Estimating soil electric properties from monostatic ground‐penetrating radar signal inversion in the frequency domain

Abstract

[1] A new integrated approach for identifying the shallow subsurface electric properties from ground-penetrating radar (GPR) signal is proposed. It is based on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with a dielectric filled transverse electric and magnetic (TEM) horn antenna to be used off the ground in monostatic mode; that is, a single antenna is used as emitter and receiver. This radar configuration is appropriate for subsurface mapping and allows for an efficient and more realistic modeling of the radar-antenna-subsurface system. Forward modeling is based on linear system response functions and on the exact solution of the three-dimensional Maxwell equations for wave propagation in a horizontally multilayered medium representing the subsurface. Subsurface electric properties, i.e., dielectric permittivity and electric conductivity, are estimated by model inversion using the global multilevel coordinate search optimization algorithm combined sequentially with the local NelderMead simplex algorithm (GMCS-NMS). Inversion of synthetic data and analysis of the corresponding response surfaces proved the uniqueness of the inverse solution. Laboratory experiments on a tank filled with a homogeneous sand subject to different water content levels further demonstrated the stability and accuracy of the solution toward measurement and modeling errors, particularly those associated with the dielectric permittivity. Inversion for the electric conductivity led to less satisfactory results. This was mainly attributed to the characterization of the frequency response of the antenna and to the high frequency dependence of the electric conductivity.

Mapping the spatial variation of soil water content at the field scale with different ground penetrating radar techniques

“…The radar system was set up using a Vector Network Analyzer (VNA) (FSH6 equipped with a VSWR bridge and power provider, Rohde & Schwarz, measurement is analyzed. A full description of the modeling of the radar signal is given by Lambot et al (2004aLambot et al ( , 2004bLambot et al ( , 2005. Due to the poor raw data quality in the higher frequency range, which might be caused by surface roughness, the fitting procedure using inverse simulation was performed on the frequency window of 800 to 1600 MHz only.…”

“…Following the approach of Lambot et al (2004aLambot et al ( , 2004bLambot et al ( , 2004cLambot et al ( , 2005, an ultra wideband stepped-frequency continuous-wave radar combined with an off-ground monostatic transverse electromagnetic (TEM) horn antenna (BBHA 9120 A, Schwarzbeck Mess-Elektronik, Schönau, Germany) was used. The radar system was set up using a Vector Network Analyzer (VNA) (FSH6 equipped with a VSWR bridge and power provider, Rohde & Schwarz, measurement is analyzed.…”

“…Alternatively, it would have been possible to determine the gravimetric water content (e.g. gram water per 15 gram soil) of this shallow upper layer. However, to convert the gravimetric water content to volumetric water content (the soil property sensed by electromagnetic methods) information on the dry bulk density of the upper layer is required.…”

“…The first approach is based on the analysis of reflected waves ( general, the ground wave is the part of the radiated energy that travels between the transmitter and receiver through the top of the soil. The third approach is based on analyzing the (sub-) surface reflections of an off-ground GPR system, as described by Davis and Annan (2002), Redman et al (2002) and Lambot et al (2004aLambot et al ( , 2004bLambot et al ( , 2006a) amongst others. The advantage of the monostatic off-ground GPR method proposed by Lambot et al (2004aLambot et al ( , 2004bLambot et al ( , 2004c 2006a) is the possibility of a full wave inversion of the radar signal which takes into account the antenna effects and does not require an antenna height specific calibration above a perfect electric conductor (PEC, e.g.…”

“…In general, the antenna elevation above the soil inherently varies during field experiments, which may lead to significant errors using the traditional off-ground methods. In our study, the ground wave approach and the monostatic 15 off-ground approach were tested on a field site where both texture and soil water content are highly variable.…”