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  • 1
    Publication Date: 2014-12-09
    Description: We examined the sensitivity of the electrochemical spectral induced polarization (SIP) model developed by Wong to the oxidation extent of pyrite and pyrrhotite minerals disseminated in silica sand. The sensitivity of this model to the oxidation of sulfide minerals was mainly related to the model parameters defining the ratio of the active to the inactive passive ions $$({c}_{2}/{c}_{o})$$ dissolved in the pore water, and the variation of the current reaction parameters $$\alpha $$ and $$\beta $$ . The increase in these parameters as well as in the associated exchange current densities, $${i}_{o}(\alpha )$$ and $${i}_{o}(\beta )$$ was consistent with an increase in the activation of the charge transfer at the metal-electrolyte interface, resulting in the decrease in polarization of such an interface, which was reflected by a decrease in the SIP phase response as previously argued by Wong. Under this premise, the model described fairly well measurements below 500 Hz from a laboratory experiment, being consistent with the depletion of the SIP phase response associated with the oxidation degree promoted on the disseminate sulfides analyzed here. This suggested that electrochemical modeling of SIP measurements can provide information to assess the oxidation state of sulfides and also to infer the formation of passivating layers coating the metal minerals during oxidation-dissolution processes. Our results suggested a possible alternative for the monitoring of mine waste deposits producing acid mine drainage and the stability of sequestered harmful metals during remedial treatments by means of the SIP method.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2015-01-21
    Description: Conceptual models for the geophysical responses associated with hydrocarbon degradation suggest that the long-term evolution of an oil plume will result in a more conductive anomaly than the initial contamination. In response to the Deepwater Horizon (DH) oil spill into the Gulf of Mexico in 2010, an autonomous resistivity monitoring system was deployed on Grand Terre, Louisiana, in an attempt to monitor natural degradation processes in hydrocarbon-impacted beach sediments of this island. A 48-electrode surface array with a 0.5-m spacing was installed to obtain twice-daily images of the resistivity structure of the shallow subsurface impacted by oil. Over the course of approximately 18 months, we observed a progressive decrease in the resistivity of the DH spill-impacted region. Detailed analysis of pixel/point resistivity variation within the imaged area showed that long-term decreases in resistivity were largely associated with the DH-impacted sediments. A microbial diversity survey revealed the presence of hydrocarbon-degrading organisms throughout the test site. However, hydrocarbon degradation activity was much higher in the DH-impacted locations compared to nonimpacted locations, suggesting the presence of active hydrocarbon degraders, supporting biodegradation processes. The results of this long-term monitoring experiment suggested that resistivity might be used to noninvasively monitor the long-term degradation of crude oil spills.
    Print ISSN: 0016-8033
    Electronic ISSN: 1942-2156
    Topics: Geosciences , Physics
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  • 3
    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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  • 4
    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.
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    Electronic ISSN: 1942-2156
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  • 5
    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.
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  • 6
    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.
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  • 7
    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.
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  • 8
    Publication Date: 2016-10-08
    Description: Accurate estimation of the hydrological properties of near-surface aquifers is important because these properties strongly influence groundwater flow and solute transport. Laboratory-based investigations have indicated that induced polarization (IP) properties of porous media may be linked, through either semiempirical or fully mechanistic models, to hydrological properties including hydraulic conductivity. Therefore, there is a need for field assessments of the value of IP measurements in providing insights into the hydrological properties of aquifers. A cross-borehole IP survey was carried out at the Boise Hydrogeophysical Research Site (BHRS), an unconsolidated fluvial aquifer that has previously been well-studied with a variety of geophysical and hydrogeologic techniques. High-quality IP measurements were inverted, with careful consideration of the data error structure, to provide a 3D distribution of complex electrical conductivity values. The inverted distribution was further simplified using k -means cluster analysis to divide the inverted volume into discrete zones with horizontal layering. Identified layers based on complex electrical conductivity inversions are in broad agreement with stratigraphic units identified in previous studies at the site. Although mostly subtle variations in the phase angle are recovered through inversion of field data, greater contrasts in the IP data are evident at some unit boundaries. However, in coarse-grained aquifers, such as the BHRS, the discrimination of mildly contrasting lithologic units and associated changes in hydraulic conductivity of one or two orders of magnitude are unlikely to be achieved through field IP surveys. Despite the difficulty of differentiating subtle differences between all units, overall estimates of hydraulic conductivity purely from our field IP data are typically within an order of magnitude of independently measured values.
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  • 9
    Publication Date: 2003-09-01
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  • 10
    Publication Date: 2013-09-01
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