David E. Stillman

Low-Frequency Electrical Properties of Ice-Silicate Mixtures

The low-frequency electrical properties of mixtures of silicates and saline H(2)O were measured over broad ranges of temperature and frequency to assess the subfreezing interactions between these materials synoptically, particularly the effects of adsorbed, unfrozen water. Adsorbed water content was determined using nuclear magnetic resonance. Materials were chosen to control effects of grain size and mineralogical complexity, and the initial salt content was also specified. The temperature-dependent DC conductivity of a sand-salt-H(2)O mixture was found to be described well by Archies law, with either brine or salt hydrate (above and below the eutectic, respectively) as the conductive and partially saturating phase. For materials with pore sizes less than a few micrometers, the brine/hydrate channels become disconnected, and the DC conductivity is controlled by the surrounding ice. High DC conductivity in a montmorillonite-H(2)O mixture is attributed to proton mobility in interlayer adsorbed water. The ice content of the sand mixture was recovered from the static dielectric permittivity using a power-law mixing model. Ice relaxation frequencies were higher than those observed in defect-saturated saline ice, indicating that additional defects are able to form in proximity to silicate surfaces. Five dielectric relaxations related to H(2)O were identified: two orientation polarizations (ice and adsorbed water), two Maxwell-Wagner interfacial polarizations (because of the conductivity differences between hydrate and silicate and adsorbed water and ice, respectively), and a low-frequency dispersion, probably caused by charge hopping. Thicknesses of a few H(2)O monolayers and the preference of hydronium for surface sites, making adsorbed water slightly acidic, favor protons as the mobile charges responsible for these adsorbed-water interfacial polarizations.

Low-frequency electrical properties of polycrystalline saline ice and salt hydrates.

We measured the 1 mHz-1 MHz electrical properties of ice-hydrate binary systems formed from solutions of NaCl, CaCl(2), and MgSO(4), with supplementary measurements of HCl. Below the eutectic temperature, electrical parameters are well described by mixing models in which hydrate is always the connected phase. Above the eutectic temperature, a salt concentration threshold of approximately 3 mM in the initial solution is required for the unfrozen brine fraction to form interconnected, electrically conductive networks. The dielectric relaxation frequency for saline ice increases with increasing impurity content until Cl(-) reaches saturation. Because there is insufficient H(3)O(+) for charge balance, salt cations must be accommodated interstitially in the ice. Dielectric relaxations near the ice signature were identified for CaCl(2).6H(2)O and MgSO(4).11H(2)O but not for NaCl.2H(2)O. Ionic and L-defect concentrations in salt hydrates up to approximately 10(-4) and 10(-3) per H(2)O molecule, respectively, follow from the electrical properties, Jaccard theory, and the assumption that protonic-defect mobilities are similar to ice. These high defect concentrations-up to a few orders of magnitude greater than saturation values in ice-indicate that intrinsic disruption of hydrogen bonding in salt hydrates is common.

Radar attenuation and temperature within the Greenland Ice Sheet

©2015. American Geophysical Union. All Rights Reserved. The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheets present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

Frequency and temperature dependence in electromagnetic properties of Martian analog minerals

[1] Ground-penetrating radar (GPR) has the potential to image the Martian subsurface to give geological context to drilling targets, investigate stratigraphy, and locate subsurface water. GPR depth of penetration depends strongly on the electromagnetic (EM) properties (complex dielectric permittivity, complex magnetic permeability, and DC resistivity) of the subsurface. These EM properties in turn depend on the mineralogical composition of the subsurface and are sensitive to temperature. In this study, the EM properties of Martian analog samples were measured versus frequency (1 MHz-1 GHz) and at Martian temperatures (180–300 K). Results from the study found the following: gray hematite has a large temperature-dependent dielectric relaxation, magnetite has a temperature-independent magnetic relaxation, and JSC Mars-1 has a broad temperature-dependent dielectric relaxation most likely caused by absorbed water. Two orbital radars, MARSIS and SHARAD, are currently investigating the subsurface of Mars. On the basis of the results of our measurements, the attenuation rate of gray hematite is 0.03 and 0.9 dB/m, magnetite is 0.04 and 1.1 dB/m, and JSC Mars-1 is 0.015 and 0.09 dB/m at MARSIS and SHARAD frequencies, respectively, and at the average Martian temperature of 213 K. With respect to using GPR for subsurface investigation on Mars, absorbed water will be a larger attenuator of radar energy as high concentrations of magnetite and gray hematite are not found globally on Mars. Citation: Stillman, D., and G. Olhoeft (2008), Frequency and temperature dependence in electromagnetic properties of Martian analog minerals, J. Geophys. Res., 113, E09005, doi:10.1029/2007JE002977.

Electrical response of ammonium-rich water ice

Abstract The electrical properties of water ice impact the study of diverse frozen environments, in particular the radar sounding of ice masses. The high-frequency response of meteoric polar ice depends partly on the bulk concentration of ammonium (NH4 +), but the nature of this response has been unclear. Here we use broadband dielectric spectroscopy to investigate the electrical response of laboratory-frozen solutions. By analyzing the relaxation frequency of these samples and its temperature dependence, we show that the mobility of Bjerrum D-defects formed in the ice lattice by ammonium is 1.4 ±0.8 x 10–9m2 V–1 s–1 at -20°C, or about an order of magnitude smaller than that of Bjerrum L-defects formed by chloride. However, co-substitution of both ions increases the ice-lattice solubility of chloride by a factor of ∼7, causing an enhanced conductivity response due to greater concentrations of Bjerrum L-defects. Thus, despite its low mobility, ammonium can also affect the high-frequency electrical response of polar ice, but its covariance with chloride must be considered.

Hydrogeologic heterogeneity of faulted and fractured Glass Mountain bedded tuffaceous sediments and ash-fall deposits: The Crucifix site near Bishop, California

Lithologic, macrostructural, microstructural, geophysical, and in situ gas permeability data from a natural exposure of highly porous, faulted and fractured tuffaceous sediments and interbedded ash-fall deposits near Bishop, California, are presented and analyzed in relation to published geologic information. This natural analog study was motivated by the need to evaluate potential length scales over which lateral flow diversion might occur above and within the nonwelded Paintbrush Tuff at Yucca Mountain, Nevada. Lateral diversion of flow in the overlying Paintbrush Tuff was previously proposed by others as a natural barrier that might protect a proposed high-level radioactive waste and spent nuclear fuel repository from percolating water. Because the length scale for capillary barrier breakthrough and leakage is decreased in the presence of subvertical structural heterogeneities, we characterized a horst-bounding fault, small-displacement normal faults within a footwall deformation zone, and secondary heterogeneities within two beds dissected by the faults. Critical deformation-related features that may influence fluid flow within bedded tuffaceous sediments include (1) permeability anisotropy imposed by steeply dipping faults and stratigraphic layering; (2) fault zone widths and styles, which are dependent on bed thickness and ash, glass, and clay content; and (3) fracture intensities and overprinting mechanisms (associated with fault deformation and vertical and nonvertical fracture orientations), which strongly influence the hydrogeologic heterogeneity of units they dissect. Microstructural analysis reveals structurally induced porosity variations at the micrometer to millimeter scale, gas permeability data show the influence of deformation on permeability at the centimeter to tens of centimeters scale, and resistivity and ground-penetrating radar data show lateral variations on the meter to tens of meters scale in horizontally bedded layers. All together, these observations and data show heterogeneity over seven orders of magnitude of length scale. Structurally enhanced porosity and permeability heterogeneities will tend to limit the length scale of lateral flow diversion, redirect flow downward, and enhance vertical fluid movement within the vadose zone.

Hot and cold lava tube characterization with ground penetrating radar

Lava tubes are of interest because they dramatically alter the hazard assessment of volcanos in several different ways and because they have been and are potential places to live. Ground penetrating radar has been used to locate and characterize lava tubes since 1978 in Hawaii. GPR surveys have been done over hot lava tubes where the dielectric properties modeled from the radar data give an indication of temperature and the imaging provides information about geometry. GPR data have also been acquired over, inside, around, and through cold lava tubes, and between lava tubes and the surface. Such data yield information about location, size and shape of lava tubes as well as number, distribution and occurrence. Over hot tubes containing flowing lava, commercial radar systems require modification to survive the heat. Case histories from Hawaii are illustrated with data acquisition, display, processing, and modeling.

Unraveling the Mysteries of Recurring Slope Lineae

Abstract Recurring slope lineae (RSL) are narrow dark features that incrementally lengthen down dust-poor steep slopes when temperatures are warm. They subsequently fade during cooler seasons and recur annually. Here, we discuss possible formation mechanisms based on observations of 748 candidate and confirmed RSL sites. RSL occur on well-preserved craters, canyon walls, light-toned layer deposits, fractures, central peaks, and pits from 42°N to 53°S. We have constrained the seasonality of five RSL regions as a function of slope orientation showing that RSL are always actively lengthening somewhere on the surface of Mars. Dry granular flow, wet-triggered debris flow, and wet-dominated flow formation mechanisms have been proposed, but no mechanism can adequately describe all the RSL observations. More advanced modeling combined with data from future remote sensing or in situ instrumentation is needed to fully discriminate between these formation mechanisms. Furthermore, we acknowledge that RSL may be formed via multiple mechanisms.