Keywords: georadar, archaeology, excavation, testing, mapping, fortification structures, buried ditch
IntroductionThe Conservation Department of the Institute of Archaeology, RAS invited a group of the experts from IZMIRAN for the inspection of the excavation area of ancient settlement Nastasjino, located near Kolomna in Moscow Region. The excavation was carried out at the site of planned construction of a highway. By archaeological estimations, the age of ancient settlement was 2.5 thousand years. This period belongs to the epoch of early iron century. Human settlements of that time, as a rule, were surrounded by earthen fortifications for defense from militant forays of the neighbours. According to the archaeological assumptions, there were probably earthen ditches around the excavation buried by centuries-old agricultural activity. This version has been confirmed by an aerial photograph, on which concentric strips allocated by more saturated colour of the vegetation are distinctly visible (Fig.1). Archaeologists have suggested to investigate the northern sector of the territory with the help of GPR try, in order to find out the borders of buried ditches and to make a map of the underground structures.
- Working frequency range 30 - 500 MHz.
- Pulse voltage on the transmitter's output 5 kV.
- Receiver sensitivity 75 mkV.
- Time resolution 1 ns.
Analysis of the resultsTwo-layer geological structure, with an interface at the depth about one meter, was registered in all georadar scans executed across the excavation. The interface between two layers fell below this level on each GPR profile only in two places. The areas of the border descent, according to a radar-tracking image, were similar to a buried ditch. The sections of the "ditches" putted on the excavation scheme form sectors of circular strips with the width 5-6 meters and the radius from the settlement centre 20 meters, for the internal, and 30 meters for the external ditch. Preliminary analysis has given us a base to assume that the border registered by the georadar divides the upper cultural layer and the lower continental one, (according to archaeological terminology). The areas of the interface descent evidently correspond to the bottom of buried earthen fortifications ditches. This courageous assumption was confirmed during the subsequent excavation. (Fig. 3). The structure of the buried ditch with the depth of 1.7 meters from the present earth surface is well visible in the photograph. The bottom contour is distinguished by a layer of the condensed clay. It is possible to assume that "the ancient builder" laid these clay locks for water accumulation in the ditches. This trick of the ancient builder keeps its properties after 2.5 thousand years preserving the increased humidity of the ground which is detected by the vegetation and by GPR sounding. According to the results of georadar investigations and the excavation data, an artist has reconstructed the relief of this area and settlement constructions (Fig. 4). By the way, near the external ditch the archaeologists found a big quantity of arrowheads and lance tips during the excavation. All the arrowheads and lance tips had similar direction - from northeast to southwest - towards the fortress wall. This fact convincingly supports the above hypothes and our interpretation of the georadar data: the found objects were buried ditches, and the space between them was the base of earthen bulwarks.
ConsulationThe results of GPR study of an archaeological excavation have shown that the georadiolocation can be useful for detection of such low contrasting interface, aged by several thousand years. The GPR application to archaeological research will essentially reduce time and volume of the excavation. The authors express their gratitude to the employees of the Conservation Department and Moscow Region archaeological expedition of the Institute of Archaeology RAS for supplying the experimental data and for the opportunity to test the georadar on an archaeological excavation.
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Fig. 1. Aerial photograph of the excavation area of ancient settlement Nastasjino
Fig. 2. Results of GPR inspection of the area around the excavation of settlement Nastasjino (4-3 centuries BC), Kolomna district, Moscow
Fig. 3. Excavation of buried ditch.
Fig. 4. Reconstruction of the relief near ancient settlement Nastasjino