Archaeological Prospection of Iron Age Migration Period ringforts in Sweden, GPR and LiDAR detect structures inside the ringforts, Elemental analysis of soil samples indicate activity areas

This article presents the Stockholm Volterra Project and its developments since 2013. This project, run by Stockholm University and the Department of Archaeology and Classical Studies, has carried out geophysical prospections in Volterra in collaboration with Soprintendenza Archeologia, Belle Arti e Paesaggio per le province di Pisa e Livorno. The aims and methods of the project are outlined together with a closer presentation of key sites from 2014 and 2015: the ‘Football Pitch’, the area in front of the church of San Giusto, the ruined church of Santo Stefano, the amphitheatre and Ortino sites.

In September 2016 geophysical surveys were conducted at Korshamn, as one of the main harbour bays of the island of Bjorko, situated outside the town boundaries of the Viking town of Birka. The investigation of a solitarily raised plateau at Erik Steffanssons hemland revealed the outline of a large Vendel-period house. Together with further anomalies at a one-sided terrace at Kalvhagen a whole manor complex might be seizable predating the Viking-Age settlement activities on the island. The latter dwelling is superimposed by a major Viking-Age hall connecting to a fenced special area as known from e.g. Lejre and Tisso and linked to cult activities. Both the structures and the chronological depth correlate well with the ancestral property of Birka's royal bailiff Herigar as mentioned in Rimbert's Vita Anskarii. If this assumption is correct even the whereabouts of Scandinavia's first church should be located in the immediate vicinity. The consequences of this identification cannot be overestimated: In terms of the emergence of the Viking town, its royal administration and the earliest Christian mission in Scandinavia.

In 2007 a magnetometer survey was carried out at the ring fort Gråborg on the Swedish island of Öland. The results were interpreted as indicating the remains of over 60 buildings, several roads, wells and a large moat outside the fort's northwestern gate. In 2011 these interpretations were severely criticised, and it was suggested that the moatlike anomaly in the geophysical data had actually been caused by a lightning strike. It was also suggested that none of the other interpreted features were actually supported by the presented magnetometry data.

This paper presents the results of a groundpenetrating radar (GPR) survey of the same area. The GPR data were collected in 2014 using themultiantenna Malå Imaging Radar Array (MIRA) system, covering an area of approximately 3.8 ha. The results show that the ground inside and outside the fort's walls is heavily disturbed by farming. Most of the underground features visible in the data can be interpreted as drainage ditches and power cables, but a fewlinear features are identified as being of possible archaeological interest. When comparing the radar data to the buildings, roads andwells suggested in the magnetometry interpretation, no apparent correlation can be established. There is furthermore no sign of any moat in the suggested area. The GPR results therefore support the idea that this moatlike feature is indeed the remains of a lightning strike.

This article aims at providing evidence for the usefulness of combining data from both above and below the ground in order to provide a more complete understanding of an archaeological site. For this purpose a Ground-penetrating radar (GPR) survey was carried out next to the standing ruins of the west tower of the church of St. Lawrence in Sigtuna, Sweden. The tower ruins were also documented using photogrammetry providing an accurate 3D-model of the site. The result of the GPR survey clearly images the buried wall foundations of the church but it is only when this data is combined with the photogrammetric 3D-model of the tower ruins that the spatial layout becomes complete. The results clearly provide evidence of the benefits of using such an integrated approach. The available evidence suggests that the tower, nave and choir (with a possible apse) were constructed during the 12th century. During the 15th century the church porch was built and arches added to the nave. The building history of the church is thus rather ordinary compared to other contemporary Swedish churches and, as a consequence, it is likely that that the church was built for the city congregation.