Strozzi, Tazio

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.

Land subsidence occurred at the Venice coastland over the 2008-2011 period has been investigated by Persistent Scatterer Interferometry (PSI) using a stack of 90 TerraSAR-X stripmap images with a 3-m resolution and a 11-day revisiting time. The regular X-band SAR acquisitions over more than three years coupled with the very-high image resolution has significantly improved the monitoring of ground displacements at regional and local scales, e.g., the entire lagoon, especially the historical palaces, the MoSE large structures under construction at the lagoon inlets to disconnect the lagoon from the Adriatic Sea during high tides, and single small structures scattered within the lagoon environments. Our results show that subsidence is characterized by a certain variability at the regional scale with superimposed important local displacements. The movements range from a gentle uplift to subsidence rates of up to 35 mm/yr. For instance, settlements of 30-35 mm/yr have been detected at the three lagoon inlets in correspondence of the MoSE works, and local sinking bowls up to 10 mm/yr connected with the construction of new large buildings or restoration works have been measured in the Venice and Chioggia historical centers. Focusing on the city of Venice, the mean subsidence of 1.1±1.0 mm/yr confirms the general stability of the historical center.

Anthropogenic land subsidence has widely been affecting the Veneto Region, northern Italy, since the past century. Groundwater withdrawals for industriai, domestic, and agri cultura! uses, exploitation of minerai water, thermal water for health treatment, methane-bearing water, and peat oxidation in reclaimed farmlands produced a land settlement varying in time and space throughout the area. Moreover, natural consolidation of the Quaternary deposits and tectonics of the pre-Quaternary basement contribute to increase ground surface lowering. Different survey techniques, with different characteristics; have been adopted to contro! land subsidence. To overcome the limits that characterize each single method and to enlarge the knowledge on regional land subsidence, an integrated monitoring method has been designed to accurately and reliably keep land movements under contro! in the study area. We combine five earth observation techniques, i.e. spirit leveling, Continuous Global Positioning System (CGPS), Differential GPS (DGPS), Interferometric Synthetic Aperture Radar (InSAR), and Interferometric Point Target Analysis (IPTA), together over about the last ten-years, and homogenized and integrated their results in both the time and space domains. The application of this Subsidence Integrated Monitoring System (SIMS) provides a new complete and dependable picture ofthe vertical displacements in the Veneto Region never available before.

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.

Our knowledge of the dynamics of vertical ground movements plays a fundamental role in the study of the geomorphological processes, particularly for determining sediment deposition and erosion causes. The conservation of lagoon morphological structures such as salt marshes and tidal flats, besides erosion due to waves and currents, depends on their surface height with respect to the sea level that, for short periods, is primarily related to the land subsidence process and secondarily to eustasy. Ground vertical variations with respect to the sea level, even if small, can, in fact, trigger sediment erosion and deposition processes. To determine land displacement rates in small islands of the Venice Lagoon not covered by traditional surveys (levelling and differential GPS), SAR-based monitoring techniques, i.e. differential SAR interferometry (INSAR) and interferometric point target analysis (IPTA) from ERS-1 and ERS-2 satellites, were performed. We will use the Murano Island for our example in this contribution.

Dopo un breve richiamo all’abbassamento del suolo di Venezia, il lavoro illustra gli studi più recenti sulla subsidenza del comprensorio lagunare dove agiscono diversi fattori valutabili su scala regionale. E’noto che alla perdita altimetrica totale avvenuta a Venezia nel secolo scorso (ca 23 cm) hanno contribuito, con diversa incidenza, la subsidenza geologica, la subsidenza indotta dai pompaggi di acque artesiane e l’innalzamento del livello del mare. Nell’ultimo decennio si sono approfonditi ed ampliati gli studi del processo subsidenziale riguardo alle sue cause quali: a) le componenti naturali a breve ed a lungo termine, ovvero la consolidazione dei depositi recenti e la neotettonica; b) l’eustatismo; c) l’influenza di potenziali sfruttamenti dei giacimenti gassiferi offshore; e d) la subsidenza geochimica dovuta all’ossidazione dei suoli organici ed alla salinizzazione dei terreni argillosi che interessa in modo particolare il bacino scolante meridionale, incidendo sull’abbassamento del suolo con tassi che raggiungono anche 2 cm/anno. Per l’analisi delle cause della subsidenza, per le conseguenze sull’ambiente e per gli sviluppi metodologici, la ricerca si è ampliata a livello regionale, dapprima verso il comprensorio meridionale Veneziano-Padovano, dove, per le caratteristiche geomorfologiche del territorio largamente bonificato ed in buona parte oggi sotto il livello del mare, la subsidenza in atto induce un serio impatto ambientale e socio-economico. 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 (ca 4 mm/anno). Si presentano infine i risultati di un metodo originale di monitoraggio integrato, elaborato per ottimizzare le informazioni derivanti dalle livellazioni, GPS, Interferometria SAR convenzionale e su riflettori permanerti, in grado di fornire mappe sinottiche dei movimenti del suolo su scala regionale ad alta risoluzione spaziale (pixel 20 × 20m) e precisione verticale millimetrica.

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.

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.

Digital Elevation Models (DEMs) of flat lowlying coastlands are becoming even more important for environmental risk analyses, for example the development of effective plans for flooding protection. The need of information at high spatial resolution over very large areas, of the order of 100×100 km2, practically precludes the use of traditional methods (e.g., leveling and DGPS) due to their intrinsic limitation in covering wide zones and reduces the possibility of using Lidar because of its high cost. Starting from the last decade, space-borne radar sensors have been used extensively for this purpose. The most known is the SRTM (Shuttle Radar Topographic Mission) DEM which covers most of the land area between ±60° latitude. This DEM was generated using singlepass interferometry and is widely available at 3-arc-seconds. In this work, we present the results obtained for the Veneto plain, Italy, using the spaceborne SAR (Synthetic Aperture Radar) crossinterferometry.

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.