Magnetic imaging of a late Bronze Age tumulus in France before and during excavation

Abstract

Geophysical surveying is taking place in the Chatillon-sur-Seine area in France to examine and map prehistoric settlements and the structure of tumuli (grave-mounds which originally reached a height of up to 5 m but which are now almost level). The magnetic survey discussed here was conducted to detect archaeological structures within a late La Tene (100-50 bce) necropolis, in particular the preexcavation recording of a late Bronze Age round barrow. During the excavation, additional surveys were carried out to analyze the influence of immediate subsurface soil layers on the magnetic anomalies originating from the deeper archaeological features. Additional radar surveys made use of a 500 MHz antenna and a SIR-2 system from GSSI which clearly showed the archaeological structures, but the results presented here will focus on the magnetic data. Two family graves within a larger late La Tene necropolis dating to between 300 and 75 bce were archaeologically examined in 1967. The objective of the present geomagnetic survey was to find more graves and record the structure of the whole necropolis. A Geoscan FM36 gradiometer was used, the upper probe being at a distance of 0.9 m from the surface, the lower probe at 0.4 m. The use of a gradiometer is preferable in order to reduce or cancel the effects of background noise and of metallic objects near the area of investigation. Most archaeological anomalies hardly achieve a value of more than 10 nanotesla (nT); therefore, sample and traverse intervals of 0.25 m were chosen …