ENVISAT SAR Data Research Articles

Resolving Three-Dimensional Surface Motion with InSAR: Constraints from Multi-Geometry Data Fusion

Interferometric synthetic aperture radar (InSAR) technology has been widely applied to measure Earth surface motions related to natural and anthropogenic crustal deformation phenomena. With the widespread uptake of data captured by the European Space Agency’s Sentinel-1 mission and other recently launched or planned space-borne SAR missions, the usage of the InSAR technique to detect and monitor Earth surface displacements will increase even more in the coming years. However, InSAR can only measure a one-dimensional motion along the radar line of sight (LOS), which makes interpretation and communication of InSAR measurements challenging, and can add ambiguity to the modelling process. Within this paper, we investigate the implications of the InSAR LOS geometry using simulated and observed deformation phenomena and describe a methodology for multi-geometry data fusion of LOS InSAR measurements from many viewing geometries. We find that projecting LOS measurements to the vertical direction using the incidence angle of the satellite sensor (and implicitly assuming no horizontal motions are present) may result in large errors depending on the magnitude of horizontal motion and on the steepness of the incidence angle. We quantify these errors as the maximum expected error from simulated LOS observations based on a Mogi deformation model. However, we recommend to use LOS observations from several image geometries wherever data are available, in order to solve for vertical and E–W oriented horizontal motion. For an anthropogenic deformation phenomenon observed in seven independent InSAR analyses of Envisat SAR data from the Sydney region, Australia, we find that the strong horizontal motion present could lead to misinterpretation of the actual motion direction when projecting LOS measurements to vertical (uplift instead of subsidence). In this example, the difference between multi-geometry data fusion and vertical projection of LOS measurements (at an incidence angle of 33.8°) reach up to 67% of the maximum vertical displacement rate. Furthermore, the position of maximum vertical motion is displaced horizontally by several hundred metres when the LOS measurements are projected.

New estimation of Nile Delta subsidence rates from InSAR and GPS analysis

Increase of water level in most of the world’s oceans and seas, including the Mediterranean, due to global warming may have a serious impact on the Nile Delta. The Nile Delta is considered as one of the most important regions in Egypt as more than 50% of the Egyptians are living there. To study the thread of a potentially sinking and flooding of the Nile Delta, here, we estimate present vertical motion by use of the Persistent Scatterer InSAR (PSI) analysis in addition to the available GPS stations in the Nile Delta. The subsidence rate of the Nile Delta is estimated by applying the PSI processing for all archived Envisat SAR data from the descending tracks 207 and 436. From these tracks, 36 and 37 SAR scenes were collected, respectively. Moreover, the results of a time series analysis for six GPS permanent stations in the Nile Delta were used to validate the estimated subsidence rates from PSI analysis. The PSI and GPS solutions show that the estimated ground deformation is strongly localized at the big cities. From time series for selected PS pixels from Cairo, Tanta, Mahala, Mansoura, Damietta, and Port Said, the estimated subsidence rates from Envisat scenes are around − 6.4 ± 0.4 mm/year, − 4.0 ± 0.6 mm/year, − 4.8 ± 1.0 mm/year, − 10.0 ± 1.2 mm/year, − 10.3 ± 1.6 mm/year, and − 4.9 ± 1.6 mm/year, respectively. The localization of subsidence rates at big cities implies that the detected signals may be due to human activities, such as ground water pumping, and that the Delta as a whole presently is not subsiding.

Quantifying groundwater exploitation induced subsidence in the Rafsanjan plain, southeastern Iran, using InSAR time-series and in situ measurements

Decades of groundwater overexploitation for agriculture and industrial development have resulted in substantial land subsidence in the Rafsanjan plain of southeastern Iran. This work presents the results of an InSAR time series analysis obtained by the exploitation of Envisat, ALOS and Sentinel-1 (S1) SAR data archives between June 2004, and May 2016, to investigate land subsidence in the plain. The InSAR analysis revealed an area of approximately 1000km2 within the study area showing subsidence of >5cm/year and locally exceeding 30cm/yr in the last decade. This area of significant subsidence is limited in its spatial extent to the agricultural land and is partly influenced by Quaternary faults. The temporal and areal relationships of subsidence and groundwater level data suggest that a significant part of the observed subsidence in the Rafsanjan region is caused by intense groundwater extraction that has led to widespread compaction within the upper parts of the up to 300m thick unconsolidated sediments, causing irreversible and inelastic deformation of the aquifer. The average volume storage loss of the aquifer system due to overexploitation is estimated to have been approximately 300millioncubicmetre (mcm)/yr over the last decade.

StaMPS Improvement for Deformation Analysis in Mountainous Regions: Implications for the Damavand Volcano and Mosha Fault in Alborz

Interferometric Synthetic Aperture Radar (InSAR) capability to detect slow deformation over terrain areas is limited by temporal decorrelation, geometric decorrelation and atmospheric artefacts. Multitemporal InSAR methods such as Persistent Scatterer (PS-InSAR) and Small Baseline Subset (SBAS) have been developed to deal with various aspects of decorrelation and atmospheric problems affecting InSAR observations. Nevertheless, the applicability of both PS-InSAR and SBAS in mountainous regions is still challenging. Correct phase unwrapping in both methods is hampered due to geometric decorrelation in particular when using C-band SAR data for deformation analysis. In this paper, we build upon the SBAS method implemented in StaMPS software and improved the technique, here called ISBAS, to assess tectonic and volcanic deformation in the center of the Alborz Mountains in Iran using both Envisat and ALOS SAR data. We modify several aspects within the chain of the processing including: filtering prior to phase unwrapping, topographic correction within three-dimensional phase unwrapping, reducing the atmospheric noise with the help of additional GPS data, and removing the ramp caused by ionosphere turbulence and/or orbit errors to better estimate crustal deformation in this tectonically active region. Topographic correction is done within the three-dimensional unwrapping in order to improve the phase unwrapping process, which is in contrast to previous methods in which DEM error is estimated before/after phase unwrapping. Our experiments show that our improved SBAS approach is able to better characterize the tectonic and volcanic deformation in the center of the Alborz region than the classical SBAS. In particular, Damavand volcano shows an average uplift rate of about 3 mm/year in the year 2003–2010. The Mosha fault illustrates left-lateral motion that could be explained with a fault that is locked up to 17–18 km depths and slips with 2–4 mm/year below that depth.

InSAR velocity field across the North Anatolian Fault (eastern Turkey): Implications for the loading and release of interseismic strain accumulation

AbstractWe use the Persistent Scatterer Interferometric Synthetic Aperture Radar (PS‐InSAR) technique with the European Space Agency's Envisat and ERS SAR data acquired on three neighboring descending tracks (T350, T078, and T307) to map the interseismic strain accumulation along a ~225 km long, NW‐SE trending section of the North Anatolian Fault that ruptured during the 1939, 1942, and 1943 earthquakes in eastern Turkey. We derive a line‐of‐sight velocity map of the region with a high spatial resolution and accuracy which, together with the maps of earthquake surface ruptures, shed light on the style of continental deformation and the relationships between the loading and release of interseismic strain along segmented continental strike‐slip faults. In contrast with the geometric complexities at the ground surface that appear to control rupture propagation of the 1939 event, modeling of the high‐resolution PS‐InSAR velocity field reveals a fairly linear and narrow throughgoing shear zone with an overall 20 ± 3 mm/yr slip rate above an unexpectedly shallow 7 ± 2 km locking depth. Such a shallow locking depth may result from the postseismic effects following recent earthquakes or from a simplified model that assumes a uniform degree of locking with depth on the fault. A narrow throughgoing shear zone supports the thick lithosphere model in which continental strike‐slip faults are thought to extend as discrete shear zones through the entire crust. Fault segmentation previously reported from coseismic surface ruptures is thus likely inherited from heterogeneities in the upper crust that either preexist and/or develop during coseismic rupture propagation. The geometrical complexities that apparently persist for long periods may guide the dynamic rupture propagation surviving thousands of earthquake cycles.

Predictability of hydraulic head changes and characterization of aquifer‐system and fault properties from InSAR‐derived ground deformation

AbstractWe evaluate the benefits of space‐derived ground deformation measurements for basin‐wide characterization of aquifer‐system properties and groundwater levels. We use Interferometric Synthetic Aperture Radar (InSAR) time series analysis of ERS, Envisat, and ALOS SAR data to resolve 1992–2011 ground deformation in the Santa Clara Valley, California. T‐mode principal component analysis successfully isolates temporally variable deformation patterns embedded in the multidecadal time series. The data reveal uplift at 0.4 cm/yr between 1992 and 2000 and < 0.1 cm/yr during 2000–2011, illustrating the end of the aquifer‐system's poroelastic rebound following recovery of hydraulic heads after the 1960s low stand. In addition, seasonal elastic deformation with amplitude of up to 3 cm, in phase with head fluctuations, is observed over the confined aquifer sharply partitioned by the Quaternary Silver Creek Fault (SCF). Integration of this deformation with hydraulic head data enables characterization of the aquifer‐system storativity and elastic skeletal specific storage. Modeling of the deformation partitioning across the SCF constrains the fault's last tectonic activity, hydraulic conductivity, and material composition. The SCF likely cuts the shallow confining clays and was last active since ~140 ka, it has a horizontal hydraulic conductivity several orders of magnitude lower than the surrounding aquifer‐system, and is likely composed of clays, making it an effective barrier to across‐fault fluid flow. Finally, we show that after a period of calibration, InSAR can be used to characterize basin‐wide water level changes without well measurements with an accuracy of 70%, which demonstrates that it provides useful data for groundwater management.

Shallow creep on the Haiyuan Fault (Gansu, China) revealed by SAR Interferometry

Interferometric synthetic aperture radar data are used to map the interseismic velocity field along the Haiyuan fault system (HFS), at the north‐eastern boundary of the Tibetan plateau. Two M ∼ 8 earthquakes ruptured the HFS in 1920 and 1927, but its 260 km‐long central section, known as the Tianzhu seismic gap, remains unbroken since ∼1000 years. The Envisat SAR data, spanning the 2003–2009 period, cover about 200 × 300 km2 along three descending and two ascending tracks. Interferograms are processed using an adapted version of ROI_PAC. The signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals. Mean line‐of‐sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise. These maps show a dominant left‐lateral motion across the HFS, and reveal a narrow, 35 km‐long zone of high velocity gradient across the fault in between the Tianzhu gap and the 1920 rupture. We model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion. The inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments. The average creep rate is ∼5 mm yr−1, comparable in magnitude with the estimated loading rate at depth, suggesting no strain accumulation on this segment. The modeled creep rate locally exceeds the long term rate, reaching 8 mm yr−1, suggesting transient creep episodes. The present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment.

Extension on the Tibetan plateau: recent normal faulting measured by InSAR and body wave seismology

We use InSAR and body wave modelling to determine the faulting parameters for a series of five Mw 5.9–7.1 normal faulting earthquakes that occurred during 2008, including the March 20 Yutian earthquake (Mw 7.1), one of the largest normal faulting events to have occurred recently on the continents. We also study three earlier normal faulting earthquakes that occurred in southern Tibet between 1992 and 2005. Coseismic deformation for each of these eight events is measured with ascending and descending interferograms from ENVISAT, ERS and ALOS SAR data. Elastic dislocation modelling of the line-of-sight InSAR displacements and body wave seismological modelling of P and SH waves are used to estimate fault parameters and are found to be in good agreement for all the events studied. The use of InSAR to measure deformation allows a relatively precise determination of the fault location in addition to resolving the focal plane ambiguity. Only five of the eight events are associated with a clear surface topographic feature, suggesting that an underestimation of the amount of extension would result from using the surface expressions of normal faulting alone. The observations, in all cases, are consistent with slip on planar surfaces, with dips in the range 40–50°, that penetrates the uppermost crust to a depth of 10–15 km. We find no evidence for active low-angle (dip less than 30°) normal faulting. The contribution of the normal faulting to overall extension estimated by summing seismic moments over earthquakes for the past 43 yr is 3–4 mm yr−1, or 15–20 per cent of the rates of extension measured across the plateau using GPS. 85 per cent of the moment release in normal faulting over the past 43 yr has occurred in regions whose surface height exceeds 5 km. This observation adds weight to the suggestions that the widespread normal faulting on the plateau is the result of variations in the gravitational potential energy of the lithosphere.

Measurements and predictions of subsidence induced by soil consolidation using persistent scatterer InSAR and a hyperbolic model

A space‐borne SAR interferometric technique is presented for measuring and predicting ground subsidence associated with soil consolidation. Instead of a conventional constant velocity model, a hyperbolic model is introduced for persistent scatterer SAR interferometry (PSI) processing. Twenty three JERS‐1 SAR acquired between 1992 and 1998 were used to measure land subsidence in Mokpo city, Korea which had been primarily built on land reclaimed from the sea. Two subsidence field maps were derived and compared: a constant velocity model and a hyperbolic model. Non‐linear components depending on the stage of soil consolidation are well represented by the hyperbolic model. The maximum subsidence velocity reaches over 6 cm/yr, while the maximum acceleration is about −0.3 to −0.4 cm/year2. The predicted subsidence rate with the new model was validated by using later ENVISAT SAR data for 2004–2005. Prediction accuracy with the non‐linear model is improved significantly, indicating the importance of a physically‐based deformation model.

Coseismic Three-Dimensional Displacements Determined Using SAR Data: Theory and an Application Test

We present a methodology to construct three-dimensional deformation maps using different parameters that can be extracted using SAR data. We apply the methodology using ENVISAT SAR data before and after the December 26 th , 2003 Mw 6.6 Bam earthquake in Iran to determine spatial quasi-continuous three- dimensional coseismic deformation maps. Two near vertical deformation offset components are computed using Envisat ASAR differential interferometry (DInSAR), while two horizontal deformation offset components are measured using sub-pixel correlation techniques applied to ASAR amplitude images. Using the presented methodology, we combine four unevenly precise independent projections of surface coseismic deformation to obtain the full three-dimensional coseismic deformation field caused by an earthquake. The full 3-D coseismic displacement vector is modeled using elastic deformation models constraining details of the fault geometry and slip distribution at depth. Results from the inversion are interpreted in the framework of the Iranian present-day tectonism. Full exploitation of dense 3-D coseismic surface deformation using SAR data, even for moderate earthquakes, should facilitate distinguishing between different interpretations of the mechanical properties of seismically active areas and within the inherent ambiguity of the geophysical inversion solutions.

InSAR-based mapping of surface subsidence in Mokpo City, Korea, using JERS-1 and ENVISAT SAR data

Abstract Mokpo City, located on the southwestern coast of the Korean Peninsula, has been built on one of the largest areas of reclaimed coastal land in Korea. This reclaimed land is currently experiencing significant ground subsidence due to soil consolidation. We have estimated the subsidence rate of Mokpo City (8 × 8 km) using the synthetic aperture radar interferometry (InSAR) and InSAR permanent scatterer (PSInSAR) techniques to analyze 26 JERS-1 SAR images acquired between 1992 and 1998 and six ENVISAT ASAR images acquired in 2004–2005. Mean subsidence velocity, which was clearly related to reclaimed land, was computed from the JERS-1 PSInSAR analysis. The results indicate a continuous and significant subsidence at three sites (Dongmyung, Hadang and Wonsan), where the subsidence velocity has reached more than 5–7 cm/yr in the area of maximum subsidence. The subsidence rate was found to have decreased in Wonsan and Hadang between 1992 and 1998, while it remained steady or increased in Dongmyung during the same period. The subsidence extended to the period of 2004–2005, and the subsidence rate predicted by the JERS-1 PSInSAR analysis using a linear model was confirmed by the ENVISAT ASAR InSAR results. Our results show that InSAR/PSInSAR-based subsidence maps are useful for the long-term monitoring of soil consolidation and for defining risk zones in coastal reclaimed regions.

The 2004–2006 uplift episode at Campi Flegrei caldera (Italy): Constraints from SBAS‐DInSAR ENVISAT data and Bayesian source inference

We investigate the 2004–2006 uplift phase of Campi Flegrei caldera (Italy) by exploiting the archive of ascending and descending ENVISAT SAR data acquired from November 2002 to November 2006. The SBAS‐DInSAR technique is applied to generate displacement mean velocity maps and time series. An appropriate post‐processing step is subsequently applied to map the areas whose temporal deformation behavior is correlated with that of the maximum uplift zone. Our results show that the deformation also extends outside the volcanological limits of the Neapolitan Yellow Tuff caldera, without significant discontinuities. The DInSAR data are inverted by considering a finite spheroid and an isotropic point‐source. The inversion results suggest that the new uplift is characterized by a source location similar to the previous small uplift event of 2000 and to the long term subsidence of the 1990's. In particular, the source is located at a depth of about 3.2 km and very close to the city of Pozzuoli (about 800 m offshore, to the SW); the associated volume variation is about 1.1 106 m3/year.

Uncertainty analysis of earthquake source parameters determined from InSAR: A simulation study

We assessed the accuracy of earthquake source parameters inverted from simulated Interferometric Synthetic Aperture Radar (InSAR) data. Using focal mechanisms of Australian earthquakes (1959 to the present), we simulated synthetic two‐pass InSAR observations with realistic spatial noise derived from the characteristics of actual ERS‐2 and ENVISAT InSAR data observed over Australia. The precision of two‐pass satellite SAR interferometry with ERS‐2 and ENVISAT SAR data in the Australian region can approach ±2 mm (1σ) and is routinely at the ±4 mm level. The use of spatially correlated observational weights has minimal impact on the accuracy of earthquake source parameters inverted from InSAR data. In most cases single geometry (i.e., ascending or descending) InSAR observations can be used to accurately determine earthquake source parameters, although typically a combined geometry reduces the source parameter uncertainties by a factor of 1.5. In general, earthquakes of magnitude <4.8 are unlikely to be observable by InSAR although very shallow events would be detectable. InSAR is insensitive to magnitude 6.2 earthquakes deeper than 10 km, and magnitude 5.5 deeper than 6 km. For earthquake magnitudes ≥5.8 (average depth 6.5 km) we could estimate the epicenter of the rupture with an average accuracy of 0.25 km, depth to within 0.5 km and the fault orientation to better than 2°. Our findings, based on simulated Australian earthquakes, are representative of typical intraplate earthquakes and would be valid in many other regions. To date no actual earthquakes have been observed by InSAR in Australia.