Avoiding miss-interpretation of GPR measurements in forested areas

Ground-penetrating radar (GPR) is a common method for investigating the shallow underground. Subsurface reflections are usually associated with variations in dielectric properties. Unfortunately, reflections and diffractions from above-surface objects may contaminate GPR data. We have conducted a dual-component GPR survey across a shallow aquifer situated beneath a forest. Strikingly different reflection patterns are obtained for the two antenna orientations. To understand better the observed patterns, we first describe key characteristics of the dipole radiation emitted by GPR antennas. This is followed by the results of a simple modelling exercise designed to determine the effects of diffractions from numerous trees on GPR data recorded with the two antenna configurations. Based on these results, it is then possible to distinguish between subsurface and above-surface features recorded in a field data set, thus leading to a more reliable image of the shallow subsurface.