Imaging Tree Roots with Borehole Radar

Ground-penetrating radar has been used to detect and map tree roots using surface-based antennas in reflection mode. On amenable soils these methods can accurately detect lateral tree roots. In some tree species (e.g. Pinus taeda, Pinus palustris), vertically orientated tap roots directly beneath the tree, comprise most of the root mass. It is difficult if not impossible to vertically delineate these roots with surface-based radars. To address this problem, a collaborative project between the USDA Forest Service, Southern Research Station, Radarteam AB and the Swedish University of Agricultural Sciences (SLU), was undertaken in August 2003 to assess the potential of high-frequency borehole radar to detect vertical, near-surface reflectors (0-2 m) resulting from tree roots. A set of controlled experiments on buried logs were used to test the efficacy of crosshole and borehole to surface travel time data to model near surface woody targets with tomography. Using these results, five Pinus sylvestris trees were scanned with borehole to surface radar and tomograms of their root systems were created. Three of the five tomograms compared favorably with root distribution maps made using destructively sampled data. However, the other two trees were misinterpreted, one was sharply underestimated, the other overestimated. This is the first report of using borehole radar to study vertical tree roots. Crosshole tomography provided excellent information on the depth of tree roots, but was less useful for imaging near surface features. Borehole to surface measures provided the best information on the near surface, where the bulk of roots are found (0-0.3 m). The technique has promise in forest research, but the development of new high-frequency borehole antennas, and forward modeling software that allows concurrent processing of travel-time and amplitude data is necessary to further this research.