archaeologia maritima mediterranea

ARCHAEOLOGIA MARITIMA MEDITERRANEA An International Journal on Underwater Archaeology

Direttore Roberto Petriaggi Comitato scientifico Francisco J. S. Alves (Portogallo), David Blackman (Gran Bretagna), Katerina Delaporta (Grecia), Maria Antonietta Fugazzola Delpino (Italia), Ehud Galili (Israele), Piero Alfredo Gianfrotta (Italia) Smiljan Gluš©ević (Croatia), Xavier Nieto Prieto (Spagna), Francisca Pallarés (Italia), Patrice Pomey (Francia), Gianfranco Purpura (Italia), Eric Rieth (Francia), Edoardo Tortorici (Italia) Segreteria di redazione Barbara Davidde Petriaggi * «Archaeologia Maritima Mediterranea» is an International Peer-Reviewed Journal. The eContent is Archived with Clockss and Portico; it is Indexed in Scopus.

ARCHAEOLOGIA MARITIMA MEDITERRANEA An International Journal on Underwater Archaeology 12 · 2015

PISA · ROMA FA B R I Z I O S E R R A E D I TO R E MMXV

Amministrazione e abbonamenti Fabrizio Serra editore Casella postale n. 1, Succursale n. 8, i 56123 Pisa, tel. +39 050 542332, fax +39 050 574888, [email protected] I prezzi ufficiali di abbonamento cartaceo e/o Online sono consultabili presso il sito Internet della casa editrice www.libraweb.net. Print and/or Online official subscription rates are available at Publisher’s web-site www.libraweb.net. I pagamenti possono essere effettuati tramite versamento su c.c.p. n. 17154550 o tramite carta di credito (American Express, Eurocard, Mastercard, Visa) * Autorizzazione del Tribunale di Pisa n. 21 del 15 settembre 2004 Direttore responsabile: Fabrizio Serra A norma del codice civile italiano, è vietata la riproduzione, totale o parziale (compresi estratti, ecc.), di questa pubblicazione in qualsiasi forma e versione (comprese bozze, ecc.), originale o derivata, e con qualsiasi mezzo a stampa o internet (compresi siti web personali e istituzionali, academia.edu, ecc.), elettronico, digitale, meccanico, per mezzo di fotocopie, pdf, microfilm, film, scanner o altro, senza il permesso scritto della casa editrice. Under Italian civil law this publication cannot be reproduced, wholly or in part (included offprints, etc.), in any form (included proofs, etc.), original or derived, or by any means: print, internet (included personal and institutional web sites, academia.edu, etc.), electronic, digital, mechanical, including photocopy, pdf, microfilm, film, scanner or any other medium, without permission in writing from the publisher. * Si invitano gli autori ad attenersi, nel predisporre i materiali da consegnare alla redazione e alla casa editrice, alle norme specificate nel volume Fabrizio Serra, Regole editoriali, tipografiche & redazionali, Pisa-Roma, Serra, 20092 (Euro 34,00, ordini a: [email protected]). Il capitolo Norme redazionali, estratto dalle Regole, cit., è consultabile Online alla pagina «Pubblicare con noi» di www.libraweb.net. Proprietà riservata · All rights reserved © Copyright 2015 by Fabrizio Serra editore, Pisa · Roma. Fabrizio Serra editore incorporates the Imprints Accademia editoriale, Edizioni dell’Ateneo, Fabrizio Serra editore, Giardini editori e stampatori in Pisa, Gruppo editoriale internazionale and Istituti editoriali e poligrafici internazionali. www.libraweb.net Stampato in Italia · Printed in Italy issn 1724-6091 issn elettronico 1825-3881

SOMMARIO Roberto Petriaggi, Editoriale

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Marco Bonino, Il Thalamegos di Tolomeo IV Filopatore (216 circa a.C.) Ehud Galili, Baruch Rosen, Protecting the ancient mariners, cultic artifacts from the holy land seas Justin Leidwanger, Sebastiano Tusa, Marzamemi II ‘Church wreck’ excavation: 2014 field season Luc Long, Giorgio Spada, Le port fluvial antique d’Arles et son avant-port maritime en Camargue. Derniers résultats des recherches Stefano Medas, Riccardo Brizzi, La Spera, tra antichità e tradizione nautica. Strumento per la salvezza di una barca che naviga o si avvicina a terra con mare tempestoso Eric Rieth, Des bateaux, des milieux et des hommes: un regard anthropologique sur l’archéologie nautique à travers «l’héritage intellectuel» de François Beaudouin (1929-2013) Maria Geraga, George Papatheodorou, George Ferentinos, Elias Fakiris, Dimitris Christodoulou, Nikos Georgiou, Xenophontas Dimas, Margarita Iatrou, Stavroula Kordella, Gerasimos Sotiropoulos, Vasilis Mentogiannis, Katerina Delaporta, The study of an ancient shipwreck using marine remote sensing techniques, in Kefalonia Island (Ionian Sea), Greece

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acta diurna Stefano Medas, Eros Turchetto, Venezia: un’àncora “bizantina” dimenticata

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recensioni L’épave de la première moitié du xv ème siècle de la Canche à Beutin (Pas-de-Calais), a cura di Eric Rieth (Marco Bonino)

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THE STUDY OF AN ANCIENT SHIPWRECK USING MARINE REMOTE SENSING TECHNIQUES, IN KEFALONIA ISLAND (IONIAN SEA), GREECE Maria Geraga* · George Papatheodorou* George Ferentinos* · Elias Fakiris* Dimitris Christodoulou* · Nikos Georgiou* Xenophontas Dimas* · Margarita Iatrou* Stavroula Kordella* · Gerasimos Sotiropoulos* Vasilis Mentogiannis* · Katerina Delaporta** 1. Introduction n 1978, Muckelroy K. stressed that “maritime archaeology is concerned with all aspects of maritime culture; not just technical matters, but also social, economic, political, religious and a host of other aspects” (Muckelroy, 1978). By this meaning a wreck site acts as a time capsule, providing a complete snapshot of the life on board at the time of sinking as well as a testimony to trade and cultural dialogue between peoples (unesco). Through time, the studying of wreck sites of different ages and in particular that of ancient world has illuminated many unknown aspects of human kind such are shipbuilding, navigation, sea routes and issues regarding trade and transportation of materials. For these reasons, wreck sites and in particular the ancient ones have attracted the attention of global and European committees and organizations which aspect them as cultural heritage sites and they work on the development of strategies for their preservation and management. In this direction, the surveying with marine remote sensing techniques has been proved a meaningful tool for the studying of the wreck sites. First of all, marine geophysical surveys designed on this orientation or incidentally are responsible for the detection of numerous wreck sites regardless the water depth of the site. Secondary these techniques have been used as prior investigation of the environmental regime of known wreck sites. At the time of the wrecking, the ship organized as a mobile entity begins an irreversible process of change leaving the Cultural world and entering the Natural world (Muckelroy, 1978, Martin, 2011). This transformation

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* Laboratory of Marine Geology and Physical Oceanography, Department of Geology, University of Patras, Greece; e-mail: [email protected]; e-mail: [email protected]; e-mail: fakiris@ upatras.gr; e-mail: [email protected]; e-mail: [email protected]; e-mail: [email protected]; e-mail: [email protected]; [email protected];

e-mail: [email protected]; e-mail: [email protected]; e-mail: [email protected] ** Byzantine and Christian Museum, Ministry of Culture, Education and Religious Affairs, Athens, Greece; e-mail: katerinadellaporta@ yahoo.com

«archaeologia maritima mediterranea» · 12 · 2015

184 maria geraga et alii which is indefinite in time is influenced by numerous factors involving cultural parameters (i.e. human behavior prior to wrecking, shipbuilding, transported material) and natural parameters (i.e. weather conditions at the time of wrecking, seafloor and water column characteristics). By entering the natural world a series of processes begin leading to the scattering and/or floating of parts of the ship, biological and sedimentological transformations of the seafloor substrates, degradation and destruction of the ship and the ship’s components. Systematic marine geophysical surveys can provide information regarding the environmental setting of the wreck site by nondestructive procedure (Quinn et al., 2002, Papatheodorou et al., 2005). For example the use of acoustic devices focus on the insonification of the surface of the seafloor (single and multi beam echosounders and side scan sonar) can provide detailed mapping of the wreck site, evaluation of the surrounding seafloor texture and possible seafloor transformation after the wrecking. On the other hand the use of sub bottom profilers can provide evaluations for the seafloor stratigraphy and the possible expansion of the wreck beneath the seafloor and information regarding the quality of the sedimentary layers in which the wreck has been found. All these acquired data is valuable for the potential excavation methodological scheme, the monitoring of the site and the strategies that will be followed aiming to the preservation and management of the site. The number of the ancient wreck sites on the seafloor of the Greek Seas, official reported and/or published, is rather limited in relation to the undeniable intensive seafaring during antiquity in these seas. Even more limited are the wreck sites for which the detection and/or the investigation have been involved marine geophysical surveys (Sakellariou et al., 2007). During a habitat mapping project (apreh, Aquarium for the Promotion of the Environment and History, European Territorial Co-operation Programme, Greece-Italy 2007-2013) at the Kefalonia Island (Ionian Sea) focused on mapping of P. oceanica meadows and coralligene formations, an ancient shipwreck was detected. The present study presents the results of the small-scale remote sensing surveying conducted on the wreck site area. Although the existence of an ancient wreck in the wider area was known from a geophysical survey conducted previously by the Greek Ephorate of Underwater Antiquities (eea), the Norwegian University of Science and Technology (ntnu) and the Norwegian Institute at Athens (nia) under the directionship of Dr. Delaporta K., its exact position was not (Delaporta et al., 2006). An oval-shaped assemblage of amphorae constitutes the visual presentation of the wreck with dimensions similar to that detected by Delaporta et al. (2006). Based on the type and decoration of the amphorae, the ship was most probably of the Roman period and dated between the 1st century bc and the 1st century ad (Delaporta et al., 2006). The present study utilized a variety of marine geophysical devices included a multibeam echosounder, three types of side scan sonar and a high resolution profiling system in collaboration with a variety of sophisticated software. The synthesis of all the acquired and processed data aimed in the detection of the wreck site, the detailed mapping of the wreck site and the surround seafloor and the examination of possible alternations of the seafloor after the wrecking. Furthermore the paper discusses difficulties which might rise in the detection of ancient wrecked ships in the shallow waters of the Greek seas.

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2. Methods The survey utilized an elac Nautic Seabeam 1185 multibeam echosounder system (mbes) for the acquisition of the bathymetric data. The operational frequency of the system was 180 kHz with a beam width of 108°, providing a theoretical vertical resolution of 0.1 m. mbes track lines having a total length of 13.9 km covered a total area of 0.32 km2 (Fig. 1). For the acquisition and the processing of the mbes data the hypack/hysweep 2014 software was used. The processing of the data incorporated the extraction of extreme bathymetric values. For the examination of the seafloor stratigraphy a Kongsberg GeoPulse Plus (GeoAcoustics Universal) Chirp sub-bottom profiler system was used operated at 1.5-11.5 kHz. A pulse length of 20ms and transmit rate of 6 pulses per second was used throughout the survey. Profiling track lines had a total length of 13.9 km (Fig. 1). The acquisition of the data achieved by the Sonarwiz (Chesapeake Fig. 1. Map showing the track lines Technology Inc) and the processing of the profiles by the sb-Interpreter (Triton Im- conducted with the multibeam echo sounder, the sub-bottom profiler aging Inc) software. and the side scan sonar systems, Side scan sonar data acquired by three operated at the same time. systems, a dual frequency (100 and 500 kHz) e.g&g. 272 td, an Edgetech 4200 sp operated at 100 kHz and 400 kHz and a Klein 3900 operated at 900 kHz. The acquisition of the data performed by the Edgetech 4100P, Edgetech 4200 and SonarPro® software, respectively. Slant-range correction was applied to the data during survey. The line spacing provided a 120% range overlap. All systems covered the same area which was of about 0.3 km2 in extent (Fig. 1). The acquired side scan sonar data were processed using isis Sonar (Triton Imaging Inc.) software and then were mosaicked with 0.1 m resolution using TritonMap (Triton Imaging Inc.) software. In the present survey, high backscatter, which is attributed to hard substrate, is presented by light tones on sonographs. Low backscatter (soft substrate) is presented by dark tones and acoustic shadow by black tone. The positioning and navigation conducted with a g.p.s. Hemisphere VS101 (d.g.p.s. type) providing an accuracy less than 0.6 m. The survey conducted on board the vessel “Socrates” N.P. 4880. All the acquired and processed data sets were georeferenced. The comparison and synthesis of the results performed in gis environment.

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Fig. 2. Side scan sonographs acquired at 100 kHz (a) and 500 kHz (b) frequencies, in the examined area. (1): sedimentary seafloor substrates (medium to low reflectivity), (2): rocky substrates with elevation and microrelief (high reflectivity), (3): ancient shipwreck.

In addition, the post processing involved the application of an image-based Automatic Classification System (acs), the Matlab Package SonarClass used for the automatic recognition and separation of the ancient shipwreck from the neighboring seafloor features (Fakiris and Papatheodorou, 2007, 2009). SonarClass performs image classification by extracting features that describe the local image texture and applies well documented classification algorithms to them. 3. Results 3. 1. Side scan sonar imaginary Figure 2a and b presents the sonographs retrieved from 100 kHz and 500 kHz frequencies, respectively. On the 100 kHz sonograph, the seafloor is representing by extensive areas of medium to low reflectivity and areas with high reflectivity (Fig. 2a). The acoustic character of the former areas suggests the prevalence of sedimentary seafloor substrates covered most probably by sandy material and/or seagrass. The

study of an ancient shipwreck in kefalonia island (ionian sea) 187 latter areas present strong patchy acoustic character and in combination with the detection of acoustic shadow zones (dark areas) within and at the boundaries, of them, which usually appear as lacy, suggest the presence of rocky substrates with elevation and microrelief (Fig. 2a). These areas detected mostly at the western-northwestern side of the studied area. Curved linear features characterized by alternations of high and low reflectivity at the southeastern side of the studied area can be attributed to the impact of anchoring on the seafloor. The 500 kHz sonograph is characterized by higher resolution showing the rocky outcrops in more details (Fig. 2b). On the other hand, it presents weaker reflectivity contrasts compared to the 100 Fig. 3. High resolution side scan sonar kHz records (Fig. 2b). sonograph acquired at 900 kHz frequency Although the geomorphological setting in the examined area. (1): ancient shipwreck, of the area which includes numerous rock (2): sedimentary seafloor substrates outcrops complicates the detection of po(medium to low reflectivity), tential archaeological sites, the detection (3): rocky substrates with elevation of the wreck site achieved on both sono- and microrelief (high reflectivity), (4) target graphs (100 and 500 kHz). The potential of unknown origin, (5): anchoring marks. wreck site was detected towards the southeastern end of the extensive rocky seafloor showing a different acoustic pattern compared to that of the rock outcrops. The wreck site portraits a weak patchy texture with sharp limits. Moreover, the wreck site has an oval shape with certain geometrical characteristics resembling the shape and the dimensions of an ancient shipwreck. Based on the above backscatter and shape characteristics that site was considered to be the ancient shipwreck and led the following field surveying to focus there. Figure 3 shows the sonograph of the studied area retrieved with 900 kHz frequency. The site considered as the most possible for the ancient wreck appears here with a more clearly acoustic signal (Fig. 3). The large acoustic shadow zone which accompanies the area implies elevation in relation to the surround seafloor and the strong patchy acoustic character suggests microrelief and material aggregation within the area. Photos obtained by drop camera confirmed the existence of the shipwreck at the examined area and ascribed the acoustic patchy character to the assemblage of amphorae of various shapes and dimensions (Fig. 4). 3. 2. Side scan sonar survey configuration and ancient shipwreck detectability The side scan sonar surveying in the present study area incorporated various configuration settings as well as various sonar devices and operational frequencies (100, 400,

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Fig. 4. Photos selected from drop camera showing amphorae stacked on the seafloor.

500 and 900 kHz), thus providing the opportunity to examine the side scan sonar survey parameters that favor the detectability of potential ancient shipwrecks on sonographs. The ancient shipwrecks, like other low elevation-high texture features, it is expected to be better distinguished on the side scan sonar images when the sonar tow-fish is moving closely above the seabed and to the target and it is transmitting high acoustic frequencies. However, these statements are not thoroughly verified in ancient shipwreck sites. A number of side scan sonar survey configuration parameters were investigated regarding the way they influence ancient shipwreck detectability. These parameters were: (a) the height of the tow-fish above the seafloor, (b) the ground range to the shipwreck, (c) the acoustic incidence angle (Ia: angle between the beam and the seafloor surface) and (d) the transmission of the acoustic frequency. To avoid subjective criteria in assessing detection significances, the SonarClass software has been used to measure the precision in which the shipwreck can be separated from other proximal seafloor features (rock outcropping, scattered rocks, sand), using objective computer vision criteria. Ten (10) different sonographs were collected from the survey area, using different side scan sonar survey configurations, originated from three different side scan sonar models and giving the choice of transmitting in four separate pulse frequencies (Fig. 5). All sonograph images were classified trough SonarClass software tool using a Naïve Bayes Classifier ( John and Langley, 1995), trained with image samples from the  ancient shipwreck interior and samples randomly picked from the rest of the survey area. The classification precision (Pc) in each case has been estimated as Pc = tp/(tp + fp), where tp is the number of true positive (expected) and fp the number of false positive (unexpected) predictions. Figure 6 shows the results of classification precisions (Pc) versus side scan sonar survey configuration parameters. A clear correlation has been revealed between all considered parameters and classification precision. The incidence angle (Ia) proved to be the most significant parameter for ancient shipwreck detectability, in a strong positive correlation to classification precision, as it leaded to P > 90% for Ia > 80° and P < 20% for Ia < 20°. Height of the side scan sonar tow-fish above the seafloor and ground range of the tow-fish to the wreck exhibited strong negative and positive respectively correlations to the classification precision, while pulse frequency, although reliable, exhibited much weaker correlation in comparison to other parameters.

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Fig. 5. Sonographs showing the ancient shipwreck and the neighboring seafloor features (rock outcrops) selected with different sonar survey configurations, side scan sonar systems and acoustic frequencies. (a),(b): 100 kHz, (c): 400 kHz, (d),(e): 500 kHz, (f ): 900 kHz.

3. 3. Bathymetry The processing of the mbes bathymetric data developed the detailed bathymetric map in the examined area, gridded at 1 m isobaths, presented in Figure 7. The water depth at the survey area varied between 44 and 73 m. From the western part and up to water depth of about 59 m, within the area attributed to rocky seafloor based on the sonograph mosaics, the water depth variation presents uneven seafloor morphology. Seawards from this area the seafloor presents relatively flat surface and a gently deepening (Fig. 7a, b). The resolution of bathymetric data was sufficient to reveal the detailed characteristics of the wreckage and the surrounding seafloor. The shipwreck is located between 58 and 60 m isobaths and is bathymetrically complex consisting of three almost separate mounds (Fig. 7c). 3. 4. Seismic Profiling The examination of the acquired seismic profiles showed two acoustic echo types as the dominant in the surveyed area (Fig. 8). The first type is characterized by a sharp,

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Fig. 6. Plots showing the correlation between classification precision (Pc) and side scan sonar survey configuration parameters.

Fig. 7. (a) 3D and (b) 2D general multibeam bathymetric map of the survey area. The ancient shipwreck is detectable on the general bathymetric map. (c) Detailed bathymetric map of the wreckage suggests that the shipwreck consists of three almost separate mounds.

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Fig. 8. High resolution Chirp seismic profiles selected from the survey area. (a) The upper unit corresponds to sedimentary deposits (1) over the bedrock (2), (b) the rocky bed is exposed on the seafloor.

Fig. 9. High resolution Chirp seismic profile acquired along the wreck site. The wreck site is represented by a chaotic acoustic pattern overlying an acoustically transparent unit and a high amplitude reflector (bedrock).

continuous surface reflector. Underneath an acoustically transparent unit has been detected without and/or very weak internal reflectors. The acoustic basement recorded as a prolonged, continuous and in general flat reflector. The second type is characterized by a surface reflector without any evidence of a deeper signal penetration and thus has no sub-bottom reflections. The acoustic signature of the surface reflector is prolonged to semi-prolonged with microrelief and rarely discontinuous. Based on their acoustic signatures, the first type represents hard substrate and the second loose sediments above the hard bedrock. The spatial distribution of the occurrences of the acoustic types are comparable with the interpretation of the sonographs and suggest that the high reflectivity extensive areas on sonographs are attributed to rocky substrates and the low reflectivity areas to the sedimentary deposition upon the hard bedrock.

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Fig. 10. Maps showing (a) the bathymetry around the ancient shipwreck and (b) the sediment thickness at the wreckage area.

The wreck site recorded on the Chirp profiles by a concentration of weak hyperbolic reflections overlapping each other and forming a chaotic acoustic pattern (Fig. 9). This acoustic signature has been detected within the first acoustic type and interrupts the continuity of the surface acoustically transparent unit. The wreck acoustic pattern detected up to 1.0-1.2 m above the surround the seafloor and the maximum thickness of the underneath high reflection was up to 1.7 m. 3. 5. Wreck site formation processes The mbes data show that the wreck forms a mound of 256 m2 and 1.0-1.2 m in elevation (Fig. 10a). The shipwreck itself and the accumulation of the sediment around the wreck site seems that cause this bathymetric change. Figure 10b presents the spatial distribution map of the sediment thickness at the examined area, as this estimated by the processing of the seismic profiles. Based on this map the sediment thickness varies between 1.0 and 5.0 m and the deposition of the sediments do not present any particular trend with the exception of the area around the shipwreck. The sediments seem to accumulate in small basins of indistinct shapes and dimensions of which the development seems to follow the uneven surface of the rock seabed. The maximum sediment thickness detected at the western and the southern sides of the wreck while the minimum thickness at the north and the east sides (Fig. 10b). The establishment of marine structures on the seafloor frequently provokes local changes in the accretion-erosion patterns of the seafloor (Sumer et al., 2001). The magnitude and the type of these changes are controlled by numerous parameters as the shape and size of the structure, the intensity and direction of the local currents, the texture of the seafloor. Similarly, the monitoring of wreck sites by remote sensing techniques has documented that these areas are vulnerable to both erosional (i.e. scouring) and accretion processes (Quinn and Bolland, 2010). In the phase of the present study no evaluations in this direction can be made since there is no data available for comparison and examination of possible seafloor changes through time. However, the sediment accumulation appears increased at the wreck site in comparison to the surround seafloor. Furthermore the accumulation rate of sedi-

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Fig. 11. (a) Sonograph of the ancient shipwreck showing the three mounds of amphorae (i, ii, iii) amalgamate into an oval-shaped mound. (b) Detailed sonograph of the wreck site showing (1), (2): spilled items from the shipwreck, (3): small-scale tongue-like debris of amphorae, (4): a gap at the ship, barren of amphorae, separating mound -i- from mound -ii-.

mentation seems that it was accelerated at the lee side of the wreck. Taking into consideration the maximum sediment thickness of the surround seafloor and the age of the shipwreck, then the excess sedimentation rate around the wreck site is at least 1.0 m/kyrs. On the sonograph mosaic the visual part of the wreck is represented by an oval shaped area of 256 m2 (Fig. 11). The wreck is to 30 m long and up to 12 m wide. The acoustic signature of the wreck site which point out to a dense and aligned assemblage of amphorae suggests that most probably the wooden ship reached the sea bottom in a more or less intact condition and probably upright. The well preserved assemblages of the amphorae is a frequent observation seen on the ancient shipwreck and is attributed to the nature of the amphorae which they had been designed especially for the nautical transportation and to the cargo-stowing abilities of the ancient mariners (Wacshmann, 2011). A detailed examination of the sonograph and the bathymetric data of the shipwreck showed that the assemblages of the amphorae can be divided in three mounds (Figs. 7, 11). These three mounds amalgamate into an oval-shaped mound which constitutes the visual part of the shipwreck above the seafloor. In the study wreck site only few acoustic anomalies linked most probably with spilled items are detected close to the main assemblage (Fig. 11). Among them the most pronounced detected west-southwards the main assemblage. It is recorded as small-scale tongue-like debris of amphorae (Fig. 11). Also, this debris seems that is related to a narrow zone of low reflectivity detected within the main assemblage which

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Fig. 12. (a) Photomosaic and (b) detailed sonograph showing the shipwreck. (1): small-scale tongue-like debris of amphorae, (2): a gap at the ship, barren of amphorae, separating mound -i- from mound -ii-, (3): a small part of the ship without amphorae.

is attributed to a gap at the ship, barren of amphorae, separating mound -i- from central mound (-ii-) (Fig. 11). The central mound (-ii-) with the mound -iii- are not clearly separated and no one zone barren of amphorae is visible between these two mounds. The separation of the vessel cargo into three mounds, and especially into mounds -i- and -ii-, could be the result of the collision of the vessel with the seafloor. When the vessel collided with the seafloor broke and spilled this small quantity of amphorae. Therefore, this tongue-like debris of amphorae could be the result of a partial displacement of the cargo items during the wrecking event when the ship reached the seafloor. There is no doubt that a gap at the hull of the ship during her collision with the seafloor might be the result of the manner that the vessel sunk (upright, bow or stern first etc) and/or of the type of the seafloor upon which it landed. Another explanation of the separation of the cargo into to three mounds could be the deterioration of the shipwreck in the course of the time due to geomorphologi-

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Fig. 13. Map showing the bathymetry of the bedrock. This palaeobathymetric map is designed by extracting the thickness of the surface sediments above the bedrock.

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Fig. 14. (a) High resolution Chirp seismic profile and (b) sonograph showing a coralligene formation at the survey area.

cal/environmental processes and human activities. The high resolution chirp survey showed the existence of rocky bedrock few meters below the present day seafloor. That bedrock is covered by a thin veneer (1-3 m) of loose sediments and is characterized by uneven morphology, particularly at the wreck site (Fig. 13). Taking into consideration the uneven morphology of the hard bedrock and the thinner sedimentary cover at the time of the wrecking, the seafloor was more uneven than today. The bedrock follows the bathymetric deepening seawards (Fig. 13). However, in the examined area and around the wreck site it presents an uneven top and it develops forming local and small-scale depressions and mounds regardless its appearance, on or beneath the seafloor (Fig. 13). Moreover, some rocks could be outcrop at the period of wrecking that now are covered by loose sediments. Based on this scenario, the vessel was landed on an uneven seafloor which it could provoke the break of its hull. On the other hand the displacement of the amphorae along the gap between mounds -i- and -ii- could be the result of human intervention in the wreck remains. An anchoring in the past could have drifted down slope these amphorae. The detection of trenching marks due to anchoring on the seafloor, very close to the shipwreck, further suggests that modern vessels and their anchoring could damage the wreck site (Fig. 3). In order to verify all the above hypothesis regarding the wreck formation processes, a systematic archaeological excavation is required. 4. Detecting Shipwreck Remains In Shallow Waters The surveying for the detection of the wreck site in the present study emerged the difficulties raising in similar surveys in the shallow zone of the Greek Seas. This zone in the Greek seas usually is composed of rocky seafloor, scattered outcrops and sedimentary seafloor of limited sediment thickness. The acoustic character of the rocky seafloor presents similarities with that of the ancient shipwrecks and thus it produces weak argumentation for the findings (Fig. 14). Furthermore, a rocky seafloor provokes breaks at the wooden hull of the vessels and subsequently drifting of the cargo items, reducing additionally the detection feasibility of the potential archaeological

196 maria geraga et alii site in such environments. Profiles selected from high resolution profilers could be decisive for the detection determining the local and confined rocky outcrops and the potential targets associated with ancient shipwreck on the sedimentary parts of the seafloor. This zone also covers the water depth range for the growth of the coralligène formations, known as “tragana”. These benthic organisms form colonies by hard, usually calcified, casings, completely covering the substrate on which they develop. They can develop on hard and soft seafloor substrates. Due to their nature these reefs area easily recognizable on the sonographs by localized strong backscatter facies (Georgiadis et al., 2009) (Fig. 14). On the seismic profiles they are presented by a lowamplitude reflector on the top which it might include hyperbolic and single domeshaped almost acoustically transparent structures due the cavernous internal structure of the reefs (Fig. 14). The acoustic signal which these biological reefs present on both side scan sonar imagery and on profiling is similar to those of the remains of an ancient shipwreck therefore reducing the detection feasibility of the potential archaeological target. In such cases, the disjuncture of the possible shipwreck remains from the other reefs should be incorporate inspection with visualization methods and further data processing with calculation of reflection coefficient (Quinn et al., 1997, Plets et al. 2009), or the performance of more sophisticate software orientated on texture analysis (Fakiris and Papatheodorou, 2012). However, the possible existence of coral aggregations close to underwater archaeological sites should be reserved additional attention regarding the management of the site. These biological reefs are important fishing grounds by fishermen which often they use trawling and dredging fishery methods on these fields (Brennan et al., 2012). These fisheries methods could cause irretrievable destruction on the wreck site, therefore the development of specific strategy aiming in the protection of the site i.e. the covering of artificial mats is required. 5. Conclusions The present study aimed on the examination of an ancient shipwreck located on the shallow waters of the Ionian Sea, using a combination of marine remote sensing techniques included multibeam, side scan sonar and sub-bottom profiler systems. This procedure proved to be essential in the detection, localization and mapping of potential archaeological sites at the shallow waters where isolated rocky seafloor substrates and biological formations composed of hard material are emerging as limitations for the recording and the disjuncture of the sites of interest. In the present study emphasis was given in the side scan sonar surveying which proved to be more efficient in the detection of the wreck site when the transducer towed close to the seafloor, transmitting high frequencies with high incidence angles. These techniques also provided information for the state of the shipwreck and triggered possible scenarios for the time of the wrecking. Furthermore they provided data for changes of the seafloor after the wrecking. In the present study area the reef of the wreck seems that increased the sedimentation at its lee side. In addition, the detection of anchoring or trawling marks on the surround seafloor reinforces the necessity of taking measures to ensure the protection and conservation of the site.

study of an ancient shipwreck in kefalonia island (ionian sea)

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Abstract In the present paper we present the marine remote surveying conducted aiming to investigate an ancient shipwreck offshore Kefalonia island, in Ionian Sea, Greece. Within the framework of the survey efforts were given to examine the site formation of the wreck

and to define the best practices for the detection and mapping of similar potential archaeological targets located on the shelf zone, where the geological and biological regime of the seafloor complicates the detection.

Keywords: ancient shipwreck, side scan sonar, sub-bottom profiler, multi beam, site formation, Greece.

References Brennan, M. L., Ballard, R. D., Roman, C., Bell, K. L. C., Buxton, B., Coleman, D. F., Inglis, G., Koyagasioglu, O., Turanli, T. 2012, Evaluation of the modern submarine landscape off southwestern Turkey through the documentation of ancient shipwreck sites. Continental Shelf Research 43, pp. 55-70. Delaporta, K., Jasinski, M. E., Soreide, F. 2006, The Greek-Norwegian Deep-Water Archaeological Survey. «The International Journal of Nautical Archaeology» 35,1, 79-87. Fakiris, E., Papatheodorou, G. 2007, Calibration of textural analysis parameters towards a valid side-scan sonar imagery classification. In 2nd International Conference & Exhibition on Underwater Acoustic Measurements, Crete, Greece, J. S. Papadakis and L. Bjørnø, (Eds.), pp. 1253-1264. Fakiris, E., Papatheodorou, G. 2009, Sonar Class: A matlab toolbox for the classification of side scan sonar imagery, using local textural and reverberational characteristics, in 3rd International Conference & Exhibition on Underwater Acoustic Measurements, Nafplion, Greece, J. S. Papadakis and L. Bjørnø, (Eds.), pp. 1445-1450. Fakiris, E., Papatheodorou, G. 2012, Quantification of regions of interest in swath sonar backscatter images using grey-level and shape geometry descriptives - The TargAn software, «Marine Geophysical Research» 33, 2, pp. 169-183. Georgiadis M., Papatheodorou G., Tzanatos E., Geraga, M., Ramfos, A., Koutsikopoulos C., Ferentinos G. 2009, Coralligène formations in the eastern Mediterranean Sea: Morphology, distribution, mapping and relation to fisheries in the southern Aegean Sea (Greece) based on high-resolution acoustics, «Journal of Experimental Marine Biology and Ecology» 368, pp. 44-58. John, G., Langley H., P. 1995, Estimating Continuous Distributions in Bayesian Classifiers in Eleventh Conference on Uncertainty in Artificial Intelligence, San Mateo, pp. 338-345. Martin, C. 2011. Wreck site formation processes in A. Catsambis, B. Ford & D. L. Hamilton (Eds.) «The Oxford handbook of maritime archaeology», Oxford and New York, Oxford University Press. pp. 47-67. Muckelroy, K., 1978, «Maritime Archaeology», Cambridge University Press, uk, Unesco. Papatheodorou, G., Geraga, M., Ferentinos, G., 2005, The Navarino Naval Battle site, Greece - an integrated remote sensing survey and a rational management approach, «International Journal of Nautical Archaeology» 34, 1, pp. 95-109. Plets, R. M. K., Dix, J. K., Adams, J. R., Bull, J. M., Henstock, T. J., Gutowski, M., Best, A. I., 2009, The use of a high-resolution 3D Chirp sub-bottom profiler for the reconstruction of the shallow water archaeological site of the Grace Dieu (1439), River Hamble, UK, «Journal of Archaeological Science» 36, pp. 408-418.

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Quinn, R., Boland, D., 2010, The role of time-lapse bathymetric surveys in assessing morphological change at shipwreck sites, «Journal of Archaeological Science» 37, pp. 2938-2946. Quinn, R., Bull, J. M., Dix, J. K., 1997. Imaging wooden artefacts using Chirp sources, «Archaeological Prospection» 4, pp. 25-35. Quinn, R., Breen, C., Forsythe, W., Barton, K., Rooney, S., O’Hara, D. 2002, Integrated Geophysical Surveys of the French Frigate La Surveillante (1797), Bantry Bay, Co. Cork, Ireland, «The Journal of Archaeological Science», 29, pp. 413-422. Sakellariou, D., Kourkoumelis, D., Georgiou, P., Micha, P., Mallios, A., Theodoulou, T., Kapsimalis, V., Dellaporta, K., 2007, Searching for ancient shipwrecks in the Aegean Sea: The discovery of Chios and Kythnos Hellenistic wrecks with the use of marine geologicalgeophysical methods. International Journal of Nautical Archaeology. 36.2, pp. 365-381. Sumer, B. M., Whitehouse, R. J. S., Torum, A., 2001, Scour around coastal structures: a summary of recent research, «Coastal Engineering» 44, pp. 153-190. Wachsmann, S., 2011, Deep-Submergence Archaeology in The Oxford Handbook of Marine Archaeology, A. Catsambis, B. Ford and D. Hamilton, (Eds.), New York, Oxford University Press. pp. 202-231.

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