A marine geoarchaeological investigation for the cultural anthesis and the sustainable growth of Methoni, Greece

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Original article

A marine geoarchaeological investigation for the cultural anthesis and the sustainable growth of Methoni, Greece Panagiotis Gkionis a,∗ , George Papatheodorou a , Maria Geraga a , Elias Fakiris a , Dimitris Christodoulou a , Konstantinia Tranaka b a b

Laboratory of Marine Geology and Physical Oceanography, University of Patras, 26504 Rio, Greece ‘Andreas Syggros’ Hospital of Cutaneous & Venereal Diseases, 16121 Athens, Greece

a r t i c l e

i n f o

Article history: Received 9 February 2019 Accepted 28 August 2019 Available online xxx Keywords: Maritime archaeology Marine geophysics Hydrography Bathymetry Site evolution Underwater cultural heritage

a b s t r a c t The ‘Evolved GE.N.ESIS Project’ highlights the underwater cultural heritage resources off the coast of Methoni, Greece that could locally drive sustainable socioeconomic growth. An integrated marine geophysical survey, a hydrographic survey, and a GNSS survey were conducted off Methoni, recording six historic wreck sites, artefacts, the ruins of a submerged prehistoric settlement, and the town’s ancient harbour/breakwater, as well as the geophysical properties of the underwater environment. The preliminary project results present bathymetric surfaces, backscatter intensity and magnetic maps, drawings, and seismic reflection profiles of the underwater antiquities and of the seabed, all fused in a 3D geographical platform. The results also shed light on the archaeological potential of the site, the nearshore physical processes, and their effect on the underwater archaeological resources. The project outcomes have shown that the establishment of an underwater archaeological park and diving sites at the cultural heritage sites will support cultural tourism development in the area and will have a positive impact on local socioeconomic development. The underwater archaeological park should comply with the basic principles of a site management plan – one that is established in the context of an integrated coastal management plan that identifies the maritime synergies or conflicts among human activities, archaeological resources, and the local environment, and utilises the 3D synthesis of marine knowledge from the project outcomes as a decision-making tool. © 2019 Elsevier Masson SAS. All rights reserved.

1. Introduction Methoni is a coastal town lying in the southwestern extremity of Greece (Fig. 1). Through the millennia, Methoni played a strategic role in the geopolitical developments across the Mediterranean Sea as a significant maritime trade node. Today, the area is of high archaeological importance: prehistoric settlement ruins are lying submerged on the seabed of Methoni Bay, and ruins of shipwrecks are lying on the Methoni Bay/Strait seabed, together with other underwater antiquities [1]. Significant coastal archaeological monuments also exist in the area. In the historic periods that followed prehistory, the town’s harbour became strategically significant. This is evident by the construction of an ancient breakwater, the successive improvement works on it [2], and the fact that during the first Venetian period until the late 15th century CE, Methoni was a mandatory port of call for Venetian ships travelling

∗ Corresponding author. E-mail address: [email protected] (P. Gkionis).

to the Eastern Mediterranean [3]. Today, the ancient breakwater is lying submerged just below the sea surface, posing a hazard to navigation. Maritime-related business in the area and commercial port operations are rare because of the construction of a new breakwater and the blockade of the harbour’s entrance in the 19th century. Local people are facing a socioeconomic crisis, and the need to reconcile sustainable economic growth with social progress is a necessity, according to the European Commission [4]. In 2014, a team of geoscientists from the University of Patras created the ‘Evolved GE.N.ESIS Project’ (the acronym is derived from the words ‘A marine GEophysical investigatioN for marine knowledge and the anthESIS of Methoni, Greece’), with a vision of highlighting the underwater archaeological resources off Methoni that could locally drive sustainable socioeconomic growth, following the development of cultural and recreational maritime tourism. The project applies methods in hydrography and marine geophysics to provide marine knowledge within the project area (Fig. 1b) to the stakeholders, the scientific community, and society in general. Marine knowledge is necessary for the design of a plan for the sustainable development and management of the area, based on the

https://doi.org/10.1016/j.culher.2019.08.009 1296-2074/© 2019 Elsevier Masson SAS. All rights reserved.

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detection, mapping, and study of underwater archaeological sites, state-of-the-art marine geophysical techniques [6–10], underwater photography, and photogrammetry [11] have been used as standalone techniques or in compilation, depending on the nature and the environmental conditions of the sites. The combined application of the marine geophysical and ground-truthing techniques (multibeam echosounders, side-scan sonars, sub-bottom profilers, marine magnetometers, and photogrammetry) provides an integrated information environment that is crucial for the understanding of archaeological deposits, in the management of sites, and in planning strategies for site excavation. Moreover, a ‘downscaling’ methodological approach, working from a large regional scale (remote sensing techniques) to a small local and detailed scale, has been proposed as a best practice for the investigation of the archaeological sites by the SASMAP Project (the development of tools and techniques to Survey, Assess, Stabilise, Monitor, and Preserve underwater archaeological sites) [12]. The Evolved GE.N.ESIS Project, through the synthesis of hydrographic, geophysical and geospatial data, aims at presenting in all four dimensions the underwater cultural heritage resources that have the potential for being drivers of sustainable growth in the project area, as well as determining the processes that pose a threat to these resources or the environment. To that end, the main project objectives are as follows:

Fig. 1. Location of the project area. Left (a): the location of Methoni in Greece. Right (b): outline of the project area (within the yellow polygon). Background map data source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN and the GIS User Community.

exploitation of its underwater cultural heritage. It gives a better understanding of the spatial distribution and condition of the local underwater cultural heritage resources, and a better understanding of the maritime synergies and conflicts in the area that affect the underwater cultural heritage. Synergies and conflicts must be identified among a number of factors: the local physical processes; the impacts of climate change; the benthic habitats; the development and hazards of the underwater environment; as well as among the human activities and their impact on the ecosystem, and the cultural heritage resources. Stakeholders can then avoid, reduce, or minimise maritime conflicts through the application of maritime spatial planning and the use of geographic information systems (GIS) [5]. Since the Valletta Treaty in 1992, European and global councils and committees (i.e. UNESCO’s Convention for the Protection of the Underwater Cultural Heritage, ICOMOS – the International Council on Monuments and Sites – and SPLASHCOS – Submerged Prehistoric Archaeology and Landscapes of the Continental Shelf) have proposed and developed effective strategies for the scientific investigation, protection, preservation, and management of underwater cultural heritage sites. The in situ preservation of underwater cultural heritage sites, proposed in these strategies as first option, requires the application of non-destructive and cost-effective methods to first locate and assess the archaeological sites, and then monitor and preserve them. For the

• to provide a high-resolution baseline bathymetric surface of Methoni Bay, for comparison with past or future surfaces and for the monitoring of local sediment transport processes and coastal erosion/accretion patterns; • to document in three dimensions the ancient/medieval Methoni harbour and submerged breakwater, as well as the submerged prehistoric settlement off Methoni, assessing its full extent, including its buried part under the seafloor, and investigating the reasons it became submerged; • to map in three dimensions the historic shipwrecks already known, and explore shipwrecks not previously documented, and features of potential archaeological interest; • to visualise, synthesise, and analyse in four dimensions the available marine geophysical data and geoarchaeological survey findings, through spatiotemporal correlation of datasets in GIS environment; • to provide consultancy about maritime spatial planning and underwater cultural heritage management to the stakeholders. This article presents the methods, preliminary results, and archaeological findings of the marine geoarchaeological survey that took place in 2015 for the Evolved GE.N.ESIS Project, and highlights the value of the 3D mapping of underwater cultural heritage as a potential driver for sustainable socioeconomic growth in the Methoni area. In this article, ethical and legal issues apply to the spatial reference of underwater antiquities; wreck sites that are not archaeologically surveyed thoroughly are potential maritime graves from where artefacts may have not been recovered. Hence, horizontal coordinates of these wreck sites are not displayed in maps, and a maximum scale has been set for the presentation of datasets in the GIS environment so that the project objectives are not affected. 2. Regional setting 2.1. Coastal and submarine morphology The project area lies in the southwestern extremity of Peloponnese Peninsula in southern Greece, off Methoni, extending from the Methoni Bay shoreline in the north to the north coast of Sapientza

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Island in the south. Sapientza Island stands 1.5 nautical miles off the Methoni coast, forming the seaward edge of the Methoni Strait. The Methoni cape and coast are the prominent natural features in the project area. Southward, the Methoni Bay/Strait seabed deepens gradually from the bay head down to about a 40 m water depth along the Methoni Strait, and then shallows abruptly to the sea level at the north coast of Sapientza Island. A north-south trending underwater ridge with a least depth of about 5 m extends from the Methoni cape to the northern extreme of Sapientza, forming an underwater barrier across the strait in the west. This barrier makes the strait navigable to only ships with small drafts. The Methoni coast is subject to wave exposure from the southeast/southwest, and there is archaeological evidence that indicates coastline recession locally since the Middle Ages [1]. Potential causes of the coastline erosion are the increased wave attacks from southeast/southwest directions, the modified sediment budget, and the interception of longshore drift by the breakwater/jetty constructions [13]. 2.2. Archaeological background Underwater archaeological surveys in the project area started back in 1962 [14], bringing to light a wreck site with a heap of four granite sarcophagi off the east coast of Sapientza Island, dated to the third century CE. During the same expedition, another heap of granite columns, dated to or before the 15th century CE, was surveyed off the north coast of Sapientza, and traces of shipwrecks were located close to the Methoni harbour. Later, Kraft et al. [15] published a geoarchaeological study on the area, resulting in a paleogeographic reconstruction of Methoni Bay. Since 1993, the Hellenic Ephorate of Underwater Antiquities (EUA) has been conducting underwater archaeological surveys in Methoni Bay, studying the ruins of a prehistoric settlement, which were discovered in the mid-1980s and are lying submerged in the shallow nearshore zone of the bay. Shards of prehistoric pottery have also been excavated [16], as well as artefacts, in addition to shipwreck ruins that have been detected in the area [17]. Some of the findings from the above surveys have been described through the literature and were sketched on paper topographic plans at various scales, using obsolete local coordinate reference systems, and without background maps – hence without correlating the findings with the environment. In 2012, the Plymouth University and the Hellenic EUA conducted the ‘GE.N.ESIS Project’ (the acronym then stood for ‘GEoreferenced depictioN and synthESIS of marine archaeological survey data in Greece’) [18], a marine geoarchaeological reconnaissance survey off Methoni. At that time, a team of archaeologists and geophysicists surveyed the prehistoric settlement ruins, the ancient breakwater/harbour, historic wrecks, and artefacts. The collected data and archaeological data from EUA records were fused in a geographical software platform, noting the need for the area to be studied further. 2.3. Socioeconomic background The population of the wider region of Methoni is 2500, following a negative rate of change over time [19]. The region is a rural area [20] where agriculture and animal production are the predominant economic activities. According to the Hellenic National Center for Social Research [21], although almost 30 percent of the economically active population is currently engaged in these sectors, the rate has decreased by almost 50 percent in the last 20 years. Construction, accommodations, and food services are economic activities related to the tourism product and are now prominent in the area, with a cumulative employment rate among the economically active population of about 30 percent, showing a positive

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trend over the years (a 17 percent employment rate in all three sectors in 2001). Fishing and aquaculture are economic activities with less than a 10 percent engagement. The trend in local employment over the last 20 years is negative and can be partially attributed to the negative rate of local population change over time. In 2016, more than 35 percent of local young people aged 18–24 were neither employed nor attending educational institutions or in training, and in 2011 only 10 percent of the local population had a higher education degree [22]. The gross domestic product (GDP) per inhabitant is regionally the lowest in the EU, while the inhabitants suffer from the greatest decrease of GDP over EU from 2007 to 2016. According to a study on the development of tourism in the region [23], Methoni typologically belongs to the dynamic zones of tourism that are characterised by moderate tourist activity, seasonality in tourism demand, and a sufficient supply of accommodation services, with low impact and a high potential for the development of tourism. The study highlights the demand for cultural and alternative tourism in the area, as well as the need for developing sectors of tourism that correspond to the local tourism product. Given the strong cultural identity of Methoni and the high sociocultural value of the local underwater cultural heritage, it is believed that cultural heritage resources not widely known to the public, as well as the local natural resources, can attract visitors to Methoni [24]. The marine ecosystem off Methoni hosts at-risk mammal species and supports sensitive habitats. The sea area is a Site of Community Importance, a Special Area of Conservation, and part of the Natura 2000 Network [25]. The Mediterranean endemic species Posidonia oceanica (a seagrass sensitive to pollution and thus a bioindicator of local ecological conditions) forms extensive, high-density underwater meadows. Sapientza Island has also been declared as a natural monument. The local natural resources can contribute to the attraction of visitors in the area, and tourism in turn may also have positive effects on the local environment. However, the impact from human activities related to tourism development, such as sea pollution and marine traffic congestion, may have negative effects on both natural and cultural heritage resources [26]. Anchoring has sporadically proved to disturb the underwater cultural heritage in Methoni Bay [18]. A socioeconomic impact assessment has shown that establishing an underwater archaeological park off Methoni for guided demonstration of the underwater cultural heritage would give rise to cultural tourism in the area, which in turn would contribute to local socioeconomic growth. The social assessment was based on the description of the local/regional social environment [24,25] and market research [27,28], and it showed that the local society will strengthen its local cultural identity, civic pride and tradition, and will benefit from educational opportunities. Negative impacts on the condition of antiquities and the environment can be mitigated by applying measures designated by the stakeholders. The economic assessment was based on market research, and on an explanatory rather than a quantitative approach to the direct, indirect, and induced effects of such an investment, since the fundamental question of ‘who pays for what’ must be answered first. According to the assessment, a moderate direct and indirect employment and income generation is expected, together with new investments. The allocation of initial resources for the implementation of a dive park is of moderate cost, though the spatial distribution of antiquities allows for a progressive implementation.

3. Methods 3.1. Survey design Fieldwork tasks involved surveying the Methoni ancient, ultrashallow harbour that is almost enclosed today, and archaeological

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remnants in shallow, not recently charted water. For that reason, data were acquired aboard a 7 m rigid hull inflatable boat granted by the EUA. The project fieldwork was carried out in two phases, following a cost-effective methodological approach for rapidly surveying areas of archaeological and historical interest [29]. During the first phase, the research team conducted a marine geophysical survey using a sub-bottom profiler, a magnetometer, and a side-scan sonar to synthesise the full picture of features of archaeological interest over and under the seafloor, and to study the physical setting, processes, and geological evolution of Methoni Bay. Data from the side-scan sonar provided seabed classification information for the project area, maps of reported and not previously reported shipwrecks, and maps of the surficial prehistoric settlement ruins. A chirp sub-bottom profiler was used for a sub-seabed investigation of buried shipwrecks, a reconstruction of the geological evolution of Methoni Bay and an estimation of the total extent of the partially buried prehistoric settlement. A 3D reconstruction of buried shipwrecks was feasible thanks to the acquisition of high-density seismic data. A high-resolution marine magnetic survey was carried out with the use of a marine magnetometer for the detection of buried or surficial antiquities. During the first fieldwork phase, a hydrographic survey was conducted, using bathymetric sonar over the 6.2 km2 project area for 3D visualisation of the Methoni ancient harbour, the submerged ancient breakwater, the submerged prehistoric settlement, shipwrecks and features of potential archaeological interest. Hydrographic methods were also used for the definition of a highresolution baseline bathymetric surface of Methoni Bay to be used for assessing and monitoring the natural processes and erosion patterns along the Methoni coast, as well as for assessing the local navigational hazards and the potential for exploitation of nearshore maritime routes. Fig. 2 shows the lines that were run by the boat for the integrated geophysical investigations and the hydrographic survey. In the second fieldwork phase, real-time kinematic (RTK) point positioning data were acquired along the shoreline for its delimitation in the current epoch and for an analysis of shoreline evolution. RTK methods were also used for mobile mapping of selected archaeological features. Moreover, the research team conducted preliminary examination of the acquired acoustic and magnetic data and features of potential archaeological interest were selected for evaluation. These features were recorded with the use of an ultra-high definition drop-down camera and further examined. Image and video captures were also taken in pre-planned positions for seabed classification support. 3.2. Instrumentation For bathymetric data acquisition, the research team utilised a Kongsberg GeoSwath Plus Compact 250 kHz phase measuring bathymetric sonar, due to its ease of deployment over the side of the boat, its splash protection and efficiency to provide wide data coverage. The bathymetric sonar provided also co-registered sidescan data for the project area. Side-scan data was also acquired with an EG&G 100/500 kHz side-scan sonar and with a towfish deployed over the side of the boat. Side-scan data acquisition parameters were dealt according to specific operational needs and tasks. Seismic data was acquired with the use of a Kongsberg GeoAcoustics GeoPulse Plus digital sub-bottom profiler. It satisfied the need for optimising resolution for archaeological investigations and the need for sub-seabed penetration for geological investigations, enabling the selection of appropriate waveform to be used for each task, in the frequency band 1.5 to 18 kHz. The system’s transducers, housed in a pod, were pole-mounted over the side of the boat. The marine magnetic survey was conducted with a light, overhauser Marine Magnetics SeaSpy magnetometer, performing

at 0.1 nT absolute accuracy, carrying a highly sensitive sensor. The magnetometer was deployed at an astern tow, 20 m behind the boat and at the sea surface with the use of an attached floating aid. Accuracy of fieldwork measurements was improved with the use of auxiliary sensors. For correct assumption of bathymetric sonar acoustic ray path, casts were conducted for acquisition of water column sound velocity values, utilising a Sea&Sun Marine Tech CTD48M Multi-Parameter probe. Compensation of the sound velocity influence on the directivity of bathymetric sonar beams close to the sonar head was achieved with the acquisition of onthe-fly sound velocity data from a Valeport miniSVS sound velocity sensor. Motion compensation of the boat was managed with an SMC IMU-108-30 motion sensor. For short period monitoring of the water level, a vertical datum was established and an In Situ MP TROLL 9500 probe was installed ashore as a tide gauge, attached on a tide pole next to a benchmark. Positioning, navigation and timing information onboard was acquired with the use of a Hemisphere VS101 GPS Compass for the geophysical survey and a Hemisphere Vector V103 GNSS for the hydrographic survey. Compensation of GNSS disadvantage in terms of accuracy was made possible with the reception of broadcasted EGNOS corrections [30]. Network RTK positioning solution was applied for mobile mapping of underwater antiquities and for coastline definition, with the use of a Leica GS14 dual frequency GNSS receiver. Bathymetric sonar acquisition parameters were controlled through the Kongsberg GS4 software. Bathymetric data was processed with the GS4 and the CARIS HIPS and SIPS software, which was also used for bathymetric data quality control and gridding, auxiliary sensor data post-processing, side-scan data processing. The Marine Magnetics SeaLink software was used for magnetic data acquisition and the iXBlue Delph software was used for side-scan, seismic and magnetic data processing. The Hypack software was used for navigating the boat and for controlling the parameters of seismic data acquisition through the Kongsberg Geoacoustics GeoUTS software. All available datasets were fused for analysis in the iXBlue Delph Roadmap 3D geographical platform and the ESRI ArcMap GIS. Final project results are to be published and disseminated through the ArcGIS Pro platform and the project website (https://methoni-genesis-evo.blogspot.gr/).

4. Results Following the survey data process, bathymetric and seafloor maps were created for the project area (Fig. 3). The acquisition and processing of high-density bathymetric data resulted in the definition of a high-resolution baseline bathymetric surface of Methoni Bay/Strait (Fig. 3a). The acquisition and processing of backscatter data resulted in side-scan sonar mosaics, presenting the acoustic backscatter variability of the seafloor and providing object detection and seafloor classification information. The acquisition of high-density seismic data along the runlines shown in Fig. 2a resulted in the creation of a dense grid of georeferenced seismic profiles across the project area. The grid allows for a detailed subseabed investigation and documentation of large-to-medium scale antiquities in 3D, as well as a reconstruction of the paleosurface over which the prehistoric settlement was built. Temporal analysis of the 2012 and 2015 seismic datasets provides information on the short time scale littoral transport processes. The magnetic data acquisition and processing resulted in the generation of magnetic anomaly maps over areas of archaeological interest. The maps significantly support the detection of archaeological remains at wreck sites. However, the existence of modern debris and contamination outspread on the seafloor hindered the exploration of cultural features over larger areas, such as the prehistoric settlement site.

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Fig. 2. The survey lines that were actually run for the project fieldwork. Left (a): lines run for the geophysical investigations. Right (b): lines run for the hydrographic survey. Calibration, cross-check lines for quality control are not shown. Background coastline data source: HNHS.

Fig. 3. Bathymetric and seafloor maps of the project area. Left (a): bathymetry of the project area. Data gridded at 1 m. Right (b): side-scan mosaic of the project area. The blue ellipsis outlines the submerged ancient breakwater and the red circle outlines the surficial ruins of the submerged prehistoric settlement. Background map data source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN and the GIS User Community.

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Fig. 4. 3D visualisation of the submerged ancient breakwater and Methoni harbour. Bathymetry is shown vertically exaggerated and gridded at 0.2 m. Background map data source: Google.

4.1. Seafloor morphology and reflectivity The bathymetric surface reveals a smooth seafloor with a gentle southwestward slope, and water depths up to 15 m in Methoni Bay. The most prominent underwater features in Methoni Bay are the ancient breakwater and the basin of the enclosed old harbour, which is showing evidence of sedimentation. Across the Methoni Strait, the dominant underwater feature is the linear rocky ridge extending from the mainland to the north, down to Sapientza Island to the south, having a significant local relief and a very steep slope. Along the Methoni Strait, the bathymetric surface presents a deep basin where the local relief is low. The southwestern section of the project area off Sapientza is rugged, with a very steep slope. To the southeast, a shoal area of less than 15 m water depth occurs, forming a bank extending up to 0.25 nautical miles off Sapientza. To the east, off the steep section of the coast in the mainland, around and south of the islet called Kouloura, the seabed is coarse, sloping seaward. Fig. 3b shows the side-scan mosaic of the whole project area, where relatively high backscatter is represented in relatively dark tones, and relatively low backscatter in relatively light tones. Accordingly, the basin along the Methoni Strait is covered by finegrained sediments of low reflectivity, and the underwater ridge across the strait is rugged and rocky. Posidonia oceanica meadows exist over the bank off the northeastern coasts of Sapientza and in the southwestern section of Methoni Bay. The seafloor of the nearshore zone of Methoni Bay is covered by soft sediment. The mosaic highlights the submerged ancient breakwater and the ruins of the prehistoric settlement. High-resolution side-scan deliverables allow for the precise mapping of shipwrecks and artefacts.

4.2. Underwater cultural heritage The acquisition of ultra-high-resolution bathymetric data allowed for detailed seafloor mapping, investigation of wrecks, submerged installations and features of archaeological interest, and for visualisation/documentation of antiquities in three dimensions through point-cloud data and bathymetric grids ranging from 0.15 to 1 m. The dense grid of collected seismic profiles supported the detailed sub-seabed investigation and the 3D reconstruction

of buried shipwrecks. The shipwreck investigations were largely supported by the produced magnetic maps and side-scan mosaics. The interpretation, synthesis and preliminary analysis of bathymetric and geophysical data have shown eight sites of great archaeological importance within the project area. Two sites represent submerged coastal installations, namely the ancient harbour/breakwater and the submerged prehistoric settlement. The remaining six are historic/ancient shipwreck sites, two of which have never been documented before.

4.2.1. Submerged historical and prehistoric Installations A 3D model of the submerged ancient breakwater and harbour was created from gridded bathymetric data (Fig. 4). The bathymetric surface depicts the underwater part of the latest breakwater and how it blocks the entrance of the harbour, leaving it commercially obsolete. It also reveals the siltation in the harbour entrance and basin. The submerged prehistoric settlement site was investigated with the use of side-scan sonars. Following sonar data acquisition and processing, backscatter intensity mosaics were created for the survey area, allowing for an assessment of the variability of the seafloor morphology and the variability of the roughness of objects on the seafloor. Fig. 5 shows a side-scan sonar mosaic of the prehistoric settlement site. On the mosaic, scattered settlement ruins, pebbles, and artefacts made of hard material show high backscatter and are represented in relatively dark tones, while fine-grained sediment and objects made of soft material show low backscatter and are represented in relatively light tones. The settlement ruins were classified from acoustic types associated with documented settlement ruins from archaeological surveys [19,20] and ground-truthing methods. In addition, data from seismic profiles over Methoni Bay document the extension of the prehistoric settlement beyond the surficial ruins (Fig. 6) and support the assessment of the total extent of the submerged and partially buried prehistoric settlement through the interpretation of acoustic types related to documented settlement ruins. Fig. 6a illustrates the section of a seismic profile where the interpreted acoustic type referring to the submerged settlement (along the green line) extends beyond the documented surficial ruins as seen in Fig. 6b, delineating the actual settle-

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Fig. 5. Side-scan sonar mosaic showing high backscatter intensity (darker tones) from the prehistoric settlement surficial ruins (in the yellow circle) off Methoni, as well as from pebbles, artefacts and shipwrecks.

Fig. 6. Chirp seismic reflection profile documenting the prehistoric settlement extent along a boat transect. Left (a): section of a seismic profile with interpretation showing the acoustic horizon referring to the prehistoric settlement (green line) to be extending beyond the surficial ruins and the acoustic horizon (yellow line) representing the palaeosurface over which the settlement was built. Right (b): section location, over side-scan mosaic, showing the acoustic horizon referring to the surficial and buried prehistoric settlement ruins (green segment). Topographic plans of the settlement ruins (beige drawings) from archaeological surveys in 1999 [16] have been georeferenced and draped on the mosaic.

ment extent/extremities. The yellow line on the profile, defines the palaeosurface over which the prehistoric settlement was built. Fig. 6b shows the location of the profile section on the side-scan mosaic. 4.2.2. Ancient and historical shipwrecks A feature of archaeological interest (Fig. 7), not previously publicly documented, was detected off Methoni at a 7.5 m water depth with the use of the bathymetric sonar (Fig. 7a). According to ground-truthing data (Fig. 7c), this feature can be attributed to the cargo of a wrecked ship, consisting of two marble blocks, two marble columns (6.5–9.5 m in length), and a marble fragment. Archaeological investigations are needed for confident wreck dating. Fig. 7b shows the feature through co-registered side-scan data draped onto the 3D bathymetric surface.

An integrated geophysical investigation was carried out over a shipwreck in close proximity to the prehistoric settlement. Its partially buried, wooden ruins had been detected with the use of side-scan sonar during the 2012 campaign. Fig. 8 shows acoustic imagery data over the wreck site, where relatively dark tones represent relatively high backscatter from the surficial wreck ruins or from pebbles around it. Magnetic anomalies over the wreck site are also shown, colour-coded. A dense grid of collected seismic profiles supports its sub-seabed investigation and 3D reconstruction. No reliable dating of the wrecked ship is feasible without further archaeological research. Investigations were also carried out over artefacts and scattered material of a shipwreck first detected during the 2012 campaign, lying semi-buried on the seafloor, across a very shallow area affected by the littoral drift. In 2015, the investigated wreck site

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Fig. 7. Recordings of a not previously publicly documented feature of archaeological interest off Methoni. Top (a): 3D visualisation of the wrecked cargo, through bathymetric data, gridded at 0.25 m. Bottom left (b): side-scan data of the wreck site, co-registered to the 3D bathymetric surface (the wrecked cargo inside the yellow ellipsis). Bottom right (c): underwater camera capture of the actual feature. Photo credits: P. Gkionis, EUA.

was found totally buried under up to 1 m of fine-grained sediment depositions, at 1.7 m water depth (Fig. 9). Following information about the existence of archaeological material at the ultra-shallow nearshore zone of Methoni Bay, a wreck investigation was conducted that resulted in the discovery of a large number of artefacts and the documentation of major archaeological features spread over an area of approximately 1000 m2 . Fig. 10 illustrates the 3D synthesis of hydrographic, geophysical, and geospatial data over the wreck site. High bathymetric relief represents wreck timbers, ballast material, and a cannon (Fig. 10a). A ship’s wheel was detected next to the shipwreck and another cannon was recorded on top of a gunwale section (Fig. 10b, c) 8 m and 15 m away from the wreck ballast, respectively. A time-series plot of magnetic field data along a boat transect over the artefacts is also shown in purple. The shipwreck and the artefacts were precisely positioned with RTK methods. The resulting digital drawings are shown as white polygons fused with the bathymetric and geophysical results on the 3D geographic platform. Sub-bottom profiles and a magnetic map were also fused on the platform. The wide distribution of antiquities, magnetic anomalies, and acoustically detected targets denote that the archaeological potential of the site is high. Archaeological research is needed for acquiring further knowledge of the wrecked ship/ships.

Two more shipwreck sites were surveyed off Sapientza Island: the ‘shipwreck of columns’, dated to or before the 15th century CE and the ‘sarcophagi shipwreck’, dated to the third century CE. Both of these highlight the limitations of geophysical methods on shipwreck investigations over a rugged seafloor.

5. Discussion The spatial distribution of cultural heritage across Methoni Bay/Strait, following the preliminary survey results, is shown in Fig. 11. All wreck sites and submerged installations are lying along the nearshore zone, in water depths of less than 10 m, in close proximity to each other within two groups, north and south. The establishment of diving sites in the area would benefit from the distribution of the cultural heritage items, since the northern group of antiquities is very close to the shore and sheltered from the north winds, while the southern group lies one nautical mile off Methoni, sheltered from the south winds. An analysis of geophysical data and archaeological evidence [9,10] at short temporal scales show that the alongshore sediment transport processes affect the exposure of archaeological material close to the Methoni shore in a way that the burial of wreck sites and parts of settlement ruins in shifting sand

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Fig. 8. Side-scan mosaic of a wreck site (outlined by the blue ellipsis) in close proximity to the prehistoric settlement. The magnetic anomaly map of the site is shown draped on the acoustic imagery highlighting material of archaeological interest. On the mosaic pebbles (a) and shipwreck ruins (b) are showing high backscatter and are represented with relatively dark tones, while sand (c) is showing low backscatter and is represented with relatively light tones.

Fig. 9. Seismic profile across a wreck site and the interpretation (in yellow) of buried shipwreck ruins/artefacts.

coincides with the exposure of other remnants. Thus, the overall potential of exploitation of sites remains high. The preliminary outcomes from the hydrographic survey define a high-resolution baseline bathymetric surface of the Methoni Bay/Strait. High-resolution bathymetry is fundamental for the

investigation of shipwrecks and features of archaeological interest. It provides archaeological feature mapping and visualisation in three dimensions and supports feature evolution analysis or degradation monitoring [31]; it is thus a tool for indexing the potential for site exploitation, for exploring the opportunities for invest-

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Fig. 10. 3D synthesis of hydrographic, geophysical and geospatial data over a wreck site. Bathymetry is shown colour-coded highlighting archaeological features and scattered debris on the seafloor. Time-series plot of magnetic data along a transect is shown in purple colour. RTK positioning of a shipwreck, cannons and other artefacts resulted in the white survey drawings. Photo credits: P. Gkionis, EUA.

ment in the underwater cultural heritage tourism sector, and for assessing the human impact on the archaeological resources and the seafloor/ecosystem. The need for high-resolution bathymetric outcomes in the big data era raises considerations regarding data management, especially when working on a large project area, big datasets, heterogeneous bathymetry, and various project objectives. These considerations were dealt with using objectivebased data processing and gridding methods, and the need to work on variable resolution surfaces was highlighted [32]. Apart from quantitative outcomes, data from bathymetric sonars portray seafloor ruggedness and backscatter, which are physical characteristics used for seabed classification and habitat mapping [33]. Through the monitoring of the seabed habitats, specifically the Posidonia oceanica meadows, evidence is presented for the evolution of the local ecological conditions. Moreover, the availability of highresolution bathymetry, promotes the safety of navigation through chart updating, which is necessary for the dynamic underwater environment off Methoni and supports the development of port infrastructures, which in turn is a prerequisite for the development of maritime transportation, tourism, and economic growth [34]. The project fieldwork was conducted in challenging environmental conditions. The nearshore zone of Methoni Bay is an

ultra-shallow, not recently charted environment, subject to adverse sea conditions, where the risk for running the boat and the sonar transducers aground was high. The detection and appropriate recording of historic shipwrecks on the rugged seafloor was similarly challenging. Through the project, the acoustic backscatter response of wrecks lying over a heterogeneous seafloor, the sonar limitations and detection performance, and the data processing techniques for optimising the detection procedures and the data rendering were evaluated. Co-registering the bathymetry and backscatter amplitude proved to be an excellent tool for shipwreck detection and analysis. In addition, the integration of magnetometer data with all other geophysical datasets in a 3D geographical platform supports the assessment of the archaeological potential and highlights the disturbance of cultural and environmental resources from human activities/behaviour such as anchoring and contamination. The spatiotemporal analysis of fused seismic, backscatter, and bathymetric data supported the detection, documentation, and interpretation of archaeological assets [35], and is expected to give information on the littoral sediment transport processes over the settlement and the shipwrecks. Ultimately, the analysis of integrated data is expected to contribute to the assessment of

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Fig. 11. Distribution of cultural heritage in the Methoni Bay/Strait. Wreck sites and submerged installations are shown draped on a fused bathymetric, backscatter surface. Wreck sites (a), (b), (c) and (d) are shown in Figs. 7, 8, 9 and 10 respectively. Background map data source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN and the GIS User Community.

site evolution and exploitation potential. The preliminary results provide evidence for both short- and long-term trends in the nearshore distribution of sediments. The periodic sand depositions locally over a number of antiquities are expected to help their preservation while not hindering the overall exploitation potential due to the uniform nearshore distribution of archaeological resources. However, there is evidence through the side-scan mosaics and archaeological records that the littoral processes are causing seafloor erosion, affecting or scattering the exposed prehistoric settlement ruins. The analysis of bathymetric changes due to accretion/erosion in time, conducting high-resolution time-lapse hydrographic surveys over the nearshore zone, can be used to better understand and monitor the sediment transport processes, their time scales, and the implications of future site vulnerability [36], and to produce high-resolution models of wreck site formation [37]. The assessment of local physical processes will also highlight the requirement for periodic resurveys to ensure navigational safety. Earlier research in the area [15] resulted in a palaeogeographic reconstruction of Methoni Bay. The project research deals with the synthesis and temporal analysis of new evidence from later archaeological records and the project’s geophysical results, in order to review and update the conceptual geologic evolution model. The interpretation of unconformities and depositional units in the seismic records is expected to locally highlight Holocene geologic events, to provide information for reconstructing the prehistoric

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settlement extent and the palaeosurface on which the settlement was built, and to provide evidence for the settlement submersion. The proposed implementation of an underwater archaeological park in Methoni should follow a management plan based on the principles of heritage preservation and public access in order for heritage resources to contribute to the development of tourism and socioeconomic growth [38]. Worldwide, there are many examples of the contribution of underwater cultural heritage sites for tourism development and sustainable growth, and the level of contribution has been proven to depend on the site’s management rather than on the historical importance of the heritage site. The Scylla shipwreck in the UK is an example where the positive socioeconomic impact of diving tourism is measurable, and demonstrates the potential of shipwrecks for local development by means of return on investment, local business growth, job creation, and net income generation. The site is an underwater laboratory with a 10year monitoring programme. On the other hand, Alexandria Eastern Harbour site in Egypt is a historically significant but scarcely visited site because of the absence of infrastructure/facilities for tourist access, site promotion, and protection [39]. In Greece, a Joint Ministerial Decision was issued in 2013 to declare several underwater archaeological sites off Greek coasts as ‘underwater museums’, including wreck sites in the project area. However, since then, there has been no private or public investment in the promotion of underwater antiquities in Greece. A management plan for a heritage site off Methoni should combine general strategies and policies (preservation, safety, accessibility, sustainability, etc.) with specific objectives such as experience-based tourism development or public involvement, acknowledging the local geomorphological and socioeconomic conditions and their changes through time, especially in an era of economic recession [40] and climate change. Therefore, the development of an ex-ante socioeconomic impact assessment before the implementation of such a plan should be followed by mid-term and ex-post impact evaluation models [41] for ensuring its sustainable development after the establishment of an underwater archaeological park. The site management plan should describe its basic characteristics (administrative details, delimitation, ownership/responsibility structure and coordination, inventory, access, and the need for periodical monitoring), its status assessment (significance, impacts, potential, threats, and opportunities), as well as a master plan with time-scheduled interventions and monitoring through scientific evaluation, an awareness/communication plan, resources (staff/budget), risk assessments, and a vision for the future [42]. The coastal and underwater archaeological resources off Methoni are a prominent and sensitive component of the local coastal system. They are vulnerable to climate change and to the impact from human activities, and they interact with the physical processes. These interactions, long-term vulnerabilities, and socioeconomic impacts need to be studied, documented, and monitored by means of an environmental impact assessment. The need for mitigating the vulnerabilities, coordinating the interactions, and promoting sustainable development in the coastal/nearshore zone, places the underwater cultural heritage management in a wider spatial, environmental, economic, and societal context [43], and implies the need for site management plans to be integrated into coastal management plans. An essential element of an integrated coastal management plan is maritime spatial planning – the process of creating a local conflict map, analysing and allocating the spatial and temporal distribution of human activities in marine areas, to achieve ecological, economic, and social objectives [44]. The hydrographic and marine geophysical data from the project outcomes contribute to the implementation of both site management and integrated coastal management, and have a direct economic impact [45] through the provision of marine knowledge.

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Specifically, a good knowledge of bathymetry is a prerequisite for any boat operation and for the safe marine transportation of visitors/tourists across the site. It is also crucial for planning safe diving operations. A good understanding of the physical processes that interact with the antiquities and that pose a threat to the coastal system and its sustainable development depends on the availability of high-resolution baseline marine data, and on resurveys being conducted for site evolution monitoring. The construction of waterfront facilities for public/tourist access to the heritage resources and the establishment of coast protection works require the availability of marine data. The data from the project outcomes also support decision-making regarding the development of suitable aquaculture sites, the choice of cable-laying routes, and the evaluation of risks from natural hazards. The project uses data fusion methods for the 3D rendering of survey and other geospatial data on geographical platforms, supporting the requirement of management plans for site delimitation and law enforcement, the spatial definition of underwater antiquities, the creation of an inventory of the local underwater antiquities, site promotion, maritime spatial planning, and ultimately decision-making for stakeholders. The 3D data fusion methods can also have educational benefits for visitors, providing a better understanding of the site before diving or through virtual diving [46] using a 3D reconstruction of the marine environment and of the antiquities, thus empowering the social impact of underwater cultural heritage for experience-based tourism development.

6. Conclusions The Evolved GE.N.ESIS Project provides marine geoarchaeological knowledge and highlights the underwater cultural heritage resources off Methoni, with the potential for being drivers of sustainable socioeconomic growth in the area through a 3D synthesis of acquired bathymetric, backscatter intensity, seismic, magnetic, and geospatial data with past geoarchaeological records. According to the preliminary results, there is a dense distribution of underwater antiquities over Methoni Bay/Strait, and the archaeological potential in the area is high. Information is given on archaeological feature evolution for degradation monitoring, for assessment of the human impact on the underwater cultural resources, and for an assessment of the implications for future site vulnerability. The results provide information for a better understanding and monitoring of the local physical processes, and of the human impact on the seafloor. The requirement for periodic resurveys for improving navigational safety is also addressed. The underwater archaeological findings constitute a prominent element of the local coastal resources. An underwater archaeological park should be established off Methoni and a site management plan should be locally implemented, acknowledging the heritage resources as a driver for tourism and local socioeconomic development. An underwater archaeological park in the area is expected to support new investments, income generation, social cohesion, civic pride, tradition enhancement, and educational opportunities. For ensuring the sustainability of the site development, an integrated coastal management plan should be established in the area so that maritime synergies are highlighted and negative socioeconomic impacts or maritime conflicts among human activities, archaeological resources, and the local environment are identified, avoided, or reduced through an environmental impact assessment and through the application of maritime spatial planning. Stakeholders must clarify the roles of maritime transportation and underwater cultural heritage towards socioeconomic growth, as they seem to compete with each other. To that end, all underwater archaeological sites in the project area could be officially declared an ‘underwater museum’ and outlined on the charts, while

marine/coastal infrastructures could be developed to support the maritime transportation needs without affecting the underwater antiquities or the environment.

Acknowledgements The authors extend special thanks to Aggeliki Simosi who, as Head of the Greek Ephorate of Underwater Antiquities at the time of fieldwork, supported the project, Ilias Spondylis, underwater archaeologist who contributed to the project with his deep archaeological knowledge and providing archaeological data ingest into the project database, as well as Despoina Koutsoumpa, underwater archaeologist who showed us the way through the governmental functions and clearances. Many thanks go to Martin Gutowski for being an excellent fieldwork companion and for supporting the hydrographic tasks by means of sonar instrumentation, Peter Schwarzberg for providing hydrographic data processing solutions and training and Philippe Alain for providing geophysical data processing and fusion solutions. Accordingly, we wish to acknowledge the support of Kongsberg Maritime, Teledyne Caris and iXBlue towards the project objectives.

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