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

There is a growing demand for nondestructive geophysical investigation in a rchaeology, especially in an urban context. This is a result of taking our heritage more seriously than in the past. Since excavations are possible on ly over a very limited area, any a priori information brought by geophysica l methods can help to focus these excavations. The classical geophysical me thods 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 x 1.3 m electrostatic quadrupole was towed continuously over a set of parallel profiles to produce a resistivity map for a 20 x 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 for mer buildings. A set of 450-MHz GPR profiles were collected and common midpoint (CMP) soun dings 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 resistiv ity map. Two different physical properties are measured (electrical resisti vity and a reflectivity coefficient that is mainly a function of the contra st 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 tw o sets of data enables us to infer the location and geometry of the Romanes que building which stood previously on the site of the present cathedral of Girona. Excavations support the interpretation.