| 1. |
- Amin, Hadi
(författare)
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Study on the Earth’s Surface Mass Variations using Satellite Gravimetry Observations
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Our complex planet is continuously undergoing temporal and spatial changes. In this context, ongoing processes in the Earth subsystems (geosphere, biosphere, cryosphere, hydrosphere, and atmosphere) cause changes in the gravity field of the Earth across a wide range of temporal and spatial scales. Accordingly, by both spatially and temporally tracing our planet’s ever-changing gravity field, scientists can better constrain the underlying processes contributing to such dynamic changes of mass distribution within the Earth system. Monitoring the Earth’s gravity field and its temporal variations is essential, among others, for tracking disasters and specifying land areas with a high risk of flooding, earthquakes, and droughts, movements of tectonic plates, and providing accurate positioning through satellite positioning technology. On short-term timescales, temporal variations in the Earth’s gravity field are mainly caused by the movement of water in its various forms. Accordingly, sea-level variations and ice-sheet and glacier changes, which are known as critical indicators of global warming and climate change, can be accurately monitored by tracking the Earth’s gravity field changes. Since there is a close link between water redistribution and the Earth’s energy cycle, climate system, food security, human and ecosystem health, energy generation, economic and societal development, and climate extremes (droughts and floods), it is essential to accurately monitor water mass exchange between the Earth system components. Among all observational techniques, satellite gravimetry has provided an integrated global view of ongoing processes within the Earth system. The current generation of satellite gravimetry missions (the Gravity Recovery and Climate Experiment (GRACE) mission and its successor, GRACE Follow-On) has dramatically revolutionized our understanding of dynamic processes in the Earth’s surface and, consequently, has significantly improved our understanding of the Earth’s climate system. By considering different aspects of studying the Earth’s gravity field, this thesis brings new insights to the determination and analysis of the mass change in the Earth system. First, by studying the shortcomings of the common techniques of estimating the geoid potential, a new approach is examined that simultaneously estimates the geoid potential, W0, and the geometrical parameters of the reference Mean Earth Ellipsoid (MEE). In this regard, as the geoid needs to be considered as a static equipotential surface, the sensitivity of the estimations to the time dependent Earth’s gravity field changes is studied. Secondly, relying on the GRACE monthly gravity fields and the complementary observational techniques, and by pushing the limit of GRACE, mass redistribution over land and ocean is investigated. Within the ocean, satellite altimetry and Argo products are utilized along with the GRACE monthly solutions for quantifying the global barystatic sea-level change and assessing the closure of the global mean sea level budget. Over land, a region with relatively high temporal mass change (oil and water extraction) is chosen in which by taking advantage of having in-situ observations and hydrological models, the ability of GRACE products in quantifying the changes in groundwater storage is studied. In this frame, for both the ocean and land studies, different aspects of the processing of GRACE monthly gravity fields are investigated and GRACE inherent errors are addressed appropriately to arrive at reliable and accurate estimates of the Earth’s surface mass change. As the final contribution in this thesis, a rigorous analytical model for detecting surface mass change from the time-variable gravity solutions is proposed and examined in different case studies of surface mass change. Since the launch of the GRACE twin satellites, the GRACE(-FO) time-varying gravity fields are conventionally converted into the surface mass change using a spherical analytical model that approximates the Earth by a sphere. More recently, the analytical mass change detection model has been improved by considering an ellipsoid as the shape of the Earth, which improved the previous estimations of surface mass change, especially over high latitudes with relatively large mass change signals. However, by taking into account the real shape of the Earth and considering more realistic assumptions, a new analytical solution for the problem of surface mass change detection from the time-varying gravity fields is proposed in this thesis. It is shown that the simplistic spherical and ellipsoidal geometries are no longer tenable and the new model surpasses the common spherical approach and its ellipsoidal version.
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| 2. |
- Bagherbandi, Mohammad, Professor, et al.
(författare)
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Deflection of Vertical Effect on Direct Georeferencing in Aerial Mobile Mapping Systems : A Case Study in Sweden
- 2022
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Ingår i: Photogrammetric Record. - : Wiley. - 0031-868X .- 1477-9730. ; 37:179, s. 285-305
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Tidskriftsartikel (refereegranskat)abstract
- GNSS/INS applications are being developed, especially for direct georeferencing in airborne photogrammetry. Achieving accurately georeferenced products from the integration of GNSS and INS requires removing systematic errors in the mobile mapping systems. The INS sensor's uncertainty is decreasing; therefore, the influence of the deflection of verticals (DOV, the angle between the plumb line and normal to the ellipsoid) should be considered in the direct georeferencing. Otherwise, an error is imposed for calculating the exterior orientation parameters of the aerial images and aerial laser scanning. This study determines the DOV using the EGM2008 model and gravity data in Sweden. The impact of the DOVs on horizontal and vertical coordinates, considering different flight altitudes and camera field of view, is assessed. The results confirm that the calculated DOV components using the EGM2008 model are sufficiently accurate for aerial mapping system purposes except for mountainous areas because the topographic signal is not modelled correctly.
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| 3. |
- Bagherbandi, Mohammad, Professor, et al.
(författare)
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Importance of precise gravity field modeling in direct georeferencing and aerial photogrammetry : a case study for Sweden
- 2022
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Ingår i: The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B2-2022XXIV ISPRS Congress (2022 edition), 2022, Vol. XLIII-B2. - : International Society for Photogrammetry and Remote Sensing. ; , s. 15-20
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Konferensbidrag (refereegranskat)abstract
- Direct georeferencing of airborne mobile mapping systems is developing with unprecedented speed using GNSS/INS integration. Removal of systematic errors is required for achieving a high accurate georeferenced product in mobile mapping platforms with integrated GNSS/INS sensors. It is crucial to consider the deflection of verticals (DOV) in direct georeferencing due to the recently improved INS sensor accuracy. This study determines the DOV using Sweden's EGM2008 model and gravity data. The influence of the DOVs on horizontal and vertical coordinates and considering different flight heights is assessed. The results confirm that the calculated DOV components using the EGM2008 model are sufficiently accurate for aerial photogrammetry purposes except for the mountainous areas because the topographic signal is not modeled correctly.
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| 4. |
- Darvishi, Mehdi, et al.
(författare)
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InSAR-based Ground Motion Service of Sweden: evaluation and benefit analysis of a nationwide InSAR service
- 2022
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Konferensbidrag (refereegranskat)abstract
- Space-geodetic techniques such as Global Navigation Satellite Systems (GNSS) and Syntenic Aperture Radar interferometry (InSAR) are powerful tools to measure and monitor ground surface motion. InSAR has widely been used for the detection and quantification of slow mass movements over the past three decades mainly at the local and regional scales. The high performance and millimeter-level measurement accuracy of radar satellite to provide a dense deformation map at different spatial and temporal resolutions are the key factors to think of using SAR data and InSAR technique as an efficient tool for geohazards motoring system at the nationwide scale.Sweden has recently joined to the countries having InSAR Ground Motion Service (GMS) at a nationwide scale. The InSAR service of Sweden, which will soon be freely available for users, provides the displacement time-series of measurement points for the entire country. The Swedish GMS project was started last year and is an ongoing collaboration between the Geological Survey of Norway (NGU) and several Swedish organizations (led by the Swedish National Space Agency (SNSA)). The InSAR-based GMS of Sweden has been generated by NGU using Sentinel-1 data (2015–2020) and the Persistent Scatterer Interferometry (PSI) technique. The web-based GMS of Sweden consists of ~1,5 billion time-series measurement points obtained from both descending and ascending satellite orbital modes.Currently, the Swedish GMS is under evaluation and validation phase and the given plan has been designed to assess the quality or validate the GMS products. We plan to conduct the data validation through two main phases: 1) a cross-comparison between InSAR measurement points and ancillary data such as GNSS, Corner Reflectors (CR), Electronic Corner Reflectors (ECR) and leveling data, and 2) assessment of tropospheric and ionospheric effects on InSAR measurement points. Specifically, we will evaluate different approaches and data for the InSAR tropospheric corrections, such as Very-Long-Baseline Interferometry (VLBI), Water Vapour Radiometry (WVR), and GNSS data at the Onsala Space Observatory (OSO).In the first phase of validation, leveling data collected in Gothenburg and Stockholm cities, mainly over the residential areas and public transport infrastructures compared to the corresponding InSAR measurements points (vertically converted) for a five-year period. The initial results present a high correlation between two sets of the vertical displacements. The same procedure will be performed for the Kiruna city where the mining activities resulted in adrastic urban land subsidence. Since the CRs and ECRs have recently been installed in different parts of Sweden, we do not have them as PS points in the current version of the GMS. Therefore, those CR-based measurement points will be used in future accuracy assessments. In the second phase, we investigated the effects of phase delay induced by troposphere on displacement time-series using two approaches, i.e., time-space filtering and using external data (e.g., atmospheric reanalysis data, GNSS, VLBI and Water Vapor Radiometer (WVR)). Recently, European GMS (EGMS) has been released and the Ortho displacement map is now available for users freely. We also evaluated and compared the EGMS-Ortho displacement map with our independent InSAR processing and GNSS data over the Kiruna.As the InSAR-based GMS can be used to monitor and identify the potential risk of geo-related hazards in Sweden, the society will directly benefit from the outcomes of this project. This open access product will help the stakeholders with decision support for prioritization of risk-reducing measures, and identification of the need for further investigations for areas in danger. The service could also assist municipalities and county administrative boards to have an update information regarding urban areas which are more prone to land subsidence and disruption urban infrastructure.
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| 5. |
- Gruber, Thomas, et al.
(författare)
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Geodetic SAR for Height System Unification and Sea Level Research - Observation Concept and Results in the Baltic Sea
- 2022
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Konferensbidrag (refereegranskat)abstract
- Traditionally, sea level is observed at tide gauge stations, which usually also serve as height reference stations for national leveling networks and therefore define a height system of a country. Thus, sea level research across countries is closely linked to height system unification and needs to be regarded jointly. One of the main deficiencies to use tide gauge data for geodetic sea level research and height systems unification is that only a few stations are connected to permanent GNSS receivers next to the tide gauge in order to systematically observe vertical land motion. As a new observation technique, absolute positioning by SAR using active transponders on ground can fill this gap by systematically observing time series of geometric heights at tide gauge stations. By additionally knowing the tide gauge geoid heights in a global height reference frame, one can finally obtain absolute sea level heights at each tide gauge. With this information the impact of climate change on the sea level can be quantified in an absolute manner and height systems can be connected across the oceans.The paper presents the results of a project, which was conducted in the years 2019 to 2021 in the frame of ESA´s Baltic+ initiative. Within this project a test network of electronic corner reflectors (ECR) as targets for Sentinel-1 was realized in the Baltic Sea area. The ECR locations were either co-located with tide gauges or with permanent GNSS stations in order to observe systematically the ellipsoidal heights of the tide gauges and possibly also any vertical land motion at the stations. Data for the year 2020 were collected at 10 stations in Estonia, Finland, Poland and Sweden and jointly analyzed with GNSS data, tide gauge records and regional geoid height estimates. The obtained results are promising, but also exhibit some problems related to the ECR´s and their performance. At co-located GNSS stations the estimated ellipsoidal heights agree in a range between about 2 and 50 cm between both observation systems. From the results it could be identified that most likely variable systematic electronic instrument delays of the ECR´s are the main reason for these differences and that each instrument needs to be calibrated individually. Nevertheless, the project provides a valuable data set, which offers the possibility to enhance methods and procedures in order to develop the geodetic SAR positioning technique towards operability. All data and reports are accessible at the following web site: https://www.asg.ed.tum.de/iapg/baltic/
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| 6. |
- Jivall, Lotti, et al.
(författare)
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Analysis of 20 years of GPS data from SWEREF consolidation points – using BERNESE and GAMIT-GLOBK software
- 2022
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Rapport (populärvet., debatt m.m.)abstract
- The SWEREF 99 national geodetic reference frame has been used in Sweden since 2007 and it was adopted by EUREF in 2000 as the national realisation of ETRS89 in Sweden [Jivall and Lidberg, 2000]. The SWEREF 99 reference frame is defined by an active approach through the 21 original (fundamental) SWEPOS GNSS stations, hence relying on positioning services such as the network real time kinematic (NRTK) and post processing services. The SWEREF 99 coordinates are assumed to be fixed in time and no temporal variations are expected. However, the stability of the stations and their coordinates can be altered due to equipment change or software as well as local movements at the reference stations.To be able to check all alterations mentioned above and having a backup national network of GNSS points, approximately 300 passive so-called consolidation points are used. The consolidation points are a subset (the main part) of the so-called SWEREF points established from 1996 and onwards. All 300 points are remeasured with static GNSS for 2x24 hours using choke ring antennas on a yearly basis with 50 points each year. The original data processing was done with the Bernese GNSS software in a regular basis and the reprocessing was carried out with both the Bernese and the GAMIT-GLOBK software packages during 2017-2018.The resulting coordinates in SWEREF 99 from GAMIT and Bernese processing are equal at 1–2 mm level for the horizontal and 4 mm for the vertical components (1 sigma) when using almost the same models and processing strategy. The result from the original processing, which partly is based on other models and parameters, differs slightly more for the north component compared to the reprocessing results (RMS of 2 mm compared to 1 mm).Our analysis both of Bernese and GAMIT results shows that the standard uncertainties for a single SWEREF 99 coordinate determination (with 2x24 hrs observation) is about 2 mm for the horizontal components and 6 mm in height. It is interesting to note that the coordinate repeatability is on the same level also for the original processing, where we have differences in models and parameters used during the years. This indicates that our concept for determining SWEREF 99 coordinates has worked well on the mentioned uncertainty level.We performed trend analysis and statistical tests for the points having minimum three observations to investigate the stability of the estimated SWEREF 99 coordinates. The low rate of redundancy (just one redundant observation in case of three observations) was a problem so a modified version of the F-test was developed which gave good agreement with visual interpretation of the time series. This strategy showed that about 10% of the points had trends (with notable movements), but we should be aware of the low redundancy. With more observations in the future, we can determine trends more reliably.We will continue to analyse the point coordinate repeatability and trends when we get more data. Further on, some reprocessing is needed to be compatible with the SWEREF 99 update 2021 at SWEPOS. We will also study the effect of using different satellite systems and finally prepare for the publication of updated coordinates in the Digital Geodetic Archive (DGA) provided by Lantmäteriet.
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| 7. |
- Jouybari, Arash, et al.
(författare)
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Impact of GNSS Signal outage on EOPs using forward Kalman filter and smoothing algorithm
- 2022
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Ingår i: 2022 | XXIV ISPRS Congress “Imaging today, foreseeing tomorrow”, Commission II. - : Copernicus GmbH. ; , s. 59-64, s. 59-64
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Konferensbidrag (refereegranskat)abstract
- The global navigation satellite system (GNSS) has been playing the principal role in positioning applications in past decades. Position robustness degrades with a standalone receiver due to GNSS signal outage in mobile mapping systems. The GNSS and inertial measurement unit (IMU) integration is used to solve positioning degradation. This article studies the GNSS/IMU integration processing (i.e., forward Kalman filter (KF) and smoothing algorithm) using a single or a network of nearby GNSS reference stations. In addition, we analyze the impact of simulated GNSS signal outage on exterior orientation parameters (EOPs). The outcomes confirm that the smoothing algorithm works better than the forward KF and improves the accuracy for position and orientation in the case when there is no GNSS signal outage. Also, it improves the position and orientation accuracy by about 95% and 60% when there is a 180 second GNSS signal outage, respectively.
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| 8. |
- Khoshlahjeh Azar, Mahdi, et al.
(författare)
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Integrated analysis of Hashtgerd plain deformation, using Sentinel-1 SAR, geological and hydrological data
- 2022
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Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Due to its proximity to Tehran, the Hashtgerd catchment in Iran is an important region that has experienced alarming subsidence rates in recent years. This study estimated the ground surface deformation in the Hashtgerd plain between 2015 and 2020 using Sentinel-1 SAR data and InSAR technique. The average LOS displacement of the ascending and descending tracks was -23 cm/year and -22 cm/year, respectively. The central area of the plain experienced the greatest vertical subsidence, with a more than -100 cm cumulative displacement. The Karaj-Qazvin railway and highway that pass through this area have been damaged by subsidence, according to an analysis of profiles drawn along the transportation lines. The southern sections of Hashtgerd city have experienced a total displacement of -30 cm/year over the course of about six years. The relationship between changes in groundwater level and subsidence rate in this region was examined using piezometer and precipitation data. Geoelectric sections and piezometric well logs were also utilized to investigate the geological characteristics of the Hashtgerd aquifer. According to the findings, the leading causes of subsidence were uncontrolled groundwater abstraction. This research highlights the need to comprehend the spatial distribution of confined aquifers and their effect on subsidence, which can aid in the development of a suitable management strategy to restore these aquifers.
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| 9. |
- Nilfouroushan, Faramarz, Senior Lecturer, 1968-, et al.
(författare)
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Establishment of a new geodetic infrastructure in Sweden using SAR Corner Reflectors: Progress report
- 2022
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Konferensbidrag (refereegranskat)abstract
- Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing and geodetic technique for ground deformation measurements using radar images of the Earth's surface that are collected regularly by orbiting satellites equipped with SAR sensors. Natural reflectors on the ground which backscatter the radar signal to the satellite are monitored in different time and changes of the line of sight (LOS) distances can detect ground surface movements. Natural reflectors are abundant in many places specially in urban areas. However, areas that are densely vegetated or covered by snow have limitations for InSAR technique and therefore artificial corner reflectors, passive or active ones, can be used instead. These corner reflectors are devices which provide precise measurement points and can be installed at desired locations. These devices can measure temporal LOS changes of the measurement point precisely using InSAR technique.Since 2020, Lantmäteriet has installed three active electronic corner reflectors (ECR) and several types of passive reflectors (CR) in different locations of Sweden. The plan is to continue and complement the national geodetic infrastructure with at least 20 passive reflectors which are collocated with permanent GNSS stations and/or tide gauges. Among others, these colocated permanent GNSS stations and corner reflectors can potentially contribute to the development and validation of the national and European ground motion services. Moreover, the colocation helps to map the relative ground motions estimated with InSAR to an absolute geodetic refercne frame.
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| 10. |
- Nilfouroushan, Faramarz, Senior Lecturer, 1968-
(författare)
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Komplettering av den Nationella Geodetiska infrastrukturen för InSAR-tillämpningar
- 2022
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Konferensbidrag (refereegranskat)abstract
- Markrörelsetjänster baserade på InSAR-teknik finns redan tillgängliga för allmänheten via olika web-GIS. I dessa applikationer redovisas hur ett stort antal punkter på marken i Sverige (~1.5 miljarder) rör sig relativt varandra. Tekniken bygger på att såväl artificiella som naturliga objekt i terrängen reflekterar radarsignaler från satelliter och efter upprepade passager av satelliterna kan objektens relativa rörelser skattas. Lantmäteriet kompletterar nu den nationella geodetiska infrastrukturen med aktiva och passiva reflektorer för InSAR-mätning. Reflektorerna samlokaliseras med våra fasta referensstationer för GNSS i Swepos-nätet. Genom att korrelera markrörelse mätta med olika tekniker (GNSS och InSAR) får vi mer tillförlitliga resultat och markrörelserna skattade med InSAR kan transformeras från relativa rörelser inom en satellitscen till absoluta tal i ett geodetiskt referenssystem, t. ex. SWEREF 99.Vi kan dessutom komplettera den högupplösta informationen i satellitscenerna med mer långvågiga rörelser som t. ex. landhöjning.
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| 11. |
- Nilfouroushan, Faramarz, Senior Lecturer, 1968-, et al.
(författare)
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Status report on the installations of geodetic SAR corner reflectors in Sweden
- 2022
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Konferensbidrag (refereegranskat)abstract
- Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing and geodetic technique for ground deformation measurements using radar images of the Earth's surface that are collected regularly by orbiting satellites equipped with SAR sensors. Natural reflectors on the ground which backscatter the radar signal to the satellite are monitored at different time and changes of the line of sight (LOS) distances can detect ground surface movements. Natural reflectors are abundant in many places, especially in urban areas. However, areas that are densely vegetated or covered by snow have limitations for InSAR technique and therefore artificial corner reflectors, passive, or active ones, can be used instead. These corner reflectors are devices which provide precise measurement points and can be installed at selected locations. These devices can measure temporal LOS changes of the measurement point precisely using the InSAR technique.Since 2020, Lantmäteriet has installed three active electronic corner reflectors (ECR) and several types of artificial passive reflectors (CR) in different locations of Sweden. The corner reflectors are anchored to the bedrock like GNSS stations using metal masts. The installation is in progress and the plan is to continue and complement the national geodetic infrastructure with at least 20 passive corner reflectors which are collocated with permanent GNSS stations and/or tide gauges and/or absolute gravity points. Among other applications, these collocated points link different geodetic measurement techniques, including InSAR and GNSS, and contribute to the development and validation of the national and European ground motion services. Moreover, the GNSS derived velocities and the LOS temporal variations measured on the nearby corner reflectors are useful to transform the relative ground motions estimated with InSAR to an absolute geodetic reference frame.
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| 12. |
- Shami, Siavash, et al.
(författare)
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Assessments of ground subsidence along the railway in the Kashan plain, Iran, using Sentinel-1 data and NSBAS algorithm
- 2022
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Ingår i: International Journal of Applied Earth Observation and Geoinformation. - : Elsevier. - 1569-8432 .- 1872-826X. ; 112
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Tidskriftsartikel (refereegranskat)abstract
- The 110-kilometer-long Qom-Kashan railway is one of the busiest lines in Iran, passing through the Kashan plain. The majority of Iran's plains have subsided in recent years as a result of uncontrolled groundwater extraction, and the Kashan plain is no exception. In this study, ground surface displacement in the Kashan plain region and its impact on the railway were investigated using New Small Baseline Subset (NSBAS) in up-down and east–west directions using descending and ascending Sentinel-1 data collected between 2015 and 2021. Our results indicate that the Kashan plain is subsiding more than 90 mm/year. The study of the local areas around the railway which passes through the study area revealed that the rate of vertical velocity in some locations reaches –23 mm/year, while the rate of east–west velocity is insignificant and is approximately ±2 mm/year. Additionally, a method for analyzing the railway's stability based on longitudinal profiles along the railway is presented. Our findings suggest that more than 60% of the railway line is subject to variable amounts of subsidence. Additionally, a region of approximately one kilometer of the railway has been classified as a risk zone due to relatively fast local deformation. After examining the effect of various factors, it was determined that uncontrolled groundwater extraction in agricultural areas contributed to the subsidence in this area. Our results show that the presented stability control approach in this study is highly reliable for creating hazard profiles for linear structures, such as railways.
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