Effects of Electromagnetic Stimulation on Soil’s Hydraulic Conductivity

Our research involves the identification of the different effects that electromagnetic (EM) stimulation has on varying soil properties; properties such as hydraulic conductivity. This work could prove to be of importance in furthering our understanding of the effects of EM stimulation with regard to the hydraulic conductivity of soil. A positive correlation between EM stimulation and an increase in hydraulic conductivity could have broad applications for environmental contaminant mitigation in soils and for various geotechnical construction applications such as minimizing soil setup during pile driving operations. EM waves can be used to enhance soil and groundwater remediation in a way that no heat is generated, yet the desired mechanisms in soil are stimulated. Our approach in this project involved the construction of a customized permeameter that enabled us to measure the change in hydraulic conductivity given a tuned EM wave from an antenna. An EM wave with a fixed frequency and varying power output was sent through the permeameter while the hydraulic conductivity was measured in real time. Tests performed for the research project were successful in showing a correlation between hydraulic conductivity and EM stimulation

Cross-Well Radar I: Experimental Simulation of Cross-Well Tomography and Validation

This paper explains and evaluates the potential and limitations of conducting Cross-Well Radar (CWR) in sandy soils. Implementing the experiment and data collection in the absence of any scattering object, and in the presence of an acrylic plate (a representative of dielectric objects, such as DNAPL (dense non-aqueous phase liquid) pools, etc.), as a contrasting object in a water-saturated soil is also studied. To be able to image the signature of any object, more than one pair of receiving and transmitting antennas are required. The paper describes a method to achieve repeatable, reliable, and reproducible laboratory results for different transmitter-receiver combinations. Different practical methods were evaluated for collecting multiple-depth data. Similarity of the corresponding results and problems involved in each method are studied and presented. The data show that the frequency response of a saturated coarse-grained soil is smooth due to the continuous and dominant nature of water in saturated soils. The repeatability and potential symmetry of patterns across some borehole axes provide a valuable tool for validation of experimental results. The potential asymmetry across other borehole axes is used as a tool to evaluate the strength of the perturbation on the electromagnetic field due to hidden objects and to evaluate the feasibility of detecting dielectric objects (such as DNAPL pools, etc.) using CWR. The experimental simulation designed for this paper models a real-life problem in a smaller scale, in a controlled laboratory environment, and within homogenous soils uniformly dry or fully water-saturated, with a uniform dielectric property contrast between the inclusion and background. The soil in the field will not be as homogenous and uniform. The scaling process takes into consideration that as the size is scaled down; the frequency needs to be scaled up. It is noteworthy that this scaling process needs to be extensively studied and validated for future extension of the models to real field applications. For example, to extend the outcome of this work to the real field, the geometry (antennas size, their separation and inclusion size) needs to be scaled up back to the field size, while soil grains will not scale up. Therefore, soil, water and air coupling effects and interactions observed at the laboratory scale do not scale up in the field, and may have different unforeseen effects that require extensive study

Laboratory Study of the Effect of Electromagnetic Waves on Airflow during Air Sparging

Air sparging is a technique that uses the injection of a gas (e.g., air, oxygen) into the subsurface to remediate saturated soils and groundwater contaminated with volatile organic compounds (VOCs). Contaminant-removal efficiency and air-sparging performance are highly dependent on the pattern and type of airflow. Airflow, however, suffers from air channel formation (i.e., preferential paths for airflow), limiting remediation to smaller contaminated zones. This paper presents the results of experimental work investigating the possibility of controlling and improving airflow patterns through a saturated glass-bead medium using electromagnetic (EM) waves to enhance air sparging. The test setup consists of a resonant cavity made of an acrylic tank covered with transparent, electrically conductive films. Experimental measurement of the electric field component of EM waves is performed at different frequencies. Airflow pattern is also studied at different air-injection pressure levels with/without EM stimulation. The zone of influence (ZOI) during air sparging is monitored using digital imaging. A quantitative approach is then taken to correlate the characteristics of EM waves and airflow patterns

Interaction Between Electromagnetic Waves and Transport in Saturated Media

Air sparging is one of the most popular remediation technologies. However, it is limited to a small radius of influence (ROI) surrounding the air injection well. Hence, there have been several efforts to improve its effectiveness. To study the possibility of improving the effectivity of air sparging electromagnetic (EM) waves, an easily visible analogous problem (dye transport in water) is studied in this paper. In order to quantify the effects of EM stimulation on flow of an inert, nonreactive dye in water, EM-stimulated and unstipulated dye transport experiments tests were performed and compared. To quantify this interaction, both dye transport and EM wave propoagation (only the electric field component Z) are quantified experimentally in lab-scale. In addition to the experimental mapping of the electric field at limited location on depth (i.e., vertical) slices, the electric field is simulated in COMSOL Multiphysics 4.1 in three dimensions (3D) for accurate field analysis. Transport analysis of the dye was performed using digital imaging to determine temporal and spatial concentration variations. The results show a visible effect on the dye transport mechanisms (i.e., fingering and diffusion). However, further study is needed to validate the proposed correlation between the electric field and the transport mechanisms

Cross-Well Radar II: Comparison and Experimental Validation of Modeling Channel Transfer Function

Close agreement between theory and experiment is critical for adequate understanding and implementation of the Cross-Well Radar (CWR, otherwise known as Cross-Borehole Ground Penetrating Radar) technique, mentioned in a previous paper by the authors. Comparison of experimental results to simulation using a half-space dyadic Green’s function in the frequency domain requires development of transfer functions to transform the experimental data into a compatible form. A Channel Transfer Function (CTF) was developed to avoid having to model the transmitting and receiving characteristics of the antennas. The CTF considers electromagnetic (EM) wave propagation through the intervening media only (soil in this case), and hence corresponds to the simulation results that assume ideal sources and receivers. The CTF is based on assuming the transmitting antenna, soil, and receiving antenna as a cascade of three two-port microwave junctions between the input and output ports of the Vector Network Analyzer (VNA) used in the experimental measurements. Experimentally determined CTF results are then compared with computational model simulations for cases of relatively dry and saturated sandy soil backgrounds. The results demonstrate a reasonable agreement, supporting both the model and CTF formulation

Electromagnetically Induced Transport in Water for Geoenvironmental Applications

Air sparging is a popular soil remediation technique that enables the removal of contaminants through diffusing air into soil. The removal process is, however, slow. The goal of this work is to study the effect of electromagnetic (EM) waves —with minimal heat generation— on transport mechanisms such as diffusion, in order to improve airflow or contaminant transport in order to expedite the cleanup process using air sparging or similar technologies. This effect is studied through an experimental setup that examines the diffusion of a nonreactive dye in water under EM waves at a range of frequencies (50-200 MHz). The electric field was simulated using COMSOL Multiphysics for better three-dimensional (3D) visualization and analysis and then validated using the experimental measurements. A dielectrophoretic study was then performed using the simulated electric field. Various dye flows under EM stimulation at different frequencies were compared. At 65 MHz and 76 MHz, the dye flow was in the direction of the dielectrophoretic forces, which are believed to be the governing mechanism for the EM-stimulated dye transport

Study of Mechanisms Governing Electromagnetic Alteration of Hydraulic Conductivity of Soils

Hydraulic conductivity is a measure of the rate at which water flows through porous media. Because of the dipole properties of water molecules, electric field can affect hydraulic conductivity. In this study, the effect of radio-frequency (RF) waves on hydraulic conductivity is investigated. This is important both for the geophysical measurement of hydraulic conductivity as well as remediation using electromagnetic waves. Bentonite clay and sandy samples are tested in rigid-wall, cylindrical permeameters and stimulated using a CPVC-cased monopole antenna vertically centered in the permeameters. The permeameters are encased within RF cavities constructed of aluminum mesh in order to prevent interference from the outside and to confine the RF wave to the medium. Falling-head and constant-head tests are performed to measure the hydraulic conductivity of the clayey and sandy soil samples, respectively. The results show a correlation between the change in the hydraulic conductivity and various characteristics of the RF stimulation. The change is, however, different for sandy and clayey soils

Experimental Validation of a Numerical Forward Model for Tunnel Detection Using Cross-Borehole Radar

The goal of this research is to develop an experimentally validated twodimensional (2D) finite difference frequency domain (FDFD) numerical forward model to study the potential of radar-based tunnel detection. Tunnel detection has become a subject of interest to the nation due to the use of tunnels by illegal immigrants, smugglers, prisoners, assailants, and terrorists. These concerns call for research to nondestructively detect, localize, and monitor tunnels. Nondestructive detection requires robust image reconstruction and inverse models, which in turn need robust forward models. Cross-Well Radar (CWR) modality is used for experimentation to avoid soil-air interface roughness. CWR is not a versatile field technology for political boundaries but is still applicable to monitoring the perimeter of buildings or secure sites. Multiple-depth wideband frequency-response measurements are experimentally collected in fully water-saturated sand, across PVC-cased ferrite-bead-jacketed borehole monopole antennae at a pilot scale facility (referred to as SoilBED). The experimental results are then compared with the 2D-FDFD model. The agreement between the results of the numerical and experimental simulations is then evaluated. Results of this work provide key diagnostic tools that can help to develop the algorithms needed for the detection of underground tunnels using radar-based methods

Multiphysics Numerical Modeling of Transient Transport of PFAS

The need to understand the fate and transport of Per- and Polyfluoroalkyl substances (PFAS) has grown due to the widespread contamination of the environment by them. PFAS are persistent, mobile, toxic manmade chemicals of great concern that contribute to the contamination of soil and groundwater. The presence of PFAS in unsaturated soil complicates their transport due to the impact of the air–water interface and solid-phase adsorption. The air–water interface can significantly increase the retention of PFAS during its transport. In this paper, a numerical model has been developed to study the transport of PFAS by coupling transient seepage and advection-dispersion, also accounting for the air–water interface and solid-phase adsorption. The numerical model was then used to study various scenarios

Predicting Power-Transformer Bushings’ Seismic Vulnerability: Mounting Stiffness and Coupling

Large power transformers are both expensive and vulnerable to seismic failure. Excessive amplification of earthquake loading to power transformer bushings beyond the bushings’ rated strength can occur if the system’s structural dynamics produce unpredictable and unfavorable resonance behavior. Determining the degree of vulnerability to large seismic amplifications is not straight-forward, and general design recommendations do not always resolve problems that can potentially arise. This paper considers a number of Case studies in an attempt to postulate a set of critical factors, which might facilitate better prediction of bushing amplification. Further, several additional Case studies are summarized in order to add to the evaluated dataset and provide more validity for drawing broader conclusions