Radar reflections from sedimentary structures in the vadose zone

Ground penetrating radar (GPR) is a suitable technique for imaging sedimentary structures in the vadose zone because small texture-related capillary-pressure variations lead to changes in water content and electromagnetic properties. To study exactly how GPR reflections are generated by sedimentary structures, GPR profiles of an aeolian sedimentary succession are combined with measurements of textural, electromagnetic and water-retention characteristics from a trench. Time domain reflectometry indicates that small variations in texture in the high-angle dune sediment are associated with changes in water content. Synthetic modelling shows that these changes cause clear GPR reflections. In an experimental approach to estimate the radar response of structures below the wave resolution, i.e. features smaller than λ/4, variations in grain-size distribution and porosity in a thin section were used to reconstruct water-retention curves and impedance models of the thinly layered sediment. Synthetic radar records calculated from the impedance models show that reflections from the studied subcentimetre-scale structures are composites of interfering signals. Although these low-amplitude interfering signals will commonly be overprinted by more prominent reflections, they may cause reflection patterns that change with frequency and do not represent primary bedding.