Health monitoring of trees and investigation of tree root systems using ground penetrating radar (GPR)

Evidence suggests that trees and forests around the world are constantly being threatened by disease and environmental pressures. Over the last decade, new pathogens spread rapidly in European forests, and quarantine measures have mostly been unable to contain outbreaks. As a result, millions of trees were infected, and many of these have already died. It is therefore vital to identify infected trees in order to track, control and prevent disease spread. In addressing these challenges, the available methods often include cutting of branches and trees or incremental coring of trees. However, not only do the tree itself and its surrounding environment suffer from these methods, but they also are costly, laborious and time-consuming. In recent years the application of non-invasive testing techniques has been accepted and valued in this particular area. Given its flexibility, rapidity of data collection and cost-efficiency, Ground Penetrating Radar (GPR) has been increasingly used in this specific area of research. Consequently, this PhD Thesis aims at addressing a major challenge within the context of early identification of tree decay and tree disease control using GPR. In more detail, two main topics are addressed, namely the characterisation of the internal structure of tree trunks, and the assessment of tree root systems’ architecture. As a result, a comprehensive methodology for the assessment of both tree trunks and roots using GPR is presented, which includes the implementation of novel algorithms and GPR signal processing approaches for the characterisation of tree trunks’ internal structure and the three-dimensional mapping of tree root systems. Results of this research project were promising and will contribute towards the establishment of novel tree evaluation approaches

Recent advances in tree root mapping and assessment using non-destructive testing methods: a focus on ground penetrating radar

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

Tree monitoring using ground penetrating radar: two case studies using reverse-time migration

Non-destructive testing (NDT) for health monitoring of trees is a suitable candidate for detecting signs of early decay [1]. Recent developments [2,3,4] have highlighted that ground-penetrating radar (GPR) has the potential to provide with a robust and accurate detection tool with minimum computational and operational requirements in the field. In particular, a processing framework is suggested in [2] that can effectively remove ringing noise and unwanted clutter. Subsequently, an arc length parameterisation is employed in order to utilise a wheel-measurement device to accurately position the measured traces. Lastly, two migration schemes; Kirchhoff and reversetime migration, are successfully applied on numerical and laboratory data in [3]. In the current paper, the detection scheme described in [2,3] using reverse-time migration is tested in two case studies that involve diseased urban trees within the greater London area, UK (Kensington and Gunnersbury park). Both of the trees were cut down after the completion of the measurements and furthermore cut into several slices to get direct information with regards to their internal structure. The processing scheme described in [3,4] managed to adequately detect the internal decay present in both trees. The aforementioned case studies provide coherent evidences to support the premise that GPR is capable of detecting decay in diseased trunks and therefore has the potential to become an accurate and efficient diagnostic tool against emerging infectious diseases of trees

Signal Processing for Tree-Trunk Investigation Using Ground Penetrating Radar

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Reverse-Time Migration for Evaluating the Internal Structure of Tree-Trunks Using Ground-Penetrating Radar

The authors would like to express their sincere thanks and gratitude to the following trusts, charities, organizations and individuals for their generosity in supporting this project: Lord Faringdon Charitable Trust, The Schroder Foundation, Cazenove Charitable Trust, Ernest Cook, Sir Henry Keswick, Ian Bond, P. F. Charitable Trust, Prospect Investment Management Limited, The Adrian Swire Charitable Trust, The John Swire 1989 Charitable Trust, The Sackler Trust, The Tanlaw Foundation and The Wyfold Charitable Trust. This paper is dedicated to the memory of Jonathon West, a friend, a colleague, a forester, a conservationist and an environmentalist who died following an accident in the woodland that he loved.Peer reviewedPostprin

Mapping the root system of matured trees using ground penetrating radar

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

The use of ultrasonic tomography for the non-destructive assessment of tree trunks

Assessing internal decay in tree trunks can be of crucial importance for industrial, environmental and public safety reasons [1]. To this effect, non-destructive testing (NDT) methods can provide information on the structural condition of trees with minimum intrusion. In this work, authors have analysed the capabilities of ultrasonic tomography in evaluating the internal structure of living trees, with a special focus on the identification of internal decay areas and tree bark inclusions. The presented ultrasonic tomography provides an image of the distribution of the ultrasonic velocity of propagation within the investigated section of a mature horse chestnut (Aesculus hippocastanum). This technique has proven its viability to detect fungal decomposition [2]. However, there exist some open issues with regard to: a) the coupling of the transducers to the tree, b) the anisotropy of the wood, c) the signal attenuation and the resolution of the tomographic inversion. To overcome these challenges, research is underway to explore the integration and new data-fusion strategies with other NDT methods, such as ground penetrating radar (GPR), which have proven their effectiveness within this area of endeavour [3]. Within this context, data have been obtained from a “diseased” horse chestnut tree located at the Kensington Gardens – The Royal Parks – in London, UK, using two different ultrasonic equipment, i.e., the PICUS Sonic Tomograph and the Arbotom Sonic Tomograph. After compilation of data, the tree was felled and cut at the two sections where ultrasonic tomography tests were performed. In more detail, 12 sensors were arranged around the perimeter of the tree in compliance with the manufacturer’s recommendations concerning the inspection methodology (sensors installed within the bark of the tree without any intrusion to the core of the tree). The adopted methodology takes to account the shape and size of the trunk [1]. The processed data were mapped against the cut sections of the tree for validity purposes. Results presented in this abstract are part of a major ongoing research project that the authors have undertaken for the last three years

Use of ground penetrating radar for assessing interconnections between root systems of different matured tree species

This study presents recent advances achieved in the use of ground penetrating radar (GPR) for the health monitoring of tree root systems. The main objectives of the research were to provide an effective and high-resolution mapping of the root systems belonging to different species of matured trees, as well as to investigate areas of roots interconnection. To this purpose, a data processing methodology based on three main stages was developed. A pre-processing algorithm was first proposed to remove noise-related information from the raw data and to enhance deep reflections from attenuated targets. Afterwards, an algorithm for identification of targets (i.e. the vertices of the reflection hyperbolas) and their automated tracking in a three-dimensional environment was developed. A third stage was focused on estimating tree root density with emphasis on the interconnection area. To test the feasibility of the proposed methodology, the soil around two different tree species (i.e. maple and ash trees) was investigated using a ground-coupled GPR system equipped with a 700 MHz central frequency antenna. The method has proven to identify distinctive features of both trees, in terms of shallow (i.e. within the first 25 cm from the soil surface) and deep (i.e. deeper than 25 cm from the soil surface) root systems. In addition, results have allowed to assess how different root systems interact with each other

Diagnosing emerging infectious diseases of trees using ground penetrating radar

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

Health monitoring of tree-trunks using ground penetrating radar

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