A sensitivity analysis of dual-polarization features to damage due to the 2016 Central-Italy earthquake

ABSTRACT

In this study, the sensitivity of dual-polarization features extracted from dual-polarimetric coherent synthetic aperture radar (SAR) imagery to urban damage due to the earthquake occurred in Central -Italy in 2016 is addressed. First, a conventional single-polarization feature, based on the coherence between pre- and post-event co-polarized imagery, is analysed using SAR scenes collected by the Copernicus Sentinel-1 mission over the area of Amatrice, an Italian city significantly damaged by the 2016 earthquake that affected Central Italy. Then, the same analysis is undertaken using the cross-polarized channel to point out that the joint use of co- and cross-polarized channels can help in better observing the damaged area. Finally, a coherent dual-polarimetric feature, based on the inter-channel coherence, is proposed. This feature results in a sensitivity to damaged areas comparable to the one provided by conventional single-polarization features. However, the interchannel coherence has the benefit to be based only on two SAR acquisitions, i.e. before and after the earthquake and it is less affected by false alarms. SAR, polarimetry, earthquake, losses     

星载SAR干涉技术获取DEM及其精度分析

星载合成孔径雷达干涉（InSAR）技术是一种数据覆盖范围广、廉价、高效、方便的数字高程模型（DEM）获取方法,但在地面植被覆盖广、大气水汽含量高的地区其影像相干性随时间基线的增加迅速降低;同时,SAR卫星的轨道误差也影响DEM精度。利用ERS-1/2卫星串行模式SAR数据获取镇江地区DEM,分析了轨道误差对DEM精度的影响;根据干涉相位的统计特性,从理论上给出干涉相位噪声与相干系数和视数之间的关系。实验结果表明就干涉像对的卫星轨道误差和相位噪声而言,在小区域内DEM精度优于3.5 m。     

Analysis of ERS SAR coherence images acquired over vegetated areas and urban features

Synthetic Aperture Radar (SAR) coherence images were analysed over vegetated areas and urban features. Coherence images were formed from interferometric SAR data acquired 1 day or 35 days apart by two European Remote sensing Satellites (ERS). Forested areas are discriminated very well from cultivated fields using 1-day SAR coherence data taken in the winter when the temperatures were below freezing. This is because fields under these conditions decorrelate much less than forest. Open sandy fields gave high coherence for both winter and summer acquisitions. All vegetated areas experienced a strong temporal decorrelation over a 35-day period. This is mainly due to changing wind, precipitation and temperature conditions, but could also be due to vegetation growth or man-made changes. Many urban objects were found to decorrelate slowly with time, regardless of changing weather conditions.     

Uplift and subsidence due to the 26 December 2004 Indonesian earthquake detected by SAR data

The Indonesian earthquake took place on 26 December 2004 at 00:58 GMT (moment magnitude 9.3) in the Indian Ocean, offshore the west coast of Sumatra, at a depth of about 30 km. This earthquake is one of the largest of the past 100 years, comparable only with those in Chile (1960) and Alaska (1964). The earthquake originated in the subduction zone of the Indian and Burma plates, moving at a relative velocity of 6 cm/year. The aftershocks were distributed along a plate boundary of about 1000–1300 km between Sumatra and the Andaman Islands. Some hours after the earthquake a destructive tsunami followed and hit the coastlines of the surrounding regions, causing widespread destruction in Indonesia, India, Thailand and Sri Lanka. The European Space Agency (ESA) made available a data package composed of European Remote Sensing Satellite Synthetic Aperture Radar (ERS‐SAR) and Environment Satellite Advanced SAR (ENVISAT‐ASAR) data covering the affected area, acquired before (four acquisition dates) and after (five acquisition dates) the earthquake. A total of 26 frames were analysed. We used this dataset to evaluate the effects of the earthquake and tsunami on the human settlements and the physiographic conditions along the coast. The proposed method is based on a visual comparison between pre‐ and post‐seismic SAR intensity images, and on an analysis of their correlation coefficients. No complex data were made available by the ESA to exploit phase coherence. Analysis of pre‐ and post‐earthquake SAR backscattering showed wide uplift areas between the Andaman Islands and Simeulue Island, and large modifications of the coastline of Sumatra. Subsiding areas were detected along the southeast coast of Andaman up to the west coast of Nicobar Island. Tidal effects were filtered out of the SAR images to identify the consequences of the earthquake. Global Positioning System (GPS) stations in the Andaman provided results confirming the surface displacement pattern detected by SAR. The analysis enabled us to draw a boundary line separating the uplift and subsidence.     

Detecting a moorland wildfire scar in the Peak District,UK, using synthetic aperture radar from ERS-2 and Envisat ASAR

Wildfires occur annually in UK moorland environments, especially in drought years. They can be severely damaging to the ecosystem when they burn deep into the peat, killing ground-nesting birds and releasing CO₂ into the atmosphere. Synthetic aperture radar (SAR) was evaluated for detecting the 18 April 2003 Bleaklow wildfire scar (7.4 km²). SAR’s ability to penetrate cloud is advantageous in this inherently overcast area. SAR can provide fire scar boundary information which is otherwise labour intensive to collect in the field using a global positioning system (GPS). This article evaluates the potential of SAR intensity and InSAR coherence to detect a large peat moorland wildfire scar in the Peak District of northern England. A time-series of pre-fire and post-fire ERS-2 and advanced synthetic aperture radar (ASAR) Single Look Complex (SLC) data were pre-processed using SARScape 4.2 to produce georeferenced greyscale images. SAR intensity and InSAR coherence values were analysed against Coordinate Information on the Environment (CORINE) land‐cover classes and precipitation data. SAR intensity detected burnt peat well after a precipitation event and for previous fire events within the CORINE peat bog class. For the 18 April 2003 fire event, intensity increased to 0.84 dB post-fire inside the fire scar for the peat bog class. InSAR coherence peaked post-fire for moors and heathland and natural grassland classes inside the fire scar, but peat bog exposed from previous fires was less responsive. Overall, SAR was found to be effective for detecting the Bleaklow moorland wildfire scar and monitoring wildfire scar persistence in a degraded peat landscape up to 71 days later. Heavy precipitation amplified the SAR fire scar signal, with precipitation after wildfires being typical in UK moorlands. Further work is required to disentangle the effects of fire size, topography, and less generalized land‐cover classes on SAR intensity and InSAR coherence for detecting fire scars in degraded peat moorlands.     

基于NSCT变换的红外空中小目标检测方法研究

针对空中远距离红外小目标检测的实际问题,提出了一种基于非抽样轮廓波变换的检测算法。首先利用非抽样轮廓波变换的优良t}质,通过分析噪声系数、背景边缘系数和目标系数在尺度间的不同特性,计算各个信号在尺度间的相关系数并归一化。接下来,按照自适应阂值法抑制噪声和背景边缘系数,然后通过反变换得到抑制背景增强目标的图像。最后,结合目标面积信息选择适当阂值,对重构图像进行分割,生成单帧检测结果并进一步利用帧间目标位置的相关性完成小目标检测过程。试验结果表明,提出的算法能够准确地检测目标。相对于通常的小目标检测算法,本算法在背景抑制方面具有一定的优势,能够获得相对较高的信噪比。     

Fast damage mapping in case of earthquakes using multitemporal SAR data

This work shows that earthquake damages in urban areas can be determined with an acceptable accuracy through the exploitation of multitemporal SAR data and ancillary information defining urban blocks. In this article, two different methodologies are presented: an unsupervised statistical analysis of the parameters of the models representing backscatterer intensity or coherence values for each block of the urban area under analysis, and a supervised approach which involves a multi-band/multi-temporal classification, performed using a Markov Random Field (MRF) classifier or a spatial Fuzzy ARTMAP (FA) classifier. The two procedures are compared by using ERS images acquired before and after the earthquake of Turkey in 1999.

A survey of temporal decorrelation from spaceborne L-Band repeat-pass InSAR

In this paper we quantify the effects of temporal decorrelation in repeat pass synthetic aperture radar interferometry (InSAR). Temporal decorrelation causes significant uncertainties in vegetation parameter estimates obtained using various InSAR techniques, which are desired on a global scale. Because of its stochastic nature temporal decorrelation is hard to model and isolate. In this paper we analyze temporal decorrelation statistically as observed in a large swath of SIR-C L-Band InSAR data collected over the eastern United States, with a repeat pass duration of one day in October 1994 and a near zero perpendicular baseline. The very small baseline for this particular pair makes the effect of volumetric scattering on correlation magnitude statistics nearly imperceptible, allowing for a quantitative analysis of temporal effects alone. The swath analyzed in this paper spans more than a million hectares of terrain comprised primarily of deciduous and evergreen forests, agricultural land, water and urban areas. The relationships of these different land-cover types, phenology and weather conditions (i.e. precipitation and wind) on the measures of interferometric correlation is analyzed in what amounts to be the most geographically extensive analysis of this phenomenon to date.     

Correlation Analysis between L-band SAR and ForestStem Volume in Northeast China

Because Synthetic Aperture Radar(SAR)can penetrate into forest canopy and interact with the primary stem volume contents of the trees (trunk and branch),SAR data are widely used for forest stem volume estimation.This paper investigated the correlation between SAR data and forest stem volume in Xunke,Heilongjiang using the stand-wise forest inventory data in 2003 and ALOS PALSAR data for five dates in 2007.The influences of season and polarizations on the relationship between stem volume and SAR data were studied by analyzing the scatterplots;that was followed by interpretation of the mechanisms primarily based on a forest radar backscattering model-water cloud model.The results showed that the relationship between HV polarization backscatter and stem volume is better than HH polarization,and SAR data in summer dry conditions are more correlated to stem volume than the data acquired in other conditions.The interferometric coherence with 46-day temporal baseline is negatively correlated to the stem volume.The correlation coefficients from winter coherence are higher than those from summer coherence and backscatter.The study results suggest using the interferometric coherence in winter as the best choice for forest stem volume estimation with L-band SAR data.     

Satellite radar and optical remote sensing for earthquake damage detection: results from different case studies

In case of a seismic event, a fast and draft damage map of the hit urban areas can be very useful, in particular when the epicentre of the earthquake is located in remote regions, or the main communication systems are damaged. Our aim is to analyse the capability of remote sensing techniques for damage detection in urban areas and to explore the combined use of radar (SAR) and optical satellite data. Two case studies have been proposed: Izmit (1999; Turkey) and Bam (2003; Iran). Both areas have been affected by strong earthquakes causing heavy and extended damage in the urban settlements close to the epicentre. Different procedures for damage assessment have been successfully tested, either to perform a pixel by pixel classification or to assess damage within homogeneous extended areas. We have compared change detection capabilities of different features extracted from optical and radar data, and analysed the potential of combining measurements at different frequency ranges. Regarding the Izmit case, SAR features alone have reached 70% of correct classification of damaged areas and 5 m panchromatic optical images have given 82%; the fusion of SAR and optical data raised up to 89% of correct pixel‐to‐pixel classification. The same procedures applied to the Bam test case achieved about 61% of correct classification from SAR alone, 70% from optical data, while data fusion reached 76%. The results of the correlation between satellite remote sensing and ground surveys data have been presented by comparing remotely change detection features averaged within homogeneous blocks of buildings with ground survey data.     

Mapping damage during the Bam (Iran) earthquake using interferometric coherence

We use changes in the interferometric coherence to map earthquake damage that occurred in the city of Bam during the Bam earthquake on 26 December 2003. The approach presented here defines a coherence change index that can be interpreted quantitatively in terms of damage during a catastrophic event like a major earthquake. Using five differential interferometric synthetic aperture radar images, we compute maps of interferometric coherence. Three coherence change images are computed from these based on different interferograms. These three damage assessments yield very similar results despite the range of times and interferometric baselines spanned. This coherence‐based damage assessment also agrees closely with independent damage maps derived from other types of imagery. Using existing or planned synthetic aperture radar (SAR) satellites, coherence‐based damage assessments can be obtained within days after a catastrophic event, provided the necessary reference images are prepared ahead of time. As SAR sensors can operate independently of weather conditions and daylight, this may present a reliable and robust means of damage assessment.     

Properties of X-, C- and L-band repeat-pass interferometric SAR coherence in Mediterranean pine forests affected by fires

Synthetic Aperture Radar (SAR) data has been investigated to determine the relationship between burn severity and interferometric coherence at three sites affected by forest fires in a hilly Mediterranean environment. Repeat-pass SAR images were available from the TerraSAR-X, ERS-1/2, Envisat ASAR and ALOS PALSAR sensors. Coherence was related to measurements of burn severity (Composite Burn Index) and remote sensing estimates expressed by the differenced normalized burn ratio (dNBR) index. In addition, the effects of topography and weather on coherence estimates were assessed. The analysis for a given range of local incidence angle showed that the co-polarized coherence increases with the increase of burn severity at X- and C-band whereas cross-polarized coherence was practically insensitive to burn severity. Higher sensitivity to burn severity was found at L-band for both co- and cross-polarized channels. The association strength between coherence and burn severity was strongest for images acquired under stable, dry environmental conditions. When the local incidence angle is accounted for the determination coefficients increased from 0.6 to 0.9 for X- and C-band. At L-band the local incidence angle had less influence on the association strength to burn severity.     

Polarimetric SAR statistical analysis using alpha-stable distribution and its application in optimal despeckling

In this article, the statistical model of the polarimetric synthetic aperture radar (SAR) single-look complex image is analysed using alpha-stable distribution. It is better to use alpha-stable distribution than Gaussian distribution to represent the statistical characteristics of the polarimetric SAR image. A polarimetric SAR covariance matrix estimation method based on fractional lower-order statistics (FLOS) is proposed. Based on this model, an adaptive polarimetric SAR optimal despeckling method based on FLOS is developed. This algorithm adaptively estimates the characteristic exponents of each channel and uses these estimated alphas to calculate the parameters for the optimal despeckling adaptively. The experiments using polarimetric SAR data demonstrate that the proposed method not only reduces the blurs that occur in the area of impulsive reflectors in the result of the original optimal despeckling method, but also maintains the speckle reduction ability (equivalent number of looks).     

雷达遥感建筑物震害信息提取方法综述

合成孔径雷达遥感具有全天时、全天候工作的能力,在地震灾害应急中发挥了重要作用。回顾了雷达遥感建筑物震害信息提取技术的发展历史,总结了各种用于建筑物震害信息提取的雷达卫星的参数特征以及使用雷达遥感提取建筑物震害信息的震例。介绍了目前应用的各种雷达遥感建筑物震害信息提取方法,包括目视解译和计算机自动信息提取两种,其中后者包括基于纹理特征和极化特征的单时相方法、基于强度特征和相干特征的多时相方法。比较了这些方法的适用性和不足,并探讨了雷达遥感建筑物震害信息提取方法的发展趋势。     

Damage assessment in urban areas using post-earthquake airborne PolSAR imagery

Synthetic aperture radar (SAR) has often been used in earthquake damage assessment due to its extreme versatility and almost all-weather, day-and-night capability. In this article, we demonstrate the potential to use only post-event, high-resolution airborne polarimetric SAR (PolSAR) imagery to estimate the damage level at the block scale. Intact buildings with large orientation angles have a similar scattering mechanism to collapsed buildings; they are all volume-scattering dominant and reflection asymmetric, which seriously hampers the process of damage assessment. In this article, we propose a new damage assessment method combining polarimetric and spatial texture information to eliminate this deficiency. In the proposed method, the normalized circular-pol correlation coefficient is used first to identify intact buildings aligned parallel with the flight direction of the radar. The ‘homogeneity’ feature of the grey-level co-occurrence matrix (GLCM) is then introduced to distinguish building patches with large orientation angles from the severely damaged class. Furthermore, a new damage assessment index is also introduced to handle the assessment at the level of the block scale. To demonstrate the effectiveness of the proposed approach, the high-resolution airborne PolSAR imagery acquired after the earthquake that hit Yushu County, Qinghai Province of China, is investigated. By comparison with the damage validation map, the results confirm the validity of the proposed method and the advantage of further improving the assessment accuracy without external ancillary optical or SAR data.     

Classification of multi-look polarimetric SAR imagery based on complex Wishart distribution

Multi-look polarimetric SAR (synthetic aperture radar) data can be represented either in Mueller matrix form or in complex covariance matrix form. The latter has a complex Wishart distribution. A maximum likelihood classifier to segment polarimetric SAR data according to terrain types has been developed based on the Wishart distribution. This algorithm can also be applied to multifrequency multi-look polarimetric SAR data, as well as 10 SAR data containing only intensity information. A procedure is then developed for unsupervised classification.

The classification error is assessed by using Monte Carlo simulation of multilook polarimetric SAR data, owing to the lack of ground truth for each pixel. Comparisons of classification errors using the training sets and single-look data are also made. Applications of this algorithm are demonstrated with NASA/JPL P-, L- and C-band polarimetric SAR data.     

Radar stereo- and interferometry-derived digital elevation models: comparison and combination using Radarsat and ERS-2 imagery

In this experiment, we derive and compare radar stereo and interferometric digital elevation models (DEMs) of a study site in Djibouti, East Africa. A Radarsat stereo pair, as well as Radarsat and ERS-2 interferometric data, comprise the test images. The primary objective of the study was to analyse and compare the results obtained by the two techniques and explore possible synergisms between them. We find that in regions of high coherence, the DEMs produced by interferometry are of much better quality than the stereo result. However, the corresponding error histograms also show some pronounced errors due to decorrelation and phase-unwrapping problems on forested mountain slopes. On the other hand, the more robust stereo reconstruction, with an error standard deviation of 45 m, is able to capture the general terrain shape, although finer surface details are lost. In the second part of our experiment, we demonstrate that merging the stereoscopic and interferometric DEMs by applying a user-defined weighting function to a filtered coherence map can significantly improve the accuracy of the computed elevation maps.     

Surface displacement of the Mw 7 Machaze earthquake (Mozambique): Complementary use of multiband InSAR and radar amplitude image correlation with elastic modelling

In this paper we investigate the surface displacement related to the 2006 Machaze earthquake using Synthetic Aperture Radar Interferometry (InSAR) and sub-pixel correlation (SPC) of radar amplitude images. We focus on surface displacement measurement during three stages of the seismic cycle. First, we examined the co-seismic stage, using an Advanced SAR (ASAR) sensor onboard the Envisat satellite. Then we investigated the post-seismic stage using the Phase Array L-band SAR sensor (PALSAR) onboard the ALOS satellite. Lastly, we focussed on the inter-seismic stage, prior to the earthquake by analysing the L-band JERS-1 SAR data. The high degree of signal decorrelation in the C-band co-seismic interferogram hinders a correct positioning of the surface rupture and correct phase unwrapping. The post-seismic L-band interferograms reveal a time-constant surface displacement, causing subsidence of the surface at a ∼ 5 cm/yr rate. This phenomenon continued to affect the close rupture field for at least two years following the earthquake and intrinsically reveals a candidate seismogenic fault trace that we use as a proxy for an inversion against an elastic dislocation model. Prior to the earthquake, the JERS interferograms do not indicate any traces of pre-seismic slip on the seismogenic fault. Therefore, slip after the earthquake is post-seismic, and it was triggered by the Machaze earthquake. This feature represents a prominent post-seismic slip event rarely observed in such a geodynamic context.     

Multi-look versus single-look processing of synthetic aperture radar images with respect to ocean wave spectra estimation

Abstract

Full-bandwidth C-band synthetic aperture radar (SAR) data are compared with 7-look and 3-look data. The peak-to-background ratio of the image intensity power spectrum describing the wave detectability is found to be on average 8-9dB higher for the 7-look data and 2-5dB higher for the 3-look data than the single-look data. This is mainly due to the decrease in the speckle noise level when going from single-look to multi-look processing. In addition, look-sum processing is evaluated against spectral-sum processing for various temporal look separations. A significant improvement in image spectral peak contrast is observed for the spectral-sum data versus the look-sum data, with increasing temporal separations between the looks. No such improvement is observed in the corresponding image spectral noise contrast parameter. These observations are in agreement with the spatial misregistration inherent in look-sum data. Finally, the acceleration contribution to the observed aximuth smearing in the spectra is found to be negligible compared with the velocity smearing contribution.     

Landslide monitoring by corner reflectors differential interferometry SAR

Although the differential interferometric synthetic aperture radar (DInSAR) technique has the potential for monitoring ground deformation with millimetric accuracy, it still suffers from temporal and geometric decorrelation. In this paper, the corner reflectors differential interferometry synthetic aperture radar (CRDInSAR) technique was used to overcome the limitations of conventional DInSAR. We studied the basic principles of CRDInSAR, discussed the calculation of the flat earth and topographic phases based on the geometry of satellite and corner reflectors, presented the phase unwrapping approach for the sparse grid of corner reflectors, then investigated the construction of, and solution to, the unwrapped phase system equation. Subsequently, we applied CRDInSAR to monitor the displacement of the Shuping landslide in Hubei Province, China. In this study, we processed five SAR images on a descending pass acquired by the Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (ASAR) sensor from September 2005 to March 2006, and compared the achieved results of CRDInSAR with Global Positioning System (GPS) measurements at the same time of the SAR data acquisition assumed as reference. The result indicates a good agreement between the measurements provided by the two different techniques, which shows that CRDInSAR allows monitoring of slow landslide deformation in low coherence areas and provides accurate results.