Array GPR investigation of the cathedral of Uppsala

We present an example of a so-called array GPR investigation outside the Cathedral of Uppsala, Sweden. The aim of the investigation was to reveal historically interesting features in surroundings where excavations are not allowed. In the investigation, 17 different GPR antennae of the same frequency were used to obtain measurements in 16 parallel profiles simultaneously. When several separate transmitter and receiver antennae are combined into one single antenna array unit, exactly positioned parallel profiles are obtained, resulting in a seamless high-resolution 3D picture of the subsurface. Processing the radar data into the resulting images involves several steps, such as aligning traces, removing static shifts and matching the mean response. Radar data are merged with geometry data from a total station (used to track the position of the antenna array) and then gridded and migrated. In the Uppsala case presented here, the resulting pictures in the form of time slices gave archaeologists very valuable help in understanding the subsurface and mapping historical anomalies. The findings indicate former paths and early medieval streets, among other features.