remote sensing Article Satellite Remote Sensing for the Analysis of the Micia and Germisara Archaeological Sites 1, 2 1 3 Iulia Dana Negula *, Cristian Moise , Andi Mihai Lazăr , Nicolae Cătălin Ris, cut, a , 3 1 2 Cătălin Cristescu , Andreea Luminit, a Dedulescu , Cristina Elena Mihalache and Alexandru Badea 1 1 Romanian Space Agency, 21–25 Mendeleev Street, 010362 Bucharest, Romania; [email protected] (A.M.L.); [email protected] (A.L.D.); [email protected] (A.B.) 2 Department of Mathematics, Physics and Land Measurements, Faculty of Land Reclamation and Environment Engineering, University of Agronomic Sciences and Veterinary Medicine of Bucharest, 59 Mărăs, ti Boulevard, 011464 Bucharest, Romania; [email protected] (C.M.); cristina.mihalache@fifim.ro (C.E.M.) 3 Museum of Dacian and Roman Civilisation, 39 1 Decembrie 1918 Boulevard, 330005 Deva, Romania; [email protected] (N.C.R.); [email protected] (C.C.) * Correspondence: [email protected] Received: 20 May 2020; Accepted: 16 June 2020; Published: 22 June 2020 Abstract: The capabilities of satellite remote sensing technologies and their derived data for the analysis of archaeological sites have been demonstrated in a large variety of studies over the last decades. Likewise, the Earth Observation (EO) data contribute to the disaster management process through the provision of updated information for areas under investigation. In addition, long term studies may be performed for the in–depth analysis of the disaster–prone areas using archive satellite imagery and other cartographic materials. Hence, satellite remote sensing represents an essential tool for the study of hazards in cultural heritage sites and landscapes. Depending on the size of the archaeological sites and considering the fact that some parts of the site might be covered, the main concern regards the suitability of satellite data in terms of spatial and spectral resolution. Using a multi–temporal Sentinel–2 dataset between 2016 and 2019, the present study focuses on the hazard risk identification for the Micia and Germisara archaeological sites in Romania as they are endangered by industrialisation and major infrastructure works and soil erosion, respectively. Furthermore, the study includes a performance assessment of remote sensing vegetation indices for the detection of buried structures. The results clearly indicate that Sentinel–2 imagery proved to be fundamental in meeting the objectives of the study, particularly due to the extensive archaeological knowledge that was available for the cultural heritage sites. The main conclusion to be drawn is that satellite–derived products may be enhanced by integrating valuable archaeological context, especially when the resolution of satellite data is not ideally fitting the peculiarities (e.g., in terms of size, underground structures, type of coverage) of the investigated cultural heritage sites. Keywords: remote sensing; Earth Observation; disaster management; Copernicus Programme; Sentinel–2; vegetation indices; cultural heritage; archaeological heritage; Roman forts 1. Introduction The Earth Observation (EO) field is expanding at a fast pace, creating both new research opportunities and challenges [1]. Since more EO satellites equipped with different imaging sensors in terms of acquisition type and resolution are launched each year, the satellite–derived products and applications are also diversifying. The sectors that benefit from the information collected by the EO Remote Sens. 2020, 12, 2003; doi:10.3390/rs12122003 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 2003 2 of 36 satellites include agriculture, forestry, environment and climate change, urban and regional planning, water management, construction, transport, energy and natural resources, disaster management, civil protection, safety, health, humanitarian aid, security and defence, cultural and natural heritage, tourism and others [2]. As grandly expressed by the United Nations Educational, Scientific and Cultural Organization (UNESCO), “Heritage is our legacy from the past, what we live with today, and what we pass on to future generations. Our cultural and natural heritage are both irreplaceable sources of life and inspiration”[3]. The cultural identity is highly important for the local communities and also valuable at global scale, hence the safeguarding of the cultural heritage sites is of utmost relevance. According to UNESCO, amongst others, the dangers that might be faced by the cultural heritage sites are represented by the physical deterioration of the materials, structure or natural environment as well as the impact of regional planning and climatic, bio–geomorphologic or other environmental factors [4]. The applicability of EO data for the documentation, mapping, monitoring and management of cultural heritage has been demonstrated by plenty of research studies. Moreover, EO contributes also to the promoting and valuing of cultural heritage [5]. Generally, the use of imagery provided by EO satellite missions supports the cultural heritage management for a variety of activities, such as the identification and measurement of the natural subsidence, ground movements and building displacements; the quantification of the damage extent and severity in case of harmful events (e.g., flooding, landslides, soil erosion, earthquakes, etc.); land use changes; assessment of the pollution that might alter the artefacts; detection of buried archaeological sites; identification and monitoring of the cultural heritage sites for destruction/looting; urban sprawl; and climate change [6–9]. In this context, the most important aspect is the unequivocal efficiency of EO–derived products in addressing early warnings for potential threats and degradation risks. Focusing on the risk assessment based on different remote sensing data and methodologies, Agapiou et al. [10] proposed a non–destructive and cost–effective methodology for the management and monitoring of cultural heritage sites. Another research study conducted by Hadjimitsis et al. [11] demonstrated the potential of satellite remote sensing for cultural heritage mapping and monitoring activities, namely the identification of buried archaeological remains and risk assessment for a large number of threats (e.g., urban expansion, air pollution, land use change). Further, Biagetti et al. [12] showed the benefits brought by EO satellite data (i.e., medium– and high–resolution satellite images) in identifying, classifying, mapping and monitoring archaeological features, especially for those located in non–accessible regions (e.g., geographically remote areas or regions affected by military conflicts). Using change detection techniques, Cerra et al. [13] also focused on the use of EO data for monitoring the cultural heritage sites located in armed conflict areas and assessing the damage extent. The study underlined the importance of long–term monitoring of cultural heritage sites for the development of risk–preparedness strategies. Likewise, the multitemporal analysis of large series of satellite data proved to be an efficient tool for identifying the threats to cultural heritage sites and providing guidance for public policy making [14]. In what regards the type of EO data, both optical and radar remote sensing imagery proved their potential in support of cultural heritage. The research carried out by Stewart [15] concentrated on the identification of buried archaeological structures using synthetic aperture radar (SAR) satellite imagery and concluded that SAR analysis could successfully complement the existing methods for archaeological survey. Furthermore, Tapete and Cigna [16] demonstrate that high temporal revisit SAR time series support condition assessment of archaeological heritage due to anthropogenic processes. Copernicus, the European Union’s Earth Observation Programme (previously known as the Global Monitoring for Environment and Security programme), delivers free, full and open access to near–real–time satellite data on a global level. Within the Copernicus Programme, the Sentinel satellites and the contributing missions enable innovative global observations of the Earth by ensuring the continuous monitoring of the changes and creating improved predictions. Currently, the Sentinel family comprises of dedicated satellite missions (i.e., Sentinel–1, –2, –3, –5P and –6) and instruments Remote Sens. 2020, 12, 2003 3 of 36 (i.e., Sentinel–4 and –5) onboard weather satellites [17]. In brief, Sentinel–1 is equipped with a C–band SAR; Sentinel–2 carries a high–resolution multispectral imaging sensor; Sentinel–3 embodies a radar altimeter, an infrared radiometer and an imaging spectrometer; Sentinel–4 and –5 are spectrometers that will be embarked upon the Meteosat Third Generation Sounder satellites and the MetOp Second Generation satellites, respectively; Sentinel–5P carries a spectrometer; and Sentinel–6 will be equipped with a radar altimeter [18]. As part of the Copernicus evolution that aims to broaden the current capabilities offered by the Programme, High Priority Candidates are already identified based on the user needs and in accordance with the European Union’s policies [19,20]. The wealth of data acquired by the current Copernicus missions is analysed, integrated with other sources (e.g., data collected by in–situ sensors, ancillary data), processed, validated and provided as meaningful information through the six thematic Copernicus services (i.e., Atmosphere, Marine, Land, Climate Change, Security and Emergency) thus