oa Forward modeling contribution in the GPR signatures interpretation of urban heterogeneities

BRGM and the french Projet National Microtunnel funded a research project where Ground Penetrating Radar (GPR) was used to characterize the test-site of the Laboratoire Central des Ponts et Chaussées (France). The objective of this project was to find a method to better interpret the radar signatures by combining data processing to forward modeling, especially in the case of urban heterogeneities. We focused a large part of our work to adapt a sufficient quick and accurate algorithm to be efficient in industrial applications. Modeling algorithm, adapted from radar waves propagation theory in 2D heterogeneous dispersive media, is based on the upward extrapolation of a downgoing field in the frequency-wavenumber domain. Like for other modeling techniques such as ray-tracing (Goodman, 1994; Cai and McMechan, 1995; Powers, 1995), FDTD (Moghaddams et aL, 1991; Roberts, 1994) or pseudospectral methods (Carcione, 1996; Casper and Kung, 1996) some compromises have been introduced and must be kept in mind. In the following, we will first recall the modeling method, and after an example description from the LCPC test site, where real processed data are compared to synthetics, we will insist on the advantages and the weakness of the method.