Simultaneous use of electrostatic quadrupole and GPR in urban context: Investigation of the basement of the Cathedral of Girona (Catalunya, Spain)

Abstract

There is a growing demand for nondestructive geophysical investigation in archaeology, especially in an urban context. This is a result of taking our heritage more seriously than in the past. Since excavations are possible only over a very limited area, any a priori information brought by geophysical methods can help to focus these excavations. The classical geophysical methods used in archaeology (resistivity, magnetism) are not applicable in an urban context with problems of accessibility and inherent electromagnetic noise. The potential of the combined use of ground‐penetrating radar (GPR) and electrostatic (ES) quadrupole data is demonstrated in the investigation of the floor of the cathedral of Girona in northern Spain. A 1.3 × 1.3 m electrostatic quadrupole was towed continuously over a set of parallel profiles to produce a resistivity map for a 20 × 60 m area. A set of resistive anomalies corresponds with known structures (probably graves). The largest observed anomalies appear to be related to foundations of former buildings. A set of 450-MHz GPR profiles were collected and common midpoint (CMP) soundings were performed to convert from time to depth. The time slice centered at 14 ns (at 0.9-m depth) shows anomalies similar to those in the resistivity map. Two different physical properties are measured (electrical resistivity and a reflectivity coefficient that is mainly a function of the contrast in dielectric permittivity); both methods may be sensitive mainly to the water content in the volume under investigation. The improved confidence in an interpretation obtained by combining these two sets of data enables us to infer the location and geometry of the Romanesque building which stood previously on the site of the present cathedral of Girona. Excavations support the interpretation.