APPLICATION OF ADVANCED-DINSAR DATA TO ...

APPLICATION OF ADVANCED-DINSAR DATA TO LAND SUBSIDENCE PHENOMENA AT RAVENNA, PO PLAIN, ITALY Ahmed Ibrahim University of Qena, Egypt [email protected] Diego Di Martire, Domenico Calcaterra Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse [email protected], [email protected] Serena Tessitore, Massimo Ramondini Dipartimento di Ingegneria Civile, Edile ed Ambientale [email protected], [email protected]

Sommario Ravenna is one of the most important cities along the Po plain and Adriatic Sea coastline. Its importance is due to its historical, industrial, touristic and agricultural value. Over the last decades Ravenna and the surrounding coastline areas have been subjected to natural and anthropogenic land subsidence. Natural subsidence represented by tectonic factors, in addition to the natural compaction of Quaternary sediments, while the anthropogenic one is being represented by groundwater and gas extraction. Many studies were carried out on the whole Po plain area and its coastline along the Adriatic Sea, Including geodetic, geological and geotechnical studies, to standup on the principal factors behind the unequal and continuous subsidence of the whole area, in attempt to decrease subsidence rate, quantify and differentiate between natural and anthropogenic land subsidence. This study uses a set of 28 ascending ENVIronmental SATellite-Advanced Synthetic Aperture Radar (ENVISAT-ASAR) images acquired for the study area, covering the period between 1st of Mars 2003 and 17th of July 2010. For data processing, analysis and interpretation this study uses one of the most advanced DInSAR techniques, the so-called SUBSOFT software, developed by the Remote Sensing Laboratory (RSLab) group from the Universitat Politècnica de Catalunya (UPC), which is based on the use of Coherent Pixels Technique (CPT) algorithm, in addition to previous published data about the subsidence at the study area to validate the DInSAR results.

1. Introduction Ground deformation of Po Plain and its coastline attract increasing attention of many authors since the last decades. due to its economic, agricultural and coastline touristic importance. Referring to Ravenna city, land subsidence result from natural and anthropogenic factors. Natural factors include the compaction of the Quaternary sediments (Gianbastiani et al., 2007 and Teatini et al., 2011), the tectonic setting of the study area (Carminati et al., 2003; Finetti, 2005; Picotti and Pazzaglia, 2008 and Mantovani et al., 2009). Anthropogenic factors are represented by extraction of gas and underground water (Gambolati & Teatini., 1998; Teatini et al., 2005). Pirazzoli (1996), Carminati and Di Donato (1999), Stocchi et al., (2005) and Stocchi & Spada (2007) added another factor for natural subsidence, which represented by a short term component likely controlled by climatic changes (glaciation cycles) acting on periods 103-104 yr,induced by the melting of Alpine and remote glaciers. Subidence rate of about 1-2.5 mm/yr was given by some

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authors (Gambolati & Teatini., 1998; Carminati et al., 2003; Gianbastiani et al., 2007; Picotti and Pazzaglia, 2008) for the natural subsidence at the study area. Teatini et al., (2005) tried to trace the evolution of Ravenna coast plain during the last millennium by using the historical maps of Fabbri (1974). The beach propagation has continued in fairly natural conditions until the end of 1st World War, with the anthropogenic influence on the coastland morphodynamics becoming evident since 1920s. River channeling, banking, damming and stream bed mining as a consequence of the progressive reclamation of swampy lagoon areas for agricultural purposes, the rapid urbanization of the shore for tourist summer recreation and the development of an industrial area between the historical center and the sea have been responsible for a significant reduction in the volume of sediments transported by water resources to the coast. This in turn has prevent the occurrence of the deposition during the most significant floods and decreased the beach nourishment (Morelli, 1998). After the 2nd World War, Ravenna experienced a new flourishing era. The discovery of several underground gas pools and the construction of big commercial port promoted rapid industrial and agricultural development in the neighboring area. The major environmental cost paid for this outstanding growth has been a pronounced land settlement as a direct consequence of groundwater and gas overdraft. Groundwater has been extensively withdrawn since 1950 to cope with the growing demand for freshwater for civil, agricultural and industrial use. The problem become critical in the middle 1970s, because of obvious large damage to infrastructures and monumental heritage which was increasingly subject to flooding during the most intensive metrological events, the highest Adriatic tides and the longest rainy periods (Teatini et al., 2005). 1.1 Geology Ravenna region belongs to the southeastern portion of the Po River plain, which presently encompasses an area of about 38,000 km2 bounded by the Apennine range to the south and the Alps to the north (Teatini et al., 2005). These mountain belts develop on the top of two subduction zones. The Alps are related to the southward dipping subduction of the European plate beneath the Adriatic plate, whereas the Apennines are generated by the south-westward dipping subduction of the Adriatic plate under the Tyrrhenian lithosphere (Doglioni, 1993). The Po River basin is a foreland sedimentary basin consisting of a sequence of deposits of Alpine and Apennine origin laid down during the Quaternary and the Pliocene in different environments, from continental, lagoonal, and deltaic in the upper zone to littoral and marine in the lower one. This sequence is made of alternating sands, silts and clays, or a combination of these lithologies it has a maximum thickness of about 2000 m (Dondi et al., 1985). The Quaternary sequence is normally consolidated and normally pressurized, and exhibits a maximum thickness of about 2000 m. The sediment accumulation is more pronounced in the areas of large tectonic subsidence, the so-called depocenter, which corresponds to the Po River delta. Active thrusts develop parallel to the Apennine alignment and are buried beneath the Quaternary sediments. In the upper 350–450 m of the sedimentary stratigraphy some multi-aquifer freshwater systems are well developed (see Fig. 1 & 2). The recharge area is located in the thick coarse Apennine foothills as well as in the beds of the major rivers crossing the plain. The large thickness of the Quaternary sediments indicate that in the past the geologic subsidence was quite pronounced in this area and still rather active (Salvioni, 1957). A number of Pliocene and Pleistocene gas fields are located in thrusted anticlines, with depths ranging between 1000 and 4500 m, simple drape structures and stratigraphic traps. A typical feature of this basin is that gas accumulation occurs in multiple zone reservoirs with thickness ranging from centimeters to a few tens of meters (Mattavelli et al., 1991). 1.2 SAR dataset and DInSAR processing The dataset available for this study is composed by 28 ENVISAT images in ascending orbit acquired by the European Space Agency (ESA), covering the period 2003-2010 and provided by the Ahmed, Di Martire, Tessitore, Ramondini, Calcaterra


Italian Ministry for the Environment, Land and Sea. The dataset is identified by the following Track and Frame numbers T172-F29785. Only 22 images have been processed of the images set. Two principal areas have been cropped from original SLC images. The first one extended from Casal Borsetti to Lido di Dante (covering the area between Ravenna coastline and Ravenna city, about 19 Km X 15 Km). The second one extended from Lido di Dante to Cervia city (covering the area between coastline and Cervia city, about 19 Km X 3 Km). It is important to highlight that all SLC images have been co-registered to make the dataset appropriate to its later DInSAR processing (Gabriel et al., 1989). At the end of the first processing step (PRESAR) this study has about 378 interferograms with spatial baseline below 250 m, and temporal base line below 1000 days.

Fig. 1: Geological map of Ravenna area (Modified from geological map of Italia 1:100000, sheet No. 89 & 100).

Fig. 2: Schematic cross-section of the Ravenna aquifer system (after Carbognin, et al., 1978).

1.3 Results and Discussion Working on the studies conducted by Polo (2002) and Teatini et al., (2005), Houtenbos et al., (2005) found that the anthropogenic subsidence values, induced by gas extraction, gave a contribution of 2.9 mm/yr at Fiumi Uniti river mouth (Angela-Angelina gas field) and of 2.4 mm/yr at Lido di Dante (Ravenna) for the period 1982-2002. The comparison of the data cited above with a numerical

Ahmed, Di Martire, Tessitore, Ramondini, Calcaterra


model proposed by Schroot et al., (2005) (Fig. 3) predicts an increase in subsidence velocity in the period between 2003-2015 in the middle of the Angela-Angelina offshore gas field, followed by a rebound in the next years. Moreover, Barends et al., (2005 a, b) highlight the role of the future subsidence increment on the coastline due to planned gas withdrawal and other causes of subsidence (5-10 cm in about 15 years), together with an expected sea level rise. Thus, some additional shore protection measures could be required in the area between Lido di Dante and Lido Adriano (Vicinanza et al., 2009). Cumulative subsidence rate of about 12 cm at Angela-Angelina gas field (Lido Adriano) during the period 2003-2010 was provided by Houtenbos et al., (2005) and Schroot et al., (2005). For the same area and period the present study gives a cumulative subsidence rate of about 14.46 cm. Gambolati & Teatini (1998) used 3D nonlinear finite element model to simulate land subsidence caused by gas withdrawal from the Angela-Angelina gas reservoir. They concluded that the maximum land subsidence due to reservoir compaction has been found to be approximately 20 cm over the center of the field at the end of its productive life in 2014. At the coastline overlying the reservoir the subsidence rate has been found to vary from 12 to 14 cm., Based on the results from the major field of Dosso degli Angeli, which has a structure and burial depth similar to those of Angela Angelina, the Authors reported that the contribution to land subsidence from the water drive compaction has been assumed to be of the same order as the settlement caused by the reservoir compaction. Hence the maximum land sinking over Angela-Angelina may be estimated in 40 cm in 2014. The present study recorded subsidence rates between 5.3 and 17.4 mm/yr along Ravenna coastline, while subsidence rates of about 3.75 and 21.0 mm/yr were recorded at the area between marina di Ravenna and Ravenna city (see Fig. 4). This is primarily due to the extraction of both groundwater and gas from the underground aquifers and reservoirs. This is indicated by velocity maps provided by the present study. In alluvial aquifer systems, especially those that include semi-consolidated silt and clay layers (aquitards) of sufficient aggregate thickness, long-term groundwater-level declines can result in a vast one-time release of pore water from consolidation of aquitards, which manifests itself as land subsidence. Accompanying this release of water is a largely non-recoverable reduction in the pore volume of the compacted aquitards, and thus an overall reduction in the total storage capacity of the aquifer system. This amount of pore water cannot be reinstated by allowing water levels to recover to their predevelopment status (USGS report, 2000). Tolman & Poland (1940) mentioned that,

Fig. 3: Comparison of the analysis of geodetic measurements (Houtenbos et al., 2005) and numerical prediction (Schroot et al., 2005) of the subsidence at Angela-Angelina gas field (modified after Barends et al., 2005a)



Permanent subsidence can occur when groundwater is being removed by pumping or drainage. The reduction of fluid pressure in the pores and cracks of aquifer systems, especially in unconsolidated rocks, is inevitably accompanied by some deformation of the aquifer system. Because the granular structure the so-called "skeleton" of the aquifer system is not rigid, but more or less compliant, a shift in the balance of support for the overlying material causes the skeleton to deform slightly. Both the aquifers and aquitards that constitute the aquifer system undergo deformation, but to different degrees. Almost all the permanent subsidence occurs due to the irreversible compression or consolidation of aquitards during the typically slow process of aquitard drainage.

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