Description: The scope of the project “VENEZIA - Subsidence monitoring service in the Lagoon of Venice for regional administrative and water authorities” was to define and implement a land subsidence monitoring service in the Lagoon of Venice for regional and administrative authorities. In order to provide the best knowledge of the land subsidence process around the Lagoon of Venice, SAR-based monitoring techniques (differential SAR interferometry and interferometric point target analysis) were integrated with levelling and GPS surveys into an overall information system. Land subsidence due to natural and anthropogenic causes has represented one of the most serious environmental problems for the Lagoon of Venice and its catchment (Tosi et al., 2002, Carbognin and Tosi, 2003, Carbognin et al., in press). Land subsidence has increased the vulnerability and the geological hazard (i.e. river flooding, riverbank stability, intrusion of seawater in the aquifer system, deteriorating of the littoral sectors with a general coastline regression and an increment of the sea bottom slope close to the shoreline) of these areas, a large portion of which lies below the mean sea level. After the regulation of groundwater exploitation from the Venetian aquifer-aquitard system, a remarkable slowing down of the induced subsidence in Marghera (industrial zone), in the historical center of Venice and along the littorals was ascertained in the 1970’s. Recent studies (Carbognin and Tosi, 2003) have shown that land subsidence is still in progress in the southern and northern coastal areas and in the nearby mainland, where groundwater is extracted from artesian wells, thicker and more compressible Holocene sediments are present, and organic soil oxidation takes place in reclaimed areas. Until 1999, levelling of the Venice region was carried out only along the coast and the lagoon edges and the monitoring network was composed by benchmarks along single levelling lines; a fine grid network was established only in the city of Venice. In recent years, the levelling network has been updated to cover all the southern part of the Lagoon, and plans exist to cover also the northern sector. The same network used for the levelling surveys is also considered for differential GPS measurements. In addition to these ground-based methods, differential SAR interferometry using long series of SAR data (Wegmüller and Strozzi, 1998; Strozzi et al, 2001) and interferometric point target analysis (IPTA) (Wegmüller et al., 2003, Werner et al., 2003) have emerged as very promising tools for the monitoring of land subsidence at high spatial resolution. The VENEZIA project was organized along a service definition phase, a service implementation phase and a service quality assessment phase. Important elements of the project were the definition, implementation and validation of the service for interferometric point target analysis and the integration of the SAR-based monitoring techniques with levelling and GPS surveys into an overall information system capable to provide the best knowledge of the subsidence process to the authorities that manage the Po Plain area around the Lagoon of Venice.

Description: Gamma Remote Sensing; Istituto di Scienze Marine - Consiglio Nazionale delle Ricerche; ESA-ESRIN

Description: Published

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open

Description: Land subsidence is a severe geologic hazard threading the lowlying coastal areas worldwide. Monitoring land subsidence has been significantly improved over the last few years by space borne earth observation techniques based on SAR (Synthetic Aperture Radar) Interferometry. Within the INLET Project, funded by Magistrato alle Acque di Venezia – Venice Water Authority (VWA) and Consorzio Venezia Nuova (CVN), we have used the Interferometric Point Target Analysis (IPTA) to characterize the ground displacements within the Venice Lagoon. IPTA measures the movement of backscattering objects (point targets, PT) at the ground surface which persistently reflect radar signal emitted by the SAR antenna. For this study 80 ERS-1/2 and 44 ENVISAT scenes recorded from 1992 to 2005 and from 2003 to 2007, respectively, have been processed by IPTA. High reliable land subsidence data have been detected for thousands of PT located on the lagoon margins, along the littorals, in major and small islands, and on single anthropogenic structures scattered within the lagoon. On the average, land subsidence ranges from less than 1 mm/year to 5 mm/year, with some PT that exhibit values also larger than 10 mm/year depending on both the local geologic conditions and anthropogenic activities. A network of few tens of artificial square trihedral corner reflectors (TCR) has been established before summer 2007 to monitor land subsidence in the inner lagoon areas where natural reflectors completely lack (e.g., on the salt marshes). The first interferometric results on the TCR appear very promising.

Description: Published

Description: Venice, Italy

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open

Description: The Venice Lagoon, Italy, is a unique worldwide environment which is presently vulnerable due to loss in surface elevation as a result of land subsidence referred to the mean sea level. Land displacements in the Venice coastland have been determined over time by traditional monitoring techniques (i.e., spirit leveling and GPS). Recently, SAR-based analyses have been used to complement the ground-based methods. Interferometric analysis on persistent point targets has been proved to be very effective in detecting land displacement in the coastal environment. ERS SAR and ENVISAT ASAR images spanning the time period 1992-2005 and 2003- 2006, respectively, have been processed at regional and local scale and on “natural” as well as “artificial” reflectors.

Description: Published

Description: Montreux, Switzerland

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open

Description: We present the result of a test aimed at evaluating the capability of RADARSAT-2 and COSMO-SkyMed to map the natural subsidence and ground movements induced by anthropogenic activities in the historical center of Venice. Firstly, ground movements have been retrieved at quite long- and short-term by the Persistent Scattered Interferometry (PSI) on 2008-2015 RADARSAT-2 and 2013-2015 COSMO-SkyMed image stacks, respectively. Secondly, PSI has been calibrated at regional scale using the records of permanent GPS stations. Thirdly, considering that over the last two decades “in the historical center of Venice” natural land movements are primarily ascribed to long-term processes, and those induced by human activities act at short-term, we have properly resampled 83-month RADARSAT-2 C-band and 27-month COSMO-SkyMed X-band interferometric products by a common grid and processed the outcome to estimate the two components of the displacements. Results show that the average natural subsidence is generally in the range of 0.9 – 1.1 mm/yr and the anthropogenic ground movements are up to 2 mm/yr.

Description: Published

Description: Prague, Czech Republic,

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open

Description: Remote-sensing techniques represent suitable tools for geological cartography and sustainable management of natural resources. Spaceborne Synthetic Aperture Radar (SAR) systems offer the possibility, through differential SAR interferometry (DInSAR), to map surface displacements at mm to cm resolution. Spectacular results are obtained for geophysical sciences with earthquake displacement, volcano deformation, glacier dynamics and land subsidence being mapped. With regard to land subsidence, SAR interferometry exhibits complementary characteristics to the levelling surveys, because it has the capability to map large urban areas at low cost and high spatial resolution. The high precision levelling surveys, on the other hand, are used outside of the cities and to set up a reference point for the SAR subsidence values. In the case of the south-eastern Veneto region, where till to 2000 high precision levelling surveys are available only (i) around the Lagoon margin and along few lines from (ii) Venezia to Rua di Feletto (Treviso), from (iii) Mestre to Padova, (iv) form Padova to Rovigo, and (v) from Chioggia to Adria, SAR interferometry has the capability to monitor the vertical displacements of all the built-up or sparsely vegetated areas (i.e. where stable structures permit the formation of a coherent phase signal over time) not fully covered with levelling results. DInSAR makes use of two SAR images acquired from slightly different orbit configurations and at different times to exploit the phase difference of the signals. The phase signal derived from an interferometric image pair relates both to topography and line-of-sight surface movement between the acquisitions, with atmospheric phase distortions, signal noise and inaccuracy in the orbit determination as main error sources. The basic idea of DInSAR is to subtract the topography related phase (for instance simulated from a Digital Elevation Model) from the interferogram to derive a displacement map. In this study a time series of six interferometric radar images of the European Remote Sensing Satellites ERS-1 and ERS-2 from 1993 to 2000 was used. In order to generate a single subsidence map with reduced errors, the interferometric radar images were combined. In the following, displacement in the vertical direction was assumed, the land subsidence map was transformed to the Italian cartographic system with a spatial resolution of 30 m, and the pixel corresponding to the benchmark Nodale 63 (ex 24’) in Treviso (that has been already considered stable for the levelling surveys) was considered the stable reference. The high accuracy of the 1993-2000 DInSAR survey is confirmed by a quantitative validation with the 1993-2000 levelling data. For the 87 benchmarks where values from both surveying techniques are available a zero average difference of the vertical displacement velocity is prescribed and a standard deviation of 0.9 mm/year is found; the minimum and maximum differences are –3.1 mm/year and +2.5 mm/year, respectively. From this number and previous works we conclude that the accuracy of the SAR interferometric subsidence rates is on the order ±1 mm/year, which is also the expected accuracy of the levelling surveys. An example of the DInSAR capability to monitor the evolution of land subsidence in small urban area is Abano – Montegrotto Terme where clearly appears the influence of the drastic reduction of groundwater pumping used for geothermal purpose. DInSAR results match very well the available leveling data. DInSAR mapping in large cities such as Padova, Treviso, and Mestre is given. These applications indicates the potentiality of the radar satellite analysis in built-up areas characterized by a general stability, with displacement rate within the range 1 mm/year. DInSAR use looks very useful to perform detailed investigations in coastal areas too. E.g., Bibione – Lignano, Chioggia, Jesolo and Caorle reveal the presence of a significant seaward gradient in land subsidence practically impossible to be detected by other traditional monitoring techniques like geometric leveling and DGPS. Hence, the proposed methodology provides a fundamental support for coastal management projects and environmental risk analyses. SAR interferometry has been applied in other minor cities of the Veneto plain characterized by very different conditions form the displacement point of view. Castelfranco and Conegliano located close to the Alpine foothills appear quite stable; land settlement in Rovigo appears very changeable with high values (2-3 mm/year) downtown and lower rate (about 1 mm/year) in the outskirts; in Cavarzere, and Oderzo, San Donà the subsidence velocity ranges between 3 and 4 mm/year. The more critical situation is found in Adria, Taglio di Po, and Portogruaro where the sinking velocity reaches 5 mm/year in large portion of these towns. Although in 2000 the old leveling network has been extended within the framework of the ISES Project (the present overall length is about 1000 km), a detailed monitoring of several villages is impossible though leveling and DGPS. Because in these urban areas the hydrogeological risk is high due to the presence of rivers and the critical elevation, since they often lie below the mean sea level, the areally distribution of the subsidence rate has to be well known. Results shown by the present work prove that this integration in the knowledge of the subsidence process can be efficiently obtained by DInSAR. This technique represents an effective methodology for both a back-analysis during the last decade and the future monitoring in the portion of territory covered by large and small built-up areas.

Description: Published

Description: Bologna, Italy

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open

Description: In the Interferometric Point Target Analysis (IPTA) point-like targets that remain phase coherent over time are identified in a sequence of satellite Synthetic Aperture Radar (SAR) images and used to estimate the progressive deformation ofthe terrain with millimetric accuracy. Building upon conventional interferometric SAR techniques, IPT A overcomes atrnospheric delay anomalies and tempora! and geometrie decorrelation by exploring the tempora! and spatial characteristics of radar interferometric signatures collected from point targets widely available over urban areas but that can be also found scattered outside cities and villages. In this contribution the application of IPT A to the monitoring of land' subsidence in the urban and littoral environments of the Venice Lagoon is described. The results achieved using ali the available ERS SAR images acquired between 1992 and 2000 are very significant due to the achieved target spatial and tempora! coverage.

Description: Published

Description: 417-425

Description: 5IT. Osservazioni satellitari

Description: open

Description: E’noto che alla riduzione altimetrica del veneziano avvenuta nel corso del XX secolo, e valutata  23 cm rispetto al livello del mare, hanno contribuito, con diversa incidenza, la subsidenza geologica, la subsidenza indotta dai pompaggi di acque artesiane e l’innalzamento del livello del mare. La ricerca sulla subsidenza di Venezia, iniziata nel 1969 con la nascita dell’Istituto per lo Studio della Dinamica delle Grandi Masse del CNR, oggi Istituto di Scienze Marine, concluse la prima complessa fase negli anni ’90, dimostrando, tra le altre cose, l’arresto del processo antropico indotto dagli sfruttamenti artesiani intensivi operati per uso industriale, soprattutto nel ventennio 1950-1970, e registrando nel 1975 un esiguo ma significativo rebound a Venezia. L’avvenuta subsidenza geomeccanica aveva comunque provocato 10-14 cm di irreversibile abbassamento del suolo, con le relative conseguenze. Gli studi a carattere geologico (litostratigrafici, sedimentologici, idrogeologici, ecc.), geofisico, le analisi modellistiche, le misure in situ ed i rilievi altimetrici condotti, se da un lato rassicurarono sull’arresto della subsidenza indotta e sulla stabilità della città lagunare, dall’altro attestarono la necessità di approfondire alcuni aspetti della ricerca ed evidenziarono che il processo subsidenziale era ancora in atto con tassi non trascurabili lungo certi tratti litoranei, alle estremità lagunari e zone limitrofe. Si sono quindi approfonditi ed ampliati gli studi riguardo alle cause naturali e antropiche della perdita altimetrica del suolo. La subsidenza naturale é stata quantificata con maggiore accuratezza individuandone le cause agenti sul breve e lungo periodo; il tasso medio di lungo periodo, calcolato sull’intera serie quaternaria, variabile da 0.3 a 0.5 mm/anno (Kent et al., 2002), è principalmente ascrivibile all’attività tettonica regionale, mentre il tasso medio di breve periodo stimato in circa 1.3mm/a per gli ultimi 40'000 anni è imputabile alla consolidazione naturale dei sedimenti di apporto recente (Bortolami et al., 1984). Negli ultimi secoli il valore della consolidazione naturale veneziana è sensibilmente diminuito fino a ≤0.5mm/anno (Gatto & Carbognin, 1981; Brambati et al., 2003; Carbognin et al., 2005a). Stime attendibili sul trend eustatico sono state calcolate su serie storiche secolari, cioè sufficientemente lunghe da non risentire dei "cicli di breve periodo", non significativi nella valutazione di tendenza (Carbognin & Taroni, 1996). L’innalzamento del livello del mare, uno dei 3 tre fattori responsabili della perdita altimetrica relativa misurata a Venezia, è stato calcolato in circa 1.2 mm/anno (depurato dall’effetto subsidenza) per il periodo 1896 – 2002. Per l’analisi delle cause della subsidenza, per le conseguenze sull’ambiente e per gli sviluppi metodologici, la ricerca si è ampliata anche al di fuori del bacino lagunare dapprima verso il comprensorio meridionale Veneziano-Padovano, dove, per le caratteristiche geomorfologiche del territorio, la subsidenza in atto induce un serio impatto ambientale e socio-economico. In particolare è stata analizzata la subsidenza geochimica che interessa il bacino scolante meridionale, la cui causa principale é da attribuirsi alla perdita di massa per ossidazione che avviene in corrispondenza delle aree bonificate ad uso agricolo in presenza di terreni torbosi superficiali. Nelle zone studiate, dove vaste zone si trovano a quote decisamente inferiori al l.m.m., fino a -4 m, solo per gli ultimi 70 anni è avvenuta una subsidenza superiore a un metro (Figura 4). Anche la contaminazione salina dei suoli, molto estesa in quest’area, è causa di compattazione geochimica dei terreni limo-argillosi. In seguito gli studi hanno interessato le aree nord-orientali della Provincia di Venezia, dove il confronto spazio-temporale dei rilievi altimetrici 2000/1993 aveva evidenziato, tra l’altro, un aumento dei tassi di subsidenza nei settori litorali a vocazione turistica di Cavallino-Jesolo-Caorle ( 4 mm/anno). La livellazione 2004, sebbene confrontabile con la precedente solo su alcuni tratti, conferma questi valori dimostrando la gravità del processo in atto in diversi centri urbani di terraferma e nei litoranei turistici. Per quanto riguarda il monitoraggio della subsidenza, che avveniva principalmente con le livellazioni geometriche, in anni recenti le tecniche basate su misure satellitari con il GPS e l'analisi di immagini radar di tipo SAR, hanno allargato il numero dei sistemi utilizzabili. Con l'intendimento di migliorare le possibilità e la qualità del monitoraggio, sia in specifiche aree urbane di interesse che a scala regionale, è stata sviluppata un'originale procedura di integrazione dei risultati forniti dalle diverse metodologie di rilevamento, cioé livellazioni, GPS differenziale ed in continuo, interferometria satellitare convenzionale (InSAR) e interferometria su riflettori permanenti (IPTA), in modo da superare i limiti di ogni singola tecnica. Tale sistema di monitoraggio integrato (SIMS) è stato applicato per ricostruire gli spostamenti verticali del territorio nel decennio 1992-2002, con elevata risoluzione spaziale e precisione verticale millimetrica.

Description: Published

Description: Mestre-Venezia

Description: 6A. Monitoraggio ambientale, sicurezza e territorio

Description: open