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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 135 (1935), S. 623-623 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] IN a letter in NATURE of February 2 (p. 265) Prof. J. Walton has remarked that infra-red photography of thin sections of coal should give interesting results. We would like to direct attention to some results of work on this subject. We should mention first that there are already some ...
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  • 3
    Publication Date: 2012-12-16
    Description: Field-scale lithologic applications of complex conductivity ( $${\sigma }^{*}$$ ) imaging have been hindered by the challenges of (1) acquiring reliable induced polarization (IP) measurements and (2) obtaining reliable $${\sigma }^{*}$$ images from the measurements. We performed a series of 2D time domain resistivity/IP surveys at the Hanford 300 Area (Richland, Washington) where the challenge was to image the spatial distribution of two lithologic units that control the exchange between groundwater and surface water of the Columbia River. Exploiting the equivalence between time domain and frequency domain measurements of polarization, a 2D $${\sigma }^{*}$$ inversion (real conductivity $${\sigma }^{\prime }$$ , imaginary conductivity $${\sigma }^{\prime \prime }$$ , and phase angle $$\phi $$ ) was used to image the spatial distribution of $${\sigma }^{*}$$ across the site. Synthetic studies were carried out to investigate the effects of noise on the resolution of $${\sigma }^{*}$$ images and to add confidence on the interpretation of possible paleochannels observed in the field data sets. The synthetic studies show that, with increasing representative noise levels, degradation of the resolution of lithologic structures in the parameters most controlled by the IP measurements ( $$\phi $$ and $${\sigma }^{\prime \prime }$$ ) is significantly greater than degradation of resolution of $${\sigma }^{\prime }$$ images. However, the acquisition of IP measurements, and the analysis of changes in $${\sigma }^{\prime }$$ and $${\sigma }^{\prime \prime }$$ constrains the lithological interpretation of the geoelectrical data set due to the strong dependency of $${\sigma }^{\prime \prime }$$ on lithological properties. A threshold based on $${\sigma }^{\prime \prime }$$ measurements from cores at the site was used to estimate the elevation of the contact between the two key units, which is consistent with boreholes at the site. Variation in the elevation of this contact provides evidence of a depression in the Hanford-Ringold contact connecting the aquifer and the Columbia River; this depression likely represents a paleochannel regulating flow and transport at the site.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 4
    Publication Date: 2012-12-16
    Description: Continuing advancements in subsurface electrical resistivity tomography (ERT) are increasing its capabilities for understanding shallow subsurface properties and processes. The inability of ERT imaging data to resolve unique subsurface structures and the corresponding need to include constraining information remains one of the greatest limitations, yet provides one of the greatest opportunities for further advancing the utility of the method. We propose a new method of incorporating constraining information into an ERT imaging algorithm in the form of discontinuous boundaries, known values, and spatial covariance information. We demonstrated the approach by imaging a uranium-contaminated wellfield at the Hanford Site in southeastern Washington State, USA. We incorporate into the algorithm known boundary information and spatial covariance structures derived from the highly resolved near-borehole regions of a regularized ERT inversion. The resulting inversion provides a solution which fits the ERT data (given the estimated noise level), honors the spatial covariance structure throughout the model, and is consistent with known bulk-conductivity discontinuities. The results are validated with core-scale measurements, indicating a significant improvement in accuracy over the standard regularized inversion and revealing important subsurface structure known to influence flow and transport at the site.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2013-07-10
    Description: Centrioles are evolutionary conserved organelles that give rise to cilia and flagella as well as centrosomes. Centrioles display a characteristic ninefold symmetry imposed by the spindle assembly abnormal protein 6 (SAS-6) family. SAS-6 from Chlamydomonas reinhardtii and Danio rerio was shown to form ninefold symmetric, ring-shaped oligomers in vitro that...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 6
    Publication Date: 2013-06-12
    Description: Bacterial toxins have evolved successful strategies for coopting host proteins to access the cytosol of host cells. Anthrax lethal factor (LF) enters the cytosol through pores in the endosomal membrane formed by anthrax protective antigen. Although in vitro models using planar lipid bilayers have shown that translocation can occur in...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 7
    Publication Date: 2013-06-04
    Description: There is a need to better characterize discrete fractures in contaminated hard rock aquifers to determine the fate of remediation injections away from boreholes and also to evaluate hydraulic fracturing performance. A synthetic cross-borehole electrical resistivity study was conducted assuming a discrete fracture model of an existing contaminated site with known fracture locations. Four boreholes and two discrete fracture zones, assumed to be the dominant electrical and hydraulically conductive pathways, were explicitly modeled within an unstructured tetrahedral mesh. We first evaluated different regularization constraints starting with an uninformed smoothness-constrained inversion, to which a priori information was incrementally added. We found major improvements when (1) smoothness regularization constraints were relaxed (or disconnected) along boreholes and fractures, (2) a homogeneous conductivity was assumed along boreholes, and (3) borehole conductivity constraints that could be determined from a specific conductance log were applied. We also evaluated the effect of including borehole packers on fracture zone model recovery. We found that the fracture zone conductivities with the inclusion of packers were comparable to similar trials excluding the use of packers regardless of electrical potential changes. The misplacement of fracture regularization disconnects (FRDs) can easily be misinterpreted as actual fracture locations. Conductivities within these misplaced disconnects were near the starting model value, and removing smoothing between boreholes and assumed fracture locations helped in identifying incorrectly located FRDs. We found that structural constraints used after careful evaluation of a priori information are critical to improve imaging of fracture electrical conductivities, locations, and orientations.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2012-10-25
    Description: We examined the dependence of imaginary conductivity ( $${\sigma }^{\prime \prime }$$ ) on pore fluid conductivity ( $${\sigma }_{w}$$ ) for an extensive database of 67 samples acquired from twelve independent studies. We compared fitting of functions describing the salinity dependence of $${\sigma }^{\prime \prime }$$ for two models of the electrical double layer (EDL) polarization, both of which predict asymptotic behavior of $${\sigma }^{\prime \prime }$$ at high $${\sigma }_{w}$$ . We define these models as the diffuse layer polarization (DLP) and Stern layer polarization (SLP) models based on the physical description of the salinity dependence of the surface polarization. We also examined the database for evidence of a high salinity decrease in $${\sigma }^{\prime \prime }$$ not predicted by either model. The dependence of $${\sigma }^{\prime \prime }$$ on $${\sigma }_{w}$$ prior to the polarization plateau predicted by both models approximates a simple empirical power law with an average exponent of 0.34. The salinity dependence predicted by the DLP model adequately describes most data sets. A fitting parameter representing the high salinity $${\sigma }^{\prime \prime }$$ asymptote is strongly correlated ( $${R}^{2}=0.822$$ ) with pore normalized specific surface ( $${S}_{\mathrm{por}}$$ ). The SLP model describes well the observations when a recently proposed additive polarization term representing the contribution of the protons is included. In this case, the SLP model provides an excellent fit to the data sets, including a low salinity asymptote (in log-log conductivity space) seen in some samples. Predicted values of the fitting parameters of the SLP model generally are consistent with the values expected based on the theory; the fitting parameter describing the high salinity asymptote of the SLP model is also strongly correlated ( $${R}^{2}=0.890$$ ) with $${S}_{\mathrm{por}}$$ . The SLP and DLP models neglect a high salinity decrease in the polarization that is observed in numerous data sets from independent studies. New data acquired on a sandstone sample demonstrate that this high salinity decrease is likely not attributable to the limited phase accuracy of earlier measurements.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2013-04-19
    Description: Alluvial riverbed elevation responds to the balance between sediment supply and transport capacity, which is largely dependent on climate and its translation into fluvial discharge. We examine these relations using U.S. Geological Survey streamflow and channel measurements in conjunction with basin characteristics for 915 reference ("least disturbed") measurement stations across the conterminous United States for the period A.D. 1950–2011. We find that (1) 68% of stations have bed elevation change ( BEC ) trends (p 〈 0.05) with median values of +0.5 cm/yr for aggradation and –0.6 cm/yr for degradation, with no obvious relation to drainage basin structure, physiography, or lithology; (2) BEC correlates with drainage basin area; (3) high-flow variability ( Q 90 / Q 50 , where Q is discharge and 90 and 50 are annual flow percentiles) translates directly into the magnitude, though not the direction, of BEC , after accounting for the scale dependence; (4) Q 90 / Q 50 declines systematically from dry to wet climates, producing disproportionately high rates of BEC in drier regions; and (5) marked increases in precipitation and streamflow occurred disproportionately at dry sites, while streamflow declined disproportionately at wet sites. Climatic shifts in streamflow have the potential to increase/decrease sediment flux and thus affect riverbed elevation by altering flood frequency. These unforeseen responses of bed elevation to climate and climate change have important implications for sediment budgets, longitudinal profiles, ecology, and river management.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 10
    Publication Date: 2013-06-29
    Description: There is an increasing need to characterize fractured rock systems and to monitor the movement of fluids in these systems. Fractured rock aquifers are increasingly exploited for water resources, and are subject to contamination from industrial activities at the Earth's surface. Deep rock repositories of hazardous waste must be carefully characterized in terms of fracture transport characteristics. More recently, there has been a surge in technologies designed to increase permeability of shale reservoirs by creating fractures to promote fluid removal. However, fractured rock systems present unique challenges for characterization and monitoring technologies. Fracturing typically generates highly heterogeneous and anisotropic systems, making the evaluation of the distribution of physical properties from sparse subsurface measurements particularly problematic. Geophysical imaging technologies are increasingly applied in an effort to overcome the limitations of sparsely located direct observations of subsurface properties. However, the application of geophysical technologies to fractured rock systems presents challenges for imaging as appropriate regularization model constraints for complex, heterogeneous systems are hard to define without additional subsurface information.
    Print ISSN: 1070-485X
    Electronic ISSN: 1938-3789
    Topics: Geosciences
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