interferometry (SBInSAR), which potentially allows to calculate the absolute interferometric phase, and Multidimensional Small Baseline Subset (MSBAS), which reconstructs the history of deformations along the horizontal and vertical directions. The aims of this thesis are the investigation of the SBInSAR and MSBAS techniques for improving the ground deformation measurements, the development of these techniques in order to achieve an operational tool of monitoring, and eventually to combine the information provided by both methods. Regarding the Split-Band Interferometry, we consider its theoretical applicability, determine optimum processing parameters and propose a method that corrects phase ambiguities by using SBInSAR as a complement to classical phase unwrapping, which we call SBInSAR-assisted phase unwrapping. We establish that the best criterion for the detection of spectrally stable pixels is the slope standard deviation. We also study the temporal behaviour of spectrally stable pixels and show that few of them persist over time. We analyze the behaviour of artificial reflectors with respect to SBInSAR and provide strong evidences that there exists a correlation between the signal-to-clutter ratio of a target and its detectability as a spectrally stable pixel. Regarding the Multidimensional Small Baseline Subset, we present the InSAR mass processing chain developed in collaboration with the European Center for Geodynamics and Seismicity for MSBAS preprocessing, and we propose to optimize the spatial coverage of MSBAS measurements by using interpolation or global masking. An MSBAS analysis is carried out over the city of Bukavu for the period from January 2016 to September 2017. We also study temporal decorrelation over the Virunga Volcanic Province and we model it using a decreasing exponential function. As a complement, we implement the Range Split-Spectrum technique in the CIS software for the ionospheric phase compensation in low frequency interferograms. We finally discuss the options for merging SBInSAR and MSBAS together. [less ▲]

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See detailOn the characterization of frequency-persistent scatterers in Split-Band Interferometry
Libert, Ludivine ULiege; De Rauw, Dominique ULiege; d'Oreye, Nicolas et al

Conference (2018, July 24)

Split-Band Interferometry (SBInSAR) requires the presence of reliable frequency-persistent scatterers within the studied scene. However, the physical nature of such reflectors is still poorly known. In ... [more ▼]

Split-Band Interferometry (SBInSAR) requires the presence of reliable frequency-persistent scatterers within the studied scene. However, the physical nature of such reflectors is still poorly known. In this work, we perform a temporal analysis over the region of the Virunga Volcanic Province (Democratic Republic of Congo - DRC) and we show that frequency-persistent scatterers are not necessarily stable over time. We study and quantify the effect of the range resolution loss due to the bandwidth degradation in the SBInSAR process. Finally, radiometric information and spectral coherence are used to show that frequency-persistent scatterers cannot be associated to a specific scattering mechanism. [less ▲]

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See detailSplit-Band SAR and Split Band InSAR principle and applications
De Rauw, Dominique ULiege; Kirkove, Murielle ULiege; Libert, Ludivine ULiege et al

Poster (2018, April 19)

Most recent SAR sensors use wide band signals to achieve metric range resolution. One can also take advantage of wide band to split it into sub-bands and generate several lower-resolution images, centered ... [more ▼]

Most recent SAR sensors use wide band signals to achieve metric range resolution. One can also take advantage of wide band to split it into sub-bands and generate several lower-resolution images, centered on slightly different frequencies, from a single acquisition (Bovenga & al. 2014). This process, named Multi Chromatic Analysis (MCA) corresponds to performing a spectral analysis of SAR images. From this spectral analysis, three potential applications are shown. First, this splitting allows performing a spectral analysis of observed scatterers. Spectral coherence is derived by computing the coherence between sub-images issued from a single SAR acquisition. It was shown that in the presence of a random distribution of surface scatterers, spectral coherence is proportional to sub-band intersection of sub-images. This model is fully verified when observing spectral coherence on open seas areas. If the scatterers distribution departs from this distribution, like for manmade structures, spectral coherence may be preserved to a certain degree and allows discriminating vessels from see clutter even in case of high sea state. This property can be used to perform vessel detection (Derauw & al., 2010). Second, Split-Band SAR interferometry (SBInSAR) is also based on this spectral analysis performed on each image of an InSAR pair, yielding a stack of sub-band interferograms. Scatterers keeping a coherent behaviour in each sub- band interferogram show a phase that varies linearly with the carrier frequency, the slope being proportional to the absolute optical path difference. This potentially solves the problems of phase unwrapping on a pixel-per-pixel basis (Libert & al.). Third, unwrapping classically two sub-band interferograms allows getting two phases of a same scene and same ionospheric components. Since these two components behave differently with frequency, SBInSAR allows discriminating both and remove the ionospheric artifacts if presents (Gomba & al. 2016, Furuya & al. 2016). [less ▲]

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See detailFormation FabSpace 2.0 : Imagerie Sentinel-1, interférométrie et ses applications
Libert, Ludivine ULiege

Diverse speeche and writing (2018)

Formation sur le radar, l'interférometrie SAR et ses applications, données dans le cadre du projet FabSpace 2.0

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See detailOptimum split-band parameters for phase offset retrieval
Libert, Ludivine ULiege; De Rauw, Dominique ULiege; Barbier, Christian ULiege

in IEEE International Geoscience and Remote Sensing Symposium (2017, December 04)

The potential of Split-Band processing to determine the absolute phase of an interferometric pair of SAR images has been demonstrated in previous studies. In this work we propose a new approach based on ... [more ▼]

The potential of Split-Band processing to determine the absolute phase of an interferometric pair of SAR images has been demonstrated in previous studies. In this work we propose a new approach based on the joint use of Split-Band Interferometry and SAR Interferometry for the phase leveling of disconnected regions of an unwrapped interferogram. In this paper a new selector of frequency-persistent scatterers is presented. In order to demonstrate the applicability of the method we propose an artificial disconnection of the regions to validate our results without any ground based measurements. The presented method is tested on TerraSAR-X images acquired with a range bandwidth of 150 MHz over the Nyamuragira volcano in the African region of the Kivu Rift. Several combinations of split parameters are applied and investigated to determine the best configuration for this application. [less ▲]

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See detailDevelopment of an Interferometric Mass Processing Chain for Multitemporal Ground Deformation Measurements
Libert, Ludivine ULiege; De Rauw, Dominique ULiege; d'Oreye, Nicolas

Conference (2017, November 07)

The main goal of the RESIST project is to understand the mechanisms driving volcanic eruptions and landslides in the Kivu region, on the East African Rift. In order to model both volcanic and landslide ... [more ▼]

The main goal of the RESIST project is to understand the mechanisms driving volcanic eruptions and landslides in the Kivu region, on the East African Rift. In order to model both volcanic and landslide processes, it is necessary to measure ground deformations in the region accurately. For this purpose, both ground-based instruments (e.g. GPS network) and spaceborne data (e.g. optical and SAR images) are used. One aspect of the project focuses on Differential SAR Interferometry (DInSAR), which is a technique used to map ground deformations occurring between two SAR images acquired at different times. It is today a well-mastered technique that offers large spatial coverage with a typical temporal sampling of one to several days, depending on the chosen sensor. In the last years, multitemporal approaches based on DInSAR have been developed, like the Small BAseline Subset (SBAS) [1] and Multidimensional Small BAseline Subset (MSBAS) [2] techniques, or the Persistent Scatterers Interferometry (PSI) [3]. In the framework of RESIST, we use MSBAS to perform ground deformations monitoring along time. The Multidimensional Small BAseline Subset (MSBAS) technique produces 2-D time series of ground deformations by integrating data sets of SAR images acquired by different sensors, with different spatial and temporal sampling, resolutions, incidence angles, wavelengths, pass directions and other varying parameters. By combining at least two data sets with overlapping spatial and temporal coverage and an extended range of look angles, the evolution of deformations in the vertical and west-east directions can be computed by the MSBAS approach [2]. The MSBAS software feeds on a large amount of deformation maps, which are produce by DInSAR. Such an amount of data cannot be produced by hand and that is the reason why we developed an automatic interferometric processing chain meant to produce large amounts of products adequate for multitemporal methods like MSBAS. The interferometric processing is supported by the CSL InSAR Suite (CIS) software developed at Centre Spatial de Liège, which presents the advantages to be fully adaptable to the needs of the MSBAS technique and the thematic specificities. Indeed, numerous options (e.g. adaptive filtering, wide swath interferometry) have been added to the CIS software in the framework of the RESIST project. In a first time, we will briefly present the MSBAS approach and its advantages regarding the ground deformation measurements. In the second part of the presentation, we will introduce the mass processing chain step by step and its functionalities. Critical steps of the chain, like the chosen strategy for the interferometric pairs selection, the integration of Sentinel-1 data or the image interpolation approach, will be presented in more details. Finally, preliminary results of an MSBAS processing over the Bukavu area will be presented. REFERENCES: [1] Berardino, P., Fornaro, G., Lanari, R., and Sansosti, E. (2002). A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms, IEEE Trans. Geosci. Remote Sens., 40, 11, pp. 2375-2383. doi: 10.1109/TGRS.2002.803792. [2] Samsonov, S., and d’Oreye, N. (2012). Multidimensional time series analysis of ground deformation from multiple InSAR data sets applied to Virunga Volcanic Province, Geophysical Journal International, 191, 3, pp. 1095-1108. doi: 10.1111/j.1365-246X.2012.05669.x [3] Ferretti, A., Prati, C., and Rocca, F. (2001). Permanent scatterers in SAR interferometry, IEEE Trans. Geosci. Remote Sens., 39, 1, pp. 8-20. doi: 10.1109/36.898661. [less ▲]

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See detailSplit-Band Interferometry-Assisted Phase Unwrapping for the Phase Ambiguities Correction
Libert, Ludivine ULiege; De Rauw, Dominique ULiege; d'Oreye, Nicolas et al

in Remote Sensing (2017), 9(9),

Split-Band Interferometry (SBInSAR) exploits the large range bandwidth of the new generation of synthetic aperture radar (SAR) sensors to process images at subrange bandwidth. Its application to an ... [more ▼]

Split-Band Interferometry (SBInSAR) exploits the large range bandwidth of the new generation of synthetic aperture radar (SAR) sensors to process images at subrange bandwidth. Its application to an interferometric pair leads to several lower resolution interferograms of the same scene with slightly shifted central frequencies. When SBInSAR is applied to frequency-persistent scatterers, the linear trend of the phase through the stack of interferograms can be used to perform absolute and spatially independent phase unwrapping. While the height computation has been the main concern of studies on SBInSAR so far, we propose instead to use it to assist conventional phase unwrapping. During phase unwrapping, phase ambiguities are introduced when parts of the interferogram are separately unwrapped. The proposed method reduces the phase ambiguities so that the phase can be connected between separately unwrapped regions. The approach is tested on a pair of TerraSAR-X spotlight images of Copahue volcano, Argentina. In this framework, we propose two new criteria for the frequency-persistent scatterers detection, based respectively on the standard deviation of the slope of the linear regression and on the phase variance stability, and we compare them to the multifrequency phase error. Both new criteria appear to be more suited to our approach than the multifrequency phase error. We validate the SBInSAR-assisted phase unwrapping method by artificially splitting a continuous phase region into disconnected subzones. Despite the decorrelation and the steep topography affecting the volcanic test region, the expected phase ambiguities are successfully recovered whatever the chosen criterion to detect the frequency-persistent scatterers. Comparing the aspect ratio of the distributions of the computed phase ambiguities, the analysis shows that the phase variance stability is the most efficient criterion to select stable targets and the slope standard deviation gives satisfactory results. [less ▲]

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See detailThe Split-Band Interferometry Approach to Determine the Phase Unwrapping Offset
Libert, Ludivine ULiege; De Rauw, Dominique ULiege; d'Oreye, Nicolas et al

Poster (2017, June)

This poster presents an approach based on the Split-Band Interferometry to solve the ambiguities introduced during the phase unwrapping of separate regions.

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See detailSPICAM observations and modeling of Mars aurorae
Soret, Lauriane ULiege; Gérard, Jean-Claude ULiege; Libert, Ludivine ULiege et al

in Icarus (2016), 264

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the ... [more ▼]

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the altitudes and the intensities of the auroral emissions. The CO (a3P–X1R) Cameron bands between 190 and 270 nm, the CO Fourth Positive system (CO 4P) between 135 and 170 nm, the CO2+ doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137 ± 27 km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 40 and 200 eV. No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO and CO2+ emissions is in good agreement with the observations. [less ▲]

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See detailThe RESIST Project: a Study of Geohazards in the Kivu Basin Region using ground- and space borne Techniques
Libert, Ludivine ULiege; d'Oreye, Nicolas; Kervyn, François et al

Poster (2015, September)

Presentation of the goals and means of the RESIST project

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See detailObservations of Mars aurorae
Gérard, Jean-Claude ULiege; Soret, Lauriane ULiege; Libert, Ludivine ULiege et al

Conference (2015, September)

We present recent results obtained by combining remote sensing observations and in situ measurements of the Martian aurora made from Mars Express.

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See detailConcurrent observations of ultraviolet aurora and energetic electron precipitation with Mars Express
Gérard, Jean-Claude ULiege; Soret, Lauriane ULiege; Libert, Ludivine ULiege et al

in Journal of Geophysical Research. Space Physics (2015)

The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures ... [more ▼]

The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures of CO Cameron and CO<inf>2</inf>+ doublet ultraviolet auroral emissions. This study has almost doubled the number of auroral detections based on SPICAM spectra. Auroral emissions are located in the vicinity of the statistical boundary between open and closed field lines. From a total of 113 nightside orbits with SPICAM pointing to the nadir in the region of residual magnetic field, only nine nightside orbits show confirmed auroral signatures, some with multiple detections along the orbital track, leading to a total of 16 detections. The mean energy of the electron energy spectra measured during concurrent Analyzer of Space Plasma and Energetic Atoms/Electron Spectrometer observations ranges from 150 to 280eV. The ultraviolet aurora may be displaced poleward or equatorward of the region of enhanced downward electron energy flux by several tens of seconds and shows no proportionality with the electron flux at the spacecraft altitude. The absence of further UV auroral detection in regions located along crustal magnetic field structures where occasional aurora has been observed indicates that the Mars aurora is a time-dependent feature. These results are consistent with the scenario of acceleration of electrons by transient parallel electric field along semiopen magnetic field lines. © 2015. American Geophysical Union. All Rights Reserved. [less ▲]

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See detailThe Martian aurora: Spectral and spatial characteristics
Gérard, Jean-Claude ULiege; Libert, Ludivine ULiege; Stiepen, Arnaud ULiege

Conference (2014, July 05)

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See detailEtude des signatures ultraviolettes des aurores sur Mars
Libert, Ludivine ULiege

Master's dissertation (2014)

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