GPR Reflection Research Articles

Quantitative integration of seismic and GPR reflections to derive unique estimates for water saturation and porosity in subsoil

For shallow subsoil, the estimates of in‐situ porosity and water saturation are important, but until now it has been difficult to estimate them reliably. We relate seismic and GPR reflection coefficients to porosity and water saturation using a shared earth model. Using this model, we propose a method to integrate quantitatively seismic and GPR angle‐dependent reflection coefficients. The new approach has been tested through numerical simulations. The results clearly show that from either seismic or GPR data alone it is impossible to obtain unique estimates for porosity and water saturation; however, a correct integration of those two data types leads to unique and stable estimates at a subsoil layer boundary. Potential applications of this approach exist in hydrogeology and environmental, agricultural and geotechnical engineering.

Radar reflections from sedimentary structures in the vadose zone

Abstract 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.

Geostatistische Analyse von Georadar-Reflexionsmustern und Vergleich mit Korrelationsmustern hydraulischer Eigenschaften in Abraumkippen von Braunkohletagebauen

Geostatistische Analyse von Georadar-Reflexionsmustern und Vergleich mit Korrelationsmustern hydraulischer Eigenschaften in Abraumkippen von Braunkohletagebauen

Iron oxides as a cause of GPR reflections

Iron oxides frequently occur as secondary precipitates in both modern and ancient sediments and may form bands or irregular patterns. We show from time-domain reflectometry (TDR) field studies that goethite iron-oxide precipitates significantly lower the electromagnetic wave velocity of sediments. Measured variations in magnetic permeability do not explain this decrease. The TDR measurements and a dielectric mixing model also show that neither electrical conductivity nor relative permittivity of the solid material are altered significantly by the iron-oxide material. From drying during all of the measurements, the amount of iron oxides appears to correlate with the volumetric water content, which is the result of differences in water retention capacity between goethite and quartz. These variations in water content control relative permittivity and explain the observed variation in electromagnetic wave velocity. Using 2-D synthetic radar sections, we show that the pattern of iron-oxide precipitation may have a profound influence on the GPR reflection configuration and can cause major difficulties in interpretation.

Finding the strike direction of fractures using GPR

GPR reflection energy varies with antenna orientation relative to the strike and dip of the reflector. This directional dependence of GPR responses was investigated through numerical experiments and was used to estimate the azimuth of fractures and joints. Three antenna configurations were considered in this study: perpendicular‐broadside (YY mode), parallel‐broadside (XX mode) and cross‐polarization (YX mode). The reflection energy in the cross‐polarization mode shows a shape characteristic similar to the strike, regardless of the dip angle. Those in the other two modes show quite different amplitudes from the strike, depending on the dip angle. We have developed a strike‐direction‐finding scheme using data obtained from the three different modes for the same survey line. The azimuth angle of each reflector was displayed in colour on the GPR profile. This scheme was applied to a field survey at a granite quarry in southern Korea. The GPR profiles showed different images of the reflectors depending on the antenna configuration. The estimated azimuths of reflectors obtained using our scheme matched fairly well with those of known fractures and joints.