Synthetic Aperture Radar Measurements Research Articles

Toward Mitigating Stratified Tropospheric Delays in Multitemporal InSAR: A Quadtree Aided Joint Model

Tropospheric delays (TDs) in differential interferometric synthetic aperture radar (InSAR) measurements are mainly caused by spatial and temporal variation of pressure, temperature, and humidity between SAR acquisitions. These delays are described as one of the primary error sources in InSAR observations. Although independent atmospheric measurements have been used to correct TDs, their sparse spatial or temporal resolution requires interpolation, leading to uncertainties in the corrected interferograms. The performance of the conventional phase-based correction method is weakened by the presence of confounding signals (e.g., TDs, deformation, and topographic errors) and spatial variability of the troposphere. Here, we propose a method that can simultaneously estimate stratified TDs together with parameters of deformation and topographic error based on their distinct spatial–temporal correlation. Spatial variability of the relationship between TDs and topographic height is addressed through localized estimation in windows divided by quadtree according to height gradient. We demonstrate the performance of the proposed method with both simulated and real data sets. In addition, both advantages and disadvantages of this method are addressed.

On the analysis of a time series of X–band TerraSAR–X SAR imagery over oil seepages

ABSTRACTA large time series of 42 dual–polarimetric co–polarized TerraSAR-X (TSX) StripMap Synthetic Aperture Radar (SAR) measurements are exploited to monitor a well-known oil seep area, i.e., the Taylor Energy site in the Gulf of Mexico. A comprehensive scattering analysis is undertaken to assess the impact of SAR imaging parameters (polarization, angle of incidence – AOI, noise floor) and environmental conditions (wind speed – WS, oil properties) on single-polarization SAR–based sea oil seep observation. The main goal of this study is to evaluate the reliability of the scattering–based information derived from the time series of TSX SAR imagery. A two-scale backscattering model is considered to give a physical framework that supports a better understanding of the effects of the above–mentioned factors.Experimental results showed that the high TSX noise floor significantly limits a reliable interpretation of the slick–free sea surface and oil seep backscattering at AOIs larger than 34 and 26, respectively, since they are contaminated by noise. Hence, it is shown that, at larger AOIs, the joint contribution of noisy SAR measurements and low oil backscattering does not result in a larger oil/sea separability. The latter is not remarkably influenced by polarization and WS, under low–to–moderate conditions. Experiments also demonstrate that, when reliable SAR measurements are available, sea oil seep backscattering is affected by the oil’s damping properties more than its concentration in the water column. The time variability of the polluted area is also estimated using the time series of TSX imagery and the obtained results agree with independent analysis undertaken on the same test site.

Multiclass Radio Frequency Interference Detection and Suppression for SAR Based on the Single Shot MultiBox Detector.

Radio frequency interference (RFI) is known to jam synthetic aperture radar (SAR) measurements, severely degrading the SAR imaging quality. The suppression of RFI in SAR echo signals is usually an underdetermined blind source separation problem. In this paper, we propose a novel method for multiclass RFI detection and suppression based on the single shot multibox detector (SSD). First, an echo-interference dataset is established by randomly combining the target signal with various types of RFI in a simulation, and the time–frequency form of the dataset is obtained by utilizing the short-time Fourier transform (STFT). Next, the time–frequency dataset acts as input data to train the SSD and obtain a network that is capable of detecting, identifying and estimating the interference. Finally, all of the interference signals are exactly reconstructed based on the prediction results of the SSD and mitigated by an adaptive filter. The proposed method can effectively increase the signal-to-interference-noise ratio (SINR) of RFI-contaminated SAR echoes and improve the peak sidelobe ratio (PSLR) after pulse compression. The simulated experimental results validate the effectiveness of the proposed method.

3-D Positioning and Target Association for Medium-Resolution SAR Sensors

Associating a radar scatterer to a physical object is crucial for the correct interpretation of interferometric synthetic aperture radar measurements. Yet, especially for medium-resolution imagery, this is notoriously difficult and dependent on the accurate 3-D positioning of the scatterers. Here, we investigate the 3-D positioning capabilities of ENVISAT medium-resolution data. We find that the data are perturbed by range-and-epoch-dependent timing errors and calibration offsets. Calibration offsets are estimated to be about 1.58 m in azimuth and 2.84 m in range and should be added to ASAR products to improve geometric calibration. The timing errors involve a bistatic offset, atmospheric path delay, solid earth tides, and local oscillator drift. This way, we achieve an unbiased positioning capability in 2-D, while in 3-D, a scatterer was located at a distance of 28 cm from the true location. 3-D precision is now expressed as an error ellipsoid in local coordinates. Using the Bhattacharyya metric, we associate radar scatterers to real-world objects. Interpreting deformation of individual infrastructure is shown to be feasible for this type of medium-resolution data.

COSMO–SkyMed Synthetic Aperture Radar Data to Observe the Deepwater Horizon Oil Spill

Oil spills are adverse events that may be very harmful to ecosystems and the food chain. In particular, large sea oil spills are very dramatic occurrences that may affect sea and coastal areas. Hence, the sustainability of oil rig infrastructures and oil transportation via oil tankers is linked to law enforcement based on proper monitoring techniques, which are also fundamental to mitigate the impact of such pollution. In this study, a showcase referring to the Deepwater Horizon (DWH) oil incident, one of the world’s largest incidental oil pollution event that occurred in the Gulf of Mexico in 2010 affecting a sea area larger than 10,000 km 2 , is analyzed using remotely-sensed information collected by Synthetic Aperture Radar (SAR). Although, operationally, SAR sea oil slick observation is typically accomplished using C-band VV-polarized SAR imagery, during the DWH oil incident, because of their very dense revisit time, even single-polarization X-band COSMO-SkyMed (CSK) SAR measurements were collected. In this study, we exploit, for the first time, incoherent dual co-polarization SAR data collected by the Italian CSK X-band SAR constellation showing the key benefits of HH-VV SAR measurements in observing such a huge oil pollution event, especially in terms of the very dense revisit time offered by the CSK constellation.

Combining InSAR and Hydraulic Head Measurements to Estimate Aquifer Parameters and Storage Variations of Confined Aquifer System in Cangzhou, North China Plain

AbstractGroundwater storage (GWS) depletion and land subsidence caused by groundwater overexploitation threaten water supply and infrastructure security in the North China Plain. However, the aquifer system responses to groundwater changes are poorly understood in this region, although it is extremely important for groundwater management and land subsidence control. In this study, aquifer parameters and GWS variations of the confined aquifer system in central Cangzhou are quantified by using Interferometric Synthetic Aperture Radar (InSAR) and hydraulic head measurements. We apply the multitemporal InSAR method to detect land deformation time series with Envisat Advanced Synthetic Aperture Radar (ASAR) data from 2003 to 2010. Seasonal signals in land deformation and head data are isolated by multichannel singular spectrum analysis to derive the elastic skeletal storativity (Ske). The results reveal a spatially heterogeneous Ske pattern with values ranging from 1.7 × 10−4 to 4.4 × 10−3 that agree well with those derived from pumping tests. An abnormal Skezone is found in the land uplift zone, and a gradual increase of Ske is revealed in the eastern coastal plain, which are likely attributed to the hydrological properties in the aquifer systems. Additionally, we characterize spatiotemporal variations in the total GWS, recoverable GWS, and irreversible GWS. The annual depletion rates of recoverable GWS and irreversible GWS are −0.52 ± 1.27 × 107 and −7.68 ± 2.68 × 107 m3/year, accounting for 6.4% and 93.6% of annual total GWS depletion, respectively, suggesting that previous groundwater pumping practices are unsustainable in this region. Finally, we find a 1‐year lag of the IGWS depletion variations behind the head changes due to low vertical hydraulic conductivities in aquitards.

Ionospheric Scintillation Observation Using Space‐Borne Synthetic Aperture Radar Data

AbstractNetworks of ground‐based global navigation satellite system (GNSS) receivers have been widely used to monitor scintillation caused by irregularities in the disturbed ionosphere. Due to the relative sparseness of such networks, however, scintillation measurements are lacking in many regions of the globe, and even in well‐instrumented geographic areas, the spacing between receivers is often too large to study the systematic spatial changes in scintillation characteristics, for example, across the equatorial anomaly region. This paper discusses the potential of studying ionospheric scintillations using low‐frequency synthetic aperture radar (SAR). It compares standard metrics of scintillation including the amplitude scintillation index S4 and vertically integrated strength of turbulence CkL, from GNSS and SAR, on two different dates with varying ionospheric conditions. For this study, polarimetric L‐band SAR images acquired from the Phased Array‐type L‐band Synthetic Aperture Radar sensor onboard the Advanced Land Observational Satellite‐2 have been used. A number of GNSS satellites also observed the particular scintillation event that was encountered by SAR on the night of 23 March 2015 over the southern and mid‐central India. The S4 index derived from SAR are computed using previously published techniques in terms of radar backscatter (σ°) enhancement and the image contrast. The results show a favorable correlation with the GNSS observations. Along with accurate information about satellite geometry and operating frequency, few spectral properties of ionospheric irregularities, such as spectral index, anisotropy, and outer scale, have been assumed from historically available low‐latitude scintillation observations to calculate the turbulence strength parameter. The results are well corroborated by measurements from four GNSS stations in India, thus demonstrating the utility of the SAR measurements in augmenting and complementing the ionospheric scintillation diagnostics available from GNSS.

Radar Path Delay Effects in Volcanic Gas Plumes: The Case of Láscar Volcano, Northern Chile

Modern volcano monitoring commonly involves Interferometric Synthetic Aperture Radar (InSAR) measurements to identify ground motions caused by volcanic activity. However, InSAR is largely affected by changes in atmospheric refractivity, in particular by changes which can be attributed to the distribution of water (H2O) vapor in the atmospheric column. Gas emissions from continuously degassing volcanoes contain abundant water vapor and thus produce variations in the atmospheric water vapor content above and downwind of the volcano, which are notably well captured by short-wavelength X-band SAR systems. These variations may in turn cause differential phase errors in volcano deformation estimates due to excess radar path delay effects within the volcanic gas plume. Inversely, if these radar path delay effects are better understood, they may be even used for monitoring degassing activity, by means of the precipitable water vapor (PWV) content in the plume at the time of SAR acquisitions, which may provide essential information on gas plume dispersion and the state of volcanic and hydrothermal activity. In this work we investigate the radar path delays that were generated by water vapor contained in the volcanic gas plume of the persistently degassing Láscar volcano, which is located in the dry Atacama Desert of Northern Chile. We estimate water vapor contents based on sulfur dioxide (SO2) emission measurements from a scanning UV spectrometer (Mini-DOAS) station installed at Láscar volcano, which were scaled by H2O/SO2 molar mixing ratios obtained during a multi-component Gas Analyzer System (Multi-GAS) survey on the crater rim of the volcano. To calculate the water vapor content in the downwind portion of the plume, where an increase of water vapor is expected, we further applied a correction involving estimation of potential evaporation rates of water droplets governed by turbulent mixing of the condensed volcanic plume with the dry atmosphere. Based on these estimates we obtain daily average PWV contents inside the volcanic gas plume of 0.2–2.5 mm equivalent water column, which translates to a slant wet delay (SWD) in DInSAR data of 1.6–20 mm. We used these estimates in combination with our high resolution TerraSAR-X DInSAR observations at Láscar volcano, in order to demonstrate the occurrence of repeated atmospheric delay patterns that were generated by volcanic gas emissions. We show that gas plume related refractivity changes are significant and detectable in DInSAR measurements. Implications are two-fold: X-band satellite radar observations also contain information on the degassing state of a volcano, while deformation signals need to be interpreted with care, which has relevance for volcano observations at Láscar and for other sites worldwide.

Sensitivity of SAR Tomography to the Phenological Cycle of Agricultural Crops at X-, C-, and L-band

Understanding the impact of soil and plant parameter changes in agriculture on Synthetic Aperture Radar (SAR) measurements is of great interest when it comes to monitor the temporal evolution of agricultural crops by means of SAR. In this regard, specific transitions between phenological stages in corn, barley, and wheat have been identified associated to certain dielectric and geometric changes, based on a time series of fully polarimetric multibaseline SAR data and in situ measurements. The data have been acquired in the frame of DLR's CROPEX campaign on six dates between May and July in 2014. The experiments reported in this paper address the sensitivity of X-, C-, and L-band to phenological transitions exploiting the availability of multiple baselines on each acquisition date. The application of tomographic techniques enables the estimation of the three-dimensional (3-D) backscatter distribution and the separation of ground and volume scattering components. Tomographic parameters have been derived at different frequencies, namely the center of mass of the profiles of the total and of the volume-only 3-D backscatter, and the ground and volume powers. Their sensitivity and ability to detect changes occurring on the ground and in the vegetation volume have been evaluated focusing on the added value provided by the 3-D resolution at the different frequencies and polarizations available.

Ship Detection Using Compact Polarimetric SAR Based on the Notch Filter

Compact polarimetric data exploitation, especially in hybrid-polarimetric (HP) mode, is currently attracting increasing interest due to the new generation of synthetic aperture radar (SAR) systems. Recently, it has been demonstrated that the notch filter is useful for ship detection in either full- or dual-polarization (DP)-mode SAR images. In this paper, the notch filter investigation is further extended to HP SAR architecture for ship detection on the ocean surface. First, a version of the notch filter that is suitable for HP SAR is proposed based on the definition of the corresponding feature partial scattering vector from the covariance matrix of the HP SAR. Subsequently, a novel model characterizing the statistics of the notch distance of sea clutter in the HP mode is developed. Based on the statistical model, the threshold of constant false-alarm rate (CFAR) ship detection is theoretically and analytically derived, which allows the automatic and adaptive implementation for ship detection in varying sea backgrounds in practical applications. Experiments on the HP SAR data emulated from full-polarization L-band Aerospace Exploration Agency Advanced Land Observation Satellite Phased-Array type L-band SAR and C-band RADARSAT-2 SAR measurements validate not only the soundness of the proposed CFAR detection but also the high accuracy of the presented model in fitting HP SAR data. Furthermore, the notch filter and its CFAR realization provide the same benchmark for the comparison of the detectability of HP and conventional linear DP SAR data. Preliminary findings suggest that the detection performance of HP SAR is superior to that of DP SAR in ship observation. Therefore, the proposed CFAR method based on the notch filter provides a promising technique for the detection of ships using HP SAR data.

An Empirical Algorithm to Retrieve Significant Wave Height from Sentinel-1 Synthetic Aperture Radar Imagery Collected under Cyclonic Conditions

In this study, an empirical algorithm is proposed to retrieve significant wave height (SWH) from dual-polarization Sentinel-1 (S-1) synthetic aperture radar (SAR) imagery collected under cyclonic conditions. The retrieval scheme is based on the well-known CWAVE empirical function that is here updated to deal with multi-polarization S-1 SAR measurements collected using the interferometric wide (IW) and the Extra Wide-Swath (EW) imaging modes, under cyclonic conditions. First, a training dataset that consists of six S-1 SAR images collected under cyclonic conditions is exploited to both tune the retrieval function and to check the soundness of the retrievals against the co-located WAVEWATCH-III (WW3) numerical simulations. The comparison of simulation from the WW3 model and measurements from altimeter Jason-2 shows a 0.29m root mean square error (RMSE) of significant wave height (SWH). Then, a testing data-set that consists of two S-1 SAR images is exploited to provide a preliminary validation. The results, verified against both WW3 and European Centre for Medium-Range Weather Forecasts (ECMWF) data, show the soundness of the herein approach.

Symmetric Double-Eye Structure in Hurricane Bertha (2008) Imaged by SAR

Internal dynamical processes play a critical role in hurricane intensity variability. However, our understanding of internal storm processes is less well established, partly because of fewer observations. In this study, we present an analysis of the hurricane double-eye structure imaged by the RADARSAT-2 cross-polarized synthetic aperture radar (SAR) over Hurricane Bertha (2008). SAR has the capability of hurricane monitoring because of the ocean surface roughness induced by surface wind stress. Recently, the C-band cross-polarized SAR measurements appear to be unsaturated for the high wind speeds, which makes SAR suitable for studies of the hurricane internal dynamic processes, including the double-eye structure. We retrieve the wind field of Hurricane Bertha (2008), and then extract the closest axisymmetric double-eye structure from the wind field using an idealized vortex model. Comparisons between the axisymmetric model extracted wind field and SAR observed winds demonstrate that the double-eye structure imaged by SAR is relatively axisymmetric. Associated with airborne measurements using a stepped-frequency microwave radiometer, we investigate the hurricane internal dynamic process related to the double-eye structure, which is known as the eyewall replacement cycle (ERC). The classic ERC theory was proposed by assuming an axisymmetric storm structure. The ERC internal dynamic process of Hurricane Bertha (2008) related to the symmetric double-eye structure here, which is consistent with the classic theory, is observed by SAR and aircraft.

Spectrum analysis of high‐resolution SAR data to obtain Bragg signatures of power cables

A spectrum analysis method is presented that can be used on high-resolution synthetic aperture radar (SAR) measurements to concentrate the returns from overhead power cables into discrete angles, alongside the resulting images and information obtained by applying the technique to spotlight SAR data obtained at Ka-band. The angular width and position of the specular and Bragg lobes are used to estimate the periodic length of the cable structure. The results presented also show the potential of detecting variations in the tautness of a power line along its length and hence make any vulnerabilities to the supply of electrical power detectable remotely. Detailed high-resolution images constructed also support peculiarities measured in these signatures. The presented results show that the described method has the ability to detect distribution line vulnerabilities and hazards to the surrounding area. As the cable structure is also related to temperature, this technique has the potential to observe changes in the climate.

Rupture process of the 2015 Pishan earthquake from joint inversion of InSAR, teleseismic data and GPS

An earthquake of M w6.4 occurred in Pishan County in Xinjiang Province, northwestern Tibetan Plateau, on July 3, 2015. The epicenter was located on an active blind thrust system located at the northern margin of the Western Kunlun Mountain Orogenic Belt southwest of the Tarim Basin. We constructed a shovel-shaped fault model based on the layered-crust model with reference to the seismic reflection profile, and obtained the rupture process of the earthquake from the joint inversion of Interferometric Synthetic Aperture Radar (InSAR) measurements, far-field waveform data, and Global Positioning System (GPS) data. The results show that the seismic fault dips southward with a strike of 109°, and the rupture direction was essentially northward. The fault plane rupture distribution is concentrated, with a maximum recorded slip of 73 cm. The main features of the fault are as follows: low inclination angle (25°–10°), thrust slip at a depth of 9–13 km, rupture propagation time of about 12 s, no significant slip in soft or hard sedimentary layers at 0–4 km depth and propagation from the initial rupture point to the surrounding area with no obvious directionality. The InSAR time-series analysis method is used to determine the deformation rate in the source region within 2 years after the earthquake, and the maximum value is ~17 mm yr−1 in the radar line-of-sight direction. Obvious post-earthquake deformation is evident in the hanging wall, with a similar trend to the coseismic displacement field. These results suggest that the Pishan earthquake has not completely released the accumulated energy of the region, given that the multilayer fold structure above the blind fault is still in a process of slow uplift since the earthquake. Post-earthquake adjustment models and aftershock risk analysis require further study using more independent data.

Storm waves focusing and steepening in the Agulhas current: Satellite observations and modeling

Strong ocean currents can modify the height and shape of ocean waves, possibly causing extreme sea states in particular conditions. The risk of extreme waves is a known hazard in the shipping routes crossing some of the main current systems. Modeling surface current interactions in standard wave numerical models is an active area of research that benefits from the increased availability and accuracy of satellite observations. We report a typical case of a swell system propagating in the Agulhas current, using wind and sea state measurements from several satellites, jointly with state of the art analytical and numerical modeling of wave-current interactions. In particular, Synthetic Aperture Radar and altimeter measurements are used to show the evolution of the swell train and resulting local extreme waves. A ray tracing analysis shows that the significant wave height variability at scales <~100 km is well associated with the current vorticity patterns. Predictions of the WAVEWATCH III numerical model in a version that accounts for wave-current interactions are consistent with observations, although their effects are still under-predicted in the present configuration. From altimeter measurements, very large significant wave height gradients are shown to be well captured, and also associated with the current vorticity patterns at global scale.

Geodetic Constraints of the 2017 Mw7.3 Sarpol Zahab, Iran Earthquake, and Its Implications on the Structure and Mechanics of the Northwest Zagros Thrust‐Fold Belt

AbstractWe reveal transient surface deformation following the 2017 Mw7.3 Sarpol Zahab (Iran) earthquake using Interferometric Synthetic Aperture Radar (InSAR) measurements. Based on the coseismic interferograms derived from the Advanced Land Observing Satellite‐2 (ALOS‐2) data, the preferred slip model of the earthquake has a centroid depth of 14.5 ± 4 km and suggests that a basement fault is most likely responsible for the 2017 earthquake in the northwest Zagros fold‐thrust belt zone. Two slip asperities with a maximum slip of 6 m separated by 16 km are observed in the best fitting slip model. The accumulated afterslip in the first month after the mainshock determined from the Sentinel‐1 postseismic interferograms reveals a slip distribution that lies immediately updip of the coseismic slip, implying that frictional properties of the fault vary along the rupture patch. The Bamo Mount in the earthquake area was uplifted by approximately 1 m during the earthquake. We suggest that local topographic growth from both seismic and aseismic faulting behaviors is common within the Zagros fold‐thrust belt zone based on our findings regarding the 2017 earthquake and other significant events in the region.

The AfriSAR Campaign: Tomographic Analysis With Phase-Screen Correction for P-Band Acquisitions

ESA's earth explorer BIOMASS mission is a P -band (432–438 MHz) synthetic aperture radar (SAR) using a combination of polarimetry and interferometric observations to quantify the vertical structure and biomass of global forests, with the primary focus on tropical forests. The methodology to map the vertical structure of the forest is based on multibaseline tomographic measurements from space. In this paper, we use data acquired by airborne sensors during the AfriSAR campaign in humid tropical forests of Africa to examine the potential of P -band tomographic SAR measurements in estimating forest parameters. We use data acquired by ONERA's P -band SAR system over the Lope National Park in central Gabon during July 2015 to estimate vertical profiles. In processing the multibaseline data, we develop and implement a phase-screen correction methodology based on recent works by Tebaldini et al. to improve the quality of measurements by removing phase perturbations associated with platform motions and uncertainties in flight trajectories. The vertical structure estimated from the corrected tomographic measurements are then compared with small and large footprint light detection and ranging (Lidar) observations collected as part of the AfriSAR campaign. The results suggest that phase-screen correction can significantly improve the vertical profile of radar backscattered power to match the Lidar observations in detecting ground, vertical vegetation density, and total height of the forests across a variety of forest types and terrain complexity.

Marine Spatial Planning Using High-Resolution Synthetic Aperture Radar Measurements

In this paper, we highlight the importance of high-resolution wind data on the application of multicriteria evaluation technique to colocate offshore wind farms and open-water mussel cultivations. An index of colocation sustainability (SI), based on an environmental information, is constructed using remote sensing data and taking into account both physical constraints (i.e., water depth and wind speed) and environmental data (i.e., chlorophyll-a, sea surface temperature anomaly, and particulate organic carbon). To verify the proposed methodology, five showcases are presented, where SI is evaluated considering potential installation sites in Kattegat, Denmark, using both low-resolution (LR) wind reanalysis maps related to the Modern Era Retrospective-Analysis for Research and Application data set and fine-resolution wind maps obtained by processing synthetic aperture radar (SAR) data. Experimental results show that the availability of a reliable fine-resolution wind information is of great importance in coastal areas where the presence of the land and the isles limits the use of LR wind data.

Inferring Geothermal Reservoir Processes at the Raft River Geothermal Field, Idaho, USA, Through Modeling InSAR‐Measured Surface Deformation

AbstractGround surface deformations detected with interferometric synthetic aperture radar provide valuable information for inferring subsurface reservoir processes that are difficult to observe directly. This study aims at building a reservoir model that honors the available geological, hydrological, and geomechanical data and also produces ground surface deformation consistent with interferometric synthetic aperture radar measurements at the Raft River Geothermal Field. In the thermo‐hydro‐mechanical coupled model developed, the reservoir deforms as a result of the rock's poroelastic response to changes in hydrologic pressure and thermal expansion/contraction. The results indicate that the observed deformation is the result of pressure decrease in the deep production reservoir and pressure increase in a shallower reservoir that accommodates the injected fluid (likely in the Salt Lake Formation). The combination of the uplift around injection wells with subsidence around the production wells, affected by the Bridge Fault as a flow barrier, creates a complex pattern of surface deformation in which the center of subtle subsidence significantly deviates from the location of the production wells. A parametric study suggests that (a) the Bridge Fault Zone is likely a flow barrier, (b) the surface deformation appears to be insensitive to the presence of the Narrows Structure, and (c) additional flow barriers likely exist to shape the flow system. This case study demonstrates the utility of a high‐fidelity forward model that honors available known information and thermo‐hydro‐mechanical coupled processes in understanding geothermal reservoir characteristics.

Small Magnitude Co-Seismic Deformation of the 2017 Mw 6.4 Nyingchi Earthquake Revealed by InSAR Measurements with Atmospheric Correction

The Nyingchi Mw 6.4 earthquake on 17 November 2017 is the first large event since 1950 at the southeast end of the Jiali fault. This event was captured by interferometric synthetic aperture radar (InSAR) measurements from the European Space Agency (ESA) Sentinel-1A radar satellite, which provide the potential to determine the fault plane, as well as the co-seismic slip distribution, and understand future seismic hazards. However, due to the limited magnitude of surface displacements and the strong topography variations, InSAR-derived co-seismic signals are contaminated by strong tropospheric effects which makes it difficult (if not impossible) to determine the source parameters and co-seismic slip distribution. In this paper, we employ the Generic Atmospheric Correction Online Service for InSAR (GACOS) to generate correction maps for the co-seismic interferograms, and successfully extract co-seismic surface displacements for this large event. The phase standard deviation after correction for a seriously-contaminated interferogram reaches 0.8 cm, significantly improved from the traditional phase correlation analysis (1.13 cm) or bilinear interpolation (1.28 cm) methods. Our best model suggests that the seismogenic fault is a NW–SE striking back-thrust fault with a right-lateral strike slip component. This reflects the strain partitioning of NE shortening and eastward movement of the Eastern Tibetan plateau due to the oblique convergence between the Indian and Eurasian plates.