High-resolution imaging of transport processes with GPR full-waveform inversion

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. GPR FWI provides 2D tomograms of the subsurface properties, the dielectric permittivity (ε) and electrical conductivity (σ), which can be correlated with hydrological properties. In the framework of the thesis, we conducted synthetic and experimental tracer tests that were monitored using time-lapse crosshole GPR full-waveform inversion results, to test the potential and limitation to reconstruct the tracer plume. For the synthetic test, we generated a realistic high resolution aquifer model based on previous hydrological and GPR FWI data from the Krauthausen test site in order perform a transport simulation that represents reasonable heterogeneity of the tracer concentration. Using petrophysical relations, we converted the concentration distribution to dielectric properties of specific tracers: saltwater (increase σ only), desalinated water (decrease σ only) and ethanol (decrease in both σ and ε). One important aspect of the GPR FWI is to investigate an optimal way to define adequate starting models especially for the time-lapse data. Therefore, we investigated three different starting model options in the synthetic test, resulting that ε and σ models from the background provide the most accurate FWI of time-lapse data. Hereby, both ε and σ FWI results have shown the potential to derive time-lapse changes. The gained insights of the synthetic optimization tests are applied for an experimental test. To prove the potential of the crosshole GPR FWI also under realistic conditions, we performed an experimental salt tracer experiment at the Krauthausen test site. Thereby, we injected to the sandy aquifer a salt tracer, and monitored the tracer development using crosshole GPR over a timeframe of 14 days within 5 crosshole planes in an area of 11x10 m. These time-lapse data are independently inverted using the background models of each plane as starting models as proposed from the synthetic study to derive the best FWI results. We investigated the consistency of the reconstruction of the plume by temporal and spatial continuity across neighboring planes, by correlating with borehole logging data, and with expectations based on previous tracer experiments from the same site. One challenge arise from the time-lapse GPR data caused by the change of the borehole filling properties over the time and transport of the plume. The salt and freshwater mixture in the tubes couple with the borehole antennae thus influence the GPR data. Fortunately, the processing for the FWI enables accounting this effect by estimating effective source wavelets for each time step and each plane, which compensate for borehole filling effects caused by the salt tracer. If these borehole filling effects would not be considered, errors in the results would occur. Performing the FWI considering the corrected effective source wavelets allows recovery of the aquifer models independently from saltwater-antennae effects. Such effects cannot be incorporated using standard ray-based approaches. In contrast from the synthetic tracer test, investigation of the best starting model for experimental data showed that σ homogenous model rather than from FWI background provides more accurate results for FWI of time-lapse data. This can be explained that possible errors in the FWI background results caused by measurement or starting model uncertainties, are forcing the FWI with these models to be trapped in a local minimum. The time-lapse GPR FWI has shown a reliable manifestation of a tracer of about 0.2 m resolution, which was not observed before from other geophysical monitoring techniques. These improved and higher resolution images of such a tracer transport can help in future to better constraint hydrological properties of interest for hydrological models. In this thesis, we have shown for the first time the potential of the GPR FWI to characterize and monitor tracer experiments using crosshole GPR data. Especially, the application to salt tracers, which traditionally were investigated with ERT, is now also possible with GPR and higher resolution images of the tracer transport are possible to obtain

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

Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

Essentially all hydrogeological processes are strongly influenced by the subsurface spatial heterogeneity and the temporal variation of environmental conditions, hydraulic properties, and solute concentrations. This spatial and temporal variability generally leads to effective behaviors and emerging phenomena that cannot be predicted from conventional approaches based on homogeneous assumptions and models. However, it is not always clear when, why, how, and at what scale the 4D (3D + time) nature of the subsurface needs to be considered in hydrogeological monitoring, modeling, and applications. In this paper, we discuss the interest and potential for the monitoring and characterization of spatial and temporal variability, including 4D imaging, in a series of hydrogeological processes: (1) groundwater fluxes, (2) solute transport and reaction, (3) vadose zone dynamics, and (4) surface–subsurface water interactions. We first identify the main challenges related to the coupling of spatial and temporal fluctuations for these processes. We then highlight recent innovations that have led to significant breakthroughs in high-resolution space–time imaging and modeling the characterization, monitoring, and modeling of these spatial and temporal fluctuations. We finally propose a classification of processes and applications at different scales according to their need and potential for high-resolution space–time imaging. We thus advocate a more systematic characterization of the dynamic and 3D nature of the subsurface for a series of critical processes and emerging applications. This calls for the validation of 4D imaging techniques at highly instrumented observatories and the harmonization of open databases to share hydrogeological data sets in their 4D components

High-resolution imaging of transport processes with GPR full-waveform inversion

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. GPR FWI provides 2D tomograms of the subsurface properties, the dielectric permittivity (ε) and electrical conductivity (σ), which can be correlated with hydrological propertie

Benchmark study using a multi-scale, multi-methodological approach for the petrophysical characterization of reservoir sandstones

This paper presents a detailed description and evaluation of a multi-methodological petrophysical approach for the comprehensive multi-scale characterization of reservoir sandstones. The suggested methodology enables the identification of links between Darcy-scale permeability and an extensive set of geometrical, textural and topological rock descriptors quantified at the pore scale. This approach is applied to the study of samples from three consecutive sandstone layers of Lower Cretaceous age in northern Israel. These layers differ in features observed at the outcrop, hand specimen, petrographic microscope and micro-CT scales. Specifically, laboratory porosity and permeability measurements of several centimetre-sized samples show low variability in the quartz arenite (top and bottom) layers but high variability in the quartz wacke (middle) layer. The magnitudes of this variability are also confirmed by representative volume sizes and by anisotropy evaluations conducted on micro-CT-imaged 3-D pore geometries. Two scales of directional porosity variability are revealed in quartz arenite sandstone of the top layer: the pore size scale of ∼0.1 mm in all directions and ∼3.5 mm scale related to the occurrence of high- and low-porosity horizontal bands occluded by Fe oxide cementation. This millimetre-scale variability controls the laboratory-measured macroscopic rock permeability. More heterogeneous pore structures were revealed in the quartz wacke sandstone of the intermediate layer, which shows high inverse correlation between porosity and clay matrix in the vertical direction attributed to depositional processes and comprises an internal spatial irregularity. Quartz arenite sandstone of the bottom layer is homogenous and isotropic in the investigated domain, revealing porosity variability at a ∼0.1 mm scale, which is associated with the average pore size. Good agreement between the permeability upscaled from the pore-scale modelling and the estimates based on laboratory measurements is shown for the quartz arenite layers. The proposed multi-methodological approach leads to an accurate petrophysical characterization of reservoir sandstones with broad ranges of textural, topological and mineralogical characteristics and is particularly applicable for describing anisotropy and heterogeneity of sandstones on various rock scales. The results of this study also contribute to the geological interpretation of the studied stratigraphic units

Benchmark study using a multi-scale, multi-methodological approach for the petrophysical characterization of reservoir sandstones

This paper presents a detailed description and evaluation of a multi-methodological petrophysical approach for the comprehensive multi-scale characterization of reservoir sandstones. The suggested methodology enables the identification of links between Darcy-scale permeability and an extensive set of geometrical, textural and topological rock descriptors quantified at the pore scale. This approach is applied to the study of samples from three consecutive sandstone layers of Lower Cretaceous age in northern Israel. These layers differ in features observed at the outcrop, hand specimen, petrographic microscope and micro-CT scales. Specifically, laboratory porosity and permeability measurements of several centimetre-sized samples show low variability in the quartz arenite (top and bottom) layers but high variability in the quartz wacke (middle) layer. The magnitudes of this variability are also confirmed by representative volume sizes and by anisotropy evaluations conducted on micro-CT-imaged 3-D pore geometries. Two scales of directional porosity variability are revealed in quartz arenite sandstone of the top layer: the pore size scale of ∼0.1 mm in all directions and ∼3.5 mm scale related to the occurrence of high- and low-porosity horizontal bands occluded by Fe oxide cementation. This millimetre-scale variability controls the laboratory-measured macroscopic rock permeability. More heterogeneous pore structures were revealed in the quartz wacke sandstone of the intermediate layer, which shows high inverse correlation between porosity and clay matrix in the vertical direction attributed to depositional processes and comprises an internal spatial irregularity. Quartz arenite sandstone of the bottom layer is homogenous and isotropic in the investigated domain, revealing porosity variability at a ∼0.1 mm scale, which is associated with the average pore size. Good agreement between the permeability upscaled from the pore-scale modelling and the estimates based on laboratory measurements is shown for the quartz arenite layers. The proposed multi-methodological approach leads to an accurate petrophysical characterization of reservoir sandstones with broad ranges of textural, topological and mineralogical characteristics and is particularly applicable for describing anisotropy and heterogeneity of sandstones on various rock scales. The results of this study also contribute to the geological interpretation of the studied stratigraphic units

Petrophysical measurements of sandstone samples extracted from an outcrop

Petrophysical measurements of samples extracted from 3 consecutive sandstone layers from an outcrop, in lower Cretaceous sandstone from North of Israel. The data includes: Grain size distribution, mercury intrusion porosimetry, XRD, porosity and permeability, micro-CT images. Further description in read_me file

Scientific deliverable Enigma ITN:Report on process-based geophysical methodologies to monitoring subsurface processes

The identified activities are to (i) develop an upscaling framework for quantifying the impact of spreading and mixing on geophysical signals (ESR9), (ii) develop quantitative inversion of SIP signals induced by biochemical processes (ESR12) and (iii) enhance resolution of time lapse geophysical imaging of transport with new experimental and inversion strategies (ESR10, ESR11). In the following report, we start by discussing the ongoing laboratory experiments and associated theoretical developments (ESR9 and 10), before moving to crosshole time-lapse GPR (ESR10) and finally to the use of innovative traces (ESR11) to image transport processes. In this way, we will naturally move from the pore scale and the scale of representative elementary volumes (REV), to metric scale and finally to larger field scales