GPR profiling and electrical resistivity tomography for buried cavity detection: A test site at the abbaye de l´Ouye (France)

The abbaye de l´Ouye (France) presents an underground room situated under a flat graveled path perfect for testing the complementarity of two geophysical methods, ground-penetrating radar (GPR) and electrical resistivity tomography (ERT), for buried cavity detection. One GPR mono-offset profile was acquired along with a 2D ERT profile on the surface above the cavity of known dimensions. We use our field site to test 2D ERT data acquisition and inversion parameters and how to complete GPR data information with ERT and vice-versa. As expected the mono-offset GPR profile contains strong reflections on the ceiling and the floor of the room. Arrival times of those reflections are translated to depth using classical GPR data processing techniques. The measured electrical apparent resistivities were inverted using the Loke (2001) software with three different options: (i) no a priori information, (ii) a priori information included as high resistivity value in an area determined from the GPR reflections, (iii) a priori information included as boundary positions derived from the GPR reflections.