Performance of ground penetrating radar in root detection and its application in root diameter estimation under controlled conditions

A plant is stabilized by its root system. In congested urban cities such as Hong Kong, ground trenching is frequently seen due to the installation of utility lines along the roadside. Soil nailing, which involves soil coring in slopes, is a common solution to improve the slope stability. However, both activities inevitably pose a risk to the integrity of any root systems present, and thus reduce the root anchorage. To prevent or minimize such damage, a careful design of the excavation/drilling location is of prime importance. Ground penetrating radar (GPR) provides a non-destructive method for locating roots by examining the contrast between the dielectric properties of the roots and the surrounding soil. To examine the performance of GPR and promote its use in Hong Kong, a test bed was prepared using local materials to create a controlled environment in which to conduct a series of systematic tests evaluating the performance of a 900 MHz GPR. The reflected radargrams were subject to the influence of the following factors: size and depth of roots, horizontal distance between roots, and contrast between the root and soil water content. Correlations between root size and a number of waveform parameters were also explored. Limiting values for root size, root embedded depth, horizontal separation distance between roots, and water content contrast between root and soil were obtained. A significant correlation was found between the root diameter and time travel parameter T ₂ (p<0.001, r=0.795). Because GPR root detection is highly site-specific, this study provides a local reference for GPR performance in the Hong Kong environment. The findings demonstrate that the 900 MHz GPR is applicable in Hong Kong for the detection of main roots.