Steven A. Grant

Complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil

We determined the complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil. We used one clay-size fraction and three silt-size fractions, measured permittivity with low error from 25 MHz to 2–4 GHz with time-domain spectroscopy, and estimated mineral weight percentages using X-ray diffraction (XRD). The volumetric water contents were near 30%, and the temperature was 25°C . For the whole soil, standard fractionation procedures yielded 2.4% clay-size particles by weight, but XRD showed that the phyllosilicate clay minerals kaolinite, illite, and smectite made up 17% and were significantly present in all fractions. Above approximately 500MHz , all real parts were similar. Below approximately 500 MHz , the real and imaginary permittivities increased with decreasing grain size as frequency decreased, and the imaginary parts became dominated by direct-current conduction. Similarly, below approximately 500 MHz , the measured permittivity of montmorillonite, a common smectite, dominate...

Effect of Temperature on Capillary Pressure

The effect of temperature on capillary pressure is one of several fascinating problems unearthed by J.R. Philip during his long career. In his classic paper written with Daniel de Vries, he assumed reasonably, but incorrectly, that the relative change in capillary pressure with temperature was equal to that of the surface tension of water. In fact the change for capillary pressure is roughly four times as large. Four mechanisms may be proposed to explain this discrepancy: expansion of water, expansion of entrapped air, solute effects on the surface tension of water, and temperature-sensitive contact angles. None of these explanations describes all of the pertinent data. A definitive explanation appears to be as elusive today as it has been at any time.

Thermodynamic Properties of NaCl Solutions at Subzero Temperatures

Heat capacities at infinite dilution of NaCl (aq) for the temperature range 0 to −25°C and apparent molar volumes at infinite dilution for 0 to −15°C have been estimated from a synthesis of experimental data collected at subzero temperatures. The parameters of the Helgeson–Kirkham–Flowers (HKF) equation for Na+ (aq) have been obtained, from which the Gibbs energies of Na+ and Cl− have been calculated. The estimated values of Pitzer-equation parameters for thermal and activity-coefficient properties have been adjusted for subzero temperatures. The experimental phase diagram for the NaCl–H2O system could be reproduced with these data, demonstrating the low-temperature applicability of the HKF model to extrapolate thermodynamic properties of aqueous-solution species at infinite dilution.

Calculation of densities of aqueous electrolyte solutions at subzero temperatures

We developed a FORTRAN program based on the Pitzer equations to calculate densities of electrolyte solutions at subzero temperatures. Data from the published literature collected at -28.9, -17.8, -12.2, -6.7, 0, and 25°C were used to calculate the Pitzer-equation parameters and to evaluate model performance. Three approaches to estimating the molar volume of the solute at infinite dilution were evaluated: (1) extrapolation of apparent molar volumes to zero square-root ionic strength; (2) calculation with the Tanger and Helgeson model; and (3) global fit of the data in which the molar volume of the solute at infinite dilution was estimated along with the Pitzer-equation parameters. The last approach gave parameter estimates that reproduced the experimental data most accurately. The parameterized model predicted accurately densities of single-electrolyte and multielectrolyte solutions at -28.9, -17.8, -12.2, -6.7, 0, and 25°C. Available experimental data are generally quite poor. Accordingly, Pitzer-equation parameters estimated for subzero temperatures should be viewed as conditional until improved measurements of single-electrolyte solution densities at subzero temperatures are made.

Homemade explosives in the subsurface as intermediate electrical conductivity materials: a new physical principle for their detection

Detection of homemade explosive (HME) containing ammonium nitrate (AN) in the subsurface is of great interest to the US military and its coalition partners. Due to the hygroscopy of AN, this HME is expected to be an intermediate electrical conductivity material (IECM), defined here as one having electrical conductivity greater than soils, which have conductivities 0.1 to 1000 mS•m−1 but less than metals, which have electrical conductivities on the order of 10 MS•m−1. Our preliminary experimental and numerical modeling have established that AN-containing HME in the subsurface can, in all likelihood, be detected by electromagnetic exploration geophysics techniques, specifically by ground penetrating radar (GPR) and by electromagnetic induction (EMI). The electromagnetic induction signatures of HME for these techniques are distinctive. For example, in the case of EMI, the maximum quadrature response frequencies for IECM targets have been found to be greater than 100 kHz.

Maxwell–Wagner Relaxation in Two Desert Soils at Medium and High Water Contents: Interpretation From Modeling of Time Domain Reflectometry Data

Maxwell-Wagner relaxations (MWRs) are generally between 1 and 20 MHz in fine grain soils at volumetric water contents less than 10%. Isolated water inclusions appear to cause MWR. We used time domain reflectometry to measure complex permittivity from 6 kHz to 6 GHz of two, quartz-rich desert soils to test if MWR occur at a limited number of higher water contents because inclusion concentration should then decrease. One soil had lesser calcite; the other lesser gypsum. Water contents ranged from 6.2% to 33.8%. Unwanted electrode polarization (EP) dominates the real part of permittivity below 1 MHz. We interpreted MWR from complex refractive index models (CRIMs) that summed expressions for EP, Debye-type MWR, free water relaxation (FWR), and air volume. Broadband MWRs centered from 1 to 36 MHz exist up to 17% volumetric water contents and up to 196 MHz in the quartz-gypsum soil at saturation, with small to significant Cole-Cole factors that suggest a range of relaxation processes, and attenuation rates that increase to nearly 100 dBm-1 as frequency increased across the 100-1000 MHz GPR bandwidth. The Cole-Cole factor generally decreased with increasing relaxation frequency, while inclusion conductivity, derived from relaxation frequency, generally increased with increasing bulk conductivity and water content. These results suggest inclusion concentration and a spectrum of their conductivity values decreased, while ionic mobility increased. At saturation, the quartz-calcite soil provided an unexpected low relaxation frequency of 1.28 MHz, possibly caused by inclusions retained within calcite growths upon quartz grains, and within minor amount of smectite.

Broadband TDR permittivity spectra of lossy soils at medium to high water contents: Separation of electrode polarization from Maxwell-Wagner relaxation by modeling

We discuss complex permittivity spectra of two lossy soils measured from 6 kHz to 6 GHz using time domain reflectometry, in which Maxwell-Wagner relaxation (MWR) is present but also unwanted electrode polarization EP, mainly below 1 MHz, is strong. The soils are mostly quartz, with one having lesser calcite and the other lesser gypsum. Volumetric water contents ranged from 8.5-30.9%. We use a simple model that adds an EP diffusion term to Debye-type terms for the MWR and free water relaxation centered near 19 GHz, and which allows us to separate the EP from the MWR. All samples show MWRs centered from 1-196 MHz, regardless of water content, and with small to significant Cole-Cole factors. The increasing water content diminishes the effect of MWR, likely by decreasing the conductive and dielectric contrasts between isolated inclusions and the soil matrix, but still can strongly contribute to attenuation rate across the 100-1000 MHz GPR bandwidth.

To “Complex permittivity and clay mineralogy of grain-size fractions in a wet silt soil,” Arcone et al., GEOPHYSICS, 73, no. 3, J1–J13

The scanning electron microscopy (SEM) images of Figure 1 were presented inadvertently in negative form. Consequently, the detail of small clay particles, rough surfaces, edges, and luster coating on a few particles could not be discerned. We apologize for any inconvenience caused by this error.