LÖDÖSEHUS: GEOPHYSICAL METHODS FOR DESCRIBING ARCHAEOLOGY AND NEAR-SURFACE GEOLOGY AT THE SITE OF A MEDIEVAL CASTLE IN LÖDÖSE, SW SWEDEN

Abstract

Near-surface geophysics is a well established method for mapping the geological conditions in the subsurface. The use of geophysics is also a commonly used method in archaeological surveys, although traditionally not as much in Sweden. Ground-penetrating radar (GPR) is however gaining more popularity and is arguably the most used method in archaeology due to the amount of data that can be gathered in a short amount of time and with little effort. This study was done mostly with GPR together with a resistivity and induced polarization (IP) survey with the aim to compare the methods with each other. Data gathered with the GPR can be visualised in 2D and 3D and it is therefore of great interest to compare these data forms. The study area is located in Lödöse, SW Sweden, which was one of the most important cities in medieval Sweden. The survey presented in this thesis was done over the former courtyard and outer bailey of a castle with a triple moat system once called Lödösehus. Today there are no visible remains of this castle and only a small part of the outer bailey has been excavated. The 3D GPR data showed previously excavated structures along with two larger, previously undiscovered structures. The 2D GPR data showed reflections from two of the moats and identified anomalies in the 3D data as postholes and structures. The resistivity survey potentially identified an unexcavated moat and showed that the top 5 meters of the soil has unusually high resistivity values for clay. This indicates a high degree of leaching of salts or thick deposits of anthropogenic waste which could explain the exceptional penetration depth with the GPR in an area of clay. The IP survey showed an area of electrical chargeability in connection to the potential moat that was found in the resistivity survey, indicating the possibility of metallic objects in the area. This study has greatly increased the archaeological knowledge of Lödösehus and provides exact locations of previously excavated structures. Furthermore, new areas of interest have been identified for future excavation in order to learn more about the history of one of the largest cities in medieval Sweden.