Zastosowanie metod akustycznych i magnetycznych w poszukiwaniu obiektów archeologicznych na przykładzie jeziora Lednica

Noninvasive surveys and prospections are a new dynamic trend in contemporary archaeology. They represent a trend of multidisciplinary analyses of cultural heritage. Due to limited access to technology, these methods are still not widely used in underwater archaeology, in particular in inland reservoirs. For this reason a team from the Department of Underwater Archaeology of the Institute of Archaeology at Nicolaus Copernicus University in Toruń in cooperation with the Maritime Institute in Gdańsk, the Museum of the First Piasts at Lednica and the Scientific Association of Polish Archaeologists (Warsaw branch), planned and completed a project Kolebka Piastów — archeologiczne prospekcje podwodne w rejonie jeziora Lednickiego in 2017. The project was financed by the Ministry of Culture and National Heritage under the following programme: Cultural heritage, priority: The protection of archaeological monuments. In the course of the project a multibeam probe, a subbottom profiler and a magnetometer were used to conduct surveys. If the conditions and assumptions of the project allowed, a direct underwater verification was conducted. Due to multibeam sonar it is possible to obtain a reliable spatial picture of a lake bottom in high resolution, e.g. 100 or more measurement points for each m2 of the bottom, which enables one to conduct a detailed overview of a surface of the research area and objects lying on it. This method turned out to be the most effective during verification and resulted in the localization of a new medieval bridge in Lednica lake. Using a subbottom sediment profiler is at the moment the only noninvasive method of searching for non-magnetic objects sunk into the bottom. After applying it on a recognized object, such as the Poznań bridge, and after receiving the positive results, it may be assumed — with high probability — that this technology will succeed in the search for other wooden archaeological structures located in subbottom sediments. Magnetometric measurements are the next technology which was used in the researched reservoir. They are indispensable when noninvasive large scale searches of metal objects with magnetic signatures are conducted e.g. the objects made of iron or steel. In the area of Lednica lake several objects with a magnetic signature were localized. In the course of research in Lednica, three types of noninvasive prospections were applied: hydroacoustics (a multibeam probe), seismology (subbottom sediment profiler) and magnetics (Caesium magnetometer). Each of these methods helps to localize other objects and gives very interesting results; however, only after all three have been used is it possible to obtain a precise picture of the bottom of the lake together with anomalies, which to a large extent can have anthropogenic origins. It seems that the future of underwater archaeology is closely related to the described noninvasive surveys. Unfortunately it will be more difficult to achieve in shallow inland reservoirs than in sea waters, where these technologies have already been introduced for some time now.