Zerstörungsfreie Wurzelortung mit geophysikalischen Methoden im urbanen Raum

Abstract

The aim of the work was to analyse whether Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT) are suitable to detect tree roots in urban areas. The non-destructive visualisation of roots was conducted for scientific as well as for applied reasons. On the one hand information about the root distribution derived from imaging techniques can be used to interpret ecological cycles. On the other hand this information is available to protect the subsurface part of urban trees during underground construction activities. The work consists of three parts including 2D-measurements under controlled conditions, fundamental analysis for 3D-measurements and the 3D-investigation of several urban tree sites. The main focus of the work was on GPR because of the high resolution of this method. The 2D-measurements in the lysimeter-basins in the New Botanical Garden in Göttingen have shown that tree roots with a minimal diameter of 3 cm can be detected with a 400-MHz-antenna. Furthermore a new approach for the determination of root diameter in 2D-GPR measurements based on multiple reflections was used. From another experiment comparing tree roots to other typical subsurface objects (like cables) it could be concluded that 3D-measurements are not only necessary to locate single roots but also to differentiate roots and other objects by their position and angle in the soil. A methodological improvement compared to the work of other authors could be achieved by the application of a dense measurement grid with a transect separation of 15 cm. The results have been verified by excavations of tree roots. Comparing different urban tree sites the best results have been achieved on homogeneous, near-natural park areas where the root systems have been reconstructed by the results of the GPR-measurements. In contrast, sites with a high portion of coarse fragments are not suitable for root detection with GPR because the reflections of the coarse fragments overlay the reflections of the tree roots and mask them in the results. Another difficulty is the visualisation of roots growing downwards. This can be explained by the overlaying of these roots by stronger reflections from other depth when bigger depth ranges are projected in one horizontal slice. On some investigated sites the GPR measurements have been supplemented by geoelectrical surveys. Here, the statistical correlation between the specific electrical resistivity and the root mass in the underground reported by other authors could not be confirmed.