International Conference on GPR in Archaeology (Nara, February 2001)

GPR and ES Surveying for Assessment of the Remains under the Cathedral of Saint- Etienne d'Auxerre (France)

Michel Dabas

Harry Titus

During the summer of 1998 two non-destructive sensing projects were carried out in tandem in the cathedral of Saint Etienne in Auxerre, France . The first project employed measurements of Electrical Resistivity (ER) captured by an electrostatic system developed at the Centre de Recherches Géophysiques de Garchy. The second project employed a ground-penetrating radar system (GPR). The goal of the two coordinated projects was to search for information concerning eleventh-century Romanesque structures, and perhaps even earlier remains, that might exist beneath the paving of the nave and transept of the present Gothic cathedral.

The electrostatic sensing system was developed by CNRS-CRG and permits the measurement of ER in an area underlying an electrically isolated surface. The classic methods that assume an injection of current by metal electrodes are obviously not operable on these surfaces. In the case of a paved floor in a cathedral, current and potential are injected and measured by charging a pole that serves as a condenser. 6 poles permit two depth of investigation simultaneously. The acquired profiles are associated to form a map of apparent underground resistivities (reorganized on a grid of 0.2 x 0.2 m). The main clear anomaly was found crossing the nave.

A puleEkko 1000 GPR system was used with a frequency of 450 MHz. Profiles were taken at 20 cm intervals and 5cm on-line using a wheel-odometer. Time-slices were compurted between 0.30 and 1.70 meter. An ensemble of radar anomalies in the nave and aisles is perfectly well defined, especially in the zone between 0.5 and 1.1 m. The radar images confirm the majority of the anomalies detected using the electrostatic method. The voids in the side aisles are visible beginning with the first level, which confirms that they are located just below the paved surface. The main anomaly that crosses the nave is also visible at a depth of about 0.5m. In large part, the radar anomalies are located in the same areas as the electrostatic anomalies and therefore, confirm the positions of the anomalies detected by the ES method.

The plan of the gothic cathedral was scanned together with the plan of what remains of the underground Romanesque crypt. Then a third, measured plan of the nave piers was digitized and superposed on the original. Tomb slabs re-employed as side aisle paving were also located. Finally, the geophysical surveys were imported as color transparencies and positioned on the plans. All these plan were used for the final historical interpretation of the anomalies.

It is very likely that the prominent transverse anomaly is associated with the façade of the Romanesque cathedral rebuilt after the fire which is known to have occured in 1023. At 3 m thick, it is certainly large enough to be the lower section of an important enclosing wall. This façade would then have been the western terminus of the cathedral whose crypt is still extant. Moreover, it does not appear that the Romanesque cathedral could have had a projecting transept. If this is correct, the overall exterior dimensions of the eleventh-century cathedral can be projected at 81 m long and 27 m wide.

Merging the new data with historical material, the eleventh-century cathedral consisted of an axial western portal, a nave with side aisles, and a sanctuary with a single axial chapel. Towers over the chevet side aisles were added during the twelfth century. The area immediately east of the transverse element interpreted as the eleventh-century façade features a pattern of seemingly related rectilinear anomalies. The pattern suggests three possibilities. First, there could have been an internal tribune associated with the eleventh-century façade, although there is no mention of such an arrangement in historical sources. Second, the pattern could be associated with the axial tower of the Carolingian cathedral, mentioned in connection with Bishops Abbon and Gui in the ninth and tenth centuries, respectively. Third, it is possible that the 1023 fire did not completely destroy the pre-Romanesque cathedral, and that portions of its western section were retained and/or renovated when a new façade was constructed. Exploratory excavation would be necessary to confirm any of these possibilities.

In the early fourteenth century the Romanesque nave was replaced by a Gothic structure whose aisles connected with the thirteenth-century façade. Construction began in the south arm of the transept, and then along the exterior nave aisle walls.

Up to this point we have discussed the nature of anomalies and their possible associations with existing or documented structures. However, if we are correct in our assessment of the location of the eleventh-century façade, a second level of interpretation becomes possible. When the Gothic chevet was constructed between ca. 1215 and ca. 1240, the eleventh-century nave was retained and abutted the present eastern crossing piers.

In summary, this project has given us results in three important areas. First, it has shown the efficacy of the electrostatic prototype, which, in this case, yielded more evocative results than the radar technique. Second, important indications concerning the size and shape of pre-Gothic buildings were produced. It is very probable that the data indicate the exterior dimensions of the Romanesque cathedral, and locate its façade. Third, indications relating to the building history and use of the Gothic cathedral emerged from the study. Inhumation locations in the side aisles were clearly indicated, and it seems equally clear that there was not a significant program of burial in the nave proper. Relationships between the Romanesque and Gothic building programs are more speculative, but serve as working hypotheses for further study. These results clearly demonstrate the usefulness of non-destructive sensing techniques. They may be used to guide further study through excavation, but even if that step is not taken, they support new hypotheses about the history of the site.

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