Mohammad Reza Mahmoudzadeh Ardekani

Full-Wave Calibration of Time- and Frequency-Domain Ground-Penetrating Radar in Far-Field Conditions

Full-wave modeling of ground-penetrating radar (GPR) data using Greens functions for wave propagation in planar layered media and antenna characteristic global reflection and transmission functions for describing far-field antenna effects, including antenna-medium interactions, has shown a great potential for nondestructive characterization of soils and materials. The accuracy of the retrieved parameters in the GPR data inversion depends on the accuracy of the GPR external calibration. In this research we studied the stability and the repeatability of two different GPR systems, namely, frequency- and time-domain systems. A combination of a vector network analyzer and 800-5200 MHz horn antenna was used as a frequency-domain GPR (FD-GPR) whereas a GSSI GPR system using a 900 MHz bowtie antenna was used as a time-domain GPR (TD-GPR). Both GPR systems including their antennas were calibrated several times using measurements with the antennas at different heights over a perfect electric conductor (PEC) in the laboratory as well as over a water layer. In addition, measurements were performed over a thin water layer and a relatively thick sandy soil layer as validating medium. The results showed that the FD-GPR is relatively stable while the TD-GPR presents a significant drift which can be accounted for using corrections based on the air direct-coupling waves (free-space measurements). Water- and PEC-based calibrations provided very similar results for the GPR calibration functions. Inversions for the water layer and the sandy soil layer provided reliable results and showed a high degree of the repeatability for both radar systems. The error on the calibration based on inaccurate antenna heights over PEC showed the significant errors on the inversion results for the directive antenna (horn antenna) but less error for the bowtie antenna. This analysis demonstrated the general validity of the proposed far-field radar modeling approach, not only with respect to frequency and time domain radars but as well with respect to the calibrating medium.

GPR data inversion for vegetation layer

In this paper, we developed a vegetation cover model for full-wave inversion of ground-penetrating radar (GPR) data assuming multiple layers with effective electrical properties. Measurements were performed in the laboratory conditions over spelt wheat grown in a sandy soil, using a stepped-frequency continuous-wave GPR system. In order to control the sand surface water content, a water table was fixed at 20 cm depth as well as time domain reflectometry probes at 2 and 20 cm depth were used. Measurements were performed and recorded hourly for about one month and the canopy height was measured daily. The effective electromagnetic properties of the vegetation layer were modeled using a complex refractive index model combined with a single Debye relaxation model. A scattering loss parameter was included in the electromagnetic full-wave GPR model for the vegetation layer. The measurements resulted in a strong link between the scattering losses of the vegetation cover and canopy height (generally, the biomass structure), which is expected to strongly influence the soil moisture values (measured with GPR) in presence of vegetation cover.

Soil moisture variability effect on GPR data

In this paper the effect of local variability of soil moisture within the antenna footprint on GPR data is studied. A combination of GprMax3D with GPR full-wave model of Lambot and André (2013) is used for an errorless set of measurements. The GprMax3D simulations are used as real GPR measurements regarding several physical-based configurations. The Greens functions of the simulated data are extracted using calibrations based on GPR full-wave models. The inversion results of horizontal local soil moisture variability focusing on the surface wavelet reflection are compared with the averaged soil moisture values within different antenna footprints which led to the antenna footprint of -9 dB as the best. Finally, the inversion results of the vertical soil moisture variability shows significant effect of shallow soil moisture layering on the GPR-retrieved soil moisture values, which is highly correlated to the antenna height from the ground surface.

SAR-image derived soil moisture enhancement using GPR data

Measuring the spatial distribution of soil moisture is important for agricultural, hydrological, meteorological and climatological research and applications. In this study, a new technique is developed to create soil moisture maps, based on the inversion of SAR measurements (RADARSAT-2, fine quad polarization) combined with GPR measurements. The Integral Equation Model is used to invert the SAR measurements, assuming a constant surface roughness and correlation length for the entire field, while the GPR data are inverted using a full wave inversion method. High resolution GPR measurements taken at different times under different land and weather conditions are used to generate a relative soil moisture landscape. We assume that these soil moisture difference patterns show little variation over time. By combining the inverted SAR data with a transformation of the soil moisture difference landscape, a high resolution soil moisture map is generated. The high resolution soil moisture maps show good agreement with the measured GPR soil moisture maps. The advantage of this technique is that once the relative soil moisture difference landscape is created, it allows the creation of new high resolution soil moisture maps later, by only taking a SAR image.