BGS Research — Sinkholes and karst research the BGS contributed to an advisory mission in Solotvyno for the European Union Civil Protection Mechanism (EUCPT) The mission was undertaken in response to a cross-border (Ukraine/Romania) environmental pollution incident and was commissioned by the Director General of the Directorate-General of European Civil Protection and Humanitarian Aid Operations (DG ECHO) It was suspected that the cross-border environmental pollution was linked to the Solotvyno salt mine complex The mission was to generate a ‘comprehensive risk assessment at the Solotvyno salt mines area’ requiring a focus on the stability of the workings and the potential impact on the River Tisza It comprised 16 experts from eight countries and included five of the experts from the original scoping mission in July The legacy of the mines was evident in all aspects of Solotyvno life with significant implications for humanity including consideration of the economic value of the salt to the region the political complexity and the Tisza as a major tributary of the Danube The Solotvyno mine in the Zakarpatye province of Ukraine was the first (former) Russian rock-salt mine (Logunov It is situated in a southerly looping meander of the River Tisza which forms the border between Ukraine and Romania Six medieval mines were worked to a relatively shallow depth and collapse of these mines led to the formation of the saline ponds that lie at the heart of the current tourist industry Three deeper mines were commissioned between 1809 and 1975 and they extended to depths of up to 430 m The mines were constructed as a series of tall (up to 65 m high) chambers Mine 7 closed in 1953 after the collapse of one of the support pillars whereas Mine 8 and Mine 9 were closed because of flooding resulting in the formation of more than 50 sinkholes of up to 20 m diameter and up to 40 m deep The largest of the sinkholes is in the order of 200 m in diameter with mining infrastructure in the background The abandoned mines were left to flood because the potential for further dissolution and therefore the risk of further collapse is minimised During the mission the International EUCPT collected data on the mines and their history undertook a surface and groundwater survey and visually assesed the integrity of infrastructure The modern tourist industry grew up based on the health benefits of brine pools Solotvyno advertised itself as a medical resort with an allergological hospital using the former mines for speleotherapy therapies at depths up to 300 m below ground level Collapsed medieval mines as a tourist attraction The Miocene (23 to 5 million years ago) salt deposits of Solotvyno occur in the fault-bounded Solotvyno depression of the Ukranian Carpathians 2000 m-long salt diapir (salt dome) that is elongate in shape and lies along a north-west to south-east alignment The Miocene sedimentary sequence consists of sandstones and siltstones interbedded with volcanic tuffs resting on igneous and metamorphic rocks of the Ukrainian Precambrian shield The rock salt is predominantly white in colour with grey layers of clays with subordinate gypsum anhydrites (Hryniv et al. The salt has recrystallised as flow-textured halite due to metamorphism Residual clays capping the salt in the flooded abandoned medieval mines The presence of the impurities gives rise to characteristic karst processes (both void and sinkhole development) operating in the salt the clay-rich salt residue forms a naturally protective layer and the different forms of crystallisation influence the geomechanical properties of the salt River terrace deposits provide evidence of Neogene (23 to 2.5 million years ago) uplift and cover the surface of the salt dome The engineering properties of the overlying sediments (high permeability) is influential in naturally occurring salt karst Scientists focused on the development of a conceptual ground model as the basis of a risk assessment Many different types of data were collected Fieldwork was monitored and facilitated by the EUCPT technical staff and Ukrainian colleagues from the National Academy of Sciences Impact assessment was challenging in the context of anthropogenic activities pipe discharges to the sinkholes and destruction of the palag cover with the potential to exacerbate the natural processes that have resulted in a deterioration of the mines and surrounding area Risk assessment was undertaken using ‘Bowtie’ techniques comprising a diagrammatic capture of risk flow thought processes in terms of causes–risk–consequences These diagrams enabled an assessment of risk criticality in terms of the land-stability risk to the local population and potential environmental impact on the Tisza the Directorate General for European Civil Protection and Humanitarian Aid Operations (DG ECHO) has provided grant funding for the project ‘Improving disaster risk reduction in Transcarpathian region’ (ImProDIRet) and other relevant stakeholders jointly manage the risks in the Transcarpathian region with particular attention to the Solotvyno salt mines This research was collaborative it was undertaken through the European Union Civil Protection Mechanism with the support of Ukrainian technical experts from the National Academy of Science (NAS) of Ukraine and Ukraine State Rescue Service (Mountain Rescue) Hryniv, S P, Dolishniy, B V, Khmelevska, O V, Poberezhskyy, A V, and Vovnyuk, S V. Evaporites of Ukraine: a review Geological Society of London Special Publications 285 Logunov, E V. 1997. A history of salt production in Russia Stoeck, L, Banks, V, Shekhunovac, S, and Yakovlevc, Y. 2020. The hydrogeological situation after salt-mine collapses at Solotvyno, Ukraine Our research extends beyond the distribution and processes associated with sinkhole formation to the broader subject of karst This website uses cookies to improve your experience. We’ll assume you’re okay with this, but you can opt out if you wish. More information. Keep up to date with all the latest research, products and events news. Volume 11 - 2023 | https://doi.org/10.3389/feart.2023.1167672 Salt deposits were a rich source of mineral resources in the past infrastructure and the natural environment Solotvyno (Ukraine) is one of the most significantly affected areas with a deformation zone where significant year-on-year subsidences occur Mining activities have caused a disturbance of the balance in the mountain massif Deformation zone of the historical salt mine Solotvyno (Ukraine) Tyachiv district of Zakarpattia region is situated on the right bank of the Tisza river in the border area with Romania in Central Europe This paper deals with the multi-sensor monitoring of the active deformation zone over the Solotvyno salt mine using satellite radar data (Sentinel-1) Since the area represents a significant geohazard from a hydrogeological (Tisza River) and civil security standpoints (infrastructure of the inhabited regions) the design of a high-precision monitoring system to monitor and evaluate current environmental changes is proposed Multi-temporal InSAR analysis exposed steeper subsidence tendencies of >−2 cm in the central part of the monitored area Optical satellite and UAV images confirmed the increase in water surface in sinkhole areas up to 28,500 m2 and proved the emergence of new sinkholes in the central part of the Solotvyno mine The aim of this study is to describe the post-collapse deformation processes by Pleiades and SPOT multi-spectral sensors and Sentinel-1 satellite SAR sensors since the 2010 collapse in order to describe the trends of deformation due to undermining and propose a high-precision permanent monitoring system based on satellite radar interferometry (InSAR) As problems began to appear in its operating mines from the mid-1990s it led to a dangerous man-made ecological situation which reached the state level in 2010 (expert opinion of the Ministry for Emergency Situations of Ukraine dated 09.12.2010 No as well as the operation of the underground department of the spelosanatorium of the Ukrainian Remediation Facilities and the laboratory of the Institute of Physics of the National Academy of Sciences of Ukraine was also terminated The Solotvyno rock salt deposit is located in a subducted block in the south-southwestern extension of the Dibrovskaya and Solotvyno anticlines the supra salt plain is close to the brachyanticlinal fold with a salt core The thickness of salt in the anticlinal core is over 1,500 m The inclination angles of the Solotvyno and Teresvyna rocks The dip of the lateral wings of the salt massif is much steeper than the dip angles of the host rocks The zone of lateral contact between the salt and host rocks is tectonic in nature complicated by hypo- and exogenic processes and represents potential and already realized pathways of groundwater and surface water migration represented by alternating layers and lenses of pure rock salt salt with varying contents of water-insoluble residues lenses and blocks of intra-salt terrigenous FIGURE 1. Main geological units distribution in Central Europe and geological sections of the Solotvyno rock salt deposit (Jacko et al., 2016; Yakovlev et al., 2016) water-insoluble residue (i.r.) content up to 2%; 2—tereblianska suite content up to 2.5%; 3—Tereblianska suite alternating rock salt: light gray and gray content less than 3%; 4—Tereblianska suite content more than 3%; 5—Tereblianska suite sandstone and tuff; 7– Quater sediments alternating: boulder-gravel beds and clay; 8—borehole; 9—old mine outline; 10—mine no These studies to date are mainly concerned with the description of the hydrological situation after the collapse of the salt mine risk mapping and stress-strain state analysis in the surroundings of the Solotvyno Salt Mine they use only InSAR technology to numerically quantify subsidence The novelty of this research is a multi-sensor (InSAR very high-resolution multi-spectral satellite images multi-temporal (2014-2022) and graphical approach to monitor this study area with a design for future high precision monitoring using permanent artificial corner reflectors (ACRs) The particular benefit of ACR units installed in-situ would be the knowledge of their effective phase center allowing the direct co-location with other geodetic measurements (GNSS leveling) and achieving accuracy better than the measurement noise of the observation techniques at hand FIGURE 2. (A) Creation of a sinkhole and (B) Meaning of the effective area, P-point drop scheme on the effective area (Trnka et al., 1968) The dip angle γ initially defines the subsidence basin the edge of the subsidence basin is defined by the limiting angle β To determine the subsidence of any point located in the middle of the subsidence basin the magnitude of vertical movement is given as: where s is the subsidence of the observed point (mm a - is the mining coefficient (method of mining) z–is the time coefficient (indicates the full subsidence of the point in time varies in time according to the nature of the overburden the storage parameters and the method of mining) e–is the effect coefficient (the ratio of the actually mined area under the considered point to the fully effective area of this point The horizontal displacement hd is defined as: where ξ is the band angle (Figure 2B) The effects of mining are not only manifested in the diameter of the mined area on the surface but also outside it and up to a certain distance r depending on the depth of the deposit H and the limiting angle β according to the relation: The period of descent can be divided into 3 stages The stage of initial subsidence - from the appearance of the first subsidence on the surface until the time of intense subsidence The duration of this stage depends mainly on the speed of the quarrying operations Intensive subsidence stage - when the subsidence of surface points is 70%–80% of the total subsidence it is the most dangerous stage for surface objects due to rapid changes the subsidence is already negligible from a technical point of view The area of interest to investigate the deformation phenomena (Figure 3) covers approximately 5 by 4 km (20 km2) area over Solotvyno town The area corresponds mostly to urbanized zones with a higher presence of vegetated regions in the surroundings FIGURE 3. Solotvyno region (Source: authors, base maps SAS. Planet v221122.10312 Nightly; Yakovlev et al., 2016) Multi-temporal InSAR analysis (Ferretti et al., 2000; Ferretti et al., 2001; Kampes, 2006; Ketelaar, 2009) is an advanced technique of multi-temporal satellite radar interferometry which is established as an operational tool for low-cost wide-area coverage and millimetric precision ground deformation monitoring MTInSAR aims to identify coherent radar targets exhibiting high phase stability over the entire observation period and derives point data with locations corresponding mainly to the point-wise Phase stable points can be used as a “natural GNSS network” to monitor terrain motion are point clouds with precise displacement time-series over monitored objects Objects can be measured with sensitivity to millimetric changes over wide areas utilizing frequent Sentinel-1 acquisitions (12/6 days) Sentinel-1 product level Ground Range Detected (GRD) SAR images were also selected to compare temporal changes in individual Sinkholes in the study area The selection of images was matched with the reference multi-spectral data of Pléiades and SPOT and their acquisition dates To compare the effect of polarization configuration and orbit type on the accuracy of the results 2 SAR images with ascending and descending orbit tracks were selected for each time interval Both types of polarization configurations of the GRD product and shortwave infrared sensors to produce images of the earth’s surface by detecting solar radiation reflected from targets on the ground Different materials reflect and absorb different wavelengths targets can be distinguished by their spectral reflectance features in remote sensing images Depending on the number of spectral bands used in the imaging process optical remote sensing systems are divided into basic types - panchromatic For the analysis of temporal changes in the study area by multi-spectral satellite images • Pléiades-1A and Pléiades-1B The Pléiades satellite constellation was designed for military and civil use by the French-Italian program ORFEO (Optical and Radar Federated earth Observation) in 2001–2003 This constellation consists of two satellites–Pléiades-1A and Pléiades-1B Pléiades-1A and 1B were built by AIRBUS Defence and Space and launched on 16 December 2011 (1A) and 2 December2,012 (1B) Pléiades constellation delivers imagery products with 0.5 m resolution phased 180° apart They operate in the same near-polar sun-synchronous orbit at an altitude of 694 km Pléiades product consists of 5 bands–PANchromatic (0.45–0.75 µm) Blue (0.45–0.52 µm) Green (0.53–0.60 µm) Red (0.62–0.69 µm) and Near Infrared (0.76–0.89 µm) SPOT6 and SPOT7 are the latest in the SPOT (“Satellite pour l'Observation de la Terre”) satellite series and were designed to continue the SPOT4 and SPOT5 missions The first five satellites were designed by the French National Centre for Space Studies (CNES) and launched in 1986–2015 The latest SPOT6 and SPOT7 are commercial satellites owned by AIRBUS Defence and Space launched on 9 September 2012 (SPOT6) and 30 June 2014 (SPOT7) They operate on the same orbit phased 180° apart with a resolution of 1.5 m (panchromatic) and 6 m (multi-spectral) Pléiades product consists of 5 bands–PANchromatic (0.48–0.82 µm) Blue (0.45–0.53 µm) Green (0.51–0.59 µm) Red (0.62–0.70 µm) and Near Infrared (0.775–0.915 µm) To analyze temporal changes in the study area from multi-spectral images we decided to monitor surface water area at individual sinkhole/mine localities using remote sensing indices for water monitoring Considering the bands provided in products from Pléiades and SPOT images we used the Normalized Difference Water Index NDWI: NDWI index is based on the reflectance in the green and near-infrared (NIR) bands The water body has strong absorption and low emissivity in the visible to infrared wavelength range It is sensitive to built-up areas and results in sometimes overestimated water areas which had to be taken into account during the extraction of water area To eliminate the occurrence of false positives in built-up areas Multi-spectral images were obtained for the years 2014–2022 from AIRBUS OneAtlas service Living Library as pansharpened GeoTIFF format for Pléiades (“© CNES (2014, 2015, 2017, 2021, 2022), Distribution Airbus DS”) and SPOT (“© Airbus DS (2016, 2018, 2019, 2020)"). The final set of images is summarized in Table 1 Specification of the used Pléiades and SPOT multi-spectral images The FLIRT Arrow aerial survey system developed by ABRIS Design Group (Ukraine) was used to perform aerial surveying of the studied territory located in the city of Solotvyno The methodology of the aerial survey included the following four main stages: the creation of an aerial survey project; determination of the coordinates of reference and control points; performing aerial survey; the processing of aerial survey results FlightPlanner software was used to сreate an aerial survey project The key parameters for the creation of the project were the boundaries of the studied territory and the required resolution in 4 cm The project also took into account the approximate terrain The prepared project was loaded into the UAV autopilot before the start of the aerial survey The coordinates of the situational points were determined within the survey area which served as reference and control points for the aerial survey The coordinates were determined by GNSS receiver SOUTH galaxy G1 in RTK mode Coordinates of points were determined from the GNSS stations SOLT (Solotvyno) which belongs to the GeoTerrace GNSS network (Ukraine) A total of 12 reference and 47 control points were determined in the given area Reference points were used to georeference the results of aerial surveys and control points for quality control of work performed The coordinates were determined in the Ukrainian state coordinate system USK 2000 as a height system was used WGS84 (ellipsoidal) It should be noted that the mean square error of reference and control points determination does not exceed 19 mm The aerial survey was performed in November 2021 in sunny weather with moderate surface winds up to 5 m/s with the help of FLIRT Arrow aerial survey system The materials of the aerial survey were analyzed and an a priori assessment of the accuracy of obtaining the coordinates of the points of the terrain was performed Then the preliminary processing of aerial survey materials was performed which includes the preparation and introduction of survey parameters creation and export of reference and control point coordinates catalogue The final processing of the prepared data was performed in specialized software Pix4Dmapper - software for photogrammetry and professional mapping with the help of UAVs Distribution of normal (vertical axis) and temporal (horizontal axis) baselines for Ascending Track 131 (top) and Descending Track 7 (bottom) Two key parameters are estimated in a network of pre-selected points: residual height and displacement velocity As with the classical methodology of MT-InSAR analysis a linear model assumption for the deformation estimates is used a displacement time-series are estimated with respect to the local reference point identified for each AOI in a scatterer with high amplitude stability (Amplitude Stability Index >.9) Displacement maps with mean Line-of-sight velocities from ascending and descending orbit passes are shown in Section: Results The area of approximately 5x4 km around Solotvyno town has been analyzed from Sentinel-1A/B’s ascending track, with results shown in Figure 5. The estimated displacement rates are in the interval of +15 to −15 mm/year (Figure 5). Point scattering targets in Figure 5 are visualized with color proportional to line-of-sight (LOS) velocities (mm/year) The reference point for the analysis was located at 47.94724651°N 23.88053400°E for the ascending track (white label) The motions detected over subsiding zones are even exceeding −15 mm/year reaching the values of −20 mm/year over the most affected parts the color scale limit has been set to −15 mm/year to better expose the spatial distribution of obtained deformation estimates and in order to highlight the most significantly affected zones (red) together with shallow subsiding zones of approx representing 60% quality of the extracted displacement time-series have been used in order to visualize only highly reliable targets 2,845 persistent scattering targets with precise deformation time-series have been detected The positive values (blue color) of LOS velocities correspond to the decrease of the sensor-to-target distance or The red areas are showing subsiding parts of the displacement field or motion away from the satellite The velocities are homogeneously distributed suggesting that the area is sufficiently coherent with a good availability of persistent scattering points and satisfactory density of PS point aside from the very central area of Solotvyno where stronger displacement momentum of up to −20 mm year is observable around this area the area needs to be further assessed in the light of the ground-truth information best with the installation of an artificial corner reflector utilizing high-resolution X-band observations of TerraSAR-X/PAZ or Cosmo-SkyMed satellites Line-of-sight (LOS) displacement velocity map from Ascending Track No Example of the displacement time-series for the PS point with velocity of −14.3 mm/year over subsiding zone is shown in Figure 6 Displacement time-series from Descending Track No 7 shown for the subsiding area from the point over the industrial building with the velocity of −14.3 mm/year Location of monitored areas and sinkhole profiles (base image: "© CNES (2022) Detail of the point cloud and DTM of the monitored area in Solotvyno we were also able to derive water levels in individual sinkholes These data show a varying water level within all sinkholes • area 1 (black moor) - 308.520 m asl • area 2–297.130 m asl • area 3–303.700 m asl • area 4–300.260 m asl From multi-spectral images in 2014 and 2022 Also the total subsided area of sinkholes in areas 2 and 4 was derived Water surface area of individual sinkholes in 2014–2022 Using the terrain shading visualisation tool Ambient occlusion (sky-view) (Čučković, 2021) in qGIS on the DTM product from the aerial survey, we were able to detect newly created sinkholes (dashed circles in Figure 9). By combining RGB compositions from multi-spectral satellite images, orthophotomosiac and DTM from the aerial survey, a varying extent of sinkholes in 2014–2022 was determined (Figure 9) Extent of monitored areas in 2014 and 2021based on 2021 UAS orthophotomosaic (top); and newly emerging sinkholes detected on DTM (bottom) the area around the Solotvyno mine in Ukraine has been analyzed utilizing the Multi-temporal satellite radar interferometry (MT-InSAR) technique and Signal-to-Clutter Ratio (SCR) analysis Two orbit paths of Sentinel-1A/B satellites were analyzed in monitoring periods from October 2014 to May 2021 with a total amount of 331 satellite radar images from Sentinel-1’s Ascending Track 131 and 328 images from Descending Track 7 the installation of artificial corner reflectors (ACRs) in the area of interest collocated with GNSS measurements and continuous InSAR monitoring is proposed as a solution for future monitoring tasks Example of CR installation and stabilization in a landslide zone The pillar is founded in a drilled borehole with special steel construction prepared to drill the two pairs (upper and lower) of mounting bolts for the reflectors The construction is rotated so that the holes for the bolts are perpendicular with respect to the geodetic north of the conventional TRF The orientation is assisted by the pair of north/south points (with the same ellipsoidal longitude) which is consequently filled with concrete Corner reflectors are mounted on the bolts with screws and nuts for finer alignment of the reflector’s central plate with respect to the geodetic north The precise alignment is performed by tacheometry The new type of InSAR and GNSS co-location reference station (ZVOL) of the permanent SKPOS network (SKPOS 2021) Expected Signal-to-clutter Ratio (SCR) (Czikhardt et al., 2021 and; 2022) maps of stations should serve as initial guidance for deciding the proper situation of artificial radar reflectors. SCR maps were produced using simulated corner reflector response of triangular trihedral with an inner leg-length of 1 m (and corresponding RCS of 30 dB m2) (Czikhardt et al., 2021; 2022) given the condition of a single Sentinel-1 C-band phase measurement standard deviation of 0.5 mm Hence all areas with SCR >20 dB and at least a single resolution cell in both range and azimuth separated from nearby strong scatterers can be considered viable for installation one has to be aware that the applied simulation does not consider the sensor’s thermal noise and other InSAR-related noises the optimal installation location should not contain any strong point scatterers within plus/minus two Sentinel-1 IW SLC resolution cells (∼40–50 m) in azimuth and range It is strongly encouraged to carry up a final decision on installation location in accordance with in-situ information; the decision process and monitoring/collocation requirements shall be used for refining the final situation of planned artificial reflectors 131 using last-year Sentinel-1 acquisitions for estimation (202012-202112) marked critical non-usage areas with SCR <20 dB 7 using last-year Sentinel-1 acquisitions for estimation (202012-202112) From the SPOT and Pléiades multi-spectral imagery processing for the Solotvyno area, it is evident that from 2014 to 2022, there has been a significant expansion of existing sinkhole areas in the study area, as well as the creation of new ones, all of which are characterized by their inundation (Figure 7; Table 2) The most significant increase in water area occurred in Sinkhole Area 4 where the water area started to appear only in 2015 and reached over 28,000 m2 by 2022 significant increases in water surface area have occurred in areas 3 and 4 An increase in water surface area also occurred in Sinkhole area 2 with the largest increase occurring in 2016 to a level of almost 19,000 m2 the water surface area has remained at similar levels with minimal increase Total subsided areas for Areas 2 and 4 were also derived from the multi-spectral imagery (Table 5) these areas reached over 30,000 m2 they had expanded to approximately 43,000 and 47,000 m2 The evaluation of the individual Sinkholes in terms of water extraction was as follows Sinkhole 1 is formed by a water surface with different reflection characteristics than the other sinkholes This was reflected in the resulting water surface extraction demonstrably better values were obtained using the polarization configuration of the VV but still with a relatively high deviation compared to the multi-spectral processing using SPOT and Pléiades satellite imagery the resulting extraction values were underestimated from 29% to almost 100% compared to the reference images the results obtained in this way cannot be considered relevant the use of images from other SAR constellations with higher spatial resolution seems to be beneficial for this type of tracking of sinkhole changes As researched by different remote sensing methods (Section 3: Results) the area of Solotvyno mine is affected by steep subsidence associated with the ongoing formation of new sinkholes or extension of existing ones Since such effects pose a direct threat to surrounding buildings and infrastructure the proposal for frequent continuous surveillance of the area is being discussed among authorities responsible for protection of the affected zones and civil safety In order to determine the velocity rate of deformation processes which would make it possible to forecast and anticipate dangerous processes in the Solotvyno region in the future it is planned to conduct several additional cycles of aerial survey These cycles will be conducted in different periods of the year to establish the possible relationship between deformation processes and seasonality the possibility of using lidar technologies in the future is being considered For precise engineering tasks and analysis over the objects with potential deformation threats it is recommended to continue the monitoring at monthly or quarterly intervals primarily utilizing InSAR technology and higher-resolution satellite missions such as TerraSAR-X/PAZ with the installation of artificial corner reflectors (ACR) in the monitored area The assessment of the surface displacement of the undermined Solotvyn was based on a multi-sensor approach of imaging the area which was performed using data from the Sentinel-1 SAR satellites the very high-resolution multi-spectral satellites Péiades and SPOT covering the time period between 2014-2022 The analysis performed has confirmed the continuous surface displacement and subsidence over the mining area suggesting that abandoned salt mines still pose a significant threat to the Solotvyno area The MT-InSAR analysis shows that steeper trends of >−2 cm of displacement have been recorded in the central part of the Solotvyno monitoring area which has also progressed significantly in the current monitoring period the results suggest that in the central area without permanent reflectors there may be surface movements that exceed the movement detectable in 1 pixel—about 2.8 cm in 6 days for Sentinel-1 This area should be further analyzed and interpreted for any anthropological or other natural changes that may have occurred during the monitoring period The analysis of the high-resolution optical multi-spectral images of the Pléiades and SPOT over the period 2014–2022 shows that there is a significant expansion of the individual sinkholes in the monitoring area and an increase in the water area within them The largest increase in water surface area has occurred in Area 4 where over 28,000 m2 of water surface area was created from 2015 to 2022 the extent of the water surface has been almost stable since 2016 the DTM from the aerial survey shows the clear emergence of several smaller sinkholes in the central part of the monitored area confirming the constant ongoing movements in this area A comparison of the satellite RGB composite and UAV orthophotomosaics shows that the total subsided area for Sinkholes 2 (Mine no while the areas of Area 1 and 3 increased their area significantly The aerial survey data also shows that the water surface elevation varies from area to area and the maximum difference is almost 11 m It is clear from these results that continuous monitoring of the area is essential to prevent further damage to the surrounding settlements and infrastructure It is the InSAR technology that represents a high potential for the development of a suitable system for automated and high-precision monitoring and continuous risk detection This research proposed a geodetic solution for permanent satellite monitoring which will utilize multiple CRs covering the whole critical area and thus can establish a subsidence monitoring network that can provide reliable time series of displacements in critical subsidence zones strengthen the natural PS estimation network improve the positioning accuracy of nearby natural PSs and provide an absolute geodetic reference for InSAR displacement estimation it is necessary to make a final decision on the installation site in accordance with the in-situ assessment The raw data supporting the conclusion of this article will be made available by the authors KP conceived the research idea and contributed to conception and design of the study and MR revised the article and assisted with the interpretation of InSAR results DP provided consultations on InSAR data processing ĽK processed SAR GRD water extraction All authors contributed to the finalization of this paper All authors have read and agreed to the published version of the manuscript 1/0648/21 and 2/0100/20 funded by the Scientific Grant Agency of The Ministry of Education Research and Sport of the Slovak Republic (VEGA) 003 TUKE-4/2023 and 055 TUKE-4/2021 funded by the Cultural and Education Grant Agency of The Ministry of Education Research and Sport of the Slovak Republic (KEGA) This work was supported by the project HUSKROUA/1901/8.1/0072 REVITAL Sentinel-1 data were provided by ESA under free and open data policy adopted for the Copernicus program Data was processed by SARPROZ utilizing Matlab and Google Maps Pleiades and SPOT data were provided by Airbus DS Author MB was employed by the company Insar.sk Ltd Author DP was employed by the company RASER Limited The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher Monitoring mining induced subsidence's using GPS and InSAR TS 48—engineering surveys for construction works II Google Scholar 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Happy reading and let us know what else you might be interested in knowing Sighetu Marmației is a small town in northern Romania set amidst rolling hills and farmland Along its northern boundary is the Tilsa River which serves as a natural border with the Ukrainian town of Solotvyno a small wooden bridge that connects the two nations is the site of a border checkpoint adorned with children’s toys It comes after a group of strollers was left at a train station in Poland for refugees fleeing Ukraine and offers a small welcoming gift offered to refugee children Photographs of the now-nicknamed “toy bridge” were shared to Twitter on March 17 which serves as the Centre for Strategic Communication under Ukraine's Ministry of Culture and Information Policy “Each child who comes from Ukraine can take a toy from there to enter the country with a nice thought,” wrote the agency The State Border Guard Service of Ukraine also shared pictures of the bridge writing in a translated post that the Romanian border guards were taking “care to entertain Ukrainian children fleeing the war.”  Additional photographs of the toy bridge show brightly colored stuffed animals and toys placed along its walkways in the middle of Eastern Europe’s winter The agency noted that the “historic bridge” now connects the two countries and serves as a checkpoint Toys have been left by Romanian border guards and volunteers, reported the English-reporting publication The New Voice of Ukraine Estimates suggest that almost one child per second is becoming a refugee of the war in Ukraine, according to the United Nations Children’s Fund (UNICEF) in a report published in mid-March The total number of people who have fled the country passed 3 million on March 15 about half of which are believed to be children “Every day, over the past 20 days, in Ukraine more than 70,000 children have become refugees. That’s every minute, 55 children fleeing the country,” said UNICEF spokesperson James Elder in a news release The United Nations High Commissioner for Refugees (UNHCR) suggests that more refugees can be expected as the situation continues an estimated 4 million people may flee Ukraine In light of the emergency and paramount humanitarian needs of refugees from Ukraine an inter-agency regional refugee response is being carried out in support of refugee-hosting countries’ efforts,” wrote UNHCR Figures published on March 16 show that nearly a half-million Ukrainian refugees have crossed into Romania’s borders which hosts the second largest influx after Poland (1.9 million) UNHCR https://www.google.com/maps/place/Punctul+de+Trecere+a+Frontierei+Sighetul+Marma%C8%9Biei/@47.9391738,23.8681897,15.43z/data=!4m13!1m7!3m6!1s0x4737bbab9a8fb321:0xd8dbb17f72b9d7f5!2sSighetu+Marma%C8%9Biei+435500,+Romania!3b1!8m2!3d47.927707!4d23.8976506!3m4!1s0x4737bb75bc343ae7:0xb6a5c500525756cc!8m2!3d47.9385943!4d23.8771346 “https://Twitter.Com/Dpsu_ua/Status/1504450233341820932.” Twitter https://twitter.com/dpsu_ua/status/1504450233341820932 “https://Twitter.Com/Newvoiceukraine/Status/1504452444801335298.” Twitter https://twitter.com/newvoiceukraine/status/1504452444801335298 “https://Twitter.Com/Stratcomcentre/Status/1504424157303427076.” Twitter https://twitter.com/stratcomcentre/status/1504424157303427076 https://romaniatourism.com/sighetu-marmatiei.html https://data2.unhcr.org/en/situations/ukraine “Ukraine War Creating a Child Refugee Almost Every Second: UNICEF.” UN News https://news.un.org/en/story/2022/03/1113942 Madison Dapcevich is a freelance contributor for Snopes This material may not be reproduced without permission Snopes and the Snopes.com logo are registered service marks of Snopes.com You don't have permission to access the page you requested What is this page?The website you are visiting is protected.For security reasons this page cannot be displayed.