Comparative application of gravimetry ground penetrating radar and high-resolution seismic reflection surveys for the detection of shallow archaelogical vestiges

The city of Bavay, in the North of France, has been, since the Roman Empire, an important place for trade, and the main crossroads before the Boulogne’s harbour. Its population was, at that time, mainly composed of rich traders and of a garrison force. Bavay had an extended waterworks system that was supplied by a 30 kilometers long underground aqueduct built in 96 A.C. This is one of the only monuments of that kind known in the North of France. The location map of this aqueduct is shown in Fig.1 The little part of this monument that has been discovered showed that the aqueduct was constituted of a brick and stone masonry. Its section is approximatively 1 m². It is burried at depth varying from 1 to 2 meters in a clay layer which is followed by limestone. A cross-section of it is shown in Fig.2 The goal of this paper is to check the possibilty of the detection and the localisation of the aqueduct. Between the various geophysical methods, three methods were chosen in order to test their availability for the aqueduct detection: gravity, ground penetrating radar (GPR) and high resolution seismic reflection (HRS). We point out that the tested methods are non destructive methods, compared with the classical commonly used destructive archaelogical exploration. Other methods like electro-magnetics, electrical coring were tested in the past and did not give any promising results. The experiments were conducted in the site where the position of the aqueduct was partially suspected. The test site was chosen as a 8 x 12 m rectangular area (Fig.3).