M.R. Mahmoudzadeh

Evaluation of far-field layered media modeling for time-domain commercial GPR antenna

The possibility to estimate accurately the subsurface electromagnetic properties from commercial ground penetrating radar (GPR) signals using inverse modeling is obstructed by the appropriateness of the forward model describing the GPR subsurface system. In 2004 Lambot et al. proposed a promising GPR model for multilayered media characterization based on frequency-domain subsurface Greens functions and antenna effects filtering by linear transfer functions. In this study we analyzed the feasibility of full-waveform frequency-domain modeling for off-ground time-domain commercial GPR. We calculated the antenna transfer functions using measurements with a 1 GHz center frequency transmitting and receiving horn antenna situated at different heights above a copper plane. Also we validated the approach in the laboratory with measurements performed over the water as a homogeneous medium with frequency dependent electrical properties. A single Debye relaxation model was used to describe the frequency dependent water electrical properties with correcting the relaxation period and static dielectric permittivity of water for temperature. Inversion of the radar data in the frequency-domain provided relatively accurate estimations of the antenna heights and water layer thickness. These results demonstrated the ability of full-waveform modeling to be also used for commercial time-domain radars.

Road inspection using full-wave inversion of far-field ground-penetrating radar data

Inspecting how robustly and safely the roads are built in order to develop better methods for both design and quality control of materials is an important task in road engineering. In this research we studied the potential of full-wave model of Lambot et al. 2004 for far-field ground-penetrating radar (GPR) configuration in order to retrieve the physical and electromagnetic properties of the road, including both asphalt and subbase layers. Two different GPR systems, namely, stepped frequency GPR and impulse GPR systems were used to collect data along a 50 m long transect. Accurate positioning was performed using a dGPS and a survey wheel. Both GPR data sets were processed using GPR full-wave inversion in the time domain focusing on the asphalt and subbase reflections. Comparison of the results from both GPR systems showed a good agreement between them. In addition, results showed that a compacted zone on the road leads to a compaction feature influenced the retrieved road parameters. The proposed radar data processing method demonstrated a strong potential of the GPR full-wave model to be used for quantitative road inspection.

Influence of acquisition related uncertainties on ground penetrating radar inversion results

Ground penetrating radar non-destructive testing requirements become more demanding. In that respect, it is essential to understand how antenna transfer functions and real system dynamic range affect the amount of retrievable information from measurements. This paper examines the confidence level of linear system transfer functions, and practical aspects of noise floor influence on stepped frequency ground penetrating radar. Small inaccuracies in antenna equations directly reflect in Greens function errors. Degradation of the numerical signal resolution was imposed by limiting the data resolution to two-bytes. The capability of the field system to detect thin layering in building materials using numerical modeling studies has been investigated. This would lead to the assumption that the radar model is correct and the only errors are due to inaccuracies in the data and the finite precision determination of the systems linear transfer functions. Systematic errors characterization is partially possible, limiting uncertainty in the data. A link seems to exist between the data precision and the possibility of information extrapolation for different settings.

Water table detection by GPR in Sardon, Salamanca, Spain

GPR was applied in the semi-arid Sardon catchment (Salamanca, Spain) in order to analyze the distribution of the water table depth with a high spatial resolution to serve as input in the parameterization of a hydrological model. We used a pulse radar with a single 200 MHz bowtie antenna combined with a differential GPS and a survey wheel for accurate positioning. Measurements were performed following a series of transects crossing perpendicularly the bed of the Sardon streams, which were dry during that period (September 2009). We measured the depth of the water table in several observation wells to interpret and validate the GPR data. A time domain reflectometry probe was used to estimate the shallow soil dielectric permittivity and corresponding wave propagation velocity along the transects. In general, the water table was visible in the GPR data, with depths ranging from about 2 to 3 meters.

Ground-penetrating radar for correlation analysis of temporal soil moisture stability and land-slope

Knowledge of temporal surface soil moisture variability is an useful key in agriculture, surface hydrology and meteorology. In that respect, ground-penetrating radar (GPR) is a non-invasive and promising tool for high-resolution and large scale characterization. In the case of quantitative analysis, off-ground GPR signal modeling and full-waveform inversion has shown a great potential during the last decade. In this research, we applied GPR for time-laps measurements in an agricultural field along a 320 m single transect with a significant land-slope for about 3 months. A 200-2000 MHz TEM-horn antenna situated 1.1 m above the ground, connected to a vector network analyzer (VNA) was used as an off-ground frequency-domain GPR. The accurate positioning was done using a differential GPS. All systems were mounted on a 4-wheels vehicle for realtime and automated mapping. The calibration of the antenna and using the GPR signal inversion permitted to the ground surface relative dielectric permittivity. Topps model was used for transformation of the relative dielectric permittivity to soil moisture. The temporal stability of the field-average soil moisture was computed by indicators based on the relative difference of the soil moisture to the field-average. The results showed an excellent correlation amount of -0.9905 for temporal stability of soil moisture and slope variability.