P60 High Resolution Geophysics Inside Machado de Castro Museum - Coimbra, Centre Portugal

Transcription

1 P60 High Resolution Geophysics Inside Machado de Castro Museum - Coimbra, Centre Portugal C. Grangeia (University of Aveiro), M.J. Senos Matias* (University of Aveiro), F. Figueiredo (University of Coimbra), H. Hermozilha (University of Aveiro) & P. Carvalho (University of Coimbra) SUMMARY Geophysics has been used inside Machado de Castro Museum buildings (Coimbra, Portugal) to guide archeological surveys and to help building restoration. The high resolution geophysical work consisted on GPR and Resistivity survey. Data was gathered on the floor of the so called Renaissance room and on the Cryptoporticus wall, that is, perpendicular to the plane of the Renaissance room floor. Restoration works in old buildings is a sensitive task as they often lay over older buildings. Therefore, high accuracy is demanded to non invasive exploration works. Hence, the geophysical survey was designed in order to obtain very high accuracy. Both the geophysical data/interpretation and the archeological findings were organized in a GIS to provide a full picture of the area under investigation. Archeological excavations confirmed the geophysical findings as well as the accuracy obtained.

2 Introduction Geophysics can be used in the characterization of the inner structure of buildings of historical interest, masonry diagnosis, void and fracture detection and resolution to the cm or tens of cm is often required. Historical monuments are fragile but their cultural value is high. Thus the restoration of historical buildings has also been a promising application and development field for geophysical techniques. Frequently, historical buildings lay over more ancient constructions. Usually the extent, geometry and importance of these older structures are unknown. Thus restoration must take them into account and it might be necessary to carry out excavations inside the buildings. It is clear the importance of Geophysics in the guidance and strategic planning of such excavation works. Geophysical surveys inside historical buildings must consider several aspects different from operations in open field areas. Most of the measurements are done over the buildings structures and, therefore, they can not damage existing constructions. Furthermore, noise can be an important factor. Special care must be taken regarding seismic sources, electrode contacts, electrical current leaks, electromagnetic environmental noise, spurious GPR reflections from walls and ceilings, etc, (Geophysics for Cultural Heritage, 2002). The Machado de Castro Museum (Coimbra, central Portugal) occupies the former Bishop Palace constructed between the XII and the XVIII centuries. These buildings lay over roman remains constituting a cryptoporticus that supported part of the forum of the ancient roman city of Aeminium. Presently the buildings are going through large restoration works. Such works needed accurate knowledge of ancient structures, so that restoration would not deteriorate the actual buildings as well as the older so far concealed ones. Hence a geophysical high resolution survey was designed to investigate part of the building. This paper discusses a combined 3D high resolution resistivity and GPR survey carried out inside the Machado de Castro Museum. The survey was done in different levels of the building, that is, the floor of the Renaissance room and the adjacent Cryptoporticus southern wall. The results are integrated to provide a better image of the investigated area and the archeological excavation results are compared with the geophysical data interpretation. Geophysical survey The location of the geophysical survey is shown in Fig.1. The floor of the Renaissance room corresponds to the blue rectangle (A,B,C,D). This area has been surveyed with a GPR meandering acquisition mode using a shielded Ramac 500 MHz central frequency antenna. Figure 1: 3D view of the location of the geophysical survey.

3 Two different areas were surveyed on the Cryptoporticus southern wall: The red slices (Fig.1) correspond to the R0 and R4 GPR profiles carried out at different heights; on the other hand, the green slices show the dipole-dipole pseudosections (P9, P3 and P8). Renaissance room Geophysical data from the floor of Renaissance room are shown in Figs.2 and 3 together with the archeological findings. To illustrate the use of GPR only some selected results will be discussed. On the left of the Fig.2 the GPR timeslices (15.80 ns and ns) are shown. At the centre the archeological stratigraphy is displayed and, finally on the right a radargram is given for comparison. The ns timeslice corresponds to the region just above the layer 9, and the ns timeslice corresponds to the layer itself. The upper timeslice shows a high degree of phase homogeneity when compared with the irregular behavior of the deeper timeslice. This is explained by the ground homogeneity above layer 9, whilst the layer is composed by irregular shaped stones assembled in an irregular manner. The radargram on the right shows clearly events that are correlated with (top to bottom): layer 1, layer 3, layer 6, layer 9 and the undifferentiated material below layer 9. Figure 2: Renaissance room GPR data and stratigraphic archeological findings. A deeper timeslice (22.29 ns) is shown in Fig.3. At this travel time the shallower layers are not visible and the timeslice features are correlated with deeper large structures, that is, stone walls as revealed by the archeological excavations. Figure 3: Renaissance room GPR timeslice (22.29 ns) and stratigraphic archeological findings. The timeslice shows the general direction of the walls and, in some cases, the location of the limits of the walls. However, signature phase changes from wall to wall. These changes might be due to large heterogeneity of the medium and to the different depths of burial of the walls.

4 Cryptoporticus southern wall survey The geophysical data from the Cryptoporticus southern wall is shown on the Fig.4. This survey was intended to investigate the inner structure of the wall, and other structures that could be correlated with the geophysical survey carried out on the Renaissance room floor Figure 4: Geophysical data from the Cryptoporticus southern wall and archeological information. The resistivity survey was done at highs of 2.6 m (top), 1.6 m (centre) and 0.6 m (bottom). All the pseudosections were processed using Res2DInv and show the inner structure of the wall as well as, the signatures from the walls already located in the GPR timeslice in Fig. 3. There is a clear contrast between the resistive walls and the conductive ground. Thus, integration

5 was obtained. Resistivity decreases to the bottom as humidity increases (underground conditions). The radargrams show an energy decrease in agreement with resistivity data. Furthermore, a general overview of these radargrams is in close agreement with the pseudosections. Therefore the Cryptoporticus wall must have a first layer composed by two adjacent sublayers of regular stones, followed by a more conductive/less energy and undifferentiated filling material. A further inner layer similar to the first one can also be interpreted in the radargrams. Finally, a strong diffraction (red dot) is recorded over the edges of an interpreted wall. Conclusions The survey on the floor of the Renaissance room allowed the location of the archeological layers with centimetric precision. Timeslice analysis allowed the definition of a region immediately above an archeological layer and the layer itself (time lag 1.41 ns). The limits of the buried walls could also be drawn but phase differences were recorded possibly due to medium heterogeneities and to different depths of burial of the walls. The inner structure of the Cryptoporticus wall of successfully investigated. It was also possible to correlate the findings from the Renaissance room survey and the Cryptoporticus wall survey. Hence, the larger deeper structures have signatures in both surveys and a clearer overall image of the area was obtained. The geophysical data and the archeological findings were organized in a GIS project to provide overall data integration and interpretation, improve communication with archeologists, guide restoration works and, therefore, overall visualization. Hence, all the geophysical interpretation is fully supported and confirmed by the archeological data presently available, that is, the accuracy and resolution aimed for were fully achieved. Bibliography Geophysics for Cultural Heritage: Payoffs for Archaeology, EAGE Seminar 2002, Florence, Italy.