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Since the beginning of the 1960s, the urban area of Bologna has experienced land subsidence due to excessive groundwater withdrawals. Sinking reached its peak in the 70s of the last century when the subsidence rate attained the maximum value of about 10 cm/year, and significant damages to structures and infrastructures occurred. This process has been intensively monitored over the years, and extensive ground displacement data were collected employing various increasingly sophisticated techniques, ranging from topographic levelling to GNSS surveys and, since 1992, to satellite interferometry. Satellite data, in particular, allowed an accurate reconstruction of the land subsidence process. The available interferometric data are the results of three different SAR campaigns undertaken by local authorities in which the PSInSAR technique was adopted: 1992 – 2000 (ERS), 2002 – 2006 (ENVISAT) and 2006 – 2011 (RADARSAT). Within this work, a new InSAR survey from the free SENTINEL1 2014 – 2020 ascending and descending orbits data was undertaken by the UniBo spin-off “Fragile”. The software GMTSAR was used to process each interferogram and then a Small Baseline (SBAS) approach was followed to resolve the ground displacements over time. Great attention was paid to the choice of reference pixels on the existing buildings and structures, in order to maximise their density in the study area, and to the definition of the considered time span ranging from 6 to 365 days, allowing to analyse both quicker and slower ground movements. Compared to previous surveys, the displacement map obtained by Sentinel has a much higher spatial and temporal resolution, thus leading to a detailed interpretation of the ongoing subsidence. Results show that the displacement field well agrees with the 3D geological model of the area and that the temporal evolution of the subsidence rate nicely matches the piezometric level and groundwater pumping temporal series.

Landslides are widespread geological features in Italy’s Northern Apennines, with slow-moving earthflows among the most common types. They develop in fine-grained rocks and are subject to periodic rainfall-induced reactivations alternating to phases of dormancy. In this paper, we use radar interferometry (InSAR) and information about landslide activity to investigate deformation signals on an areal basis and to assess the dynamics of recently reactivated earthflows. We use traditional two-pass interferometry by taking advantage of the short revisit time of the Sentinel 1 satellite to characterize 4 years of slope deformations over the 60 km2 study area, where 186 landslides are mapped. Our results show that most intense and sustained deformation signals are associated with phenomena on the verge of reactivation, indicating that radar interferometry may have a potential for early warning purposes. By focusing on three specific earthflow reactivations, we analyze their dynamics through the years that preceded their failure. Despite inherent uncertainties, it was possible to retrieve the deformation signal’s temporal evolution, which displayed seasonally recurring accelerations, peaking during the major precipitation episodes in the area.

Landslides are widespread landscape features in the Northern Apennine mountain chain and their activity frequently cause damages to settlements and infrastructures. In such context, slow-moving landslides are very common and typically affect fine-grained weathered rocks. Long periods of sustained slow-movements (cms/year) can be interrupted by rapid acceleration and catastrophic failures (ms/day) that are caused by intense rainfall events. Space-borne synthetic aperture radar interferometry (InSAR) proved effective to detect actively deforming phenomena and monitor their evolution in the periods before and after failures. We present InSAR results derived from the Sentinel 1 satellite constellation for landslide cases that underwent reactivation during 2019. In all cases, the catastrophic failures were unexpected and no ground-based monitoring data are available. We processed pre- and post-failure interferograms of SAR images acquired by Sentinel 1 A/B with time spans ranging from 6 to 24 days, removing those having low coherence by manual inspection. The conventional 2-pass technique allowed us to obtain measurements of surface displacement despite the fact that sparse to none infrastructures nor bare rock outcrops are present on the landslide bodies. Our interferograms show that surface displacements are visible well in advance of the actual failure. They display nearly continuous downslope motion with seasonal velocity changes. Time series between 2015 and 2019 shows that surface displacements can be appreciated throughout most part of the year with snow cover and summer peak of vegetation being the most notable exceptions. Distinct accelerations can be detected in space and time during the weeks and months preceding the reactivation.We compare time-dependent deformations to precipitation patterns to explore their relationship and to document the transition from stable to unstable deformation. Our work suggests that InSAR interferometry can be successfully used to measure pre-failure displacements and detect slow-moving landslides that are more prone to reactivation in case of rainfall events.

Slow moving landslides are widespread geomorphological features in the Northern Apennines of Italy where they represent one of the main landscape forming processes. The lithology of the Northern Apennines fold and thrust belt is characterized by alternations of sandstone, siltstone and clayshales, also known as flysch, and clay shales with a chaotic block in matrix fabric, which are often interpreted as tectonic or sedimentary melanges. While flysch rocks with a high pelitic fraction host earthslides that occasionally evolve into flow like movements, earthflows are the dominant landslide type in chaotic clay shales. In the present work, we document the kinematic response to rainfall of landslides in these different lithologies using radar interferometry. The study area includes three river catchments in the Northern Apennines. Here, the Mediterranean climate is characterized by two wet seasons during autumn and spring respectively, separated by dry summers and winters with moderate precipitation. We use SAR imagery from the X-band satellite COSMO SkyMed and from the C-band satellite Sentinel 1 to retrieve spatial displacement measurements between 2009 and 2016 for 25 landslides in our area of interest. We also document detailed temporal and spatial deformation signals for eight representative landslides, although the InSAR derived deformation signal is only well constrained by our dataset during the years 2013 and 2015. In spring 2013, long enduring rainfalls struck the study area and numerous landslide reactivations were documented by the regional authorities. During 2013, we measured higher displacement rates on the landslides in pelitic flysch formations compared to the earthflows in the clay shales. Slower mean velocities were measured on most landslides during 2015. We analyse the temporal deformation signal of our eight representative landslides and compare the temporal response to precipitation. We show that earthslides in pelitic flysch formations accelerate faster than earthflows in chaotic clay shales and reach higher velocities, while the kinematic behaviour of the earthflows can be described as rather steady with only minor accelerations. Although we have no detailed pore pressure measurements for the period of interest, the observed behaviour can be explained in our view by the morphological and hydrological characteristics of the different landslide types. On the one hand landslide material and bedrock in the pelitic flysch rocks are more resistant, which is why slope angles are higher in this lithology. On the other hand, landslides in the pelitic flysch formations have often deeper slip surfaces and landslide material is more permeable. This is why long persistent rainfall is necessary to saturate the landslide material and induce pore pressures that are high enough to trigger displacement.

Landslides are common landscape features in the Northern Apennine mountain chain and cause frequent damages to human structures and infrastructure. Most landslides in the area can be classified as earthflows, where the clay-shales form the substrate, whereas complex landslides with flow and sliding components are common on the slopes where fine-grained turbidites form the substrate. Most of these landslides move periodically with contained velocities and, only after particular rainfall events, some of them accelerate abruptly. Space-borne synthetic aperture radar interferometry (InSAR) provides a particularly convenient way for studying the periods before and after failures. In this paper, we present InSAR-results derived from the Sentinel 1 satellite constellation for two landslide cases in the Northern Apennines. The first case is a complex landslide that is hosted on a pelitic flysch formation, whereas the second case is an earthflow located in chaotic clay shales. Both cases failed catastrophically and threatened or damaged important infrastructures. In the case of the complex landslide, we report spatially variations of the deformation field between repeated periods of acceleration. The data illustrate that the deformation initiated in the upper part of the slope and expanded over the whole landslide body afterward. In the case of the earthflow, we describe spatial and temporal kinematics during the period before a catastrophic failure in March 2018. We discuss the temporal deformation signal together with rainfall and snowmelt data from a nearby meteorological station. Deformation and precipitation data highlight that high total precipitation can be considered the trigger of the failure.

Slow moving earth-flows are a common geomorphological feature in the Northern Apennines and one of the main landscape forming agents. Often houses and small villages were built on these phenomena which are as consequence affected by damages that are caused by small displacements. In this short note, we present the InSAR results obtained from the Stanford Method of Persistent Scatterers (StaMPS) for Cosmo SkyMed and Envisat imagery together with a geologicalgeomorphological survey. We focus on the village of Gaggio Montano located in the Apennines south of Bologna. Here the tectonically sheared Flysch of the Monghidoro Formation structurally overlies the mélange of the Palombini shales and different complex landslides were mapped on the slope. We show that several landslide bodies are moving with steady deformation rates at least since 2003 and that the results obtained from Envisat and Cosmo SkyMed compare well to each other. Moreover the displacements measured by InSAR are in the same order of magnitude as displacements that have been measured in the past with inclinometers. Because the higher deformation rates are closely related to young houses and industrial complexes, it is possible that anthropogenic factors, like leakage from the water supply/discharge system, influence the displacements. A steady infiltration of water from the Flysch slabs into the clay shales might however represent a possible explanation.

none 3 si Advanced interferometric synthetic aperture radar (InSAR) postprocessing, like persistent scatterer InSAR (PSInSAR), offers the possibility to investigate slow moving landslides, where standard interferometry is problematic. These advanced algorithms involve the analysis of a series of SAR acquisitions in both time and space. One input that requires particular attention for landslide applications is the external digital elevation model (DEM) that is used to correct the interferograms for the topographic phase term. When multiple elevation data sets are available for a given study area, it is difficult to decide which one should be used. In this paper, we test the sensitivity of PS-InSAR/Small Baseline Subset (SBAS) results to different DEMs. The study area is located in the Northern Apennines of Italy, where chaotic clay shales and finegrained flysch host slow-moving earth flows and ancient rock slides. C-band (Envisat) and X-band (Cosmo-SkyMed) data are processed with different DEMs. We describe a simple framework to statistically analyze the influence of these models on the final PS-InSAR/SBAS results. We find that individual interferograms do not vary much depending on the DEM, while the results from PS-InSAR and SBAS analysis do vary. This is likely caused by the way the DEM error is estimated. We find also that the quality of the DEM is more important than the resolution and that Xband InSAR data are more sensitive to the choice of the DEM than C-band. The significance of the results is discussed with reference to two landslide areas. Bayer, B.; Schmidt, D.; Simoni A. Bayer, B.; Schmidt, D.; Simoni A.

Synthetic Aperture Radar (SAR) interferometry (InSAR) is used to make measurements of small surface displacements in different situations including ground-settlement, landslides and excavations. We document the deformation history of four dormant deep-seated landslides in the Northern Apennines of Italy that were reactivated by the excavation of a double road tunnel. The tunnel is part of a larger infrastructure project that involves the improvement of the A1 highway which connects Bologna and Florence. The excavation of the tunnel under the landslides’ basal shear surfaces started in 2011, and with the advance of the tunnel front, deformation began to occur, causing damages to homes and infrastructures. The tunnel passes under the small villages of Ripoli and Santa Maria Maddalena. The deformation slowed down after the tunnel was completed in November 2014, and low surface displacement rates were registered for the period between October 2014 and June 2016. We show the results of an InSAR analysis designed to measure surface movements between 2003 and 2016. The InSAR displacements are derived from the Envisat, COSMO SkyMed, and Sentinel 1 datasets and cover the main construction phase of the tunnel (2011-2015). A detailed geological and geomorphological mapping of the slope is developed based on the field evidence and on the spatial pattern of surface deformation provided by our InSAR analysis. Together with the map, a new conceptual geological model of the slope in tectonically sheared and weathered flysch is presented. The satellite InSAR data show good agreement with available ground based monitoring data that include inclinometer and GPS-measurements, while small differences occur with respect to deformation time series obtained from a ground-based InSAR instrument. In particular, the InSAR results for the X-Band data of COSMO SkyMed demonstrate the close relationship between the position of the excavation front and the displacement rates of previously dormant landslide deposits. Conversely, the concurrent rainfall does not appear to significantly influence the observed behaviour and, if any, had only a minor influence on the displacement history of the investigated landslides.

SAR interferometry has proven capable of measuring surface displacements in various situations including ground-settlement, landslides and excavations. We present a case study that consists of a tunnel excavation under a slope where dormant deep seated landslides are recognized. We take advantage of the high resolution images of the COSMO SkyMed Satellite system, that has a high sampling frequency, to measure the distribution of displacements in space and time. Our results are compared to ground-based InSAR data, surface geophysical techniques, GPS and inclinometer measurements. The data were acquired between 2011 and 2015 during the construction phase of the highway that will connect Bologna with Florence across the Northern Apennines. The tunnel passes under the small villages of Ripoli and Santa Maria Maddalena. The geological conditions are characterized by highly sheared flysch sandstones that belong to the LigurianMonghidoro Formation, a unit that is well known for its poor geomechanical properties and high landslide susceptibility. The work on the tunnel started in 2011 and, with the advance of the tunnel front, severe surface displacements were induced, causing damage to houses and infrastructure. The deformation slowed down after the tunnel was completed in November 2014, and low displacement rates were registered during the spring of 2015. The data illustrate the evolution of ground deformations as the excavation proceeds. In particular the InSAR results obtained with the Stanford Method of Persistent Scatterers (StaMPS) for the X-Band data of COSMO SkyMed give interesting insights into the nature of observed surface deformations that include clear downslope movements. 55 SAR scenes were analyzed that had been acquired in ascending and descending orbits, between December 2012 and February 2015. Together with the new findings from combined HVSR-MASW techniques and a detailed remapping of the area, a new conceptual geological model of the slope is developed.

We present an InSAR analysis of slow moving landslide in the Northern Appennines, Italy, and assess the dependencies on the choice of DEM. In recent years, advanced processing techniques for synthetic aperture radar interferometry (InSAR) have been applied to measure slope movements. The persistent scatterers (PS-InSAR) approach is probably the most widely used and some codes are now available in the public domain. The Stanford method of Persistent Scatterers (StamPS) has been successfully used to analyze landslide areas. One problematic step in the processing chain is the choice of an external DEM that is used to model and remove the topographic phase in a series of interferograms in order to obtain the phase contribution caused by surface deformation. The choice is not trivial, because the PS InSAR results differ significantly in terms of PS identification, positioning, and the resulting deformation signal. We use four different DEMs to process a set of 18 ASAR (Envisat) scenes over a mountain area (~350 km2) of the Northern Appennines of Italy, using StamPS. Slow-moving landslides control the evolution of the landscape and cover approximately 30% of the territory. Our focus in this presentation is to evaluate the influence of DEM resolution and accuracy by comparing PS-InSAR results. On an areal basis, we perform a statistical analysis of displacement time-series to make the comparison. We also consider two case studies to illustrate the differences in terms of PS identification, number and estimated displacements. It is clearly shown that DEM accuracy positively influences the number of PS, while line-of-sight rates differ from case to case and can result in deformation signals that are difficult to interpret. We also take advantage of statistical tools to analyze the obtained time-series datasets for the whole study area. Results indicate differences in the style and amount of displacement that can be related to the accuracy of the employed DEM.