3-D characterization of high-permeability zones in a gravel aquifer using 2-D crosshole GPR full-waveform inversion and waveguide detection

Reliable high-resolution 3-D characterization of aquifers helps to improve our understanding of flow and transport processes when small-scale structures have a strong influence. Crosshole ground penetrating radar (GPR) is a powerful tool for characterizing aquifers due to the method's high-resolution and sensitivity to porosity and soil water content. Recently, a novel GPR full-waveform inversion algorithm was introduced, which is here applied and used for 3-D characterization by inverting six crosshole GPR cross-sections collected between four wells arranged in a square configuration close to the Thur River in Switzerland. The inversion results in the saturated part of this gravel aquifer reveals a significant improvement in resolution for the dielectric permittivity and electrical conductivity images compared to ray-based methods. Consistent structures where acquisition planes intersect indicate the robustness of the inversion process. A decimetre-scale layer with high dielectric permittivity was revealed at a depth of 5-6 m in all six cross-sections analysed here, and a less prominent zone with high dielectric permittivity was found at a depth of 7.5-9 m. These high-permittivity layers act as low-velocity waveguides and they are interpreted as high-porosity layers and possible zones of preferential flow. Porosity estimates from the permittivity models agree well with estimates from Neutron-Neutron logging data at the intersecting diagonal planes. Moreover, estimates of hydraulic permeability based on flowmeter logs confirm the presence of zones of preferential flow in these depth intervals. A detailed analysis of the measured data for transmitters located within the waveguides, revealed increased trace energy due to late-arrival elongated wave trains, which were observed for receiver positions straddling this zone. For the same receiver positions within the waveguide, a distinct minimum in the trace energy was visible when the transmitter was located outside the waveguide. A novel amplitude analysis was proposed to explore these maxima and minima of the trace energy. Laterally continuous low-velocity waveguides and their boundaries were identified in the measured data alone. In contrast to the full-waveform inversion, this method follows a simple workflow and needs no detailed and time consuming processing or inversion of the data. Comparison with the full-waveform inversion results confirmed the presence of the waveguides illustrating that full-waveform inversion return reliable results at the highest resolution currently possible at these scales. We envision that full-waveform inversion of GPR data will play an important role in a wide range of geological, hydrological, glacial and periglacial studies in the critical zon

2.5D crosshole GPR full-waveform inversion with synthetic and measured data

Full-waveform inversion (FWI) of cross-borehole Ground Penetrating Radar (GPR) data is a technique with the potential to investigate the subsurface structures. Typical FWI applications transform the 3D measurements into a 2D domain via an asymptotic 3D to 2D data transformation, widely known as a Bleistein filter. Despite the broad use of such a transformation, it requires some assumptions that make it prone to errors. Although the existence of the errors is known, previous studies have failed to quantify the inaccuracies introduced on permittivity and electrical conductivity estimation. Based on a comparison of 3D and 2D modeling, errors could reach up to 30% of the original amplitudes in layered structures with high contrast zones. These inaccuracies can significantly affect the performance of the crosshole GPR FWI in estimating permittivity and especially electrical conductivity. We addressed these potential inaccuracies by introducing a novel 2.5D crosshole GPR FWI that utilizes a 3D finite-difference time-domain forward solver (gprMax3D). This allows us to model GPR data in 3D, while carrying out FWI in the 2D plane. Synthetic results showed that 2.5D crosshole GPR FWI outperformed the 2D FWI by achieving higher resolution and lower average errors for permittivity and conductivity models. The average model errors in the whole domain were reduced by around 2% for both permittivity and conductivity, while zone-specific errors in high contrast layers were reduced by about 20%. We verified our approach using crosshole 2.5D FWI measured data, and the results showed good agreement with previous 2D FWI results and geological studies. Moreover, we analyzed various approaches and found an adequate trade-off between computational complexity and accuracy of the results, i.e. reducing the computational effort whilst maintaining the superior performance of our 2.5D FWI scheme

The potential of time-lapse GPR full-waveform inversion as high resolution imaging technique for salt and ethanol transport

Crosshole GPR full-waveform inversion (FWI) has shown a high potential to characterize the near surface at a decimeter scale which is crucial for flow and transport. GPR FWI provide high-resolution tomograms of dielectric permittivity and electrical conductivity, which can be linked lithological properties. This study tests the potential of time-lapse GPR FWI to monitor tracers of different geophysical properties (salt, heat, ethanol). Synthetic and preliminary field results show that both properties can resolve major transport processes

Construction of Minirhizotron Facilities for Investigating Root Zone Processes

Minimally invasive monitoring of root development and soil states (soil moisture, temperature) in undisturbed soils during a crop growing cycle is a challenging task. Minirhizotron (MR) tubes offer the possibility to view root development in situ with time. Two MR facilities were constructed in two different soils, stony vs. silty, to monitor root growth, root zone processes, and their dependence on soil water availability. To obtain a representative image of the root distribution, 7-m-long tubes were installed horizontally at 10-, 20-, 40-, 60-, 80-, and 120-cm depths. A homemade system was developed to install MR tubes in the silty soil in horizontally drilled straight holes. For the stony soil, the soil rhizotubes were installed in an excavated and subsequently backfilled pit. In both facilities, three subplots were established with different water treatments: rain sheltered, rainfed, and irrigated. To monitor soil moisture, water potential, and soil temperature, time domain reflectometer probes, tensiometers, and matrix water potential sensors were installed. Soil water content profiles in space and time were obtained between two MR tubes using cross-hole ground-penetrating radar along the tubes at different depths. Results from the first growing season of winter wheat (Triticum aestivum L.) after installation demonstrate that differences in root development, soil water, and temperature dynamics can be observed among the different soil types and water treatments. When combined with additional measurements of crop development and transpiration, these data provide key information that is essential to validate and parameterize root development and water uptake models in soil–vegetation–atmosphere transfer models

Full-waveform inversion of crosshole GPR data for hydrogeological applications

High resolution and precise characterization of aquifers is needed to improve the understanding of flow and solute transport processes. Decimeter-scale and high contraststructures caused by changes in the porosity or clay content can have a dominant effect on hydraulic processes within an aquifer. Such heterogeneities or layering in aquifers can be related to preferential flow paths or impermeable clay lenses and can act as electromagnetic low-velocity waveguides. Crosshole ground penetrating radar is able to provide shallow subsurface electrical properties, viz. dielectric permittivity and electrical conductivity, and has proven a powerful tool to map and characterize aquifers due to the method’s high resolution and sensitivity to porosity and soil water content. Ray-based methods, which incorporate only a small part of the measured signal in the inversion, such as first-arrival travel times and first-cycle amplitudes, are not able to detect such layers. In contrast, the crosshole GPR full-waveform inversion, which considers the entire waveform or significant parts thereof, is able to resolve sub-wavelength high resolution images and can detect high contrast layers. Recently, a novel 2D time-domain vectorial full-waveform crosshole radar inversion was introduced that significantly improves the model resolution compared to standard ray-based techniques. This GPR full-waveform inversion is modified by allowing an optimized acquisition setup that significantly reduces the acquisition time and computational costs. The improved algorithm is employed to invert crosshole GPR data acquired within a gravel aquifer in the Thur valley, Switzerland, using the ray-based results as starting models. Compared to the ray-based inversion, the results from the full-waveform inversion show images with significantly higher resolution. The simulated traces of the final model, obtained by the full-waveform inversion, fit the observed traces very well in the lower part of the section and reasonably well in the upper part of the section. By incorporating the vadose zone and the water table in the starting models and inversion domain, we are able to improve the initial results and resolve unprecedented sub-wavelength high resolution images for permittivity and conductivity in the entire inversion domain including a high permittivity layer between 5 m - 6 m depth. This high permittivity layer acts as an electromagnetic low-velocity waveguide and is caused by an increased porosity indicating a [...