DSpace Community: Earth and Environmental Science

Remote Sensing of Fire Effects on Tamarisk Evapotranspiration and Regeneration

Thu, 04 Jun 2009 15:56:52 GMT

Title: Remote Sensing of Fire Effects on Tamarisk Evapotranspiration and Regeneration

Authors: Alkov, Nicole

Characteristics of faults in nonwelded ignimbrites from the Pajarito Plateau and implications for fluid flow

Wed, 03 Jun 2009 17:17:20 GMT

Title: Characteristics of faults in nonwelded ignimbrites from the Pajarito Plateau and implications for fluid flow

Authors: Wilson, Jennifer E.

Abstract: Faults can impact groundwater flow as well as vadose-zone processes of recharge and transport. The impact of a given fault on fluid flow and transport depends in part on geologic products of fault-zone processes. For example, the presence of fractures versus deformation bands (narrow cataclastic shear zones) and degree and nature of diagenetic alteration directly influence the hydrologic properties of a given fault. Both fractures and deformation bands occur in ignimbrite sequences at Los Alamos, New Mexico, and Busted Butte, Nevada. The primary controls on mode of failure in these ash-dominated ignimbrites are grain-contact area and strength, which are directly related to degree of welding and crystallization and inversely proportional to porosity. Low-porosity welded units deform by transgranular fracture; high-porosity, glassy, non-welded units deform by cataclasis within deformation bands. Moderately high-porosity, non-welded units that have undergone devitrification and/or vapor-phase crystallization form either deformation bands or fractures, depending on local variations in the degree and nature of crystallization (qualitatively indexed by relative amounts of glass, cristobalite, and tridymite). Grain- and pore-size reduction in deformation bands commonly produces indurated, tabular zones of clay-sized fault material. Unlike fractures, which increase saturated permeability through welded ignimbrites by at least two orders of magnitude, preliminary data suggest that deformation-band permeability can be up to an order of magnitude less than adjacent protolith. This decrease in permeability within deformation bands may introduce permeability heterogeneity into non-welded ignimbrites, which were previously considered to accommodate fluid flow by slow, matrix diffusion. If sufficiently continuous and numerous, deformation bands may cause permeability anisotropy at the scale of perched aquifers. This could affect water recovery rates in shallow wells, where short production time scales are coupled with sub-vertical zones of small reductions in permeability. Although this reduction may have minimal impact on saturated flow, preferential water retention and geochemical modification within these deformation bands in unsaturated, near-surface ignimbrite deposits suggest that they do affect fluid flow and solute transport in the vadose zone. More than two-thirds of the deformation bands identified in this study are locally rich in smectite and/or cemented by calcite. Stable isotope analyses indicate that these diagenetic minerals result from low-temperature meteoric fluid-fault interaction. The micro-structural character, REE signatures, and chemical compositions of smectite in the deformation bands suggest that it is added to fault zones by a combination of localized in situ alteration of fault gouge and translocation from the surface. In situ alteration of ignimbrite to form smectite indicates preferential water retention in the presence of unstable mafic minerals and volcanic glass. Colloidal transport of smectite into the deformation bands requires preferential fluid flow in these zones. Rod-shaped microcrystallites of calcite and increased spatial density of plant roots in deformation bands suggest pedogenic precipitation associated with repeated desiccation events and microbial activity. Since roots in this semiarid climate seek moisture, it is inferred that preferential root growth and associated calcite are indicators of episodic preferential wetting of deformation bands, which facilitates fluid flow in semiarid vadose zones. Presence of smectite and calcite increases the surface area of fault material available for contact with fluids. This is interpreted to facilitate the addition and subtraction of some trace elements and oxides in these fault zones (e.g., TiO2, Cr, Cu, and V). Collectively, these observations indicate that these faults have served as, and may still be, zones of preferential vadose-zone fluid flow. Therefore, numerous, vertically continuous deformation-band faults in non-welded ignimbrites in the vadose zone may serve as relatively fast pathways for fluids to reach the water table from the surface. Preferential fluid flow and transport lead to alteration and mineralization, further modifying fault-zone permeability (by closing pore throats) and the mechanical properties of the fault zone (through the development of relatively weak, clay-rich gouge or relatively strong calcite-cemented zones).

Theory and analysis of thermally simulated discharge currents in pure ice

Wed, 03 Jun 2009 16:57:47 GMT

Title: Theory and analysis of thermally simulated discharge currents in pure ice

Authors: Will, Robert A.

Abstract: Theory of Thermally Stimulated Discharge (TSD) is presented. TSD currents from first order dipolar relaxation mechanism and space charge drift are discussed. Methods of experimentation and analytical determination of trapping parameters E, T0, P0, and dielectric constant E s are outlined. Results from pure ice show three to four peaks in the temperature range from 100* K to 200* K. Generalized eigenvalue-eigenvector fitting of these curves yields activation energies 7.24 kcal/mole, 8.23 kcal/mole, and 9.38 kcal/mole for peaks at 128* K, 135* K and 145* K respectively. Anomalous results obtained from a single sample show only two peaks at 125* K and 145* K with activation energies 7.03 kcal/mole and 10.4 kcal/mole respectively. Interpretation of discrepancies indicate strong shifting of the 135* K peak in anomalous results. Possible mechanisms are discussed.

Bulk and surface dielectric dispersion of water ice

Wed, 03 Jun 2009 16:41:49 GMT

Title: Bulk and surface dielectric dispersion of water ice

Authors: Svec, Robert K.

Abstract: The dielectric response of ice is of interest to researchers in the fields of atmospheric physics (thunderstorm electrification) and polar studies (dielectric ice core logging). An automated measurement system, based on the lock-in amplifier, was developed in order to expedite the dielectric measurement process. A Frequency domain interpretation procedure was developed for the lock-in data which avoids some of the model assumptions common to prior methods of interpreting dielectric data. Lock-in measurements were made on pure ice and ice doped with ammonium chloride or hydrochloric acid over a wide range of concentration. The frequency range of the measurement spanned 1 Hz to 100 kHz at 31 frequencies about equally spaced in log-frequency space. Each ice sample was measured at temperatures ranging form -1*C to -80*C. Dielectric relaxation times in the ice were found to be most depressed in the samples with the highest impurity content. The dc conductivity was found to be nearly an order of magnitude greater in chloride doped samples than in ammonium chloride doped samples of similar concentration. Dielectric conductivities were most elevated in those samples with the highest impurity content. The polarization strength was generally found to be most depressed in samples with the highest impurity content. The automated lock-in measurement system and the associated frequency domain interpretation method were found to give reliable estimates of several ice dielectric parameters. Of these parameters, the relaxation time and dielectric conductivity were most diagnostic of impurity content. The lower limit of detectability for the impurities studied lies in the range of 10-7 to 10-8 N.