Different ground-penetrating radar (GFR) techniques have been tested on the same site in order to establish the performance and reliability of this method when applied to civil-engineering problems. The Laboratoire Central des Fonts et Chaussees (LCPC) test site at Nantes, France, was selected because it includes most of the underground heterogeneities commonly found in urban contexts, such as pipes, small voids, etc. The GPR survey consisted in recording measurements in tomographic (surface to horizontal borehole measurements), monostatic (2D surface profiling) and bistatic (Common Mid Point [CMP]) modes above various buried heterogeneities. Different processing techniques were also performed, such as tomographic inversion, 2D and 3D migration, velocity analysis, as well as numerical simulations, the results of which can be summarized in three points. (1) Although the different filling materials of the site can be distinguished by velocity and attenuation tomography, the buried heterogeneities are more difficult to identify because of limited resolution related to angular aperture and Fresnel zone. (2) 2D surface profiling can detect the different shallow heterogeneities, such as pipes and voids, down to a depth of several meters. Additional processing. such as forward modeling and attenuation curve analysis, provides more quantitative information related to the medium. A comparison between 2D and 3D migrated data highlights the error introduced when the structures are considered to be perfectly cylindrical. (3) CIC LP analysis gives relatively good estimations of vertical velocity contrasts when the medium is layered. A lithologic log can be derived assuming that the velocity changes are related to material variations. (C) 2000 Elsevier Science B.V. All rights: reserved.
