Direct measurements of tree root relative permittivity for the aid of GPR forward models and site surveys

Ground penetrating radar has been used extensively in near-surface studies to detect underground objects and features typically located within a few metres beneath the surface. In urban areas, ground penetrating radar is widely used to study buried utilities such as pipes and cables. A more recent and unconventional application of ground penetrating radar is the detection of tree roots, which can interact negatively with the human infrastructure in a number of ways. However, the geophysical study of tree roots has proven quite challenging and site-specific. Most tree roots (even coarse roots) have a small diameter and are hard to resolve through geophysical methods. In addition, the sheer amount of potential variability regarding the tree species, age, size, health and the subsurface environment (e.g., soil or a man-made material such as concrete or asphalt) makes it very hard to implement a one-size-fits-all approach. This is where robust, easily customizable forward models can be of assistance, indicating the range of detectable geophysical contrast and the limitations of the method, as well as the suitable antenna frequencies. Here, a vector network analyser with a commercial open-ended coaxial probe was used to take direct measurements of the relative permittivity of freshly cut tree root segments at frequencies from 50 MHz to 3 GHz. The results were used as inputs to ground penetrating radar forward modelling using gprMax open source software, depicting various realistic scenarios which could be encountered in actual field surveys. The developed models help better understand the applicability, potential and limitations of ground penetrating radar surveys for detecting tree roots in different environments, aiding the development of future surveys. The notable variability in the tree roots is a significant consideration for surveys and forward models.