We present a new technique for real-time, proximal sensing of the soil hydrogeophysical propertie... more We present a new technique for real-time, proximal sensing of the soil hydrogeophysical properties using ground-penetrating radar (GPR). The radar system is based on international standard vector network analyser technology, thereby setting up stepped-frequency continuous-wave GPR. The radar is combined with an off-ground, ultra-wideband, and highly directional horn antenna acting simultaneously as transmitter and receiver. Full-waveform forward modelling of the radar signal includes antenna propagation phenomena through a system of linear transfer functions in series and parallel. The system takes into account antenna–soil interactions and assumes the air–subsurface compartments as a three-dimensional multilayered medium, for which Maxwell’s equations are solved exactly. We provide an efficient way for estimating the spatial Green’s function as a solution of Maxwell’s equations from its spectral counterpart by deforming the integration path in the complex plane of the integration variable. Signal inversion is formulated as a complex least squares problem and is solved iteratively using the global multilevel coordinate search optimisation algorithm combined with the local Nelder–Mead simplex method. The electromagnetic model has unprecedented accuracy for describing the GPR signal in controlled laboratory conditions, providing accurate estimates for both soil dielectric permittivity and electrical conductivity. The proposed method has been specifically designed for the retrieval of soil surface dielectric permittivity and correlated surface water content, which has been validated in field conditions. We also show that constraining the electromagnetic inverse problem using hydrodynamic modelling theoretically permits retrieval of the soil hydraulic properties and reconstruction of continuous vertical water content profiles from time-lapse GPR data. The proposed method shows great promise for field-scale, high-resolution digital soil mapping, and thereby for bridging the spatial-scale gap between ground truthing based on soil sampling or local probes and airborne and spaceborne remote sensing.
Sustainable and optimal agricultural and environmental management of water and land resources par... more Sustainable and optimal agricultural and environmental management of water and land resources particularly relies on the description and understanding of soil water distribution and dynamics at different scales. We present an advanced ground penetrating radar (GPR) method for mapping the shallow soil water content and unsaturated hydraulic properties at the field scale. The radar system is based on vector network analyzer technology, for which calibration is simple and constitutes an international standard. A directive horn antenna is used as both transmitter and receiver and operates off the ground. A full-waveform model describes accurately the radar signal, and is based on a linear system of complex transfer functions for efficiently describing electromagnetic phenomena within the antenna and its interaction with soil, and a specific solution of the three-dimensional Maxwell's equations for wave propagation in multilayered media. The soil electromagnetic properties and their vertical distribution are estimated by resorting to full-waveform inverse modeling using iterative global optimization methods. The proposed methodology has been validated for a series of model configurations of increasing complexity. The method is now routinely used for real-time mapping of soil surface water content and reconstruct a few number of shallow soil layers. For more complex configurations, it is necessary to regularize the inverse problem. We have shown that constraining radar data inversion using soil hydrodynamic modeling has the potential to reconstruct time-lapse, continuously variable, vertical soil water content profiles and identify the shallow unsaturated hydraulic properties. The proposed approach shows great promise for quantitative imaging of the soil properties at the field scale. The technique will be combined with electromagnetic induction in a mechanistic data fusion framework to further extend its capabilities in a digital soil mapping context.
We present a new technique for real-time, proximal sensing of the soil hydrogeophysical propertie... more We present a new technique for real-time, proximal sensing of the soil hydrogeophysical properties using ground-penetrating radar (GPR). The radar system is based on international standard vector network analyser technology, thereby setting up stepped-frequency continuous-wave GPR. The radar is combined with an off-ground, ultra-wideband, and highly directional horn antenna acting simultaneously as transmitter and receiver. Full-waveform forward modelling of the radar signal includes antenna propagation phenomena through a system of linear transfer functions in series and parallel. The system takes into account antenna–soil interactions and assumes the air–subsurface compartments as a three-dimensional multilayered medium, for which Maxwell’s equations are solved exactly. We provide an efficient way for estimating the spatial Green’s function as a solution of Maxwell’s equations from its spectral counterpart by deforming the integration path in the complex plane of the integration variable. Signal inversion is formulated as a complex least squares problem and is solved iteratively using the global multilevel coordinate search optimisation algorithm combined with the local Nelder–Mead simplex method. The electromagnetic model has unprecedented accuracy for describing the GPR signal in controlled laboratory conditions, providing accurate estimates for both soil dielectric permittivity and electrical conductivity. The proposed method has been specifically designed for the retrieval of soil surface dielectric permittivity and correlated surface water content, which has been validated in field conditions. We also show that constraining the electromagnetic inverse problem using hydrodynamic modelling theoretically permits retrieval of the soil hydraulic properties and reconstruction of continuous vertical water content profiles from time-lapse GPR data. The proposed method shows great promise for field-scale, high-resolution digital soil mapping, and thereby for bridging the spatial-scale gap between ground truthing based on soil sampling or local probes and airborne and spaceborne remote sensing.
Sustainable and optimal agricultural and environmental management of water and land resources par... more Sustainable and optimal agricultural and environmental management of water and land resources particularly relies on the description and understanding of soil water distribution and dynamics at different scales. We present an advanced ground penetrating radar (GPR) method for mapping the shallow soil water content and unsaturated hydraulic properties at the field scale. The radar system is based on vector network analyzer technology, for which calibration is simple and constitutes an international standard. A directive horn antenna is used as both transmitter and receiver and operates off the ground. A full-waveform model describes accurately the radar signal, and is based on a linear system of complex transfer functions for efficiently describing electromagnetic phenomena within the antenna and its interaction with soil, and a specific solution of the three-dimensional Maxwell's equations for wave propagation in multilayered media. The soil electromagnetic properties and their vertical distribution are estimated by resorting to full-waveform inverse modeling using iterative global optimization methods. The proposed methodology has been validated for a series of model configurations of increasing complexity. The method is now routinely used for real-time mapping of soil surface water content and reconstruct a few number of shallow soil layers. For more complex configurations, it is necessary to regularize the inverse problem. We have shown that constraining radar data inversion using soil hydrodynamic modeling has the potential to reconstruct time-lapse, continuously variable, vertical soil water content profiles and identify the shallow unsaturated hydraulic properties. The proposed approach shows great promise for quantitative imaging of the soil properties at the field scale. The technique will be combined with electromagnetic induction in a mechanistic data fusion framework to further extend its capabilities in a digital soil mapping context.
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