Induced Polarization of Metal Grains: Simulations of Three-Dimensional Electric Fields

International audienceInduced polarization (IP) method is one of the geophysical methods that utilizes polarization of subsurface materials. This method is very sensitive to the presence of (semi) conducting materials. Previous studies revealed the underlying mechanisms of metal polarization in simple geometry. However, there is a gap between such studies and the conditions in geological materials. To fill the gap, I have established the simulation method for studying the polarization of metal grains in a three-dimensional field using the open-source computational-fluid-dynamic software OpenFOAM. The three-dimensional view of the surface potential distribution on a metal grain clarified how polarization is induced while varying geometrical constrictions. I varied the channel size, the orientation of the anisotropic grain, the elongation of the grain, and the distance of two grains. The results include: (a) a less decay of imaginary potential over distance for shorter or narrower channel, enhanced polarization in narrower channel and reduced polarization in shorter channel, (b) shift of the position of maximum imaginary potential over frequency on the surface of the oriented anisotropic grains, (c) higher peak frequency and larger polarization locally at the edge of the grains for elongated grains, and (d) enhancement of polarization by interaction of nearby grains. The underlying mechanisms were examined for each case. In addition, relationships between each scenario in my simulation and the subsurface environment were discussed. My successful implementation of the simulation for metal grains may be applied to improve the modeling, interpretation, and laboratory experimental setup

Spectral induced polarization of calcite precipitation in porous media

Precipitation and dissolution dynamics of calcite in response to variations in groundwater pH andalkalinity are important in a range of subsurface engineering applications. In the context of geotechnicalengineering and subsurface remediation applications, induced calcite precipitation is widely used. Theeffectiveness of induced calcite precipitation is typically investigated by measuring hydraulic pressure,analysing biochemical properties of sampled solution, and cone penetration tests. However, thesemethods are spatially and temporally limited, expensive and laborious. Geophysical methods havepotential to overcome these limitations. In particular, Spectral Induced Polarization (SIP) measurementshave been shown to be sensitive to calcite precipitation. However, previous experimental studiesshowed inconsistent SIP responses and have not explored what controls the SIP response of calcite. Inthis context, the overall aim of this thesis was to better understand how spatially variable and temporallydynamic calcite precipitation processes affect the SIP response.In a first step, the effect of solute concentration on the SIP response of calcite precipitation wasinvestigated by a four-phase experiment with SIP measurements on a column filled with sand. In phaseI, Na2CO3 and CaCl2 solutions were co-injected, which resulted in a calcite precipitation front thatincreased the imaginary part of the electrical conductivity ("). In phase II, several diluted (but stilloversaturated) solutions were injected into the sample with calcite precipitation from phase I, whichresulted in a decrease of ". The results suggested that the " associated with calcite depends morestrongly on the solute conductivity than in the case of sand and sandstone. In phase III, the solutionsfrom phase I were injected first and then the injection was stopped. Since calcite precipitation continuedafter stopping the flow, this resulted in a decrease of the solute concentration in the mixing zone and anassociated decrease of ". In phase IV, the injection rate of the Na2CO3 solution was reduced relativeto that of the CaCl2 solution. This shifted the mixing zone away from the calcite precipitation front andagain the " decreased. These results clearly suggested that the SIP response of calcite is sensitive tothe solute concentration near the precipitates

Oscillating reaction in porous media under saddle flow

International audiencePattern formation due to oscillating reactions represents variable natural and engineering systems, but previous studies employed only simple flow conditions such as uniform flow and Poiseuille flow. We studied the oscillating reaction in porous media, where dispersion enhanced the spreading of diffusing components by merging and splitting flow channels. We considered the saddle flow, where the stretching rate is constant everywhere. We generated patterns with the Brusselator system and classified them by instability conditions and Péclet number (Pe), which was defined by the stretching rate. The results showed that each pattern formation was controlled by the stagnation point and stable and unstable manifolds of the flow field due to the heterogeneous flow fields and the resulting heterogeneous dispersion fields. The characteristics of the patterns, such as the position of stationary waves parallel to the unstable manifold and the size of local stationary patterns around the stagnation point, were also controlled by Pe. © 2023 Author(s)

Dispersion Versus Diffusion in Mixing Fronts

International audienceMixing fronts form when fluids with different chemical compositions are brought into contact. They influence a large range of biogeochemical processes in hydrological systems. An important mechanism governing mixing rates in such fronts is stretching by non‐uniform flows that accelerates diffusive mass transfer by enhancing concentration gradients. In a range of systems, including porous media at Darcy scale, hydrodynamic dispersion dominates over diffusion to control local mixing rates. As it differs from diffusion through its velocity‐dependent dispersion tensor, it is not known how local dispersion interacts with macroscopic mixing front stretching. Here, we investigate the impact of local dispersion versus diffusion on the properties of steady mixing fronts created by both uniform and non‐uniform flows. We derive analytical solutions for the concentration profile, mixing scale and mixing rate across the fronts. We validate these predictions by comparison with numerical simulations and experiments performed in quasi two‐dimensional tanks over a broad range of Péclet numbers. Without porous media, the mixing scale is governed by local diffusion coupled with flow: it increases diffusively along streamlines in uniform flows while it is constant in converging flows due to the balance between fluid compression and local diffusion. With porous media, the Batchelor scale is no longer sustained and the mixing scale grows with dispersion in non‐uniform flows. In addition, the coupling between flow acceleration and dispersion results in a flow rate independent mixing interface, in contrast with the local diffusion scenario. We discuss the consequences of these findings on mixing rates in mixing fronts

Effect of solute concentration on the spectral induced polarization response of calcite precipitation

Induced calcite precipitation is used in geotechnics to modify the mechanical and hydrological properties of the underground. Laboratory experiments have shown that spectral induced polarization (SIP) measurements can detect calcite precipitation. However, the results of previous studies investigating the SIP response of calcite precipitation were not fully consistent. This study aims to investigate how the SIP response of calcite depends on solute concentration to explain the differences in SIP response observed in previous studies. A four-phase experiment with SIP measurements on a column filled with sand was performed. In phase I, calcite precipitation was generated for a period of 12 d by co-injecting Na2CO3 and CaCl2 solutions through two different ports. This resulted in a well-defined calcite precipitation front, which was associated with an increase in the imaginary part of the conductivity (⁠σ′′⁠). In phase II, diluted solutions were injected into the column. This resulted in a clear decrease in σ′′⁠. In phase III, the injection of the two solutions was stopped while calcite precipitation continued and solute concentrations in the mixing zone decreased. Again, this decreased σ′′⁠. Finally, the injection rate of the Na2CO3 solution was reduced relative to that of the CaCl2 solution in phase IV. This resulted in a shift of the mixing zone away from the calcite precipitation front established in phase I and an associated decrease of σ′′⁠. These results imply that the SIP response of calcite is highly sensitive to the solute concentration near the precipitates, which may explain previously reported conflicting results

Monitoring of methane emission from a landfill site in daily and hourly time scales using an automated gas sampling system

Landfill sites are significant sources of methane gas globally. Understanding the temporal variabilities of methane emissions from landfill sites is necessary for estimating such emissions. In this study, an automated monitoring system was used to monitor methane emission flux and concentration on daily and hourly time scales at a landfill site. Measured methane emission fluxes were almost negligible in the studied area. However, methane concentration at landfill surface at nighttime was significantly higher than those in the daytime, which demonstrates the importance of investigating methane emissions at an hourly time scale, including during nighttime. The daily and hourly variations in methane concentration were well correlated with either soil temperature or volumetric water content near the surface. The obtained relations indicate that the automated monitoring system measurements can facilitate a more comprehensive understanding of the methane emission mechanisms at different time scales

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

Spectral induced polarization response of calcite precipitation

Induced calcite precipitation is used in a range of geotechnical applications to improve the mechanical properties of porous media. It has been shown that spectral induced polarization (SIP) allows to monitor calcite precipitation, although results were partly inconsistent. Therefore, this study aims to investigate how the SIP response of calcite depends on solute composition, since this may explain the differences in previous studies. SIP measurements were made on a column filled with sand while calcite precipitation was created by injecting Na2CO3 and CaCl2 solutions through two different ports. The experiment consisted of five phases. In phase I, calcite precipitation was generated for a period of 12 days. This resulted in a well-defined calcite precipitation front, which was associated with an increase in the imaginary conductivity. In phase II, the injected solutions were increasingly diluted. This resulted in a clear decrease in imaginary conductivity. In phase III, the injection of the two solutions was stopped. Nevertheless, calcite precipitation continued and solute concentrations in the mixing zone decreased. As in phase II, this led to a decrease in the imaginary conductivity. In phase IV, the injection rate of the Na2CO3 solution was reduced to shift the mixing zone, which also decreased the imaginary conductivity. Finally, the column was flushed with a solution in equilibrium with calcite in phase V, which led to a very small SIP signal. These results imply that calcite only generates a SIP response when it is in contact with solution which is strongly oversaturated with respect to calcite