Quantum Systems

In 2013 large scale magnetic prospection was carried out in Franconia, Germany, to reveal remains of a canal construction and corresponding infrastructure built by order of Charlemagne in the Early Middle Ages. The extended canal structures of the so-called Fossa Carolina were expected within a part of the main European watershed between Altmühl and Rezat – tributary streams of Rhine/Main and Danube. The construction of a first navigable continuous waterway between the latter seems to be forced by Charlemagne’s military or economic strategy. Tremendous feat of engineering as well as enormous human and material resources were required. The realized geophysical investigations are part of an ongoing multidisciplinary geo-archaeological research project embedded into the German Research Foundation (DFG) Priority Programme Harbours from the Roman Period to the Middle Ages (SPP 1630). The Fossa Carolina project combines geophysics, physical geography, archaeology, history and archaeometry to contribute to fundamental questions like: What was the entire extension and hydro-engineering concept of the canal? Has it been finished and put into operation? Which structures in the hinterland belong to the building? The site has been prospected magnetically on a large scale by means of a SQUID (Superconducting Quantum Interference Device) measurement system [1]. The motorized, fast and high-sensitive system was used at a very early stage of the project to get data sets of the entire archaeological site as the basis for subsequent geophysical and geo-archaeological investigation. Thus, an area of more than 120 hectares was mapped in the vicinity of the last visible remains of the Fossa Carolina as well as in a greater distance. Most of the area was measured deep in winter of February 2013. The frozen and snow-covered surface soil enabled the access to the partially intensively used farmland. The precisely georeferenced magnetic data were used in combination with topographic information and sediment analysis of well-positioned drillings [2] to determine a promising position for a first excavation in summer 2013.This work led to the remarkable finding of a 5.3 m wide canal structure flanked by axed oak piles which were dated precisely to the year 793 AD [3]. The canal sole was found in a depth of 3 m referenced to the present-day surface. The further analysis of the immense SQUID data - which show a variety of magnetic signatures [4] [5] – was focussed on an extended linear anomaly corresponding to the canal course to the north, i.e. to the confluence with the Rezat river, see figure 1. Calculations of the magnetic source layers on the base of the magnetic information from three different SQUID gradiometers and a polyhedral inversion algorithm [6] revealed a significant change of the canal sole geometry (figure 2) hundreds of meters apart from the first excavation position. The width of the canal sole is decreasing by a factor of two within a distance of about 60 meters. This discovery was the base of a second excavation campaign carried out in summer 2016, see figure 3. The calculated sole geometries were used to select two ideal excavation positions and to estimate the required time and digging effort. The geo-archaeological results which have fully verified the magnetic inversion models will be presented and discussed. The excavations revealed canal sole layers with high organic proportion (figure 3) and an increased magnetic susceptibility which has been predicted by the inversion of the SQUID magnetic data. The calculated canal cross sections will be compared further with data of linear geophysical measurements like seismic SH-wave refraction and electrical resistivity tomography.