H23B-1132 1340h
Examining the Influence of TDR-Rod-Induced Flow Disruption on Measured Water Content during Steady State Flow
Time domain reflectometry (TDR) is used routinely to monitor volumetric water content in porous media. TDR is used widely because it can make rapid, automated measurements with little need for medium-specific calibration. Another advantage of the method for some applications is its relatively small sample volume, which allows for high spatial resolution of the water content. Previous studies have shown that within its support volume, TDR is most sensitive to the water content in the immediate vicinity of the TDR rods. However, this is the area that may be most susceptible to flow disruption due to the presence of the instrument. In this study, we examine the spatial distribution of TDR sensitivity during steady state flow through unsaturated porous media under unit gradient conditions. We use a numerical model, built in FEMLAB, to predict the changes in water content in the medium due to TDR rods of different size, separation, and orientation. We then use a FEMLAB-based model to determine the effect of the water content distribution on the TDR sensitivity. Finally, we use the spatial sensitivity to predict the TDR-measured water content and compare this with the water content with the homogeneous background water content. We examine the effects of soil type, flux, and rod design on the measurement errors and show that, for the wide range of cases examined, the measurement errors are acceptably small for most common TDR probe designs. This gives further support for the use of TDR to conduct high-resolution volumetric water content monitoring.
H23B-1133 1340h
The Study of Energetic Status of Bound Water Using Dielectric Methods
The main goal of this work is to estimate energy status of water (its mobility) in both unfrozen and frozen soil and ground on the basis of their dielectric properties. We studied the relationships between complex (E), real (E') and imaginary (E") components of dielectric permittivity and (1) volumetric water content (W) within the range of hydroscopic water to total water content, and (2) temperature (T) in the range --16 to + 2oC (for melting samples). Investigation was conducted for Soddy Podzolic, Gray Forest, and Chernozemic soils as well as the samples of bentonite, kaolin, and quartz sand. Energy status of water was determined on the basis of soil-hydrological constants and water soil potential. Dielectric properties were measured in both time and frequency domains. We used a time domain reflectometer (TDR) (pulse rising times < 100 ps), and a capacitive sensor (at 50 MHz). For both methods, slope angle of the curves of E, E' and E'' versus W changes as water status changes. It is dependent on both mineralogy of these samples and method used. Unlike TDR, the capacitive sensor (Hydra Soil Moisture Probe, Vitel Inc) was sensitive to change in water mobility at lower water content. The shape of curves of E' and E'' versus T reflects a gradual increase of water mobility as T increases, and they are different for samples having different mineralogy. We hope that a more detailed investigation of the relationships obtained will allow us to develop a technique for the determination of energy status of water in the wet soil and grounds on the basis of their dielectric characteristics. This work was support by Russian Foundation for Basic Research grants 03-04-49325 and 04-05-79077.
H23B-1134 1340h
Hydraulic Units Associated with Unconsolidated Sediments and Spectral Electrical Response Measurements.
Knowledge of the hydraulic properties of unconsolidated materials (hydraulic conductivity, porosity) from non-invasive geophysical measurements is appealing to geoscientists involved in the hydrological characterization of the earth's subsurface. There is the need to understand the quantitative relationship between measurable geophysical attributes and hydraulic parameters of sediments or soils. Clustering sediments in hydraulic units help in relating parameters characterizing spectral electrical response (SER) measurements and hydraulic conductivity. The SER of soils is modeled with a multi-Cole-cole model considering soils as a heterogeneous multi-phase system. The validity and usefulness of the relations between the electrical parameters and the hydraulic units were assessed using laboratory measurements of the spectral electrical response(0.01Hz to 10 kHz) of over 30 soil samples with wide variability in physical properties. The soils are fully characterized: hydraulic conductivity, porosity, grain size distribution and moisture content of each soil sample were measured. The SER measurements are utilized to estimate the hydraulic unit characterizing the potential flow zones the soils. The intrinsic parameters, which describe the response of the model are retrieved by inversion schemes and are used in establishing the relations. Such relationships between parameters characterizing the spectral electrical response of soils and their hydraulic units may provide versatile and relaible non-invasive methodology of obtaining hydraulic properties information of soils from geophysical measurements.
H23B-1135 1340h
Stress effects on the spectral electrical response of unconsolidated sediments.: Laboratory measurements and analyses
The relations between micro-structural changes in soils due to stress changes and modifications in electrical parameters describing the ir spectral electrical responses may be of interest to geoscientist concerned with non-invasive predictions of changes in strain and over-pressure conditions in the earth's subsurface. The spectral electrical responses and hydraulic properties of clay-sand mixtures saturated with water and subjected to varying stress levels in a laboratory environment are investigated and analyzed. The SER of soil mixtures are described with an equivalent circuit model considering the soils as heterogeneous multi-phase systems with grain contacts. The validity and usefulness of the relations between the electrical parameters and the soil texture were assessed using laboratory measurements of the spectral electrical response(0.01Hz to 10 kHz) of varying mixtures of sand and clay under varying stresses. The stress effects on the SER and how it is influenced by the micro-structural characteristics of the mixtures are also investigated and their effects on the hydraulic properties assessed.
H23B-1136 1340h
Aggregate bed Deformation and Hydraulic Conductivity
The frequent use of earth materials for hydraulic capping and waste containment requires improved understanding of relationships between soil mechanical dynamics and its hydraulic and transport properties. We study deformation of pores embedded in viscoplastic soils subjected to anisotropic remote stress using Finite Element analysis (FEA). FEA enables consideration of complex pore geometries, and provides a detailed picture of pore cross-section change and subsequent change in hydraulic conductivity. Images of deforming aggregate beds monitored using X-ray computer assisted tomography (CAT) coupled with independently measured soil rheological properties were used to test the FEA model. Pore deformation for different remote stresses was compared with data from compaction experiments using modeling clay and soil aggregates. The impact of inter-aggregate pore deformation on hydraulic conductivity was obtained from detailed FEA model and compared with standard approximations (e.g., using the hydraulic radius). The evolution of complex pore shapes during compaction on prediction of saturated hydraulic conductivity will be discussed.
H23B-1137 1340h
A Gravimetrically Based Air-Drying Method to Measure Soil Hydraulic Diffusivity
A novel technique to measure soil hydraulic diffusivity is described and initial results are presented. The method consists of suspending a wet soil column by two load cells: one for each end of the column. One end of the column is sealed (no flow boundary). A small fan is set to blow air towards the other end of the column, maintaining a dry surface (constant head boundary); thus, the Bruce and Klute boundary conditions are achieved. As the soil column dries, the load cells record the total mass of water evaporated from the column. Additionally, the centroid of the mass of water removed the column can be calculated from the ratio of the change in weight recorded by the two load cells. Previously published algorithms that describe water content along the length of the column during one-dimensional semi-infinite air-drying are optimized such that the resultant water content predictions are consistent with data collected from the load cells. Soil hydraulic diffusivity is then calculated directly from the predicted water contents. The goodness of fit of the predicted water contents is verified using absolute measurements of water content collected with a gamma-ray attenuation system. The method is relatively rapid, and it requires only a small investment for hardware and a minimal amount of technician support to perform.
http://bioengr.ag.utk.edu/jtyner/air-drying.htm
H23B-1138 1340h
An Investigation of Surface Phenomena on Electrical Properties of Aquifer Materials
Electrical property measurements of aquifer materials are important in both borehole and surface geophysical methods used to infer subsurface conditions such as porosity, clay content and water saturation. The accuracy of these estimated hydrologic parameters is strongly dependent on the theoretical understanding of the electrical behaviour of the heterogeneous system. This behaviour is known to be affected by both the shape of the constituents in the medium (e.g. porosity, rock grains, clay platelets) and the presence of surface phenomena, such as electrochemical double layers and surface conduction. Past research was commonly restricted to either an accurate treatment of the geometrical or surface effects with limiting approximations applied to treat the other component. We present an exact solution to Laplace's equation inside an ellipsoidal particle subject to boundary conditions specifying a surface conductivity and associated surface current; this result is an extension of the one obtained by O'Konski for a spherical particle. This solution can be readily generalized to spheroidal particles whose morphology can be adjusted to adequately represent components in aquifer materials ranging from elongated rods to spheres to flattened discs. This expression allows the investigation of the effects of surface phenomena on geometrically realistic aquifer constituents using inclusion based models. This new theoretical treatment accurately incorporates surface phenomena on geometrically representative aquifer constituents and is used to investigate the effects clay platelets can have on electrical property versus porosity relationships. The results are directly compared to previous data obtained on analogous systems without surface phenomena.