Applications: Subsidence and Landslides (2)

Synthetic Aperture Radar (SAR) interferometry is an already mature technique with important applications to the monitoring of geological phenomena, infrastructures and urban areas. All these applications have been developed using spaceborne SAR data regularly acquired by different missions and characterized by different radar wavelengths (L, C and X bands), spatial resolutions (from more than 20 m to less than 3 m) and revisiting frequencies (from more 40 days to less than 11 days). In the last decade, niche applications of this technique, based on data acquired by ground-based SAR (GBSAR) sensors, have been demonstrated for the monitoring of landslides and buildings and for the terrain mapping at local scale. Starting from these first works on GBSAR, research has been conducted on mitigation of atmospheric artifacts in interferometric applications and the analysis of polarimetric GBSAR images. More recently, paper have been published on focusing of GBSAR data and applications to the real-time monitoring of landslides, bridges, dams and measurements of vibration frequencies in structures such as bridges and bell towers. In this work, we concentrate on the use of SAR interferometry techniques for the monitoring of earthfill or rockfill embankments for dam impoundments. This non-invasive technique provides overall displacements patterns measured with a sub-millimeter accuracy. The need of reliable monitoring of old embankment dams is rapidly increasing since a large number of these structures are still equipped with old monitoring devices, usually installed some decades ago, which can give only information on localized areas of the embankment. This application is of great relevance for the dam safety and research on this topic can help to update the current monitoring protocols. Spaceborne and ground-based SAR interferometry can provide a non-invasive real-time monitoring techniques to test how dams react to different "events" that can occur during their lifetime. We describe strategies for an effective application of this technique to the analysis of dam displacements emphasizing limits and providing possible solutions. In particular, we study two earth dams using SAR data acquired by a Ku-band GBSAR sensor and the X-band Cosmo-Sky-Med sensor. As far as GBSAR sensors are concerned, we analyze displacement maps obtained at distances of a few months, using two acquisition geometries. This is may be the most useful interferometric application because of the installation flexibility of GBSAR sensors and the possibility to cover the whole dam structures and to measure its horizontal displacements. The analysis is performed on the Occhito and Osento dams of the Consorzio di Bonifica di Capitanata (CBC), which manages four dams in the Capitanata province in the northern part of the Apulia Regions (southern Italy). Three 1-day GBSAR campaign were carried out at the Osento dam on October, 2013 January, 2014 and September, 2014. GBSAR radar has been installed in two sites located on the upstream and down-stream sides, respectively. Two 1-day GBSAR campaign were carried out at the Occhito dam on March, 2014 and September, 2014. Furthermore a time series of Cosmo-Sky-Med SAR images acquired from August, 2013 to September, 2014 over the Occhito dam has been analyzed. For each installation site, a set of fifty SAR images has been collected. Images have been acquired with a temporal baseline of five minutes. Temperature, pressure and relative humidity of atmosphere have been measured during radar acquisition and used to mitigate atmospheric phase delay artifacts in SAR images. The time series of spaceborne SAR images has been processed using the Persistent-Scatterers (PS) technique. The outcome of the PS processing has been a map of mean velocity, precision of the estimated velocity and temporal profiles of each PS. We shows the results obtained by merging GBSAR displacement maps with displacements of Persistent Scatterers obtained by processing COSMO-Sky-Med SAR data. The aim of this analysis is to demonstrate the possibility of monitoring small dams located in a moderately roughly area and estimate the 3D displacement vector from the measurement of line-of-sight displacements along different directions.

Subsidence caused by underground water extraction increasingly affects major cities in the world. The relationship between groundwater level changes and aquifer system consolidation can be explained by means of Terzaghi’s effective stress principle. Soil deformation phenomena may produce damages on urban structures and infrastructures, causing important economic impacts on developed societies and warning about the importance of monitoring those areas. In this framework, effective and efficient subsidence monitoring is a key issue to improve prevention and mitigation within urban management strategies. Classical ground subsidence monitoring techniques are leveling topographic networks, permanent GPS measurements and extensometers. However, these techniques are only able to monitor a limited amount of points due to their cost. In order to monitor large urban areas Differential Synthetic Aperture Radar Interferometry (DInSAR) and advanced DInSAR (A-DInSAR) techniques have proven to be cost effective. PSI techniques are able to provide dense surficial displacement measurements over large areas.

The Madrid Metropolitan area is underlain by a large Tertiary detritic aquifer (TDAM), which has formed in a large tectonic depression that was filled with continental deposits of Tertiary age. These deposits result from a classic superimposed alluvial fan configuration. Most permeable band is made of arkosic sand lenses embedded in a clay and clay-sand matrix that constitute the main body of the TDAM.

Due to the continuous increasing of the Madrid metropolitan area population and the strategic nature of this aquifer as fresh water supply, the study of its behavior is critical for an optimal exploitation. This information is also important for a correct management in the framework of climate change effects in major Mediterranean cities and metropolitan areas, such as Madrid.

The purpose of this work is to analyze the quasi-elastic deformational behavior that has been induced by groundwater withdrawal of the Tertiary detrital aquifer of Madrid (Spain). The spatial and temporal evolution of ground surface displacement was estimated by processing two datasets of radar satellite images (SAR) using Persistent Scatterer Interferometry (PSI). The first SAR dataset was acquired between April 1992 and November 2000 by ERS-1 and ERS-2 satellites, and the second one by the ENVISAT satellite between August 2002 and September 2010. The spatial distribution of PSI measurements reveals that the magnitude of the displacement increases gradually towards the center of the well field area, where approximately 80 mm of maximum cumulated displacement is registered.

The correlation analysis made between displacement and piezometric time series provides a correlation coefficient greater than 85% for all the wells. The elastic and inelastic components of measured displacements were separated, observing that the elastic component is, on average, more than 4 times the inelastic component for the studied period. Moreover, the hysteresis loops on the stress-strain plots indicate that the response is in the elastic range. These results demonstrate the quasi-elastic behavior of the aquifer. During the aquifer recovery phase ground surface uplift almost recovers from the subsidence experienced during the preceding extraction phase.

Taking into account this unique aquifer system, a one dimensional elastic model was calibrated in the period 1997-2000. Subsequently, the model was used to predict the ground surface movements during the period 1992–2010. Modeled displacements were validated with PSI displacement measurements, exhibiting an error of 13% on average, related with the inelastic component of deformation occurring as a long-term trend in low permeability fine-grained units. This result further demonstrates the quasi-elastic deformational behavior of this unique aquifer system.

Sentinel 1 is a key milestone in monitoring ground subsidences overcoming traditional problems, thanks to its enhanced terms of revisit, coverage, timeliness and reliability of service. It will trigger research on aquifer deformation, allowing a quasi-real time monitoring of such systems.

InSAR allows the measurement the regular and irregular motions of the Earth’s surface with exquisite and unprecedented spatial detail, and is significantly less expensive and less labor intensive than obtaining spatially sparse measurements from leveling and GPS surveys. Schmidt et al. (2003) showed that temporally dense InSAR time-series could be used to monitor the seasonal deformation associated with Santa Clara valley aquifer. Reeves et al. (2011) showed that InSAR time-series spanning a few years could be used for constraining the hydrologic response of confined aquifers. Bawden et al. (2001), Watson et al. (2002) and Argus et al. (2005) used individual interferograms and short stacks of interferograms to identify key hydrogeological features in the Los Angeles basin. Lanari et al. (2004) correlated SBAS deformation estimates from 102 acquisitions with sinusoids to delineate the boundaries of the Santa Ana basin.

Relative to previous work, we can now exploit a much-expanded data set using improved methods.  In particular, we consider an 18-year long C-band dataset from the ERS and ENVISAT platforms over Los Angeles, CA to directly derive hydrogeological parameters of interest. Building on existing time-series InSAR approaches, we present a novel method to decouple contributions due to long term deformation and seasonal variations allowing for mapping of lateral hydrological conductivity over large areas in great detail. One of the major challenges in using InSAR data for ground water monitoring is the unavailability of detailed baseline information like distribution of amplitudes and phase of seasonal variations over a region. Our approach allows us to generalize the temporal form of seasonal deformation beyond sinusoids and establish the baseline for seasonal variations in ground deformation over the Los Angeles area including the identification of key features like barriers and aquifer boundaries in the process. We introduce the use of spatial distribution of time of maximum (or minimum) quasi-periodic seasonal deformation as a tool for ultra-fine mapping of the boundaries of shallow confined aquifers and for deriving lateral hydrologic conductivity. We validate the InSAR derived hydrological conductivity estimates against those derived from detailed wellhead pressure measurements in the Los Angeles basin.

Various geodynamical processes and anthropogenic activities can lead to surface deformation. Volcanoes and earthquakes are typical examples of geodynamical processes, whereas, exploitation of underground resources in mines and oil/gas/water reservoirs are standard cases of human activities on the Earth’s surface. In particular, natural gas production has experienced a significant growth in the past few years to meet the energy demands of rising population. Extraction of natural gas leads to a decrease in the reservoir pressure which may result in subsidence, in turn affecting ecosystems, waterways, building foundations, water supply etc. Monitoring this subsidence is important for geological and hazard analysis. This paper aims to provide a better understanding of ground movements caused by natural gas production sites and to study their spatial and temporal effects.     

    InSAR is a powerful remote sensing technique that detects surface deformation (even small-scale) for large areas. The high resolution X-band SAR satellite TerraSAR-X provides millimetric accuracy. Advanced multitemporal InSAR algorithms, for example, Persistent Scatterer Interferometry (PSI), allow long term deformation time series analysis. In PSI, permanently coherent scatterers are exploited and a deformation model is used for the estimation. Several successful demonstrations and validation on various applications, in the framework of ESA's Terrafirma project, highlight the benefits of PSI. The Federal Institute for Geosciences and Natural Resources (BGR), Germany, is collaborating with the German Aerospace Center (DLR), Germany, to make this technique operational for government regulated monitoring of surface deformation due to natural gas extraction in Germany. As part of the pilot study, active hydrocarbon fields in Lower Saxony in Germany have been monitored using TerraSAR-X data.

    A stack of stripmap TerraSAR-X data has been acquired from 23.01.2008 to 18.04.2009 comprising of 35 scenes. DLR’s Integrated Wide Area Processor (IWAP) has been used for PSI processing. PSI is most effective in urban areas because there are a lot of man-made structures, and consequently a large number of stable scatterers. Consequently, the extension of PSI to non-urban areas requires robust algorithms for PS selection, network construction and inversion and atmospheric phase removal. IWAP is a highly-automated, efficient and robust multi-sensor processor for processing high amounts of data. The IWAP has been developed at DLR for the ESA project, Terrafirma. Deformation time series results for the test site obtained using IWAP are presented in this paper and visualizations of the geocoded deformation are shown. The results demonstrate that the subsidence is concentrated around the natural gas extraction wells. This paper illustrates the potential of PSI in providing a powerful and cost-effective tool for monitoring the impact of hydrocarbon reservoirs. In the future, ERS data archives would be analyzed for determining any previous ground movements at the test site. Also, Sentinel-1 data would be acquired for large area monitoring. 

Dam failures are catastrophic events that can result in loss of life and significant destruction of property and infrastructure. For decades dam movements have been monitored using well understood and tested manual geodetic survey methods. Although well understood and recognized, these methods are time consuming, expensive, and vulnerable to errors from factors such as damage to fixpoints, gaps in the measurement series or changes in the national or local reference system.

Specifically, in subarctic countries climatological detrimental effects on the benchmarks, such as ground frost induced heave is a common problem for accuracy and reliability over time. In addition, because of the expense, monitoring of dam movements is often limited to surface readings with low resolution and frequency –especially for remote and inaccessible dams. 

Recently, a series of adaptations and trials of satellite radar interferometry(InSAR) has demonstrated the potential of this technique that is able to continuously monitor movements on dams and surrounding areas with millimetre accuracy. The key advantages of the InSAR technique are: (1) coverage of large areas (dams, reservoir slopes); (2) high spatial resolution that spot smaller subsidence; (3) frequent measurement capability at moderate cost (4) the possibility accessing historical satellite imagery databases to retrieve long time-series of movement. 

In order to demonstrate the effectiveness of InSAR to the dam industry and national authorities, four dams in Norway and Sweden have been the target of a trial where InSAR measurements have been verified against standard in-situ measurements. Each of the four dams varies in size, construction method and material, topography, as well as orientation in relation to the satellite measurements. Several of these dams have previously experienced large deformations for various reasons and are therefore more densely instrumented and monitored than many other large hydropower dams in Scandinavia. 

Findings from the trial, including possibilities and limitations in applying InSAR for monitoring dams and surrounding areas will be presented.

By applying multi-temporal InSAR techniques to a series of satellite SAR images over the same region, it is possible to detect movements of the structure systems on the ground in the millimeter/centimeter range and, therefore, to identify abnormal or excessive movement indicating potential problems requiring detailed ground investigation. The wide variety of currently available spaceborne SAR sensors allows for the use of different frequencies and spatial resolutions. Results from multiple systems operating at different frequencies will often reveal different features. Furthermore, the repeat-pass nature and different system characteristics give rise to the low coherence due to geometrical and temporal decorrelation in some areas. The success and performance of InSAR technology for structural health monitoring (SHM) is then questionable. To achieve a complete knowledge framework on the performance of InSAR in SHM we highlight the differences in exploiting of various sources of the data (ERS, ENVISAT, TerraSAR-X, COSMO-SkyMed) especially for small scale analysis as there are some unexplored circumstances regarding scattering mechanism over the objects with different dimensions, geometry, orientation, material and the amount of displacements that could cause aliasing and others. In this work we also address the fact that we often work with limited or unequally sampled data stacks which, together with unfavourable environmental conditions and non-linear or seasonal components of movements, negatively affect proper parameter estimation using standard PSInSAR algorithm. The main improvement achieved using PSInSAR technique was that uniform deformation at low rates could more accurately be assessed. However, the standard PSInSAR algorithm is typically not successful in observing higher deformation rates or non-uniform deformations. In the case of information gaps for low-coherence areas or the difficulty to resolve high-phase gradients a non-linear model for retrieving deformation signal has to be searched for. Moreover, thanks to the large dataset of frequently acquired high resolution SAR data (e.g. TerraSAR-X, COSMO-SkyMed), it is possible to properly discover various types of deformation movements. Due to their very high sensitivity, the influence of some seasonal deformation sources, such as local temperature or water level changes, can more precisely be estimated (see Appendix 1, Figure 1). Finally, with a proper approach based partially on the model assumptions, the detected deformations or movements of structures in direction of satellite line of sight can be better decomposed into its horizontal and vertical direction components. In this paper it is clearly demonstrated that InSAR techniques may be particularly useful as a hot spot indicator of relative structures deformation over large areas, making it possible to develop interferometric based methodologies for SHM. Different case studies from structural health monitoring of buildings, bridges and highways (see Appendix 2, Figure 2) and dams in Slovakia, Czech Republic, Hong Kong and Portugal processed within the scope of “RemotWatch – Alert and Monitoring System for Physical Structures” project using non-linear and other SHM-optimized algorithms of SARPROZ software, are reported. For the future investigation it is expected, that due to the faster product delivery of new missions (e.g. SENTINEL-1), it will be possible to deliver new workflows suitable for near-real time analysis aimed to better understanding of the deformation characteristics of the structures in urban and extra urban areas, important for structure stability and risk management applications.