Seismic Refraction and Electrical Resistivity Tests for Fracture Induced Anisotropy in a Mountain Watershed

The critical zone (CZ) is the earth’s layer where water, air, rock, and life meet. It is the zone with which humans interact most. The National Research council (2001) defines the CZ as a “heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life sustaining resources”. The CZ may extend roughly from the top of the vegetation canopy to the deepest part of the rock column where meteoric water circulates – this is often in the 10 – 30 m range. The upper 1-2 m of the CZ, the most weathered portion of the CZ depth profile, can be reached via soil pits or cores enabling detailed characterization. Weathering is the process in which a parent rock decays into mobile soil, through mechanical breakdown and chemical weakening. The maximum depth at which both, mechanical and chemical processes are present is referred to as weathering depth. Below 2 m, characterizing the CZ is a challenge because of the expense and logistical challenge of drilling boreholes, particularly in rugged, mountainous terrain. Geophysical methods are increasingly being used to probe deeper into the CZ and have proven to be a powerful tool. Fully characterizing the deep CZ is made even more challenging when fractures are present. Fractures may have a preferential orientation according to the local stress field which leads to both geophysical and hydraulic anisotropy. Because fractures can be hydraulically active, understanding fracture induced anisotropy retrieves information on the preferential distribution of water pathways in the subsurface. To test our ability to detect and characterize systems of deep CZ fractures with preferred orientation in a mountain watershed, I conducted a series of multi-azimuthal 2D electrical resistivity tomography and 2D seismic refraction surveys. I utilized the Dry Creek Experimental Watershed (DCEW) as a field laboratory – a previous outcrop study mapped fracture orientations throughout the watershed. I collected data, at three different sites, near or within the DCEW. For the anisotropy and fracture density case, I estimated fracture density as a function of depth at all sites, and determine that the depth at which most fractures close ranges from 13-27 m depth. I found significant P-wave anisotropy throughout the watershed with maximum values of 28%. Additionally, my results indicate that anisotropy continues to much greater depths. I infer that the observed geophysical anisotropy likely correlates with significant hydraulic anisotropy and has an important impact on deep water circulation in the DCEW. Additionally, I show that attempts to characterize this system with single azimuth data and an assumption of isotropy will lead to erroneous results – at my sites the error in estimated fracture density could be as high as 0.24. I conclude that geophysical investigations in similar terrains need to test for anisotropy and use appropriate models, particularly if the objective is quantitative estimation of hydraulic or other physical properties. General observations of the CZ were that weathering depth increased with decreasing elevations – this has been observed at other sites and is likely due to increased chemical reactivity at higher temperatures. At the low elevation site I observe a depth to bedrock at approximately 29 m depth, at the mid-elevation site I observe a depth to bedrock of approximately 23 m depth, at the high elevation site, I observe a depth to bedrock as little 5 m depth. I observed a significant increase in weathering depth on northwest facing aspects and speculate that this is a combination of lower evaporation rates, slow steady delivery of snow melt waters into the subsurface and the positive feedback of increased vegetation and root enhanced weathering

Geophysics-Informed Hydrologic Modeling of a Mountain Headwater Catchment for Studying Hydrological Partitioning in the Critical Zone

Hydrologic modeling has been a useful approach for analyzing water partitioning in catchment systems. It will play an essential role in studying the responses of watersheds under projected climate changes. Numerous studies have shown it is critical to include subsurface heterogeneity in the hydrologic modeling to correctly simulate various water fluxes and processes in the hydrologic system. In this study, we test the idea of incorporating geophysics-obtained subsurface critical zone (CZ) structures in the hydrologic modeling of a mountainous headwater catchment. The CZ structure is extracted from a three-dimensional seismic velocity model developed from a series of two-dimensional velocity sections inverted from seismic travel time measurements. Comparing different subsurface models shows that geophysics-informed hydrologic modeling better fits the field observations, including streamflow discharge and soil moisture measurements. The results also show that this new hydrologic modeling approach could quantify many key hydrologic fluxes in the catchment, including streamflow, deep infiltration, and subsurface water storage. Estimations of these fluxes from numerical simulations generally have low uncertainties and are consistent with estimations from other methods. In particular, it is straightforward to calculate many hydraulic fluxes or states that may not be measured directly in the field or separated from field observations. Examples include quickflow/subsurface lateral flow, soil/rock moisture, and deep infiltration. Thus, this study provides a useful approach for studying the hydraulic fluxes and processes in the deep subsurface (e.g., weathered bedrock), which needs to be better represented in many earth system models

Exploring the effect of the pore size distribution on the streaming potential generation in saturated porous media, insight from pore network simulations

International audienceUnderstanding streaming potential generation in porous media is of high interest for hydrological and reservoir studies as it allows to relate water fluxes to measurable electrical potential distributions. This streaming potential generation results from an electrokinetic coupling due to the presence of an electrical double layer developing at the interface between minerals and pore water. Therefore, the pore sizes of the porous medium are expected to play an important role in the streaming potential generation. In this work we use 2‐D pore network simulations to study the effect of the pore size distribution upon this electrokinetic mechanism. Our simulations allow a detailed study of the influence of a large range of permeabilities (from 10−16 to 10−10 m2) for different ionic concentrations (from 10−4 to 1 mol/L). We then use and compare two different approaches that have been used over the last decades to model and interpret the streaming potential generation: the classical coupling coefficient or the effective excess charge density, which has been defined recently. Our results show that the four pore size distributions tested in the present work have a restricted influence on the coupling coefficient for ionic concentration smaller than 10−3 mol/L while it completely drives the behavior of the effective excess charge density over orders of magnitude. Then, we use these simulation results to test an analytical model based on a fractal pore size distributions. This model predicts well the effective excess charge density for all pore size distributions under the thin double layer assumption

Can equivalent circuit models be used for the complex conductivity of clayey materials?

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Spectral Induced Polarization Characterization of Non‐Consolidated Clays for Varying Salinities—An Experimental Study

International audienceClay material characterization is of importance for many geo-engineering and environmental applications, and geo-electrical methods are often used to detect them in the subsurface. Spectral induced polarization (SIP) is a geo-electric method that non-intrusively measures the frequency dependent complex electrical conductivity of a material, in the mHz to the kHz range. We present a new SIP data set of four different types of clay (a red montmorillonite sample, a green montmorillonite sample, a kaolinite sample, and an illite sample) at five different salinities (initially de-ionized water, 10−3, 10−2, 10−1, and 1 mol/L of NaCl). We propose a new laboratory protocol that allows the repeatable characterization of clay samples. The complex conductivity spectra are interpreted with the widely used phenomenological double-Pelton model. We observe an increase of the real part of the conductivity with salinity for all types of clay, while the imaginary part presents a non-monotonous behavior. The decrease of polarization over conduction with salinity is interpreted as evidence that conduction increases with salinity faster than polarization. We test the empirical petrophysical relationship between the imaginary and real surface conductivities and validate this approach based on our experimental data and two other datasets from the literature. With this data set we can better understand the frequency-dependent electrical response of different types of clay. This unique data set of complex conductivity spectra for different types of clay samples is a step forward toward better characterization of clay formations in situ

Predicting the Electrical Conductivity of Partially Saturated Frozen Porous Media, a Fractal Model for Wide Ranges of Temperature and Salinity

International audienceThe quantitative determination of liquid water content and salinity in soils is crucial for the preservation of hydrological environments and engineering infrastructures, especially in frozen regions. Electrical conductivity, as a fundamental physical parameter in electrical and electromagnetic non‐destructive techniques, varies significantly with the physical and chemical properties, such as pore water conductivity, salinity, water saturation, and temperature. In this study, accounting for pore size and tortuous length following fractal distributions, we develop a new capillary bundle model for variation of electrical conductivity as a function of temperature in broad water saturation and salinity ranges. In this new model, we consider the contributions of bulk and surface conductivities to the total electrical conductivity. To test this model, a series of laboratory experiments were carried out for different initial water saturations and salinities using an electrical resistance apparatus and a nuclear magnetic resonance method. The experimental results show that unfrozen water saturation and ionic concentration affect the electrical conductivity of unsaturated frozen soils. Furthermore, the proposed model is capable of fitting the main trends of the experimental data from the literature and acquired in this study in unfrozen‐frozen conditions for different water contents. Relying on the proposed model, we also determine the expression of the apparent formation factor, which is significantly sensitive to porosity, water saturation, and temperature. The predicted values of the apparent formation factor also agree very well with the experimental data. This new capillary bundle model provides a new perspective in interpreting electrical monitoring to easily deduce changes in key variables in the cryosphere such as liquid water content and moisture gradients

Electroosmotic Coupling in Porous Media, a New Model Based on a Fractal Upscaling Procedure

International audienceElectrokinetic and electroosmotic couplings can play important roles in water and ions transport in charged porous media. Electroosmosis is the phenomena explaining the water movement in a porous medium subjected to an electrical field. In this work, a new model is obtained through a new up-scaling procedure, considering the porous medium as a bundle of tortuous capillaries of fractal nature. From the model, the expressions for the electroosmosis pressure coefficient, the relative electroosmosis pressure coefficient, the maximum back pressure, the maximum flow rate, the flow rate-applied back pressure relation and the product of the permeability and formation factor of porous media are also obtained. The sensitivity of the relative electroosmosis pressure coefficient is then analyzed and explained. The model predictions are then successfully compared with published datasets. Additionally, we deduce an expression for the relative streaming potential coefficient and then compare it with a previously published model and experimental data from a dolomite rock sample. We find a good agreement between those models and experimental data, opening up new perspectives to model electroosmotic phenomena in porous media saturated with various fluids

From CRITEX to OZCAR: Geophysical wandering accross the FrenchCritical Zone Observatorie

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