Ground deformation of Long Valley caldera and Mono Basin,eastern California,mapped by satellite radar interferometry

We have illustrated the key results of the Differential SAR Interferometry (DInSAR) analysis focused on the ground deformation of Long Valley caldera and Mono Basin, eastern California. In particular, we have applied the DInSAR algorithm referred to as Small BAseline Subset (SBAS) approach and processed 21 SAR images, spanning the time interval from 1992 to 2000, acquired from descending arbits by the ERS‐1 and ERS‐2 sensors of the European Space Agency (ESA). The deformation affecting the resurgent dome of Long Valley caldera has been highlighted as well as the previously unreported subsidence of the Pahoa island, located in Mono Lake.     

COSMO-SkyMed数据在常州市地表形变监测中的应用

小基线集合成孔径雷达干涉测量技术(SBAS-InSAR)已成功应用于城市地表形变监测,并表现出极大的潜力和优势。X波段高分辨率雷达卫星在地表微小形变探测方面较C波段和L波段更为敏感。选取覆盖常州地区COSMO-SkyMed高分辨率SAR影像,采用SBAS-InSAR方法获得了地表形变时间序列,对比水准观测数据,分析了干涉测量结果的精度,根据历史地下水位监测数据,分析了地下水水位变化对地表形变的影响。结果表明:干涉测量结果与水准观测数据具有很好的一致性,沉降区域主要发生在武进区,最大沉降量超过-40mm,主城区出现了轻微的回弹现象,回弹达到+5mm;地下水水位持续上升与地面沉降减缓、地面回弹趋势一致,地下水水位变化仍然是常州市地表形变的主要影响因素。     

InSAR Permanent Scatterer analysis reveals fault re-activation during inflation and deflation episodes at Campi Flegrei caldera

Permanent Scatterers Synthetic Aperture Radar Interferometry (PSInSAR) and Global Position System (GPS) are applied to investigate the most recent surface deformation of the Campi Flegrei caldera. The PSInSAR analysis, based on SAR data acquired by ERS-1/2 sensors during the 1992-2001 time interval and by the Radarsat sensor during 2003-2007, identifies displacement patterns over wide areas with high spatial resolution. GPS data acquired by the Neapolitan Volcanic Continuous GPS network provide detailed ground velocity information of specific sites. The satellite-derived data allow us to characterize the deformation pattern that affected the Campi Flegrei caldera during two recent subsidence (1992-1999) and uplift (2005-2006) phases. PSInSAR results show the re-activation of the caldera ring-faults, intra-caldera faults, and eruptive fissures. We discuss the results in the light of the available volcanological, structural and geophysical data and propose a relationship between the structures activated during the recent unrest episodes and those responsible for the recent (< 3.8-4 ka) volcanism. The combined interpretation of the collected data show that (a) the caldera consists of two sectors separated by a N-S striking faulting zone and (b) the intra-caldera NW-SE faults and eruptive fissures in the central-eastern sector re-activated during the studied unrest episodes and represent possible pathways for the ascent of magma and/or gas to the surface. In this sector, maximum horizontal strain, recent volcanism (3.8-4 ka), active degassing and seismicity concentrate. The fault re-activation is related to the dynamics of the caldera and not to tectonic stress. The deformation fields of the uplift and subsidence episodes are consistent with hydrothermal processes and degassing from a magmatic reservoir that is significantly smaller than the large (∼ 40 km³) magma chamber responsible for the caldera formation. We provide evidence that the monitoring of the horizontal and vertical components of deformation improves the identification of active, aseismic faults. Accordingly, we suggest that future ground deformation models should include the re-activation of the detected structures.     

Recent subsidence in Tianjin,China: observations from multi-looking TerraSAR-X InSAR from 2009 to 2013

Tianjin, China, has been suggested to have serious ground subsidence due to excessive extraction of groundwater. It is essential to monitor this subsidence, which has potential hazards and risks. Time series InSAR (TS-InSAR), such as small baselines subset (SBAS), is a powerful tool that can monitor ground deformation with high accuracy and at high spatial resolution over a long time interval. However, the high computational complexity may exceed computer memory limit when high-spatial resolution SAR (such as TerraSAR-X, TSX) images are used. In this article, the multi-look approach is introduced to the SBAS tool from StaMPS/MTI (Stanford method for persistent scatter/multi-temporal InSAR) in order to balance the spatial resolution and subsidence information in detection. The looks used for multi-looking are first fixed in terms of the accuracy of deformation and the density of coherent points. Then, the recent subsidence in Tianjin is extracted using multi-looking SBAS based on 48 TSX images acquired from 2009 to 2013. The results are validated by levelling measurements with a root mean square error (RMSE) of 4.7 mm year^–1, which demonstrates that SBAS analysis can effectively monitor deformation based on multi-looking TSX acquisitions in the area under investigation. Besides, the results also show that Tianjin has been suffering from subsidence during this period, and there were two separate large subsidence basins located in this study area with more than 500 mm cumulative subsidence. Moreover, the subsidence rate increased after December 2010 in Tianjin.     

Advanced InSAR techniques for deformation studies and for simulating the PS-assisted calibration procedure of Sentinel-1 data: case study from Thessaloniki (Greece), based on the Envisat/ASAR archive

Using state-of-the-art InSAR techniques, namely persistent scatterers (PSs) and small baseline subset (SBAS) approaches, this study contributes to open geotechnical questions in the area of Thessaloniki (Greece) from a remote-sensing perspective. It also demonstrates the potential of these techniques for calibration purposes, with reference to the new C-band synthetic aperture radar (SAR) sensor on board the Sentinel-1 mission satellites. By exploiting the historical archive of Envisat/ASAR data, as well as a pair of the first Sentinel-1A SAR images, recent (2004–2010) deformation rates up to 18 mm year^–1 are detected over the study area. These results are then compared to the findings of previous InSAR and geophysical observations, indicating for example, subsidence or tectonic activity. On the other hand, the usefulness of the PS technique is shown over the same region for external SAR calibration purposes. This process simulates the PS-assisted calibration procedure to be applied systematically to Sentinel-1 SAR products.     

Simulation of time-series surface deformation to validate a multi-interferogram InSAR processing technique

A refined, small baseline subset (SBAS) interferometric synthetic aperture (InSAR) method has been developed to estimate time-series surface deformation through fusion of multi-interferogram processing. Using a synthetic data set that takes into account two time-varying deformation sources, topography-induced errors, atmospheric delay anomalies, orbital errors and temporal decorrelation, all of which are based on realistic ERS-1/ERS-2 SAR image acquisition dates and baseline configuration over the Seguam volcano, Alaska, we assess the accuracy of the refined SBAS technique. Detailed comparison between SBAS-derived products, including time-series deformation maps, atmospheric delays and baseline errors with those of synthetic values, attest the robustness of the refined SBAS technique. The root mean square error of the mean deformation rate between the simulated and SBAS-retrieved is about 0.66 mm year^–1. Thus, the measurement accuracy of the refined SBAS method would be around 1 mm year^–1.     

Landslide detection in La Paz City (Bolivia) based on time series analysis of InSAR data

Geologically, La Paz City is located in an unstable area. During the history of La Paz city, many landslides have destroyed houses and valuable infrastructures. In the last decades, time series Interferometric Synthetic Aperture Radar (InSAR) technologies have demonstrated a great capacity for detecting slow ground displacement, achieving an accuracy of millimetre-level. In order to have a better landslide monitoring of La Paz city, in this study, the Sentinel-1 SAR images have been processed by Persistent Scatterer Interferometry (PSI) and the Small Baseline Subset (SBAS) techniques. The time span of the datasets is from March 2015 to August 2016. Both ascending and descending Synthetic Aperture Radar (SAR) images have been processed to obtain the line of sight (LOS) ground velocity, and then the results have been combined to estimate the up-down and east-west displacement. Several active movement areas have been identified, showing a surface velocity up to 158 mm year^?1 westward and 49 mm year^?1 eastward. Furthermore, two important findings have been discovered. First, the InSAR result has detected movement in Auquisamaa hill before the area collapsed (15 February 2017), where five houses are buried. Second, the InSAR result has identified that there are still some unstable sites in Callapa area, where a mega-landslide has destroyed more than a thousand of houses in February 2011. In conclusion, we have verified that the InSAR technology could be a very useful tool to help La Paz public institutions for a better management of urban planning, landslide areas delimitation and landslide risk mitigation.     

Surface deformation over the buried Nasr Abad salt diapir,Central Iran using interferometric synthetic aperture radar data

ABSTRACT

This study employs interferometric synthetic aperture radar (InSAR) data sets to monitor the surface deformation of the Nasr Abad buried salt diapir in the Central Basin of Iran. The Nasr Abad salt diapir is one of the largest buried salt diapirs in Iran and could be ideal site for oil/gas storage and industrial waste disposal. In this study, we use 40 advanced synthetic aperture radar (ASAR) images from Environmental Satellite (EnviSat) to analyse surface displacement rates of Nasr Abad diapir and its surrounding regions. A time series of line-of-sight (LOS) displacements on the residual cap above the buried diapir were obtained from both descending and ascending images between 2003 and 2010 by applying the Small-BAseline Subset (SBAS) technique. Tropospheric artefacts in the displacement interferograms were mitigated using the power-law correction method in Toolbox for Reducing Atmospheric InSAR Noise (TRAIN) software. Finally, the data for temperature, precipitation, and tidal forces were correlated with the time-series displacement results of four points that located on the residual cap. Our results indicate that surface above the diapir and an area of about 2 km² subsided with maximum LOS velocity of about 7 mm year^?1 for ascending images and 5 mm year^?1 for descending images. The amount of vertical subsidence derived from LOS decomposition in reactive stage of Nasr Abad salt body is about 7 mm year^?1. Comparing the temperature and precipitation data with the time series of displacement confirmed that the salt expands when temperatures increases and that salt flow accelerates when it is wet. The displacement pattern retrieved from InSAR analysis is in good agreement with intervals near maximum and minimum solid earth tides. Monitoring the activities of the Nasr Abad region over 7 years shows that the region of surface subsidence is confined to the area along the Khurabad and AbShirin-Shurab Fault zones in the southern Central Basin.     

Detecting land uplift associated with enhanced oil recovery using InSAR in the Karamay oil field,Xinjiang, China

Subsurface fluid injection is a well-established technology that is often used for enhanced oil recovery from oil fields. The injection process can often cause deformation due to changes in pore pressure, which can be measured using interferometric synthetic aperture radar (InSAR). In this study, the Karamay oil field in Xinjiang served as an example and an attempt was made to detect subsurface fluid injection-induced ground surface deformation. First, possible deformation was assessed across the entire Karamay oil field during 2006–2010 by stacking multiple interferometric pairs. Then, the spatiotemporal evolution of deformation around well Hei103 was studied using the small baseline subset (SBAS) algorithm. The results suggest that localized deformation at the Karamay oil field was related to oil production. Additionally, subsurface fluid injection caused obvious surface uplift around the Hei103 well region.     

Surface movements in Bologna (Po Plain — Italy) detected by multitemporal DInSAR

We studied the surface deformations affecting the southeastern sector of the Po Plain sedimentary basin, in particular the area of Bologna. To this aim an advanced DInSAR technique, referred to as DInSAR-SBAS (Small BAseline Subset), has been applied. This technique allows monitoring the temporal evolution of a deformation phenomenon, via the generation of mean deformation velocity maps and displacement time series from a data set of acquired SAR images. In particular, we have processed a set of SAR data acquired by the European Remote Sensing Satellite (ERS) sensors and compared the achieved results with optical levelling measurements, assumed as reference. The surface displacements detected by DInSAR SBAS from 1992 to 2000 are between 10 mm/year in the historical part of Bologna town, and up to 59 mm/year in the NE industrial and agricultural areas. Former measurements from optical levelling referred to 1897 show 2-3 mm/year vertical movements. This trend of displacement increased in the second half of the 20th century and the subsidence rate reached 60 mm/year. We compared the more recent levelling campaigns (in 1992 and late 1999) and DInSAR results from 1992 to 1999. The standard deviation of the difference between levelling data, projected onto the satellite Line Of Sight, and DInSAR results is 2 mm/year. This highlights a good agreement between the measurements provided by two different techniques. The explanation of soil movements based on interferometric results, ground data and geological observations, allowed confirming the anthropogenic cause (surface effect due to the overexploitation of the aquifers) and highlights a natural, tectonic, subsidence.     

基于多时相干涉SAR提取地表形变的关键技术探究

基于重复轨道的合成孔径雷达差分干涉测量(D-InSAR,Differential Interferometric SAR)技术自问世以来,就被立即成为研究重点并在许多领域得到应用。但是常规D-InSAR技术易受时间、空间失相干与大气延迟等影响而有时无法得到可靠的形变信息,因此难以在实践中得到常态化应用。永久散射体技术与小基线集技术的出现,有效地克服了上述问题的制约。本文融合了两种技术的优点,介绍了多时相干涉SAR技术的关键处理步骤,并基于多时相干涉SAR技术提取了太原市2005～2009年间的形变信息。     

The present status of subsiding land vulnerable to roof collapse in the Jharia Coalfield,India, as obtained from shorter temporal baseline C-band DInSAR by smaller spatial subset unwrapped phase profiling

In this paper, we identified recently subsiding areas in Jharia Coalfield, Jharkhand, India from the shorter temporal baseline Radarsat-2 C-band interferometric synthetic aperture radar (InSAR) data pairs of 2012. Although shorter wavelength C-band differential InSAR (DInSAR) is more sensitive to slow deformation and better suited for higher precision land subsidence measurement, the dynamic and adverse land cover in mining areas and resulting temporal decorrelation problem poses a serious problem for DInSAR observation in mining areas. We used smaller temporal baseline data pairs and adopted InSAR coherence-guided incremental filtering with smaller moving windows to highlight the deformation fringes over temporal decorrelation noise. We identified the deformation fringes and validated them based on ground information to prepare the land subsidence map of the coalfield in 2012. Several new, previously unreported subsidence areas were detected in the present study with a total subsiding area of 6.9 km². The recent incidence of roof collapse on 15 November 2014 at Angar Patra village in Katras region of the coalfield where 45 houses collapsed and 10 people were injured is situated in a highly subsiding vulnerable area as obtained from the present study. Due to spatial discontinuities of InSAR coherence, DInSAR phase unwrapping for the entire study area in one go did not appear feasible. To avoid this problem, we performed DInSAR processing in smaller spatial subsets and unwrapping of the subset interferograms by a ‘minimum cost flow’ algorithm. Subsequently, we plotted unwrapped phase profiles across the deformation fringes and retrieved the maximum deformation phase with respect to background phase and translated them into radar line of sight (LOS) displacement rates. For obtaining the average subsidence rates, we adopted InSAR coherence-weighted LOS displacement rates taking into account the contribution of each data pair as a function of DInSAR phase quality of the fringe areas. Ground-based subsidence measurements by precision levelling were conducted in four test sites that had been undergoing active underground mining during the observation period. We compared space-borne DInSAR-based subsidence rates obtained by the adopted technique with precision levelling measurements. Overall, the results are found to agree well. In the four test sites with gentle to flat topography, land subsidence occurs at slow to moderate rates due to compression of in-filled material (resulting from sand stowing in underground mining), without any evidence of roof collapse. In such cases, the horizontal displacement component is less significant, and overall surface displacement occurs essentially in the vertical direction. However, we assessed the nature of subtle horizontal strain to infer relative shrinkage or dilation of the land surface which could be additive or subtractive to vertical displacement in DInSAR-based LOS displacement.     

A study on measuring surface deformation of the L’Aquila region using the StaMPS technique

This research compares two time-series interferometric synthetic aperture radar (InSAR) methods, namely persistent scatterer SAR interferometry (PS-InSAR) and small baseline subset (SBAS) to retrieve the deformation signal from pixels with different scattering characteristics. These approaches are used to estimate the surface deformation in the L’Aquila region in Central Italy where an earthquake of magnitude Mw 6.3 occurred on 6 April 2009. Fourteen Environmental Satellite (ENVISAT) C-band Advanced Synthetic Aperture Radar (ASAR) images, covering the pre-seismic, co-seismic, and post-seismic period, are used for the study. Both the approaches effectively extract measurement pixels and show a similar deformation pattern in which the north-west and south-east regions with respect to the earthquake epicentre show movement in opposite directions. The analysis has revealed that the PS-InSAR method extracted more number of measurement points (21,103 pixels) as compared to the SBAS method (4886 pixels). A comparison of velocity estimates shows that out of 833 common pixels in both the methods, about 62% (517 pixels) have the mean velocity difference below 3 mm year^?1 and nearly 66% pixels have difference below 5 mm year^?1. It is concluded that StaMPS-based PS-InSAR method performs better in terms of extracting more number of measurement pixels and in the estimation of mean line of sight (LOS) velocity as compared to SBAS method.     

新型多基线DInSAR地表形变监测技术研究动态

合成孔径雷达干涉测量（InSAR）是一项广泛采用的雷达遥感测量技术,可以获取大区域、长时间、毫米级的地表形变监测,是SAR图像应用研究的热点。从InSAR技术监测地表形变时面临的问题出发,分析了近年来多基线DInSAR方法的新进展,论述了相干目标算法\,分布目标算法及SAR层析成像技术等在监测地表运动时的原理及技术应用,详细讨论了DInSAR地表形变监测由二维参数研究发展至三维、四维空间,由城区发展至广阔非城区地表监测的发展趋势。     

Surface subsidence and uplift above a headrace tunnel in metamorphic basement rocks of the Swiss Alps as detected by satellite SAR interferometry

Surface subsidence associated with the construction of a headrace tunnel in the Swiss Alps at more than 2000 m above sea level (a.s.l.) has been detected at two locations with satellite differential Synthetic Aperture Radar (SAR) interferometry. At the first location, a subsidence trough of about 4 cm in the satellite line-of-sight direction following the headrace tunnel axes has been measured between August 1995 and August 1996. Similar values from SAR data of ascending and descending orbits indicate displacements in the vertical direction of the movement. In the second case, a symmetric cone of depression with a maximum displacement of about 4 cm between 1995 and 1997 has been observed above the tunnel. Differences in the results from satellite SAR data of ascending and descending orbits indicate that the direction of displacement in this second case was not entirely vertical. Large-scale consolidation associated with pore-pressure reduction in the rock mass arising from tunnel drainage at about 200-400 m depth beneath the topographical surface is believed to be the contributing mechanism (Zangerl et al., 2008a, 2008b). Evidence for this process is based on pore pressure recordings in nearby deep wells. In both areas, the subsidence was followed by a small uplift of about one centimeter between 1997 and 1999, after the tunnel was cased with permeable concrete segments. This partial recovery is also visible in pore pressure records and can be related to the elastic components of rock mass deformation.     

Long-term ERS/ENVISAT deformation time-series generation at full spatial resolution via the extended SBAS technique

We extend the small baseline subset (SBAS) differential synthetic aperture radar (SAR) interferometry (DInSAR) approach to allow the generation of deformation time-series by processing, at the full spatial resolution scale, long sequences of European Remote Sensing (ERS-1 and ERS-2) and Environmental Satellite (ENVISAT) SAR data acquired with the same illumination geometry. In particular, we avoid the generation of ERS/ENVISAT cross-interferograms, which are severely affected by noise phenomena due to the carrier frequency separation of the two SAR systems, and we focus on single-platform interferograms only (i.e. ERS/ERS and ENVISAT/ENVISAT interferograms) that are properly combined by applying the singular value decomposition (SVD)-based SBAS approach. Moreover, we exploit the Doppler centroid variations of the post-2000 acquisitions of the ERS-2 sensor and the carrier frequency difference between the ERS-1/2 and the ENVISAT systems, in order to maximize the number of investigated SAR pixels and to improve their geocoding. The presented results, achieved on two data sets relevant to the Napoli Bay area and to the Murge region, both located in southern Italy, confirm the effectiveness of the extended SBAS technique and demonstrate the relevance of deformation analysis carried out at the scale of single buildings or human-made structures with more than 15 years of ERS and ENVISAT acquisitions.     

Annual rates of ground deformation (1993–2010) at Campi Flegrei,Italy, revealed by Persistent Scatterer Pair (PSP) – SAR interferometry

Campi Flegrei is an active volcanic district located along the Eastern Tyrrhenian continental margin, Italy, and is worldwide known for dramatic ground deformation phenomena (bradyseism) recorded over the last centuries. The purpose of this article is to present the annual rates of ground deformation fields (average velocity, average acceleration, and combined annual velocity/acceleration) at Campi Flegrei, during almost two decades (years 1993–2010). The research work was conducted based on a temporal analysis and advanced mapping of Persistent Scatterer Pair (PSP) data, obtained from interferometric processing of radar satellite European Remote Sensing-1/2 and Environmental Satellite (ENVISAT) scenes of the study area. Patterns and trends of annual PSP velocity have provided a reconstruction of the spatial and temporal variability of ground deformation in terms of uplift and/or subsidence along the satellite line of sight (LOS). The analysis of annual PSP velocity and acceleration has revealed an intense dynamics of the Campi Flegrei caldera collapse-resurgence system, testified by the high PSP velocity and acceleration values, as well as the significant changes in the rates of ground deformation through time. The main results of this research indicate that the largest ground deformation is localized within and around the structural border of the Campi Flegrei caldera and suggest a systematic recurrence of opposite trends (uplift vs. subsidence) in the ground deformation of the inner caldera region with respect to the surrounding areas throughout the analysed time period.     

Land deformation monitoring using coherent target-neighbourhood networking method combined with polarimetric information: a case study of Shanghai,China

In this article, an advanced approach for land deformation monitoring using synthetic aperture radar (SAR) interferometry combined with polarimetric information is presented. The linear and nonlinear components of the deformation, the error of the digital elevation model (DEM) and the atmospheric artefacts can be achieved by a coherent target (CT)-neighbourhood networking approach. In order to detect recent land deformation in Shanghai, China, 12 ENVISAT advanced synthetic aperture radar (ASAR) alternating polarization images acquired from January 2006 to August 2008 are employed for deformation analysis. Over a 2.5-year period, two deformation velocity fields from HH and VV modes over Shanghai are derived using the CT-neighbourhood networking SAR interferometry (InSAR), then integrated into a final deformation map by a fusion scheme. It is found that the annual subsidence rates in the study area range from??20 to 10 mm year^?1 and the average subsidence rate in the downtown area reaches??7.5 mm year^?1, which is consistent with the local government statistics published in 2007.     

Routine Monitoring of Changes in the Columbia Glacier, Alaska, with Synthetic Aperture Radar

This study focuses on the use of spaceborne Synthetic Aperture Radar (SAR) from the European Remote Sensing Satellite (ERS-1) and the Canadian RADARSAT satellite to monitor retreating glaciers, specifically the Columbia Glacier, and the hazards caused by their recession on a routine basis. The Columbia Glacier is an important tidewater glacier to monitor because of its present rapid retreat, its generation of icebergs, and its vicinity to oil-tanker traffic in and out of the Port of Valdez, Alaska. We have established routine monitoring of the Prince William Sound area using SAR aboard the Canadian RADARSAT on a semi-weekly basis. To demonstrate the use of this data, four ERS-1 and seven RADARSAT SAR images from 1992 to 1998 are used in this study to monitor the long-term retreat of the Columbia Glacier. A loss of 17.7±0.2 square kilometers is measured from 1992 to 1997, with 38.4% of this change occurring from 1993 to 1994. Also, hazards such as icebergs are monitored in near real-time. SAR is an efficient and cost-effective means of monitoring glaciers, where cloud cover and logistical costs and difficulties hamper other data-collection efforts. These data are available to U.S. Government users and would be of benefit to the Prince William Sound region if used operationally.     

Two‐scale surface deformation analysis using the SBAS‐DInSAR technique: a case study of the city of Rome,Italy

We have exploited the capability of the differential synthetic aperture radar (SAR) interferometry (DInSAR) technique, referred to as Small BAseline Subset (SBAS) approach, to analyse surface deformation at two distinct spatial scales: a low resolution, large scale, and a fine resolution, local scale. At the large scale, the technique investigates DInSAR data with a ground resolution of the order of 100 m×100 m and leads to generate mean deformation velocity maps and associated time series for areas extending to some thousands of square kilometres. At the local scale, the technique exploits the SAR images at full spatial resolution (typically of the order of 5 m×20 m), detecting and analysing localized deformation phenomena. The study is focused on the city of Rome, Italy, and we used the ERS‐1/2 satellite radar data relevant to the 1995–2000 time period. The presented results demonstrate the capability of the SBAS approach to retrieve, from the low‐resolution DInSAR data, large‐scale deformation information leading to identify several sites affected by significant displacements. Our analysis permitted us to conclude that a major contribution to the detected displacements is due to the consolidation of the alluvial soils present in the area, mostly enforced by the buildings' overload. Furthermore, in a selected area, a detailed analysis was carried out by exploiting the full resolution DInSAR data. In this case we investigated deformation phenomena at the scale of single buildings. As key result we showed that differential displacements of few mm a^?1, affecting single man‐made structures or building complexes, could be detected, thus allowing to identify sites that may potentially be involved in critical situations.