"ARCHEOEGADI": A GIS FOR THE MARINE
ARCHAEOLOGICAL SURVEY IN THE EGADI ISLANDS

Roberto Ialuna, Daniele Gravili

CEOM S.C.p.A. (Centro Oceanologico Mediterraneo) - Gruppo ENI, Palermo (IT)

1. INTRODUCTION
The underwater archaeological research is becoming every day more interesting and current, both for the possibility to find archaeological and historical data and the importance of modern techniques and instrumentation use.
Up to now marine archaeological surveys have been carried out with traditional methodology, using direct observation from divers in shallow waters or mini-submarines (AUV/ROV) in deep waters.
In the last years it has become common the use of equipment derived from geophysical surveys for scientific purposes, above all, in the offshore operation (e.g. laying and installation of submarine pipelines). Moreover, GIS tools are starting to be used for their essential role in planning and managing purposes.
Which is the goal using this kind of tools? Firstly because the main part of the archaeological world heritage is under the sea bottom. It means that the environment can create an obstacle to the direct target vision. In addition such instrumentation provides to cover very large areas in a short time and to be able to operate in deep waters, avoiding the use of submarines. In this way we can obtain a reduction of management costs compared to surveys realised with traditional visual methods only. However the use of visual inspection methods remains always indispensable, but addressed to spot checks of suspected targets only.
The most important step in successfully locating an archaeological target is the definition of the search box, the area where it's most likely to be found. Establishment of a target area needs the aggregation and comparison of information from multiple sources.
In this context the use of a GIS permits, e.g., the control of the navigation, to verify the acquired data-set and to check the coverage of the whole searching area. Moreover it also provides to aggregate, analyse and compare data, allowing to improve the confidence level in identifying archaeological targets and planning fast visual inspections.
An example in using such "non conventional" equipment and GIS system for archaeological surveys has been realised in the context of an agreement between the Cultural and Environmental Sicilian Regional Authority and CEOM (CEntro Oceanologico Mediterraneo). A Feasibility Study entitled " Research and recovering project of the archaeological heritage to realise marine archaeological routes in the Western Sicily" has been developed.
Along June 2000 to March 2001, the "ArcheoEgadi" campaign took place in the marine area of Egadi Islands (western coast of Sicily). This site was the main scene of a naval battle between Romans and Phoenicians and it is well placed to be an important centre for the trading routes in the Mediterranean Sea.
Main results achieved in "ArcheoEgadi" were a detailed map of the sea bottom to verify the application of these technologies in the archaeological field and a setup of parameters to detect the archaeological sites hidden by sediments and marine flora on the sea bottom. In particular a relevant result was the discovery of the wreck of a XI century Arabic vessel probably sank next to the Bull's Shoal during a storm. The wreck is about 25 m length and 5 m large, it is in a very good state and is located at about 70 meters of water depth.

Figure 1: Survey area

2. CONTEXT AND OBJECTIVES
The ARCHEOEGADI GIS is an application developed within a project realised by CEOM and Sicilian Regional Authority to verify the possibilities and limits in the use of elettroacoustic and magnetometric equipment normally used for geological and geophysical surveys on large areas.
In particular CEOM provided the co-operation of his geophysicists and engineers with geophysics and magnetometric equipment and with the support of GIS tools while S.C.R.A.S. (Servizio per il Coordinamento delle Ricerche Archeologiche Sottomarine of Sicilian Regional Authority) operated visual inspections.
The main objectives of the project were:

1. To assess the effectiveness of the use of GIS tools in planning and organisation of marine archaeological surveys;
2. To assess if the modern geophysical equipment is able to give information about the sea bottom morphology and bathimetry with the necessary resolution to find possible archaeological targets
3. To realise an informative system to permit both the visualisation and the cartographic production of data and to use specific tools for the analysis of the various informative levels.

The project was developed in 4 phases:

Planning (March - June 2000)
The first phase of the research was based on the study of historical data to find information about possible archaeological targets and to define the searching area. In this context the use of GIS tools permitted: the organisation and aggregation, in a geographic environment, of information from different and heterogeneous sources and therefore the planning of the underwater survey.

Survey and processing data (June - July 2000)
Once defined the searching areas (about 40 km2 around Favignana and Levanzo islands) and planned the campaign, the characteristics of the seafloor have been surveyed using acoustic and magnetic equipment. In particular, it has been used a Side Scan Sonar for geomorphologic relieves, a Sub Bottom profiler for stratigraphic analysis, a Multibeam Echosounder for bathymetric relieves and finally a differential gradiometer for differential magnetometric relieves.
At the end of this phase the GIS permitted to identify possible targets which could hide archaeological sites.

Visual inspections (December 2000)
On the basis of the GIS analysis it was possible to realise visual inspections using a class ACHILLES ROV (Remoted Operate Vehicle) equipped with a camera.

Managing data and cartography (January - March 2001)
All the data were inserted in the GIS "ArcheoEgadi" used for the production of thematic charts to evidence the archaeological presences and every information on the geomorphologic, stratigraphic, bathymetric and magnetic characteristics of the Egadi Islands area.

3. TOOLS
Bathimetry: designated areas were mapped with a SIMRAD EM3000 high frequency Multibeam Echosounder to ensure the highest possible resolution. Multiple swaths are collected across the feature to provide 100 % coverage and a minimum of 30% overlap. The multibeam data is collected using Merlin, then cleaned, processed and displayed with Neptune and exported, with Cfloor, in GIS format.

Morpho-stratigraphy: morpho-stratigraphic data were acquired with a DATASONIC SIS-1000, which is a fully integrated Seafloor Imaging sonar System that uses advanced Chirp technology to produce high resolution side scan sonar images and sub bottom profiles. All the imagery data were collected and processed using Triton ISIS Sw and mapped to support mosaic generation of the seafloor morphology. Mosaics are generated using DelphMap Sw after all sonar data has been collected and integrated with differential GPS data (which tracks the surface vessel's position and, by extension, the position of the side scan sonar vehicle). Next the mosaics are also imported in ArcView and plotted at the required scale for determining areas of interest and high importance
Magnetometry: The instrument used for the magnetometric survey is a magnetic marine horizontal transverse gradiometer, a G-881 GEOMETRICS. The gradiometer uses a couple of G-881 synchronised magnetometers. The resultant low noise output characteristics improve the efficiency in detection of targets under silky or sandy bottoms.
GIS: The GIS "ArcheoEgadi" performed to manage, analyse and present the geographic and descriptive data acquired during bathymetric and geomorphologic surveys was made up using the ESRI's geographic information system software ArcView 3.2a. The ArcView programming language Avenue and the Dialog Designer extension were used to personalise the user interface. The spatial analyse was performed with the ArcView Spatial Analyst and 3D Analyst extensions. Finally, the database Oracle 8 was used to manage the descriptive information acquired during the project (in particular historic and bibliographic information).
Next picture shows the architecture of the system. It's possible to identify:

• the acoustic and magnetic equipment used during the survey;
• the positioning system based on a Differential GPS which permits a precise georeferencing of the data;
• the Data Acquisition System to receive and archive data;
• the GIS used to process, manage and display data information acquired and to produce thematic maps.

Figure 2: ARCHEOEGADI architecture

4. FUNCTIONALITY AND EXAMPLES
One of the main characteristics in a GIS is the large availability of tools which permit to link documents by different nature (tables of a database, films, layouts) and geographic entities represented in a graphic manner (SSS and Multibeam navigation, ROV lines, immersion points) making an integrated informative system.
ARCHEOEGADI is an example of this ability. In particular, it is characterised from the following functions:

• the automatic and interactive management of the electro-acoustic and magnetic information acquired during the survey and of the recorded videos ;
• the automatic and interactive management of the cartography;
• the possibility to make queries and analysis with a high level of detail;
• the exporting and printing of cartographic data in the most common formats.

Picture 3 shows the user interface which ARCHEOEGADI gives to access to the data simply by clicking the mouse.

Figure 3: Example of ARCHEOEGADI's User Interface

5. CONCLUSION
A GIS can greatly assist in the planning of an at-sea operation, reducing costs, avoiding hazardous consequences, and ensuring that an area has been thoroughly covered.
ESRI's GIS software suite, in concert with combined geophysical, magnetic and geological methodologies, can constitute an integrated systems approach to mapping seafloor features, providing an efficient and economical way to image the seafloor and producing data which can be used to address many problems associated with deepwater search, salvage and marine archaeology.
In our project this approach has permitted the discovery of several archaeological targets, the most important of these is the wreck of a Sicilian-Arabic ship of the XI century sank about 2 miles South of Favignana Island.

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References

1. AA.VV.: "SIS1000 System Manual". Datasonics, July 1998.
2. AA.VV.: "EM3000 System Manual". Simrad, March 1998.
3. AA.VV.: "GG-881 System Manual". Geometrics, June 1995.
4. ARCNEWS - ESRI International User Conference Issue - Vol.24 No. 1 - Spring 2002;
5. ARCNEWS - ESRI International User Conference Issue - Vol.24 No. 2 - Summer 2002;
6. A. DE DOMINICIS ROTONDI: Principi di elettroacustica subacquea". Edai, 1990.
7. J.P. FISH: "Sound Underwater Images: a guide to the generation and interpretation of Side Scan Sonar Data". Lower Cape Pu. Co., November 1990.
8. MORANG, R. LARSON AND L. GORMAN: Monitoring the coastal environment; Part III. Geophysical and research methods. Journal of Coastal research, 13(4), pp. 1064-1085.
9. S. TUSA, G. LINO, S. ZANGARA, D. GRAVILI, A. URBANO: Acoustical Underwater Researches In The Egadi Islands. Acts of the 4° Colloque d'Archéométrie "ARCHEOMETRIE 2001", La Rochelle, France