Intricacies in the interpretation of vertical radar profiling caused by borehole effects

Estimation of hydraulic parameters of the vadose zone is a relevant issue in hydrological characterization and flow model calibration. Two borehole Ground Penetrating Radar (GPR) techniques are discussed: Zero-Offset Profiling (ZOP) and Vertical Radar Profiling (VRP). The field case is representative of a very common situation in vadose zone characterization: Above the water table, the permittivity inside the air-filled borehole is significantly smaller than in the embedding soil. In this case, if the first-arrivals are picked and the corresponding inversions are performed without a careful analysis, the recovered dielectric relative permittivity (år) profiles are in large disagreement. The presented analysis of VRP synthetic and real datasets clarifies that the presence of the air-filled borehole may alter the propagation of electromagnetic waves, invalidating the comparison among VRP and ZOP first-arrivals. Once the borehole effects are accounted, the comparison between the ZOP and VRP år-profiles is more reasonable and reveals the different resolution of these techniques, focusing on the information that can be inferred for hydrological characterizations. Thus, VRP surveys in vadose zone must be accurately interpreted, as the electromagnetic waves may propagate via "guided" modes along the borehole.