Full-Waveform Modeling and Inversion of Ground Penetrating Radar Data for Non-Invasive Characterization of the Soil Hydrogeophysical Properties

Lambot, Sébastien;Slob, E.C.;Minet, Julien;Jadoon, Khan Zaib;Vereecken, H.;et.al.
(2010) Proximal Soil Sensing, Developments in Soil Science Series — ISBN: [978-90-481-8858-1], p. 299-313, published

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Abstract
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.
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Lambot, S., Slob, E. C., Minet, J., Jadoon, K. Z., Vanclooster, M., & Vereecken, H. (2010). Full-Waveform Modeling and Inversion of Ground Penetrating Radar Data for Non-Invasive Characterization of the Soil Hydrogeophysical Properties. In Viscara Rossel R.A., A.B. McBratney and B. Minasny (ed.), Proximal Soil Sensing, Developments in Soil Science Series (p. p. 299-313). Springer. https://doi.org/10.1007/978-90-481-8859-8_25