Jan van der Kruk

Lysimeters are widely used to investigate soil‐plant‐atmosphere interactions and to assess the impact of management practices on water and nutrient fluxes. These interactions and fluxes are strongly impacted by local scale variability in soil water content and soil hydraulic properties. However, centimeter‐scale imaging of subsurface processes and the controlling variables such as soil water content remains challenging, and conventional approaches often rely on simplifying assumptions that smooth heterogeneities. This study presents a theoretical investigation of full‐waveform ground penetrating radar (GPR) modeling for cylindrical lysimeters, with the objective of assessing the lysimeter‐induced effects on GPR data and their implications for data analysis and inversions. Numerical experiments were conducted for three scenarios: homogeneous dry and saturated sand, heterogeneous soil, and a simplified experimental model, using different antenna frequencies and explicitly accounting for the three‐dimensional lysimeter geometry. The results demonstrate that incorporating the cylindrical geometry is essential, particularly for dry soil conditions and high‐frequency data. The modeled wavefields reveal strong interference among direct, reflected, and refracted waves, consistent with observations from experimental GPR data. While wave‐type distinction is more straightforward in saturated soils, increased attenuation limits signal penetration. A simplified layered model inside the lysimeter geometry was used to reproduce dominant observed features of experimental data from a layered sand‐filled lysimeter, providing a robust foundation for future FWI applications in lysimeter studies.

Soil hydraulic parameters (SHP) play a crucial role controlling the spatiotemporal distribution of water in the soil–plant continuum and thus affect water availability for crops. To provide reliable information on the SHP at different scales, measurement techniques with a good spatial resolution and low labor costs are required. In this study, we used crosshole ground penetrating radar (GPR)‐derived soil water contents (SWCs) measured along horizontal rhizotubes under a controlled experimental test site cropped with winter wheat to estimate the unimodal and dual‐porosity soil hydraulic characteristics with different soil layer setups. Therefore, sequential inversion of the GPR‐derived SWCs was performed using the hydrological model HYDRUS‐1D, whereby the SWC data were either averaged prior inversion or used in a spatially distributed way. To analyze if the time‐lapse gathered GPR data contain enough information to estimate the SHP, additional synthetic studies were performed increasing the data resolution to daily GPR measurements. The results showed that the time‐lapse data contained enough information to estimate the SHP accurately. Additionally, spatially distributed soil hydraulic characteristics differed from the one estimated based on averaged SWCs derived from spatially distributed GPR data. Finally, we derived spatially resolved SHP, which can be used for 3D process rhizosphere processes and root–soil interaction modeling.

Non‐invasive imaging of processes within the soil–plant continuum, particularly root and soil water distributions, can help optimize agricultural practices such as irrigation and fertilization. In this study, in‐situ time‐lapse horizontal crosshole ground penetrating radar (GPR) measurements and root images were collected over three maize crop growing seasons at two minirhizotron facilities (Selhausen, Germany). Root development and GPR permittivity were monitored at six depths (0.1–1.2 m) for different treatments within two soil types. We processed these data in a new way that gave us the information of the “trend‐corrected spatial permittivity deviation of vegetated field,” allowing us to investigate whether the presence of roots increases the variability of GPR permittivity in the soil. This removed the main non‐root‐related influencing factors: static influences, such as soil heterogeneities and rhizotube deviations, and dynamic effects, such as seasonal moisture changes. This trend‐corrected spatial permittivity deviation showed a clear increase during the growing season, which could be linked with a similar increase in root volume fraction. Additionally, the corresponding probability density functions of the permittivity variability were derived and cross‐correlated with the root volume fraction, resulting in a coefficient of determination ( R ² ) above 0.5 for 23 out of 46 correlation pairs. Although both facilities had different soil types and compaction levels, they had similar numbers of good correlations. A possible explanation for the observed correlation is that the presence of roots causes a redistribution of soil water, and therefore an increase in soil water variability.

Imaging subsurface small‐scale features and monitoring transport of tracer plumes at a fine resolution is of interest to characterize transport processes in aquifers. Full‐waveform inversion (FWI) of crosshole ground penetrating radar (GPR) measurements enables aquifer characterization at decimeter‐scale resolution. The method produces images of both relative dielectric permittivity (ε_r) and bulk electrical conductivity (σ_b) that can be related to hydraulic aquifer properties and tracer distributions. To test the potential of time‐lapse GPR FWI for imaging tracer plumes, we conducted a numerical tracer experiment by injecting saline water, desalinated water, and ethanol in a heterogeneous aquifer. The saline and desalinated tracers only changed σ_b, whereas ethanol changed both ε_r and σ_b. Tracer concentrations were retrieved from the inverted ε_r and σ_b models using information about petrophysical parameters. GPR FWI ε_r and σ_b tracer images could recover preferential paths of ∼0.2 m width, while the derived σ_b structures are smoother. FWI of 50 time‐lapse data sets demonstrated the potential of the FWI to derive spatially resolved breakthrough curves of the saline and ethanol tracer in the image plane between the boreholes. Thus, high‐resolution imaging with GPR FWI of tracers that produce a high permittivity contrast against the background has a great potential for characterization of heterogeneous transport in aquifers.

To unequivocally discover the actual presence of life or even of near surface liquid water on extraterrestrial planetary bodies would be a key scientific breakthrough for humankind. For this reason, studying similar environments on Earth is essential to understanding the processes shaping such extraterrestrial objects. The Yungay area in the Chilean part of the Atacama Desert is deemed to be particularly suitable as a terrestrial analog of Mars (TAM). In this study, we deployed multi‐frequency ground penetrating radar (GPR) and a six‐coil electromagnetic induction (EMI) system with a maximum depth of investigation of 1.8 m over an area of 0.66 hectares (110 x 60 m). By applying a LOWESS algorithm to the GPR envelope data, we aimed to extrapolate the strongest amplitudes indicating physical contrasts to 3D. The results were constrained with two existing pits 100 m apart. Whereas clay content was mostly responsible for GPR signal attenuation, changes in texture and stratigraphy were linked with strong amplitude reflections. EMI showed very low apparent electrical conductivity ( ECa ) values between 0 and 5 mS/m. The ECa variability could be linked to changes in clay content with depth. This agreed with the surface obtained from the LOWESS algorithm. Although soil samples are still necessary to constrain the measured signals, we showed the benefits of applying geophysics for large‐scale characterization and can conclude that these two methods are suitable for such hyperarid TAM environments. A similar routine if applied on the surface of Mars could deliver promising results for similar characteristics.

For high-resolution crosshole ground-penetrating radar (GPR) tomography, a wide-range of ray path angles are required, including transmitter-receiver pairs with high-angles. However, artifacts have been observed in the inverted GPR tomograms when high-angle data were incorporated in ray-based inversion (RBI) tomography due to not well-understood increasing apparent velocities for increasing ray-angles. To reduce these artifacts, it is common practice to limit the angular aperture to a threshold between 30° and 50°, which reduces the spatial resolution. We apply 3-D finite-difference time-domain GPR modeling including borehole fluid and resistive loaded finite-length antenna (FLA) models to study the increase of apparent velocity with increasing ray path angle. This study shows that the strong refraction of the electromagnetic waves at the borehole interface between water and subsurface is one of the reasons for these not well-understood phenomena. We introduce a novel borehole-fluid effect correction (BFEC) that relocates the transmitter and receiver positions to the location, where the refraction is occurring to remove any influence of the borehole such that the remaining traveltimes can be inverted using an RBI. BFEC improves the estimated apparent-velocity (relative permittivity) values and enables the incorporation of wide-angle ray paths resulting in more accurate tomograms. We verify the BFEC with a simple layered subsurface model and a realistic synthetic model. By applying curved-ray RBI without and with the BFEC, the subsurface structures are reconstructed with more details for the BFEC data and the average relative error model reduced from 13% to below 8% for the high-resolution inhomogeneous model.

The movement and spreading of contaminated groundwater plumes and their mixing with non-contaminated water is strongly influenced by the heterogeneity of the aquifer properties, which may vary strongly over small spatial scales. Thus, imaging these small-scale features and monitoring transport of tracer plumes at a fine resolution is of interest to characterize transport processes in aquifers. Full-waveform inversion (FWI) of crosshole ground penetrating radar (GPR) measurements can provide an aquifer characterization at decimeter-scale resolution. The method produces images of both relative dielectric permittivity ( ε _r ) and bulk electrical conductivity ( σ _b ), which related to hydraulic aquifer properties and tracer distributions. To test the potential of time-lapse GPR FWI for imaging tracer plumes, we conducted a numerical experiment of tracer transport in a heterogeneous aquifer. Concentration was converted to saline and desalinated tracers, which changed σ _b , and to ethanol, which changed both ε _r and σ _b . The simulated ε _r and σ _b distributions in a crosshole plane were considered to simulate GPR data. These data were subsequently used to reconstruct ε _r and σ _b distributions using the crosshole 2D GPR FWI. Tracer concentrations were retrieved from the inverted ε _r and σ _b models using information about petrophysical parameters. GPR FWI ε _r images could recover preferential paths of ~0.2 m width, while the σ _b images resolved structures up to ~ 0.2-0.3 m. The results highlight that changes in ε _r , e.g., ethanol and hot water, can be used to image transport processes with high resolution by time-lapse GPR FWI, while the accuracy of the recovery of σ _b is limited.

Detailed knowledge of the intra‐field variability of soil properties and crop characteristics is indispensable for the establishment of sustainable precision agriculture. We present an approach that combines ground‐based agrogeophysical soil and aerial crop data to delineate field‐specific management zones that we interpret with soil attribute measurements of texture, bulk density, and soil moisture, as well as yield and nitrate residue in the soil after potato ( Solanum tuberosum L.) cultivation. To delineate the management zones, we use aerial drone‐based normalized difference vegetation index (NDVI), spatial electromagnetic induction (EMI) soil scanning, and the EMI–NDVI data combination as input in a machine learning clustering technique. We tested this approach in three successive years on six agricultural fields (two per year). The field‐scale EMI data included spatial soil information of the upper 0–50 cm, to approximately match the soil depth sampled for attribute measurements. The NDVI measurements over the growing season provide information on crop development. The management zones delineated from EMI data outperformed the management zones derived from NDVI in terms of spatial coherence and showed differences in properties relevant for agricultural management: texture, soil moisture deficit, yield, and nitrate residue. The combined EMI–NDVI analysis provided no extra benefit. This underpins the importance of including spatially distributed soil information in crop data interpretation, while emphasizing that high‐resolution soil information is essential for variable rate applications and agronomic modeling.

Detailed characterization of aquifers is critical and challenging due to the existence of heterogeneous small-scale high-contrast layers. For an improved characterization of subsurface hydrologic characteristics, crosshole ground-penetrating radar (GPR) and cone penetration test (CPT) measurements are performed. In comparison to the CPT approach, which can only provide 1D high-resolution data along vertical profiles, crosshole GPR enables measuring 2D cross sections between two boreholes. In general, a standard inversion method for GPR data is the ray-based approach, which considers only a small amount of information and can therefore only provide limited resolution. In the past few decades, full-waveform inversion (FWI) of crosshole GPR data in the time domain has matured, and it provides inversion results with higher resolution by exploiting the full-recorded waveform information. However, FWI results are limited due to complex underground structures and the nonlinear nature of the method. A new approach that uses CPT data in the inversion process is applied to enhance the resolution of the final relative permittivity FWI results by updating the effective source wavelet. The updated effective source wavelet possesses a priori CPT information and a larger bandwidth. Using the same starting models, a synthetic model comparison between the conventional and updated FWI results demonstrates that the updated FWI method provides reliable and more consistent structures. To test the method, five experimental GPR cross section results are analyzed with the standard FWI and the new proposed updated approach. The synthetic and experimental results indicate the potential of improving the reconstruction of subsurface aquifer structures by combining conventional 2D FWI results and 1D CPT data.