Systematic InSAR tropospheric phase delay corrections from global meteorological reanalysis data

6p.International audienceDespite remarkable successes achieved by Differential InSAR, estimations of low tectonic strain rates remain challenging in areas where deformation and topography are correlated, mainly because of the topography‐related atmospheric phase screen (APS). In areas of high relief, empirical removal of the stratified component of the APS may lead to biased estimations of tectonic deformation rates. Here we describe a method to correct interferograms from the effects of the spatial and temporal variations in tropospheric stratification by computing tropospheric delay maps coincident with SAR acquisitions using the ERA‐ Interim global meteorological model. The modeled phase delay is integrated along vertical profiles at the ERA‐I grid nodes and interpolated at the spatial sampling of the interferograms above the elevation of each image pixel. This approach is validated on unwrapped interferograms. We show that the removal of the atmospheric signal before phase unwrapping reduces the risk of unwrapping errors in areas of rough topography

Earthquake relocations and InSAR analysis following the June 12th 2011 eruption of Nabro volcano, Afar

Nabro volcano sits on the southern part of Danakil block to the east of the Afar depression, on the Arabian plate. On the 12th June 2011, Nabro volcano suddenly erupted after being inactive for 10,000 years. The eruption caused a 17-km-long lava flow, a 15-km-high ash cloud, and ranks as one of the largest emissions of SO2 since the Mt. Pinatubo (1991) event. This eruption creates an important opportunity to use seismicity and surface deformation measurements to understand the subsurface magmatic system and deformation of a hazardous, off axis caldera during continental rupture. We installed a network of 8 seismometers around Nabro caldera which began recording on the 31st August and tasked SAR acquisitions from TerraSAR-X (TSX) and Cosmo-SkyMed (CSK) satellites. The SAR images used for this study post date the eruption. We used TSX stripmap mode images from ascending and descending orbits. Using a small baseline approach, we used 25 images acquired between the 1st July 2011 to the 5th October 2012 on descending orbit 046, to create 34 interferograms. We complemented these with 19 images from ascending orbit 130 spanning the 6th July 2011 to the 10th October 2012 from ascending orbit 130, which we used to create 21 interferograms. We produced a velocity ratemap and timeseries using π-RATE showing subsidence of up to 25cm/yr centred on Nabro. We used a Monte-Carlo hybrid downhill simplex technique to invert the dataset and found the best fitting solution as a mogi source at 6.9 ±1.1 km depth, and located at a 13.35 (lat) and 41.69 (long). The time dependence observed is consistent with a viscoelastic relaxation around the magma chamber, following depletion. Concurrent with the TSX acquisitions, CSK imaged the volcano on a descending track between 26th June 2011 and 18th July 2012 within the ASI project SAR4Volcanoes, and 64 images were used to produce 171 interferograms which were inverted to form a timeseries using a SBAS approach. This dataset has an overall subsidence signal, but the time series shows a shorter wavelength fluctuation of ground deformation, which is not apparent in the TSX data. We processed the seismic signals detected by the temporary local network and by a seismic station within a permanent regional array, to provide hypocentre locations for the period September-October, 2011. We used Hypoinverse-2000 to provide preliminary locations for events, which were then relocated using HypoDD. Absolute error after Hypoinverse-2000 processing was approximately ±2 and ±4 km in the horizontal and the vertical directions, respectively. Using HypoDD, relative errors were reduced to ±20 and ±30 m in the horizontal and vertical directions, respectively. The hypocentres show clusters of activity as well as aseismic regions. The majority of the earthquakes are located at the active vent, with fewer events located on the flanks. There is a smaller cluster of events to the south-west of Nabro beneath neighbouring Mallahle volcanic caldera, despite no eruption occurring here nor any post-eruptive deformation. This may imply some stress triggering mechanism or some pressure connection between the magma system of the two calderas. We present both the seismic and InSAR datasets as a joint approach to understand this eruption, as well as further implications for other ‘quiet calderas’

Ground deformation monitoring of the eruption offshore Mayotte

In May 2018, the Mayotte island, located in the Indian Ocean, was affected by an unprecedented seismic crisis, followed by anomalous on-land surface displacements in July 2018. Cumulatively from July 1, 2018 to December 31, 2021, the horizontal displacements were approximately 21 to 25 cm eastward, and subsidence was approximately 10 to 19 cm. The study of data recorded by the on-land GNSS network, and their modeling coupled with data from ocean bottom pressure gauges, allowed us to propose a magmatic origin of the seismic crisis with the deflation of a deep source east of Mayotte, that was confirmed in May 2019 by the discovery of a submarine eruption, 50 km offshore of Mayotte ([Feuillet et al., 2021]). Despite a non-optimal network geometry and receivers located far from the source, the GNSS data allowed following the deep dynamics of magma transfer, via the volume flow monitoring, throughout the eruption

De la source des tremblements de terre au risque sismique: apport de la géodésie spatiale

La quantification des aléas sismiques repose sur une combinaison d’approches incluant la sismologie, la géodésie, la paléosismologie et la géologie. Les récents progrès en géodésie spatiale permettent d’accéder à la distribution de glissement fini sur les systèmes de faille pendantet entre les grands séismes. En particulier, l’interférométrie radar (InSAR) à large fauchée, notamment grâce à la constellation Sentinel-1, fournit aujourd’hui des moyens d’observationà haute fréquence de revisite, capables de mesurer des déformations tectoniques à grande échelle, avec une précision et une résolution temporelle accrues. Le développement de méthodes d’inversion conjointe des données de géodésie spatiale (InSAR, corrélation optique, GNSS) et sismologiques permet dès lors de construire une représentation précise des processus de ruptures, et ainsi de tester et repousser les limites des diérents modèles existants de cycle sismique.Afin d’illustrer ces questionnements actuels, quatre cas de séismes récents observés par l’InSAR à large fauchée sont discutés: (a) le séisme himalayen de Gorkha (M7.8, 2015), (b) le séisme décrochant du Baloutchistan (M7.8, 2013), (c) le doublet sismique d’Iquique sur la subduction chilienne (M8.1+M7.6, 2014) et (d) le séisme de Pawnee (M5.8, 2016) induit par l’activité anthropique. Ces exemples démontrent l’intérêt de progresser dans la caractérisation des déformations tectoniques actuelles, afin notamment de pouvoir mettre en perspective les observations fragmentaires associées aux séismes passés, et ainsi aboutir à une meilleure évaluation des aléas sismiques.Des progrès supplémentaires dans les technologies radar sont anticipés pour les années à venir, permettant d’accéder simultanément à la haute résolution spatiale et à la haute résolution temporelle (notamment avec la future mission NISAR). Ces progrès permettront d’achever la convergence avec les approches développées à l’heure actuelle en imagerie optique à haute résolution (notamment avec la constellation Pleiades). Ensemble, ces nouvelles observations fourniront des images à haute définition des déformations en champ très proche des ruptures de surface des séismes à venir, faisant ainsi la jonction avec les observations de terrain de décalages cosismiques instantanés et cumulés.Je propose de développer l’exploitation de ces observations diverses – géodésiques, sismologiques, paléosismologiques, tectoniques – dans le but de progresser dans la quantification du rapport entre déformation élastique et anélastique, dans le temps et dans l’espace, en surface et en profondeur, ce qui nécessitera d’accomplir en parallèle des progrès en modélisation mécanique de la déformation cosismique. De manière complémentaire, je compte également prolonger mes travaux portant sur les déformations volcano-tectoniques. L’analyse conjointe des observations géodésiques, télésismiques et de télédétection de l’activité volcanique de surface (gaz, aérosols, thermique, infrason) permettra de caractériser, à distance, la dynamique éruptive de volcans isolés et non-instrumentés, et ainsi de mieux comprendre les aléas associés

De la source des tremblements de terre au risque sismique: apport de la géodésie spatiale

La quantification des aléas sismiques repose sur une combinaison d’approches incluant la sismologie, la géodésie, la paléosismologie et la géologie. Les récents progrès en géodésie spatiale permettent d’accéder à la distribution de glissement fini sur les systèmes de faille pendantet entre les grands séismes. En particulier, l’interférométrie radar (InSAR) à large fauchée, notamment grâce à la constellation Sentinel-1, fournit aujourd’hui des moyens d’observationà haute fréquence de revisite, capables de mesurer des déformations tectoniques à grande échelle, avec une précision et une résolution temporelle accrues. Le développement de méthodes d’inversion conjointe des données de géodésie spatiale (InSAR, corrélation optique, GNSS) et sismologiques permet dès lors de construire une représentation précise des processus de ruptures, et ainsi de tester et repousser les limites des diérents modèles existants de cycle sismique.Afin d’illustrer ces questionnements actuels, quatre cas de séismes récents observés par l’InSAR à large fauchée sont discutés: (a) le séisme himalayen de Gorkha (M7.8, 2015), (b) le séisme décrochant du Baloutchistan (M7.8, 2013), (c) le doublet sismique d’Iquique sur la subduction chilienne (M8.1+M7.6, 2014) et (d) le séisme de Pawnee (M5.8, 2016) induit par l’activité anthropique. Ces exemples démontrent l’intérêt de progresser dans la caractérisation des déformations tectoniques actuelles, afin notamment de pouvoir mettre en perspective les observations fragmentaires associées aux séismes passés, et ainsi aboutir à une meilleure évaluation des aléas sismiques.Des progrès supplémentaires dans les technologies radar sont anticipés pour les années à venir, permettant d’accéder simultanément à la haute résolution spatiale et à la haute résolution temporelle (notamment avec la future mission NISAR). Ces progrès permettront d’achever la convergence avec les approches développées à l’heure actuelle en imagerie optique à haute résolution (notamment avec la constellation Pleiades). Ensemble, ces nouvelles observations fourniront des images à haute définition des déformations en champ très proche des ruptures de surface des séismes à venir, faisant ainsi la jonction avec les observations de terrain de décalages cosismiques instantanés et cumulés.Je propose de développer l’exploitation de ces observations diverses – géodésiques, sismologiques, paléosismologiques, tectoniques – dans le but de progresser dans la quantification du rapport entre déformation élastique et anélastique, dans le temps et dans l’espace, en surface et en profondeur, ce qui nécessitera d’accomplir en parallèle des progrès en modélisation mécanique de la déformation cosismique. De manière complémentaire, je compte également prolonger mes travaux portant sur les déformations volcano-tectoniques. L’analyse conjointe des observations géodésiques, télésismiques et de télédétection de l’activité volcanique de surface (gaz, aérosols, thermique, infrason) permettra de caractériser, à distance, la dynamique éruptive de volcans isolés et non-instrumentés, et ainsi de mieux comprendre les aléas associés

Interferometric Processing of SLC Sentinel-1 TOPS Data

International audienceInSAR processing usually involves two successive steps: focusing and interferometry. Most public-domain InSAR processing toolboxes are capable of performing both operations with data acquired in the standard Stripmap mode, starting from raw SAR data (level 0). However, the focusing of burst-mode data, such as TOPS and ScanSAR, requires substantial modifications to standard focusing methods due to the particular spectral properties of these data. Anticipating on this potential difficulty for non-expert users, the European Space Agency has chosen to release Sentinel-1 TOPS data in a Single Look Complex format (level 1). The data are already focused using state-of-the-art processing techniques, with phase information preserved. Even so, the focusing method introduces an additional quadratic phase term in the azimuth direction. In case of a small misregistration error between a pair of images, this residual term leads to steep phase ramps in azimuth that are superimposed on the desired interferometric phase. Therefore, this quadratic phase term needs to be removed from the SLC data prior to interferogram calculation. Here, a pre-processing method allowing for compensating this phase term and simply feeding the corrected SLC data into a standard InSAR processing chain is described. The method consists of three steps. The first step uses the metadata in order to reconstruct a continuous image in the azimuth direction, accounting for the small overlap between adjacent bursts (“stitching”). In the second step, multiplication of the images by an appropriate phase screen is performed so as to cancel the azimuthal quadratic phase term (“deramping”). The deramping operation uses the metadata, as well as the azimuth time lag between the images deduced from sub-pixel image correlation, in order to determine small misregistration errors. Misregistration errors are compensated using a simple affine relation deduced from least-square fitting of the azimuth offsets. Following this second step, the azimuth phase ramps are significantly reduced in the corrected interferogram. The third step consists in refining the affine coefficients that account for the misregistration error. The refinement is achieved by differencing the backward- and forward-looking interferograms, exploiting the spectral diversity in burst overlap regions (“spectral diversity”). This final step makes it possible to remove residual phase jumps across burst boundaries with the desired level of accuracy

Interferometric Processing of SLC Sentinel-1 TOPS Data

International audienceInSAR processing usually involves two successive steps: focusing and interferometry. Most public-domain InSAR processing toolboxes are capable of performing both operations with data acquired in the standard Stripmap mode, starting from raw SAR data (level 0). However , the focusing of burst-mode data, such as TOPS and ScanSAR, requires substantial modifications to standard focusing methods due to the particular spectral properties of these data. Anticipating on this potential difficulty for non-expert users, the European Space Agency has chosen to release Sentinel-1 TOPS data in a Single Look Complex format (level 1). The data are already focused using state-of-the-art processing techniques, with phase information preserved. Even so, the focusing method introduces an additional quadratic phase term in the azimuth direction. In case of a small misregistration error between a pair of images, this residual term leads to steep phase ramps in azimuth that are superimposed on the desired in-terferometric phase. Therefore, this quadratic phase term needs to be removed from the SLC data prior to interfer-ogram calculation. Here, a pre-processing method allowing for compensating this phase term and simply feeding the corrected SLC data into a standard InSAR processing chain is described. The method consists of three steps. The first step uses the metadata in order to reconstruct a continuous image in the azimuth direction, accounting for the small overlap between adjacent bursts (" stitching "). In the second step, multiplication of the images by an appropriate phase screen is performed so as to cancel the azimuthal quadratic phase term (" deramp-ing "). The deramping operation uses the metadata, as well as the azimuth time lag between the images deduced from sub-pixel image correlation, in order to determine small misregistration errors. Misregistration errors are compensated using a simple affine relation deduced from least-square fitting of the azimuth offsets. Following this second step, the azimuth phase ramps are significantly reduced in the corrected interferogram. The third step consists in refining the affine coefficients that account for the misregistration error. The refinement is achieved by differencing the backward-and forward-looking interfer-ograms, exploiting the spectral diversity in burst overlap regions (" spectral diversity "). This final step makes it possible to remove residual phase jumps across burst boundaries with the desired level of accuracy. Azimuth spectral properties of burst-mode SAR data, such as ScanSAR [1] or TOPS [2], are significantly different from those of Stripmap. In burst-mode, the system observes a series of sub-swaths by periodically steering of the antenna in the elevation direction and transmitting a short succession of pulses gathered in the so-called " bursts ". As a consequence, a given target is imaged only during a fraction of the equivalent stripmap synthetic aperture duration, thereby reducing its duration of illumination. Therefore, the increased swath width comes at the expense of azimuth resolution (Fig. 1). Furthermore, the Doppler centroid of targets within any burst becomes variable in azimuth. Due to the latter effect, the impulse response function of standard burst-mode focusing methods includes a residual phase term that exhibits a characteristic quadratic variation of the az-imuth phase. This phase term needs to be compensated prior to using the phase for interferometric applications, a process referred to as " deramping " [3, 4]. Figure 1. Schematic representation of the TOPSAR acquisition mode (from [5]). The beam is steered periodically in range, in order to cover several sub-swaths, and progressively in azimuth, aft to fore, in order to increase the azimuth bandwidth, expand the footprint and decrease scalloping

Three-dimensional displacement field of the 2015 Mw8.3 Illapel earthquake (Chile) from across- and along-track Sentinel-1 TOPS interferometry

International audienceWide-swath imaging has become a standard acquisition mode for radar missions aiming at applying synthetic aperture radar interferometry (InSAR) at global scale with enhanced revisit frequency. Increased swath width, compared to classical Stripmap imaging mode, is achieved at the expense of azimuthal resolution. This makes along-track displacements, and subsequently north-south displacements, difficult to measure using conventional split-beam (multiple-aperture) InSAR or cross-correlation techniques. Alternatively, we show here that the along-track component of ground motion can be deduced from the double difference between backward and forward looking interferograms within regions of burst overlap. “Burst overlap interferometry” takes advantage of the large squint angle diversity of Sentinel-1 (∼1°) to achieve subdecimetric accuracy on the along-track component of ground motion. We demonstrate the efficiency of this method using Sentinel-1 data covering the 2015 Mw8.3 Illapel earthquake (Chile) for which we retrieve the full 3-D displacement field and validate it against observations from a dense network of GPS sensors

Elastic thickness control of lateral dyke intrusion at mid-ocean ridges

International audienceMagmatic accretion at slow-spreading mid-ocean ridges exhibits specific features. Although magma supply is focused at the centre of second-order segments, melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions. It has been previously suggested that an along-axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation. However, this cannot account for the poor correlation between dyke opening and surface elevation in the 2005–2010 series of 14 dyke intrusions of Afar (Ethiopia). Using mechanical arguments, constrained by both geodetic and seismological observations, we propose that the large dykes that initiate near the mid-segment magma source are attracted toward segment ends as a result of a thickening of the elastic–brittle lithosphere in the along-rift direction. This attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long-term focusing of melts, and comparatively " colder " , hence stronger segment ends. The axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along-axis elastic–brittle thickness, rather than the primary cause of lateral dyke injections