Microscopic modeling

Numerical modeling at the microscopic scale is a powerful tool for understanding the physical properties of geological materials. My current research focuses on electrical properties, including electrical conductivity and induced polarization. This type of simulation solves the relevant governing equations by explicitly considering the material microstructure and small-scale heterogeneity, and thus it can help reveal the mechanism without making many assumptions. Below are some examples.

Solving Nernst-Planck-Poisson equations to understand the induced polarization of granular material

The induced polarization of porous geological materials results mainly from the electrical double layer (EDL) formed at the solid-liquid interface in response to the charged mineral surface. Under an external electric field, the charge in EDL may change its spatial distribution and store part of the energy. While many empirical/or (simple) physics-based models are used in practice, they may not work for certain conditions or for some special materials. They also can not answer the question of "why." We are the first group to explicitly solve the coupled Nernst-Planck-Poisson equations to understand how the ions in EDL and nearby respond to external electrical fields. The above figure shows the improved understanding of ion movement around a spherical grain under an external electric field.

Pore-scale simulation of interfacial polarization in granular materials

The induced polarization of porous geological materials may also result from the interfacial polarization at some heterogenous boundaries (electrical properties contrast). We innovatively combined the Discrete Element Method (DEM) in pore-scale simulation so that the packing of granular materials can be controlled. This realistic microstructure is then explicitly considered in our numerical solutions of the Poisson equation. By changing the packing structure, we can evaluate how the interfacial polarization changes with material structure/properties. In the above figure, we should how different degree of packing affects the permittivity of the saturated granular materials in the frequency domain.

Laboratory studies

Laboratory scale studies use lab instruments to measure physical properties of soils and rocks at different hydrological, chemical, or mechanical conditions and study the factors (materials structure or environmental conditions) controlling these properties. Lab-scale studies also include the development of theoretical models (mostly physics-based) to describe soil/rock properties and the development of new data-processing techniques to extract more information from lab measurements.

Integrated hydrogeophysical soil column

In hydrogeophysical applications, it is critical to extract the hydrological information from geophysical measurements. The key in this interpretation process is the knowledge of both geophysical and hydraulic properties of geological materials. We have developed an integrated soil column system, which can be arranged to conduct both saturated and unsaturated flow tests on the same soil sample; the electrical responses of the flows can also be monitored. These data can be processed to acquire electrical resistivity, induced polarization, and hydraulic conductivity of soils under both saturated and unsaturated conditions. The soil water retention curve may also be estimated. Potential applications of this soil column include critical zone materials, aquifer materials, etc.

SP signals induced by evaporation and transpiration

In this study, we used laboratory soil column tests to simulate vadose zone hydrologic processes (drainage, evaporation, and transpiration) and to generate SP data in low water flow conditions. The measured water fluxes and SP signals in different hydrologic stages of the tests are used to study if electrokinetic coupling is still the dominant mechanism for the SP signals in unsaturated, low-flow conditions. Theoretical models of electrokinetic and electrodiffusion couplings are also used to guide the analysis. It is shown that the SP signals measured during soil evaporation and plant transpiration in this soil column test were not only caused by unsaturated water flows in the soil column through electrokinetic coupling. Instead, they are likely related to the ion concentration gradient in the soil column, which creates an electrical current of a diffusive nature.

Electrical properties measurement under controlled hydrologic conditions

Electrical properties of near-surface geological materials experience various hydrologic conditions. To interpret field-collected geoelectrical results, we need to establish the relationship between electrical properties and other key soil/rock properties from, often, laboratory experiments. Thus, we need to well control the hydrologic conditions of near surface materials. In this study, we combined pressure plate and resistivity measurement so that the moisture condition of the soil samples can be well controlled. This new lab instrumentation is currently used to study granitic regoliths and how weathering changes their electrical properties. The study may help critical zone scientists to delineate CZ structures from geophysical results.

Field geophysical tests and inversion

Incorporate subsurface structure into soil moisture estimation from resistivity

The electrical resistivity method has been frequently used to estimate the moisture content in the field. The uncertainty associated with resistivity-estimated moisture content is mainly from two sources: regularized inversion and petrophysical interpretation. In this study, we use subsurface structural information from seismic data to relax the smoothness-based regularization at structural boundaries. In addition, we also use structural unit-specific petrophysical relationships to translate resistivity into moisture content. Examples (e.g., above figure) have shown that spatial patterns and the moisture content values estimated with the new method are very close to the true model with low uncertainty.

Hydrologic modeling

Geophysics-informed hydrologic modeling

Hydrologic modeling is a useful tool for studying various hydrologic problems, from lab-scale experiments to earth systems. In mountain hydrology, it is critical to understand how precipitation is partitioned into stream flow, ET, subsurface storage, and deep drainage so that we can make predictions regarding the influence of future climate change on our valuable water resources. Hydrologic modeling could play a key role in addition to field monitoring. We have incorporated geophysical results in hydrologic modeling to represent the subsurface heterogeneity better. Our preliminary work has demonstrated that the added detailed subsurface structure info from seismic velocity can help improve the hydrologic modeling in small catchments. We are currently upscale this technique to larger hydrologic systems.

Hydrologic partitioning in mountain catchment: influence of CZ subsurface structure

We used numerical simulation to study how critical zone (CZ) structure (i.e., heterogenous soil/bedrock interface) affects the hydrologic partitioning in the mountainous catchment. The results show that runoff generation, soil storage, and rock storage are all affected by the subsurface structure. This highlights the importance of considering the below-ground heterogeneity in understanding many hydrologic processes in mountain catchment.

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