Sebastian Busch

Chlorides and moisture assessment in concrete by GPR full waveform inversion

Corrosion of rebar within reinforced concrete is a major problem for countries where salt is applied to roads for de-icing. Concrete structures are periodically inspected in order to monitor possible damage caused by chloride-induced corrosion of the reinforcement. However, the available drilling and visual inspections do not supply sufficient spatial information or can only be assessed in advanced stages of corrosion, respectively. Consequently, the condition of bridge decks can only be assessed with low certainty. Therefore, a spatially continuous and non-destructive method detecting chloride in concrete structures is desirable. This paper describes a novel method to estimate material nproperties using the full-waveform inversion of bistatic off-ground ground penetrating radar data. In this way, all information present in the ground-penetrating radar (GPR) traces is used, which enables the estimation of quantitative electromagnetic properties. A critical step for full-waveform inversion is a proper characterization of our horn antenna GPR system by estimating the phase centre and the effective wavelet using measurements over a stainless steel plate. The inversion of GPR data measured over nine concrete specimens having different moisture and chloride contents nreturned a relative dielectric permittivity and a conductivity that included a frequency-dependent ncomponent. As expected, the inversion results for almost all specimens showed for increasing chloride and humidity content specimens increasing conductivity and permittivity values, respectively. nIn contrast to traditional ray-based techniques we were able to distinguish between moisture and chloride effects and to obtain quantitative values for the permittivity and conductivity. For increasing chloride content increasing frequency-dependent conductivity values were obtained.

Coupled hydrogeophysical inversion of time‐lapse surface GPR data to estimate hydraulic properties of a layered subsurface

[1]xa0A major challenge in vadose zone hydrology is to obtain accurate information on the temporal changes of the vertical soil water distribution and its feedback with the atmosphere and groundwater. A state of the art coupled hydrogeophysical inversion scheme is applied to evaluate soil hydraulic properties of a synthetic model and a field soil in southern Ontario based on time-lapse monitoring of soil dynamics with surface ground-penetrating radar (GPR). Film flow was included in the hydrological model to account for noncapillary water flow in a sandy medium during dry conditions. The synthetic study illustrated that GPR data contain sufficient information to accurately constrain soil hydraulic parameters within a coupled inversion framework and led to an accurate estimation of the soil hydraulic properties. When film flow was not accounted for within the inversion, an equally good fit could still be achieved. In this case, errors introduced by neglecting film flow were compensated by different hydraulic parameters. For the field data, the coupled inversion reduced the overall misfit compared to an uncalibrated model using hydraulic parameters obtained from laboratory data. Although the data fit improved significantly for water content in the deeper soil layers, accounting for film flow in the uppermost subsurface layer did not lead to a better fit of the GPR data. Further research is needed to describe the processes controlling water content in the dry range, in particular coupled heat and vapor transport. This study illustrates the suitability of surface GPR measurements combined with coupled inversion for near-surface characterization of soil hydraulic parameters.

Improved Characterization of Fine-Texture Soils Using On-Ground GPR Full-Waveform Inversion

Ground-penetrating radar (GPR) uses the recording of electromagnetic waves and is increasingly applied for a wide range of applications. Traditionally, the main focus was on the analysis of the medium permittivity since estimates of the conductivity using the far-field approximation contain relatively large errors and cannot be interpreted quantitatively. Recently, a full-waveform inversion (FWI) scheme has been developed that is able to reliably estimate permittivity and conductivity values by analyzing reflected waves present in on-ground GPR data. It is based on a frequency-domain solution of Maxwells equations including far, intermediate, and near fields assuming a 3-D subsurface. Here, we adapt the FWI scheme for on-ground GPR to invert the direct ground wave traveling through the shallow subsurface. Due to possible interference with the airwaves and other reflections, an automated time-domain filter needed to be included in the inversion. In addition to the obtained permittivity and conductivity values, also the wavelet center frequency and amplitude return valuable information that can be used for soil characterization. Combined geophysical measurements were carried out over a silty loam with significant variability in the soil texture. The obtained medium properties are consistent with Theta probe, electromagnetic resistivity tomography, and electromagnetic induction results and enable the formulation of an empirical relationship between soil texture and soil properties. The permittivities and conductivities increase with increasing clay and silt and decreasing skeleton content. Moreover, with increasing permittivities and conductivities, the wavelet center frequency decreases, whereas the wavelet amplitude increases, which is consistent with the radiation pattern and the antenna coupling characteristics.

Full-waveform inversion of multi-offset surface GPR data

Common ray-based techniques for analysing common midpoint (CMP) ground penetrating radar (GPR) data use only part of the measured traces and return results with limited resolution and non-quantitative values for conductivity. Due to the fact that full-waveform inversion uses all information of the measured traces, a higher resolution image of the subsurface and quantitative conductivity values can be obtained. Using an experimental dispersive CMP dataset, the results of dispersion analysis, i.e. permittivity and thickness, define the start model parameters for full-waveform inversion. After estimating an effective source wavelet, the full-waveform inversion based on a simplex search algorithm returns reliable permittivity and conductivity values of the subsurface.