Publications by Year: 2021

Domazetović F, Šiljeg A, Marić I, Faričić J, Vassilakis E, Panđa L. Automated Coastline Extraction Using the Very High Resolution WorldView (WV) Satellite Imagery and Developed Coastline Extraction Tool (CET). Applied Sciences [Internet]. 2021;11(20):9482.
The accurate extraction of a coastline is necessary for various studies of coastal processes, as well as for the management and protection of coastal areas. Very high-resolution satellite imagery has great potential for coastline extraction; however, noises in spectral data can cause significant errors. Here, we present a newly developed Coastal Extraction Tool (CET) that overcomes such errors and allows accurate and time-efficient automated coastline extraction based on a combination of WorldView-2 (WV-2) multispectral imagery and stereo-pair-derived digital surface model (DSM). Coastline extraction is performed and tested on the Iž-Rava island group, situated within the Northern Dalmatian archipelago (Croatia). Extracted coastlines were compared to (a) coastlines extracted from state topographic map (1:25,000), and (b) coastline extracted by another available tool. The accuracy of the extracted coastline was validated with centimeter accuracy reference data acquired using a UAV system (Matrice 600 Pro + MicaSense RedEdge-MX). Within the study area, two small islets were detected that have not been mapped during the earlier coastline mapping efforts. CET proved to be a highly accurate coastline mapping technique that successfully overcomes spectral-induced errors. In future research, we are planning to integrate data obtained by UAVs infrared thermography (IRT) and in situ sensors, measuring sea and land surface temperatures (SST and LST), into the CET, given that this has shown promising results. Considering its accuracy and ease of use, we suggest that CET can be applied for automated coastline extraction in other large and indented coastal areas. Additionally, we suggest that CET could be applied in longitudinal geomorphological coastal erosion studies for the automated detection of spatio-temporal coastline displacement.
Anifadi A, Sykioti O, Koutroumbas K, Vassilakis E. . In: International Conference on Raw Materials and Circular Economy. Athens, Greece; 2021.
Nickel laterites constitute a significant proportion of world nickel reserves. In Greece, LARCO GMMSA exploits the domestic laterite deposits, operating in Locris area and Euboea island (Central Greece) and in Kastoria area (Northern Greece). During the last decades, the use of remote sensing technology in geological mapping has gained significant attention worldwide. In particular, multispectral/hyperspectral imagery is one of the most widespread and standard source of data for the recognition of spatial and/or spectral patterns in mineral/ore exploration. To this purpose, the development of suitable state-of-the-art processing algorithms, either at pixel level or at sub-pixel level, is of crucial importance. In particular, spectral unmixing (SU) is currently a very powerful method that allows sub-pixel level processing in order to assess spectral information regarding different lithologies that co-exist within a pixel. The aim of SU is the decomposition of the spectral signatures of mixed pixels onto a space spanned by a set of spectral signatures (represented as vectors) corresponding to pure physical materials (endmembers). The latter are retrieved either from spectral libraries or they are extracted from the image itself by detecting the relatively “purest” pixels via suitable algorithms. The resulting decomposition coefficients are indicative of the degree of each endmember’s presence within each pixel. It should be noted here, that in contrast to other scalar-based approaches (e.g. spectral indices), SU is a vector-based processing that provides richer representational capabilities. The most widely used assumption in SU is that each pixel’s spectral signature can be written as a linear combination of the spectral signatures of its endmembers (linear mixing hypothesis). In this study, we investigate the ability of SU to reliably map FeNi laterite outcrops. The study area is the Tsouka laterite excavation in Central Greece, operated by LARCO GMMSA. More specifically, linear SU is applied to a Sentinel-2 satellite image. The dataset is atmospherically corrected, spatially resampled to 10m pixel size and subset to the excavation area. For the needs of SU, two endmembers are used, namely a laterite endmember and a non-laterite endmember. The former corresponds to the average spectral signature of selected pixels of pure laterite composition. The latter represents both the other two existing lithologies, namely Cretaceous limestones and ophiolites. The SU resulting abundance maps show that Sentinel-2 succeeds to satisfactorily map the FeNi laterite. The proposed method is fast, low-cost and non-destructive and can contribute to the research of LARCO GMMSA for new exploitable ores.
Kaviris G, Voulgaris N, Lozios S, Soukis K, Zymvragakis A, Vassilakis E, Antoniou V. . In: 37th General Assembly (GA) of the European Seismological Commission. ; 2021.
Lasithi is the easternmost regional unit on Crete Island (Greece), south of which lies the Hellenic Arc. The broader offshore study area is dominated by reverse and strike-slip faulting, while active normal faults are mapped on land. Detailed research of the latter suggests that the northern and central parts of Lasithi are affected by arc-parallel extension, whereas the southern part by arc-normal extension. The NNE-SSW striking faults are more frequent and form horst and graben or half-graben macro-structures. These faults usually consist of two or more segments accompanied by post-glacial scarps and clear evidence of recent reactivations. The E-W to ESE-WNW striking faults are scarcer and mainly observed along the southern coastal zone or offshore. Compared to the NNE-SSW strikingfaults, they present fewer active traces. Significant events in the study area include the 8th August1303 (M≈8.0) earthquake and the 1st July 2009 (Mw=6.4) earthquake that occurred south of Crete, triggering a small tsunami. A probabilistic seismic hazard assessment is performed for Lasithi. The maximum expected ground motion parameters (PGA, PGV and PGD) for mean return periods of 475and 950 years are determined using the SHARE 2013 zones and seismicity model. Greek GMPEs that consider type of faulting and soil type were applied. Optimum results are obtained through a logic-tree approach, which considers all GMPEs. The Uniform Hazard Spectrum for Agios Nikolaos, Sitia and Ierapetra are presented and compared with the elastic design spectra proposed by the Greek National Building Code and Eurocode 8.
Karantanellis E, Marinos V, Vassilakis E, Hölbling D. Evaluation of Machine Learning Algorithms for Object-Based Mapping of Landslide Zones Using UAV Data. Geosciences [Internet]. 2021;11(8):305.
Landslides are a critical geological phenomenon with devastating and catastrophic consequences. With the recent advancements in the geoinformation domain, landslide documentation and inventorization can be achieved with automated workflows using aerial platforms such as unmanned aerial vehicles (UAVs). As a result, ultra-high-resolution datasets are available for analysis at low operational costs. In this study, different segmentation and classification approaches were utilized for object-based landslide mapping. An integrated object-based image analysis (OBIA) workflow is presented incorporating orthophotomosaics and digital surface models (DSMs) with expert-based and machine learning (ML) algorithms. For segmentation, trial and errors tests and the Estimation of Scale Parameter 2 (ESP 2) tool were implemented for the evaluation of different scale parameters. For classification, machine learning algorithms (K- Nearest Neighbor, Decision Tree and Random Forest) were assessed with the inclusion of spectral, spatial, and contextual characteristics. For the ML classification of landslide zones 60% of the reference segments have been used for training and 40% for validation of the models. The quality metrics of Precision, Recall, and F1 were implemented to evaluate the models’ performance under the different segmentation configurations. Results highlight higher performances for landslide mapping when DSM information was integrated. Hence, the configuration of spectral and DSM layers with the RF classifier resulted in the highest classification agreement with an F1 value of 0.85.
Asimakopoulou P, Nastos P, Vassilakis E, Hatzaki M, Antonarakou A. Earth Observation as a Facilitator of Climate Change Education in Schools: The Teachers’ Perspectives. Remote Sensing [Internet]. 2021;13:1587.
Climate change education (CCE) fosters the skills and behavioral patterns of students in regards to climate-related challenges and risks. Despite its importance, the integration of CCE in schools is challenging due to the interdisciplinary nature of climate science and the obstacles and demands of everyday school reality. Here, we examine the case of satellite Remote Sensing (RS) for Earth Observation (EO) as an innovative tool for facilitating CCE. We focus on Greece, a country that, despite being a hot spot for climate change, shows a low level of CCE integration in schools and awareness for EO-based educational resources. Based on interviews with in-service teachers, our research reveals the following: (a) there is a high interest in how satellites depict environmental phenomena; (b) EO is considered an efficient vehicle for promoting CCE in schools because it illustrates climate change impacts most effectively; (c) local natural disasters, such as intense forest fires and floods, are more familiar to students and, thus, preferable for teaching when compared to global issues, such as the greenhouse effect and sea level rise; and (d) educators are in favor of short, hands-on, EO-based activities (also known as “activity-shots”), as the most useful material format for integrating climate change topics in their everyday teaching practice.
Karkani E, Saitis G, Tsanakas K, Evelpidou N, Karymbalis E, Vassilakis E, Drinia, H., Batzakis D-V. Palaeogeographic evolution and sea level changes of a tectonically active area: the case of Psatha, Alkyonides Gulf, Greece. In: EGU General Assembly 2021. Vol. EGU21-13236. online; 2021.
The study of environmental changes in coastal areas provide useful information for past conditions and constitute a powerful tool for accurate palaeogeographic reconstructions. Several coastal landforms are present on the coastal zone, with different response to environmental change. Coastal wetlands and lagoons are particularly sensitive to local paleoenvironmental changes and provide an excellent opportunity to reconstruct the evolution of the coastal zone evolution and the sea level changes. In this context, the aim of this work is to elucidate the coastal evolution of Psatha bay, Alkyonides Gulf, Greece, through coastal drillings and geomorphological sea level markers. The study area is located at the eastern end of Corinth Gulf, in the Gulf of Alkyonides. Psatha is bounded by active neotectonic structures, which have been a determining factor in its development. In this work we adopt a multiproxy approach through the study of coastal drillings and beachrocks. We coupled detailed beachrock mapping, microstratigraphic analysis and luminescence dating to study beachrock outcrops found up to 1 m above the present sea-level. For the palaeoenvironmental reconstruction, multiproxy analyses were undertaken, which included sedimentological analysis of the core, paleontological analysis of macrofauna and microfauna and radiocarbon dating. The results of this work will contribute to the better understanding of a coastal site in a tectonically active area and the relative sea level changes.
Frank A, Frei R, Triantaphyllou M, Vassilakis E, Kristiansen K, Frei K. Why we need extensive, regional, bioavailable Sr isotope baselines for human mobility studies in archaeology: A case from the Peloponnese. In: Goldschmidt. Lyon, France; 2021.
Sr isotopes have been applied as a provenance tracer in archaeology for over three decades providing unprecedented insights into past human mobility at an individual level. However, their successful application requires a comparison to the bioavailable Sr isotopic composition characteristic for the areas where finds were unearthed or are suspected to originate from (referred to as baseline). Due to financial reasons or other limitations many studies only take a few baseline samples within or in close proximity to the archaeological site for reference purposes. This can introduce large uncertainties to the interpretation of human 87Sr/86Sr data especially in geologically complex areas. Here, we present Sr concentrations and isotopic data of modern environmental proxies (plants, soils and water) sampled at 52 sites across the Peloponnese, Greece, and compare them to previously published 87Sr/86Sr data to evaluate the need for extensive regional baselines. The bioavailable Sr isotopic composition of the Peloponnese is characterised by a wide range in 87Sr/86Sr values (0.7077 – 0.7237). The distribution of the 87Sr/86Sr data corresponds well with the surface lithology, suggesting a dominance of Sr derived from the bedrocks. The 87Sr/86Sr baselines defined by the environmental samples taken from areas characterised by clastic and chemical surface sediments are calculated as comparatively unradiogenic, narrow 87Sr/86Sr ranges of 0.70832 ± 0.00053 (x̅ ± 2σ) and 0.70835 ± 0.00089 (x̅ ± 2σ), respectively. In contrast, the areas characterised by metamorphic outcrops are characterised by more radiogenic 87Sr/86Sr signals with wider 87Sr/86Sr baseline ranges of 0.70906 ± 0.00116 (x̅ ± 2σ) and 0.71429 ± 0.01133 (x̅ ± 2σ) for marbles and schists, respectively. These significantly extend the bioavailable 87Sr/86Sr baseline data of the Peloponnese compared to previously published data (0.70841 ± 0.00092; x̅ ± 2σ). This emphasises the importance of regionally extensive sampling in geologically complex areas, such as the Peloponnese, to reduce uncertainties in the application of Sr isotopes as a tracer for past human mobility.
Frank AB, Frei R, Triantaphyllou M, Vassilakis E, Kristiansen K, Frei KM. Isotopic range of bioavailable strontium on the Peloponnese peninsula, Greece: A multi-proxy approach. Science of The Total Environment [Internet]. 2021:145181.
Sr isotopes are a powerful tool used for provenancing in many disciplines, but their successful application requires the availability of robust Sr baselines of potential target areas. This study presents 87Sr/86Sr signatures and Sr concentrations of water, plants and soil leachates from the Peloponnese peninsula, Greece, to establish the first comprehensive bioavailable Sr isotope baseline for this region. Additionally, this study aims to evaluate which proxy is most suitable to characterise bioavailable Sr in a geologically complex area also exposed to foreign aeolian Sr sources. Our recorded bioavailable Sr isotope signatures correspond well with the surface lithologies characteristic of the Peloponnese. Unradiogenic 87Sr/86Sr ratios and a narrow isotope range (0.70779 - 0.70955) characterise the bioavailable Sr signatures of the sedimentary deposits and more radiogenic and isotopically variable values (0.70791 - 0.72370) were measured for metamorphic and igneous rock outcrops. The differences in 87Sr/86Sr values measured between proxies of one site are comparatively low for samples from the sedimentary and igneous deposits, while the overall spread in 87Sr/86Sr values is wider for samples from metamorphic deposits. We propose to define bioavailable 87Sr/86Sr baseline ranges as the average bioavailable 87Sr/86Sr ratio of all proxies of each lithology ± its double standard deviation (x̅ ± 2σ). This results in narrow baselines for the sedimentary outcrops of 0.70832 ± 0.00053 (n=58) for clastic sediments and 0.70835 ± 0.00089 (n=29) for chemical sediments. The metamorphic deposits are characterised by wider bioavailable 87Sr/86Sr baselines of 0.70906 ± 0.00116 (n=4) and 0.71429 ± 0.01133 (n=13) for marble and schist, respectively. The bioavailable Sr baseline for igneous rock outcrops is also characterised by a comparatively wide range with 0.70950 ± 0.00259 (n=7). The wide range in inter- and intra-site specific bioavailable 87Sr/86Sr variation observed in this study emphasise the need for comprehensive multi-proxy sampling strategies within geologically-complex areas.
Vassilakis E, Foumelis M, Erkeki A, Kotsi E, Lekkas E. Post-event surface deformation of Amyntaio slide (Greece) by complementary analysis of Remotely Piloted Airborne System imagery and SAR interferometry. Applied Geomatics [Internet]. 2021;13(1):65-75.
The results of structure from motion photogrammetry and SAR interferometry as complementary techniques for measuring ground deformation induced by the massive open-pit landslide in Amyntaio, Greece (June 10, 2017), is presented in this paper. This unexpected slide damaged the entire westernmost marginal area of the pit, significant number of buildings, and infrastructures of the nearby village of Anargiri. The described methodology includes the generation of a multi-temporal dataset (from Sept. 2017 to Sept. 2018) of very high-resolution surface topography (at 10 cm), based on the analysis of imagery collected by Remotely Piloted Airborne Systems (RPASs). Satellite observations involved interferometric processing of TerraSAR-X data for a complementary estimation of ground displacement rates. Height differences from consecutive aerial photography campaigns as well as space-borne measurements provided valuable information on the evolution of the deformation and its spatial characteristics during the post-event period, an important aspect for future hazard and risk considerations.