ELECTROMAGNETIC SURVEY OF A CELTIC TUMULUS

This paper presents the results of an electromagnetic survey on a Celt ic tumulus (prince's grave) in the Canton of Fribourg, Switzerland. Th e aim of the project is to investigate the application of the electrom agnetic tool EM-31 for identifying anomalies of potential archaeologic al significance below an uneven surface. As is known from the explorat ion of other tumuli as well as from a partial investigation of the Boi s de Moncor tumulus itself in 1983, the archaeological structures can be expected to consist of loose layers of quartzite rocks arranged to form low narrow walls and covering the tomb chamber. A ground conducti vity survey was carried out over the tumulus and surroundings. The con ductivity map of the raw data has shown the importance of correcting t he measurements because of different systematic and casual errors. Two very low conductivity zones were determined in a second conductivity survey on the top of the tumulus with the instrument lying on the grou nd. Further investigation of one of these zones has shown a deep burie d object of high resistivity and of limited extent. Neither conductivi ty readings using the four different coil configurations (broadside an d inside, vertical and horizontal mode) nor the differences between th e measurements made with two coil configurations showed any visible an omaly over a known buried stone wall. Finally, the conductivity survey has shown that the EM-31 is particularly suited for locating refills of ditches and trenches. A circular zone of high conductivity surround ing the tumulus most probably represents a refilled ditch such as is k nown from other tumuli. An attempt to correct the conductivity anomali es for variations in topography was unsuccessful.