Assessing the Perspectives of Ground Penetrating Radar for Precision Farming

The United Nations 2030 Agenda for Sustainable Development highlighted the importance of adopting sustainable agricultural practices to mitigate the threat posed by climate change to food systems around the world, to provide wise water management and to restore degraded lands. At the same time, it suggested the benefits and advantages brought by the use of near-surface geophysical measurements to assist precision farming, in particular providing information on soil variability at both vertical and horizontal scales. Among such survey methodologies, Ground Penetrating Radar has demonstrated its effectiveness in soil characterisation as a consequence of its sensitivity to variations in soil electrical properties and of its additional capability of investigating subsurface stratification. The aim of this contribution is to provide a comprehensive review of the current use of the GPR technique within the domain of precision irrigation, and specifically of its capacity to provide detailed information on the within-field spatial variability of the textural, structural and hydrological soil properties, which are needed to optimize irrigation management, adopting a variable-rate approach to preserve water resources while maintaining or improving crop yields and their quality. For each soil property, the review analyses the commonly adopted operational and data processing approaches, highlighting advantages and limitations

Estimating field-scale soil water dynamics at a heterogeneous site using multi-channel GPR

We explore the feasibility to quantify the field-scale soil water dynamics through time series of GPR (ground-penetrating radar) measurements, which bridge the gap between point measurements and field measurements. Working on a 40 m × 50 m area in a heterogeneous agricultural field, we obtain a time series of radargrams after a heavy rainfall event. The data are analysed to simultaneously yield (i) a three-dimensional representation of the subsurface architecture and (ii) the total soil water volume between the surface and a reflection boundary associated with the presence of paleo sand dunes or clay inclusions in a rather uniform sand matrix. We assess the precision and the accuracy of these quantities and conclude that the method is sensitive enough to capture the spatial structure of the changing soil water content in a three-dimensional heterogeneous soil during a short-duration infiltration event. While the sensitivity of the method needs to be improved, it already produced useful information to understand the observed patterns in crop height and it yielded insight into the dynamics of soil water content at this site including the effect of evaporation

Advances in Monitoring Dynamic Hydrologic Conditions in the Vadose Zone through Automated High-Resolution Ground-Penetrating Radar Images and Analysis

This body of research focuses on resolving physical and hydrological heterogeneities in the subsurface with ground-penetrating radar (GPR). Essentially, there are two facets of this research centered on the goal of improving the collective understanding of unsaturated flow processes: i) modifications to commercially available equipment to optimize hydrologic value of the data and ii) the development of novel methods for data interpretation and analysis in a hydrologic context given the increased hydrologic value of the data. Regarding modifications to equipment, automation of GPR data collection substantially enhances our ability to measure changes in the hydrologic state of the subsurface at high spatial and temporal resolution (Chapter 1). Additionally, automated collection shows promise for quick high-resolution mapping of dangerous subsurface targets, like unexploded ordinance, that may have alternate signals depending on the hydrologic environment (Chapter 5). Regarding novel methods for data inversion, dispersive GPR data collected during infiltration can constrain important information about the local 1D distribution of water in waveguide layers (Chapters 2 and 3), however, more data is required for reliably analyzing complicated patterns produced by the wetting of the soil. In this regard, data collected in 2D and 3D geometries can further illustrate evidence of heterogeneous flow, while maintaining the content for resolving wave velocities and therefore, water content. This enables the use of algorithms like reflection tomography, which show the ability of the GPR data to independently resolve water content distribution in homogeneous soils (Chapter 5). In conclusion, automation enables the non-invasive study of highly dynamic hydrologic processes by providing the high resolution data required to interpret and resolve spatial and temporal wetting patterns associated with heterogeneous flow. By automating the data collection, it also allows for the novel application of established GPR data algorithms to new hydrogeophysical problems. This allows us to collect and invert GPR data in a way that has the potential to separate the geophysical data inversion from our ideas about the subsurface; a way to remove ancillary information, e.g. prior information or parameter constraints, from the geophysical inversion process

High-resolution imaging of transport processes with GPR full-waveform inversion

Imaging subsurface small-scale features and monitoring transport of tracer plumes at a fine resolution is of interest to characterize transport processes in aquifers. Full-waveform inversion (FWI) of crosshole ground penetrating radar (GPR) measurements enables aquifer characterization at decimeter-scale resolution. GPR FWI provides 2D tomograms of the subsurface properties, the dielectric permittivity (ε) and electrical conductivity (σ), which can be correlated with hydrological properties. In the framework of the thesis, we conducted synthetic and experimental tracer tests that were monitored using time-lapse crosshole GPR full-waveform inversion results, to test the potential and limitation to reconstruct the tracer plume. For the synthetic test, we generated a realistic high resolution aquifer model based on previous hydrological and GPR FWI data from the Krauthausen test site in order perform a transport simulation that represents reasonable heterogeneity of the tracer concentration. Using petrophysical relations, we converted the concentration distribution to dielectric properties of specific tracers: saltwater (increase σ only), desalinated water (decrease σ only) and ethanol (decrease in both σ and ε). One important aspect of the GPR FWI is to investigate an optimal way to define adequate starting models especially for the time-lapse data. Therefore, we investigated three different starting model options in the synthetic test, resulting that ε and σ models from the background provide the most accurate FWI of time-lapse data. Hereby, both ε and σ FWI results have shown the potential to derive time-lapse changes. The gained insights of the synthetic optimization tests are applied for an experimental test. To prove the potential of the crosshole GPR FWI also under realistic conditions, we performed an experimental salt tracer experiment at the Krauthausen test site. Thereby, we injected to the sandy aquifer a salt tracer, and monitored the tracer development using crosshole GPR over a timeframe of 14 days within 5 crosshole planes in an area of 11x10 m. These time-lapse data are independently inverted using the background models of each plane as starting models as proposed from the synthetic study to derive the best FWI results. We investigated the consistency of the reconstruction of the plume by temporal and spatial continuity across neighboring planes, by correlating with borehole logging data, and with expectations based on previous tracer experiments from the same site. One challenge arise from the time-lapse GPR data caused by the change of the borehole filling properties over the time and transport of the plume. The salt and freshwater mixture in the tubes couple with the borehole antennae thus influence the GPR data. Fortunately, the processing for the FWI enables accounting this effect by estimating effective source wavelets for each time step and each plane, which compensate for borehole filling effects caused by the salt tracer. If these borehole filling effects would not be considered, errors in the results would occur. Performing the FWI considering the corrected effective source wavelets allows recovery of the aquifer models independently from saltwater-antennae effects. Such effects cannot be incorporated using standard ray-based approaches. In contrast from the synthetic tracer test, investigation of the best starting model for experimental data showed that σ homogenous model rather than from FWI background provides more accurate results for FWI of time-lapse data. This can be explained that possible errors in the FWI background results caused by measurement or starting model uncertainties, are forcing the FWI with these models to be trapped in a local minimum. The time-lapse GPR FWI has shown a reliable manifestation of a tracer of about 0.2 m resolution, which was not observed before from other geophysical monitoring techniques. These improved and higher resolution images of such a tracer transport can help in future to better constraint hydrological properties of interest for hydrological models. In this thesis, we have shown for the first time the potential of the GPR FWI to characterize and monitor tracer experiments using crosshole GPR data. Especially, the application to salt tracers, which traditionally were investigated with ERT, is now also possible with GPR and higher resolution images of the tracer transport are possible to obtain

Time-Lapse Monitoring of Two-Dimensional Non-Uniform Unsaturated Flow Processes Using Ground Penetrating Radar

Unsaturated flow in the vadose zone often manifests as preferential flow resulting in transport of water and solutes through the soil much faster than would occur for uniform matrix flow. Time-lapse ground penetrating radar (TLGPR) shows promise as a non-invasive means to monitor unsaturated flow and here is used to monitor lab-scale forced infiltration events for capturing evidence of non-uniform and preferential flow phenomena directly from arrivals in the GPR images while simultaneously characterizing parameters of the flow system, such as bulk water content and rates of wetting front movement. This was accomplished by 1) directly interpreting transient arrivals in GPR profiles for evidence of ono-uniform flow and 2) with the aid of migration processing techniques to improve the quality of GPR images for identification and tracking of transient arrivals related to wetting in the soil. A novel method is described and evaluated to characterize the 2D velocity structure of a soil and used to migrate the GPR images. This method incorporates multi-offset measurements to characterize the depth to a potentially unknown static reflector and root mean square (RMS) velocity above the reflector with incremental changes in travel time to the static reflector and a transient reflector (i.e. the wetting front) determined from single-offset constant offset profiles to determine incremental changes in velocity above and below the transient arrival. The method is applied to TLGPR data during infiltration experiments in a 60 cm deep sand-filled tank and monitored with water content probes. To verify the approach the methodology is applied to GPR data simulated using transient water contents generated by the unsaturated flow simulator HYDRUS 2D given lab-measured hydraulic properties of the soil. For both the empirical and simulated data, we found that the 2D velocity analysis was effective in monitoring changes in the wetting front and that migration of the reflection profiles was able to improve the interpretation of non-uniform flow

Palm Tree Coplanar Vivaldi Antenna Array on the Same Substrate Size: Design and Performance Evaluation

This paper aims to describe the performance of the palm tree Coplanar Vivaldi Antenna Array (CVA) that was simulated from 0.25-6.25 GHz in terms of return loss and radiation pattern. Palm Tree Coplanar Vivaldi Antenna is available in four different configurations: single-element, two-element array, four-element array, and an eight-element array. We create a feeding network and radiator patch for two, four, and eight-array antennas. The simulation results demonstrate that the single-element antenna has the best return loss performance and can cover all frequency work from 0.25-6.25 GHz. In contrast, the antenna array can only cover multiband frequency. At 3 GHz, a single-element antenna has a directivity of 8.77 dBi, a sidelobe level of -2.2 dB, and a beamwidth of 63.70. In contrast, an antenna array of 8 elements has a directivity of 15.5 dBi, a sidelobe level of -12.6 dB, and a beamwidth of 80. Using the same substrate size, by configuring the Vivaldi Coplanar antenna to be an array at a frequency of 3 GHz, the 1×8 array antenna has a 6.73dBi improvement in directivity, a 10.4 dB boost in side lobe level, and a 55.70 enhanced in beamwidth performance compared to a single element. According to the simulation findings, the radiation pattern performance of the. Palm Tree CVA is greater than a single element in the same substrate size. Good directivity, SLL, and beamwidth performance make the proposed Palm Tree CVA array suitable for integration in telecommunication, radar, or cognitive radio applications

Methodology Applied to the Diagnosis and Monitoring of Dikes and Dams

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Détermination des propriétés hydrodynamiques des sols par mesures radar de surface

Soil hydraulic properties, represented by the soil water retention and hydraulic conductivity functions, dictate water flow in the vadose zone, from surface to aquifers. Understanding the water flow dynamic has important implications for estimating available water resources and flood forecasting. It is also crucial in evaluating the dynamicsof chemical pollutants in soil and in assessing the risks of groundwater pollution. Ground Penetrating Radar is a geophysical method particularly suited to measure contrasts of electromagnetic parameters such as those created by water content variations in soils.In this manuscript, we focus on Ground Penetrating Radar temporal monitoring of fluid flows in the near surface, such as infiltration in soil. We were initially curious to know to know if Ground-Penetrating Radar could be used to estimate soil hydraulic properties, more efficiently than classical soil characterization methods such as disk infiltrometer and water hanging column. We developed coupled hydrodynamic and electromagnetic numerical modeling to invert the two way travel times associated with reflections corresponding to strong dielectric permittivity contrasts, such as these involved on infiltration bulbs or fronts orwater tables.We designed two types of Ground Penetrating Radar infiltration monitoring techniques: one using a single ring infiltrometer and the other using shallow boreholes. The first type of infiltration experiment relied on uni-dimensional numerical modeling whereas the second one used a 2D-axi-symmetrical hypothesis. In each of these cases, our original methods have been numerically tested and applied to sandy soils. Beyond quantitative soil hydraulic parameter estimations, electromagnetic wave interactions with hydrodynamicprocesses lead to peculiar phenomena such as waveguide creation by infiltration bulbs.In addition, we showed that Ground Penetrating Radar monitoring was accurate enough to detect the hysteresis of the water retention function during successive drainage and wetting cycles, in a laboratory experiment using a sub-metric tank filled with Fontainebleau sand. The parameters characterizing the hysteresis of this function have been quantified through two-way travel time inversions of the bottom tank reflection during one-step and multi-step hydraulic head experiments.Les propriétés hydrodynamiques des sols, représentées par les fonctions de rétention en eau et de conductivité hydraulique, régissent les écoulements d'eau et de solutés de puis la surface jusqu'aux nappes souterraines. La caractérisation et la compréhension de cette dynamique des fluides ont énormément d’importance pour la détermination des ressources en eau disponibles, la pollution des sols et des eaux souterraines. Le radar de sol est une méthode d'imagerie géophysique particulièrement adaptée pour détecter les contrastes de paramètres électromagnétiques tels que ceux engendrés par des gradients de teneur en eau. La problématique à laquelle répond cette thèse est de savoir si le suivi temporel par radar de sol de phénomènes de dynamique des fluides en proche surface, tels que des infiltrations, est suffisant pour retrouver les paramètres hydrodynamiques du sol considéré, de manière plus efficace que les procédés d'estimation classiques.Nous avons développé des algorithmes couplant des modèles hydrodynamique et électromagnétique, afin d'obtenir les propriétés hydrodynamiques de sols, en inversant les temps aller-retour des ondes électromagnétiques correspondant à des réflexions sur des contrastes de permittivité forts comme ceux rencontrés sur des fronts ou bulbes d'infiltration.Nous avons développé le suivi radar de deux techniques d'infiltration, en simple anneau et en forage de faible profondeur. La première permet une modélisation unidimensionnelle alors que la seconde utilise une modélisation 2D-axisymétrique. Dans chaque cas, nos méthodes originales ont été testées numériquement, puis appliquées à différentes expériences de terrain, principalement sur des sols sableux. Les paramètres des fonctions de rétention en eau et de conductivité hydraulique retrouvés sont en accord avec ceux obtenus classiquement par des méthodes telles que des mesures sur échantillons en colonne suspendue et in situ par infiltromètrie à disque. Au-delà de l'aspect quantitatif des processus hydrodynamiques, l'interaction de ces derniers avec les champs électromagnétiques conduit à l'observation de phénomènes atypiques, comme des guides d'ondes créés par des bulbes d'infiltration.De plus, nous avons montré que le radar de sol est assez précis pour détecter des différences dans les profils de teneur en eau selon le cycle drainage-imbibition, causées par l'hystérèse sur les fonctions de rétention en eau et de conductivité hydraulique. Les paramètres de ces fonctions ont été obtenus par inversion de données radar lors de suivis de battements de nappe au sein de réservoirs, de l'ordre du mètre cube, en laboratoire

Hydrogeophysical investigation of water recharge into the Gnangara Mound

Increased demand for freshwater in combination with a drying climate has led to water table decline on the Gnangara Groundwater Mound north of Perth, Western Australia. For sustainable groundwater management, a regional-scale modelling system has been developed. Accurate groundwater modelling requires good estimates of aquifer recharge, which in the case of the Superficial Aquifer may be achieved by a Vertical Flux Model. Recharge studies provide this model with input parameters such as unsaturated hydraulic conductivity, soil moisture content and water retention potential. Another key component of sustainable water resource management is to understand the biophysical processes that are involved in surface- and groundwater and plant interaction in order to conserve the natural ecosystem.Hydrogeophysical measurements have the potential to provide non-invasive, in-situ physical parameter estimation for the near-surface. As such it provides a tool to quantify and monitor unsaturated zone dynamics. From hydrogeophysical observations, hydrogeologic parameters can be deduced and then used as constraints for the numerical modelling. Geophysical monitoring further provides field evidence to corroborate or reject modelling results. Some subsurface physical properties are invariable over long time-scales (e.g. depositional features, porosity) and can be mapped with geophysical measurements. Other subsurface components are subject to temporal variations. They are determined by environmental factors, for example the water content changes during the hydrogeologic cycle. Capturing those seasonal variations requires time lapse investigation..The groundwater recharge rates at the Gnangara Mound are dominated by winter rainfall in a Mediterranean climate setting. Rainwater infiltrates through a sandy soil profile that contains water retentive soil horizons. In this thesis, the physical properties of the soil and their temporal variations are explored using Ground-Penetrating Radar (GPR) and neutron logging to delineate the influence of water retentive soil horizons.The spatial distribution of indurated, friably cemented sand layers varies spatially. To delineate these layers, large-scale surface 2D common offset GPR reflection profiles that span the entire groundwater mound are examined. It is found that these layers produce strong reflections in the radargrams that suggest a strong contrast in water content; indicating water retentiveness is present. An analysis scheme is developed that allows large-scale classification of water retention potential based on spatial reflector configuration and reflection strength. The results from spatial investigation indicate that the distribution of potentially water retentive layers is patchy. Where pronounced layers exist, they commonly show dip, which in combination with pipe structures (dissolution and root channels) is likely to result in preferential flow.Laboratory dielectric experiments on samples with variable water saturation demonstrate that retentive and non-retentive soil horizons have a similar dielectric permittivity versus water content relationship which corroborates that high reflectivity indicates elevated water content.Six test sites were selected for time lapse investigation based on soil properties and hydrogeologic setting. A range of surveys were performed before, during and after the annual rainfall cycle in 2011 to capture the temporal variability of vertical water content distribution. Time-lapse crosswell- and surface-to-hole borehole radar datasets were acquired. To obtain high certainty moisture content profiles from those data, a new processing scheme is proposed based on a combined use of zero-offset profiling and vertical radar profiling. Sequential and baseline difference curves are calculated and reveal infiltration scenarios ranging from simple wetting and unsaturated flow regime, to delayed wetting and impeded flow. While some impact on infiltration can be attributed to retentive soil layers, it was found that vegetation appears to play a crucial role in determining soil moisture depletion between wetting cycles. The results from the time-lapse GPR were validated by analysis of long-term time-lapse neutron logging. Neutron logging reinforces the view that retention horizons are unlikely to store additional plant available water compared to the clean sand intervals.Very near-surface water content measurements are a challenge with commercial common offset GPR systems. I develop a new analysis methodology that enables estimation of water content as part of the spatial and temporal characterization of shallow moisture distribution. Dispersion curves are derived from shallow diffracted wavefields that appear in common offset GPR due to a waveguide structure. Inversion based on modal wave propagation in a waveguide allows derivation of waveguide parameters. Dispersion curves are demonstrated to be sensitive to small changes in waveguide properties, which are strongly dependent upon water content. Field examples illustrate the full potential of this technique in lateral near-surface water content quantification.The small- and large scale surveys presented in this thesis form the basis for examination and advancement of the radar methodology in a sandy environment as well as providing field evidence for hydrogeologic significance and distribution of water retentive soil horizons in the unsaturated zone of the Swan Coastal Plain, Western Australia