Interpretation of reflection attributes in a 3-D GPR survey at Vallee d'Ossau, western Pyrenees, France

To study the attributes of reflected radar waves, a 3-D ground-penetrating radar (GPR) data set was recorded, processed with ProMAX 3-D seismic proces sing software, and analyzed with Stratimagic 3-D seismic interpretation sof tware. The zone studied was an old glacial valley (Vallee d'Ossau, western Pyrenees) where fluvio-glacial deposits (8-10 m thick) overlay a karstic li mestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity v = 0.07 mins). Analysis of the isochron map of the bedrock surface reflec tor showed north-south undulations, corresponding to the displacement of th e old glacier, together with sharp discontinuities, interpreted as local fa ults or karstic zones. Amplitude variations along the reflector were interp reted as being from water saturation and the nearby north-south alignment o f springs at the surface. In the fluvio-glacial deposits, paths of old chan nels were also imaged using amplitude analysis. The radar-wave amplitude at tenuation was also characterized within the deposits by analyzing the avera ge absolute amplitude for different time windows. The strong amplitude atte nuations were interpreted as the result of water-saturated zones. As a conc lusion, we correlated the surface karstic springs with bedrock structures a nd followed the water path across the glacial sedimentary filling. The reco gnized features were Very consistent with a 2-D de resistivity profile made through the GPR data block.