Search

Recent developments in predicting and interpreting seismoelectric (SE) signals suggest a great potential for studying near‐surface hydrogeological properties, particularly in the vadose zone. Previous studies have revealed that the SE spectral ratios obtained from earthquake‐triggered SE data contain valuable hydrogeological information concerning porous media (e.g., permeability, porosity, fluid viscosity, and salinity). This study introduces Multi‐Channel SeismoElectric Spectral Ratios (MC‐SESRs) by considering an active seismic source acting on the ground surface. The frequency‐ and saturation‐dependent excess charge density is adopted to calculate the cross‐coupling coefficients. Applying a supervised learning task based on a flat neural network, the so‐called "broad learning (BL)" model, to map and extract the features of MC‐SESRs data, we seek to determine the permeability and the water table depth. Our results indicate that (a) MC‐SESRs are sensitive to the water table depth and permeability; (b) using more traces of SESRs data for inversion can increase accuracy; and (c) the changing water table can be rapidly determined by the MC‐SESRs by resorting to the BL inverse model, and it can attain an excellent accuracy while disturbed by data noise and misspecified model parameters (e.g., porosity and permeability) with errors of up to 20%. The proposed MC‐SESRs inversion has potential applications for non‐invasive monitoring in shallow porous media (e.g., frost thawing and geothermal upwelling). Plain Language Summary: A seismic source acting on the ground or occurring in porous materials containing water will generate seismic and electromagnetic field waves. The spectral ratios between the electric field and the seismic field are defined as SeismoElectric Spectral Ratios (SESRs), which are sensitive to physical properties' contrasts at layer boundaries (e.g., water table and hydrogeological and/or lithological layer boundaries). Applying SESRs to reconstruct hydrogeological parameters eliminates the need to know the seismic source function, which greatly facilitates quantitative interpretation. However, SESRs are often acquired by natural earthquakes in previous studies. It limits interpreting SESRs to one‐trace data. This study uses an active seismic source to obtain the Multi‐Channel SESRs (MC‐SESRs). We conduct several experiments on synthetic MC‐SESRs data by using a neural network to obtain water table depths and permeabilities for a layered Earth model. Our results show that the trained neural network can instantly predict the time‐variant water table depths accurately. This study indicates that the quantitative interpretation of MC‐SESRs data allows for effective and rapid characterization of near‐surface hydrogeological properties and also provide a possible approach for the non‐invasive monitoring of hydrogeological variations in shallow porous media by using controllable source. Key Points: Multi‐channel seismoelectric spectral ratios are sensitive to the water table depth and the permeabilities of shallow layersBroad learning neural network is introduced to perform the inversion efficientlyThis study allows us to monitor the water table depth from the ground surface for an otherwise pre‐defined model [ABSTRACT FROM AUTHOR]

Liquid water in frozen porous media provides the path for moisture and solute migration, which is one of the essential problems to study thaw‐weakening and frost‐heave in freezing region engineering. Determining and monitoring liquid water saturation and permeability in saturated frozen porous media are therefore critical issues in cold regions. To this end, geophysical methods are tools of choice given their non‐invasive nature. Electrical conductivity as a physical property is related to both the bulk characteristics of saturated porous medium submitted to frozen temperatures, like porosity as well as liquid water content, and surface properties, such as the solid surface‐liquid water and bulk ice‐liquid water interfaces. In this study, we upscale a microstructural procedure of liquid water‐saturated frozen soils to predict the electrical conductivity with different pore size distributions (PSDs). Then, we analyze the model sensitivity to the model parameters and compare the lognormal and fractal PSD models. Furthermore, the proposed model successfully predicts the experimental data of electrical conductivity for different samples from experiments that are part of this work and published data. The proposed model for characterizing electrical conductivity opens up new possibilities to describe the distribution and dynamics of liquid water with geoelectrical and electromagnetic techniques in frozen environments. Plain Language Summary: Electrical and electromagnetic methods are non‐invasive geophysical techniques, which have been applied to address complex environmental issues such as those encountered in cold regions, for example, the distribution and dynamics of liquid water/solute in frozen porous media, the monitoring the spatio‐temporal evolution of the active layer thickness, or snowmelt infiltration. However, the application of these electrical and electromagnetic methods in cold regions requires more experimental and modeling studies of the electrical conductivity of frozen soils. In this study, we establish a physically‐based capillary bundle model for describing the electrical conductivity of saturated frozen porous media with different pore size distributions. We test the accuracy of the proposed model against the experimental data described in this work and published experimental data. This physically‐based approach may provide a better theoretical means to quantitate and monitor the geo‐environmental problems in cold regions. Key Points: A novel physically‐based model for electrical conductivity of the saturated frozen soils is developed, using the Gibbs‐Thomson effect and surface complexation modelThe influences of bulk and surface conductivities are considered and surface conductivity is linked with pore size distribution and cation exchange capacityThe proposed model successfully predicts the data from the experiment described in this work and published experimental data [ABSTRACT FROM AUTHOR]

In the context of seismoelectric and self‐potential surveying, the effective excess charge density and the electrokinetic coupling coefficient are key parameters relating the measured electrical potential and the hydraulic characteristics of the explored porous media. In this work, we present a novel flux averaging approach that permits to estimate the frequency‐dependent effective excess charge density in partially saturated porous media. For this, we conceptualize the porous medium as a partially saturated bundle of capillary tubes under oscillatory flux conditions. We account for the pore size distribution (PSD) to determine the capillary‐pressure saturation relationship of the corresponding medium, which, in turn, permits to determine the pore scale saturation. We then solve the Navier‐Stokes equations within the saturated capillaries and, by means of a flux‐averaging procedure, obtain upscaled expressions for: (a) the effective excess charge density, (b) the effective permeability, and (c) the electrokinetic coupling coefficient, which are functions of the saturation and the probing frequency. We analyze and explain the characteristics of these functions for three different PSDs: fractal, lognormal, and double lognormal. It is shown that the PSD characteristics have a strong effect on the corresponding electrokinetic response. The proposed flux‐averaging approach has an excellent capability for reproducing experimental measurements and models in the literature, which are otherwise based on well‐known empirical relationships. The results of this work constitute a useful framework for the interpretation of electrokinetic signals in partially saturated media. Plain Language Summary: Seismic waves travel throughout the Earth deforming the rocks in their passage. If rocks are porous, permeable, and contain fluids in their pores, as is the case in many geological formations, the wave's passage may induce oscillatory fluid flow. Minerals composing rocks are commonly electrically charged and, thus, the flowing fluid can result in an electrical field. Interestingly, measuring this electrical field at the Earth's surface may permit to characterize the hydromechanical properties of geological formations of interest, motivating the so‐called seismoelectric method. The effective excess charge that is mobilized by the fluid motion depends on the frequency content of the wave, and models exist to estimate this dependence in terms of the rock and fluid properties. However, in many scenarios in Earth sciences, rocks contain two immiscible fluid phases, such as, water and air, for which frequency‐dependent effective excess charge density models based on pore‐scale physics are missing in the literature. In this paper, we derive such a model and show that it is able to reproduce previous estimates and experimental data. Key Points: We present a novel flux‐averaging approach to compute the dynamic effective excess charge density in partially saturated porous mediaThe model accounts for the pore size distribution of the medium and permits to estimate the dynamic electrokinetic coupling coefficientThe proposed approach has an excellent capability for reproducing previous models and experimental measurements in the literature [ABSTRACT FROM AUTHOR]

Estimating hydraulic properties and monitoring water flow in fractured rocks using self‐potential observations essentially relies on our ability to model streaming potential. One of the most promising approaches for modeling electrokinetic couplings is based on the macroscopic quantification of the excess charge which is effectively transported by pore water flow. In this study, we derive a fractal model to predict the effective excess charge density for fully and partially water saturated fractured media. Fractures are conceptualized as parallel plates with a fractal pattern described by the Sierpinski carpet. From the calculation of the excess charge in a single fracture and a flux averaging upscaling procedure, we obtain closed‐form expressions for the effective excess charge density. This new analytical model explicitly depends on the fracture water ionic concentration, interface properties, water saturation, porosity and permeability. Model predictions under saturated conditions are compared to published data measured in laboratory during hydraulic fracturing. The model development also shows the independence of the excess charge from the pore shapes: when expressed in terms of hydrological parameters, one can find an expression identical to recently published models for sedimentary porous media using capillary tubes. These results extend the validity of the existing models to fractured rocks and highlight the importance of hydraulic parameters for an accurate modeling of electrokinetic couplings. Plain Language Summary: Groundwater flow in fractured rocks can be indirectly monitored by measuring the electrical potential distribution in the medium or at its surface. The water flow within each fracture drags electrical charges that produce variations in the electrical field. Then, to quantitatively study the water flow from self‐potential measurements, we need a theoretical model that links the hydraulic and electrical properties of fractured rocks. In this study, we present a theoretical development of a key parameter called the effective excess charge density. It quantifies the electrical potential generated by water flow. To calculate this parameter, we assume a fracture network with a fractal distribution, that is, a pattern of fractures that repeats itself at different spatial scales. From the description of the drag of electrical charges in a single fracture we estimate a value of the effective excess charge density which is representative of the whole fractured rock. The model results in rather simple mathematical expressions depending on the chemical composition of water, solid‐water interface properties, porosity, saturation, and rock permeability. To validate the proposed model, our theoretical predictions are compared with experimental laboratory data. This new model opens‐up possibilities of using self‐potential data for monitoring complex processes like hydraulic fracturing. Key Points: A conceptual model is proposed to describe effective excess charge density in fractured mediaThe model is based on physical principles and a fractal description of fracture networksModel expressions in terms of hydraulic variables are identical to those obtained for classic porous media [ABSTRACT FROM AUTHOR]

Improving our understanding of the relation between the water content and the seismic signatures of unconsolidated superficial soils is an important objective in the overall field of hydrogeophysics. Current approaches to constrain the water content in the vadose zone from seismic data are based on computing the ratio between compressional and shear wave velocities Vp/Vs. While this allows for the detection of pronounced changes in saturation, such as the groundwater table, it is essentially insensitive to variations in the saturation‐depth profile. Conversely, evidence shows that surface waves are sensitive to both the location of the water table and the saturation‐depth profile. Classic rock physics models are unable to explain the corresponding observations. We propose to estimate surface‐wave signatures accounting for capillary suction effects. We extend the Hertz‐Mindlin model using Bishop's effective stress definition, thus accounting for stiffness changes associated with capillary stresses acting on the soil's frame. We then compute the elastic properties of the partially saturated medium using the Biot‐Gassmann‐Wood model. Considering a 1D unconsolidated porous medium under steady‐state saturation conditions, as given by Richards' equation, we simulate body‐wave travel times and surface‐wave dispersion characteristics for different water table depths and overlying soil textures. Our results illustrate that surface‐wave phase velocity dispersion curves are remarkably sensitive to capillary effects in partially saturated soils, exhibiting velocity changes of up to 20% in the 10–100 Hz frequency range. These effects, which are particularly important in medium‐to fine‐grained soils, are virtually nonexistent in the corresponding Vp/Vs profiles. Plain Language Summary: Seismic waves are usually employed to study the water content in the shallow subsurface for environmental purposes. Most studies estimate the water content of the soil at different depths relating compressional and shear wave velocities, which are estimated by measuring body‐wave travel times. Even though this method permits to locate the depth at which the soil becomes fully water saturated, it is rather insensitive to changes in the water content of the overlaying portion of the soil, where partial air‐water saturation prevails. Surface waves, however, appear to be significantly sensitive to changes in saturation within this partially saturated region. Classic rock physics models cannot explain this difference in sensitivity between surface‐wave velocity and body‐wave travel times. In this work, we propose to include the effects of capillary forces, which arise when the soil is partially saturated, in the rock physics models. By doing so, we show that capillary action, which acts bringing soil particles further together, can explain why surface‐wave dispersion curves are sensitive to saturation variations in the partially saturated zone while body‐wave travel times remain virtually unperturbed. These results may help to better interpret seismic data for environmental studies. Key Points: Water content variations in the partially saturated zone hardly alter Vp/Vs ratios, but have large effects on surface‐wave signaturesThe effective soil stress is strongly affected by capillary forces, which stiffen the medium at relatively low saturationsCapillary effects permit to explain observed changes in surface‐wave dispersion due to small water content variations in the vadose zone [ABSTRACT FROM AUTHOR]

Clay 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 nonintrusively 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 nonmonotonous 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 σsurf″ and σsurf′ 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. Key Points: The quadrature conductivity of clays behaves non‐monotonously with increasing salinitySome polarization mechanisms may cease to act or decrease significantly at a specific salinityThe quadrature to surface conductivity ratio is lower for clays than for other minerals [ABSTRACT FROM AUTHOR]

Understanding 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. Key Points: We simulate streaming potentials for 2‐D networks with different pore size distributionsThe pore size distribution has a very restricted influence on electrokinetic coupling coefficientsA recent effective excess charge density model accounts for all the pore size distributions [ABSTRACT FROM AUTHOR]