Interpretation of reflection attributes in a 3-D GPR survey at Vallee d'Ossau, western Pyrenees, France
Résumé
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 processing software, and analyzed with Stratimagic 3-D seismic interpretation software. The zone studied was an old glacial valley (Vallee d'Ossau, western Pyrenees) where fluvio-glacial deposits (8-10 m thick) overlay a karstic limestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity v = 0.07 m/ns). Analysis of the isochron map of the bedrock surface reflector showed north-south undulations, corresponding to the displacement of the old glacier, together with sharp discontinuities, interpreted as local faults or karstic zones. Amplitude variations along the reflector were interpreted as being from water saturation and the nearby north-south alignment of springs at the surface. In the fluvio-glacial deposits, paths of old channels were also imaged using amplitude analysis. The radar-wave amplitude attenuation was also characterized within the deposits by analyzing the average absolute amplitude for different time windows. The strong amplitude attenuations were interpreted as the result of water-saturated zones. As a conclusion, we correlated the surface karstic springs with bedrock structures and followed the water path across the glacial sedimentary filling. The recognized features were very consistent with a 2-D dc resistivity profile made through the GPR data block. 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 processing software, and analyzed with Stratimagic 3-D seismic interpretation software. The zone studied was an old glacial valley (Vallee d'Ossau, western Pyrenees) where fluvio-glacial deposits (8-10 m thick) overlay a karstic limestone bedrock. Reflections were picked to 250 ns (9 m depth with velocity v = 0.07 m/ns). Analysis of the isochron map of the bedrock surface reflector showed north-south undulations, corresponding to the displacement of the old glacier, together with sharp discontinuities, interpreted as local faults or karstic zones. Amplitude variations along the reflector were interpreted as being from water saturation and the nearby north-south alignment of springs at the surface. In the fluvio-glacial deposits, paths of old channels were also imaged using amplitude analysis. The radar-wave amplitude attenuation was also characterized within the deposits by analyzing the average absolute amplitude for different time windows. The strong amplitude attenuations were interpreted as the result of water-saturated zones. As a conclusion, we correlated the surface karstic springs with bedrock structures and followed the water path across the glacial sedimentary filling. The recognized features were very consistent with a 2-D dc resistivity profile made through the GPR data block.