Michael Foumelis

Long-and Short-Term Deformation Monitoring of Santorini Volcano: Unrest Evidence by DInSAR Analysis

Santorini Volcano is an active strato volcano, located at the central part of the Hellenic Volcanic Arc, developing ad retro of the Hellenic Arc and Trench. The explosive history of the volcano dates back to 1645 BC with the Minoan eruption, while it is documented to have produced at least ten eruptions until 1950 AD. The most recent volcanic unrest began in early 2011. Multi-reference Synthetic Aperture Radar (SAR) Interferometric techniques were applied to study the evolution of ground deformation during 1992-2011, with the use of ERS-1 and -2 and ENVISAT radar imagery. Datasets of common acquisition geometry were added into a single stack so as to obtain the linear deformation rates by means of phase averaging. However, to reveal the deformation history of the volcano, Singular Value Decomposition (SVD) method was implemented. This allowed retrieving ground deformation time-series on a pixel basis over regions with high temporal coherence levels. Results from independent tracks, agreeing with each other, suggest a deformation rate of approximately 5 mm/yr of subsidence at the southern part of Nea Kammeni Volcano, for the period 1992-2010. For the unrest period of 2011, intense uplift of 4.8 cm was observed throughout Nea Kammeni. Global Positioning System (GPS) observations from a local geodetic network confirm the DInSAR findings.

Time Series Synthetic Aperture Radar Interferometry for Ground Deformation Monitoring over a Small Scale Tectonically Active Deltaic Environment (Mornos, Central Greece)

ABSTRACT Parcharidis, I.; Kourkouli, P.; Karymbalis, E.; Foumelis, M., and Karathanassi, V., 2013, Time series synthetic aperture radar interferometry for ground deformation monitoring over a small scale tectonically active deltaic environment (Mornos, Central Greece). This study deals with the estimation of subtle ground deformation at millimetric accuracy over the broader area of the Mornos River delta in Central Greece and its spatio-temporal distribution for the period between 1992 and 2009 through Persistent Scatterers Interferometry (PSI). The results showed that the majority of the scatterers, which show subsidence, are located within the delta plain with mean subsidence rates throughout the delta varying between −7.2 and +0.7 mm/y. An attempt is made to highlight the geographic distribution, the amplitude, and the causes of the observed delta plain subsidence. The positive correlation between the thickness of the fine-grained Holocene deltaic deposits and the subsidence rates reveals that the main cause is the natural compaction of sediments. The highest subsidence is observed at Bouka Karahassani village, which corresponds to the area of the most recently abandoned river mouth, which is intensely eroded by marine processes. Apart from the dominance of fine sediments in the study area, subsidence may also be attributed to submarine gravitational mass movements along the steep slopes of the prodelta as well as to the reduction of sediment load after the dam construction in 1979. The NW part of the delta seems to have been affected by aseismic slip along a NE-SW trending normal fault buried beneath the alluvial deposits of the Skala torrent fan.

Sentinel-1 assessment of the interferometric wide-swath mode

This contribution reports on the performance investigations of the interferometric wide swath (IW) mode of Sentinel-1, which is implemented using the terrain observation by progressive scans (TOPS) mode. The key aspects of the TOPS mode that need to be considered for accurate interferometric processing will be presented, and first analyses with Sentinel-1 time series will be shown. The results focus on the pilot sites of Campi Flegrei/Vesuvius and Mexico City, as well as Greenland glaciers. Other aspects related to the interferometric performance are also presented, like the burst synchronization, the pointing accuracy, or the considerations when evaluating non-stationary scenes.

Detection and Monitoring of Active Faults in Urban Environments: Time Series Interferometry on the Cities of Patras and Pyrgos (Peloponnese, Greece)

Monitoring of active faults in urban areas is of great importance, providing useful information to assess seismic hazards and risks. The present study concerns the monitoring of the potential ground deformation caused by the active tectonism in the cities of Patras and Pyrgos in Western Greece. A PS interferometric analysis technique was applied using a rich data–set of ERS–1 & 2 SLC images. The results of the interferometric analysis were compared with the tectonic maps of the two cities. Patras show clearer uplift–subsidence results due to the more distinct fault pattern and intense deformation compared to the Pyrgos area, where more diffused deformation is observed, with no significant displacements on the surface.

Surface Motion and Structural Instability Monitoring of Ming Dynasty City Walls by Two-Step Tomo-PSInSAR Approach in Nanjing City, China

Spaceborne Multi-Temporal Synthetic Aperture Radar (SAR) Interferometry (MT-InSAR) has been a valuable tool in mapping motion phenomena in different scenarios. Recently, the capabilities of MT-InSAR for risk monitoring and preventive analysis of heritage sites have increasingly been exploited. Considering the limitations of conventional MT-InSAR techniques, in this study a two-step Tomography-based Persistent Scatterers (PS) Interferometry (Tomo-PSInSAR) approach is proposed for monitoring ground deformation and structural instabilities over the Ancient City Walls (Ming Dynasty) in Nanjing city, China. For the purpose of this study we utilized 26 Stripmap acquisitions from TerraSAR-X and TanDEM-X missions, spanning from May 2013 to February 2015. As a first step, regional-scale surface deformation rates on single PSs were derived (ranging from −40 to +5 mm/year) and used for identifying deformation hotspots as well as for the investigation of a potential correlation between urbanization and the occurrence of surface subsidence. As a second step, structural instability parameters of ancient walls (linear motion rates, non-linear motions and material thermodynamics) were estimated by an extended four-dimensional Tomo-PSInSAR model. The model applies a two-tier network strategy; that is, the detection of most reliable single PSs in the first-tier Delaunay triangulation network followed by the detection of remaining single PSs and double PSs on the second-tier local star network referring to single SPs extracted in the first-tier network. Consequently, a preliminary phase calibration relevant to the Atmospheric Phase Screen (APS) is not needed. Motion heterogeneities in the spatial domain, either caused by thermal kinetics or displacement trends, were also considered. This study underlines the potential of the proposed Tomo-PSInSAR solution for the monitoring and conservation of cultural heritage sites. The proposed approach offers a quantitative indicator to local authorities and planners for assessing potential damages as well as for the design of remediation activities.

Sentinel-1 tops interferometric time series results and validation

This paper presents results of the Sentinel-1 sensor in the interferometric wide-swath (IW) mode encompassing the first two years of operation of the mission. The paper focuses on persistent scatterer interferometric results and their validation. Further applications and investigations are also addressed, e.g., earthquakes, volcanoes and tomography.

Human induced groundwater level declination and physical rebound in northern Athens Basin (Greece) observed by multi-reference DInSAR techniques

The northern part of Athens Basin was subjected to human induced lowering of groundwater table due to over-pumping. The prolonged drought from late 1980s to early 1990s and the increased water demand resulted in significant settlement. Advanced SAR interferometric techniques were applied to investigate the time dependent ground displacements, focusing mainly on the non-linear component of motion, by utilizing ERS and ENVISAT satellite archive data. It has been shown that high displacement rates (up to -22.7 mm/yr) observed until 1995 were reduced when these areas were integrated to the national water supply system and groundwater extraction was abandoned. After 2002 large percent of the area is undergoing uplift indicating that the system is under a state of physical rebound.

Deformation monitoring in Kos Island Hellenic volcanic arc, Eastern Greece) using differential interferometry

In this study SAR interferometry technique has been applied over Kos Island (Hellenic Volcanic Arc) in order to detected pre-, coand post-seismic deformation and to compare with the deformation detected in Nisyros Island for the same period using the same interferometric pairs. The seismic unrest activity observed in Nisyros volcano (1996-1999) could produce crustal deformation in the adjacent island of Kos. The results of the interferometric processing show that coand post-seismic deformation was not possible to be detected due to the obtained low coherence. Contrary the results of the interferometric pair of the pre-seismic unrest period show two fringes of deformation in the southwest part of the island, the same amount of deformation was observed in Nisyros island.

Scientific Exploitation of Sentinel-1 within ESA's SEOM programme element

ESAs Scientific Exploitation of Operational Missions (SEOM) programme represents a pathfinder for science and innovation addressing the needs and requirements of the Earth system science community in terms of providing novel observations, new algorithms and products that will be a driver for new and innovative scientific discoveries. The current paper aims to provide a brief overview of the various SEOM activities relevant to the first Copernicus Sentinel 1 mission and to present the main achievements and discuss future opportunities for research.

A method predicting pumping-induced ground settlement using back-analysis and its application in the Karla region of Greece

In many arid planar regions of the world, ground subsidence induced by the lowering of the water table line due to pumping has recently caused damage to houses and other overlying structures. The depth of the water table lowering is usually tens of meters, the depth of the underlying soil layers may be hundreds of meters, and the region where the lowering is applied may extend tens of square kilometers. In this aspect, the problem under consideration differs drastically from other geotechnical engineering problems and the application of the physical models may have the serious deficiency that required geotechnical information may be incomplete and very costly to obtain: The change in water table variation and the depth of rock are usually known from results of pumping borings and geophysical investigations, but the location, width, compressibility and consolidation characteristics of the clay layers, are usually not known. New space technologies, such as the phase shifting interferometry radar method, provide cost-effective measurements of past displacement data. Based on past displacement measurements, an alternative approach is proposed to predict ground subsidence induced by the lowering of the water table. In particular, the work derives a simplified equation and corresponding methodology which predicts ground subsidence in terms of water table history, based primarily on data of past ground subsidence. This equation was derived and validated based on a state-of-the-art proposed model predicting one-dimensional ground subsidence induced by water level lowering in planar regions. Based on the derived simplified expression, a method predicting the risk at the built environment due to future ground subsidence induced by water level lowering was proposed and applied successfully in a well-documented case study of ground subsidence: the Niki village at Thessaly, Greece.