Ground penetrating radar (GPR) is traditionally applied to smooth surfaces in which the assumption of halfspace is an adequate approximation that does not deviate much from reality. Nonetheless, using GPR for internal structure characterization of tree trunks requires measurements on an irregularly shaped closed curve. Typical hyperbola-fitting has no physical meaning in this new context since the reflection patterns are strongly associated to the shape of the tree trunk. Instead of a clinical hyperbola, the reflections give rise to complex-shaped patterns that are difficult to be analyzed even in the absence of clutter. In the current paper, a novel processing scheme is described that can interpret complex reflection patterns assuming a circular target subject to any arbitrary shaped surface. The proposed methodology can be applied using commercial handheld antennas in real-time avoiding computationally costly tomographic approaches that require the usage of custom-made bespoke antenna arrays. The validity of the current approach is illustrated both with numerical and real experiments.

This paper provides an overview of the existing literature on the subject of assessment and monitoring of tree roots and their interaction with the soil. An overview of tree root systems architecture is given, and the main issues in terms of health and stability of trees, as well as the impact of trees on the built environment, are discussed. An overview of the main destructive and non-destructive testing (NDT) methods is therefore given. The paper also highlights the lack of available research based outputs in the field of tree roots and soil interaction, as well as of the interconnectivity of tree roots with one another. Additionally, the effectiveness of non-destructive methods is demonstrated, in particular ground penetrating radar, in mapping tree root configurations and their interconnectivity. Furthermore, the paper references recent developments in estimating tree root mass density and health. Keywords: assessment of tree roots; destructive testing methods; non-destructive testing methods; ground penetrating radar (GPR); tree root interconnectivity; tree root mass density. vertical plane, they are called buttress roots, the presence of which has been associated with soils that offer poor anchorage (Henwood, 1973). Beyond the zone of rapid taper emanates a framework of woody structural roots that gather water and nutrients from long distances to the trunk: their size is often influenced by mechanical stresses such as the wind load (Stoke, 1994).  Non-woody roots: also known as fine or absorbing roots, they are responsible for the absorption of water and nutrients (Pallardy, 2008), the synthesis of rooting hormone, root exudation, and symbiosis with soil microorganisms. As the name suggests, they do not undergo secondary thickening, are generally small in diameter (<2 mm) and their lifespan ranges from days to weeks, depending on soil conditions and temperature (Pallardy, 2008). Root architecture is quite complex and varies between and within plant species (Gregory, 2006). As far as rooting depth is concerned, it is influenced not only by the tree species but also by the type and conditions of the soil (Stone & Kalisz, 1991): in fact, the downward penetration of roots can be impeded by soils that are poorly aerated or

Aggressive fungal and insect attacks have reached an alarming level, threatening a variety of tree species, such as ash and oak trees, in the United Kingdom and beyond. In this context, Ground Penetrating Radar (GPR) has proven to be an effective non-invasive tool, capable of generating information about the inner structure of tree trunks in terms of existence, location, and geometry of defects. Nevertheless, it had been observed that the currently available and known GPR-related processing and data interpretation methods and tools are able to provide only limited information regarding the existence of defects and anomalies within the tree inner structure. In this study, we present a microwave tomographic approach for improved GPR data processing with the aim of detecting and characterising the geometry of decay and cavities in trees. The microwave tomographic approach is able to pinpoint explicitly the position of the measurement points on the tree surface and thus to consider the actual geometry of the sections beyond the classical (circular) ones. The robustness of the microwave tomographic approach with respect to noise and data uncertainty is tackled by exploiting a regularised scheme in the inversion process based on the Truncated Singular Value Decomposition (TSVD). A demonstration of the potential of the microwave tomography approach is provided for both simulated data and measurements collected in controlled conditions. First, the performance analysis was carried out by processing simulated data achieved by means of a Finite-Difference Time-Domain (FDTD) in three scenarios characterised by different geometric trunk shapes, internal trunk configurations and target dimensions. Finally, the method was validated on a real trunk by proving the viability of the proposed approach in identifying the position of cavities and decay in tree trunks.

Ash dieback, acute oak decline (AOD) and Xylella Fastidiosa are Emerging Infectious Diseases (EIDs) that have spread rapidly in European forests during the last decade. Quarantine measurements have mostly failed to repress the outbreaks and millions of trees have already been infected. Identifying infected trees in a non-destructive manner is of high importance for monitoring, managing and preventing EIDs. The aim of this paper is to examine the capabilities of Ground Penetrating Radar (GPR) on evaluating the internal structure of tree-trunks and detecting tree-decay associated with EIDs. Traditionally used processing schemes tuned for GPR line-acquisitions are modified accordingly to be compatible with the new measurement configurations. In particular, a detection framework is presented based on a modified Kirchhoff and a reverse-time migration. Both of the aforementioned methodologies are compatible with measurements taken along closed irregular curves assuming a homogeneous permittivity distribution. To that extent, prior to migration, a novel focal criterion is used that estimates the bulk permittivity of the host medium from the measured B-Scans. The suggested detection scheme is successfully tested on both numerical and laboratory measurements, indicating that GPR has the potential to become a coherent and practical tool for detecting tree-decay associated with EIDs.

This study reports a demonstration of the ground penetrating radar (GPR) potential in health monitoring of tree roots. The main aim of the research was to provide effective and high-resolution mapping of tree root systems. To this purpose, a dedicated data processing methodology, based on two main chronological stages, was developed. First, an algorithm was proposed to filter out the data from noise-related information and to enhance deep reflections from attenuated targets. At a later stage, a multi-step algorithm connecting the identified targets (i.e. the vertices of the reflection hyperbolas) in a three-dimensional environment was created. To demonstrate the viability of the proposed methodology, the soils around two different tree species (i.e. fir and oak trees) were investigated using a ground-coupled multi-frequency GPR system equipped with 600 MHz and 1600 MHz central frequency antennas. The method has allowed to identify distinctive features in terms of shallow (i.e. within the first 25 cm from the soil surface) and deep (i.e. lower than 25 cm from the soil surface) tree root systems for different species of trees.