Integrating surface georadar and crosshole radar tomography: A validation experiment in braided stream deposits

We have used a combination of surface ground‐penetrating radar (GPR) profiling, crosshole radar tomography, and natural gamma‐ray logging to characterize a gravelly braided stream deposit. In a gravel pit, we conducted a survey using 300‐MHz surface GPR, 250‐MHz crosshole radar, and densely sampled gamma‐ray logging at single‐borehole locations. After excavation, we validated the geophysical results by comparison with the sedimentological and hydrogeological information obtained from the corresponding outcrop wall. We found the visual lithofacies boundaries agreed very well with the images provided by applied geophysical techniques. Our results illustrate how GPR reflector images are improved using tomographic velocity information. In addition, the structural interpretation of tomographic velocity fields is guided by the GPR reflector images in combination with natural gamma‐ray logging results. Groundwater flow and transport modeling was also performed on different subsurface models. The hydrogeological response of parameter distributions derived from a digitized outcrop image are compared with the response of a parameter field derived from the combined geophysical data and with the response of a simple block interpolation between the boreholes. Comparison of cumulative particle arrival times (breakthrough curves) indicates that the characterization of an appropriate real aquifer would benefit from incorporating high‐resolution geophysical data into the analysis.